WO1992003833A2

WO1992003833A2 - Thin-film resistance temperature device

Info

Publication number: WO1992003833A2
Application number: PCT/US1991/005811
Authority: WO
Inventors: Thomas M. Kiec; Dennis W. Livengood; Arthur P. Haag
Original assignee: Advanced Temperature Devices, Inc.
Priority date: 1990-08-15
Filing date: 1991-08-15
Publication date: 1992-03-05
Also published as: WO1992003833A3; AU8428991A

Abstract

A temperature integrating sensor for an air duct in a heating/ventilating/air conditioning (HVAC) system, or other conduit for fluid flow, comprises a flexible resistance temperature device (RTD) that is fabricated by depositing a thin film of chrome and a second thin film of gold on a thin, flexible, thermoplastic substrate.

Description

THIN - FILM RESISTANCE TEMPERATURE DEVICE

A. Resistance Temperature Detectors

The present invention relates to thin film resistance temperature devices (RTDs) , and more particularly to thin film RTDs comprising flexible composite. In one aspect, the invention relates particularly to the use of such RTDs in an air flow temperature integrating sensor for use in applications involving dynamic air movement.

As is well known in the art, a resistance temperature device (RTD) is a device whose resistance varies according to its temperature. By measuring the resistance of a RTD at various temperatures, a Resistance vs. Temperature curve (RT curve) may be obtained for that RTD. Given the resistance of an RTD and its RT curve, the temperature of that device can be computed.

One important value that may be obtained from an RT curve, given a base resistance at 0°C, is a value known as alpha (α) . Generally, alpha (sometimes referred to as the temperature coefficient of resistance (TCR) ) represents the percent unit change in resistance per unit change in temper¬ ature (Ohms/Ohm/°C) . Graphically, the TCR at a given point represents the slope of the RT curve at that point. While any device whose resistance varies with temperature may function as an RTD, several desirable features can improve an RTD's performance.

First, it is generally desired that the RTD's RT curve be substantially linear over the temperature range to be measured. As discussed above, the TCR corresponds to the slope of the RT curve. Thus, in order to have a substantially linear curve, the TCR of the RTD should be substantially constant over the desired temperature range.

This linearity is desired because, among other thing, it simplifies the development of electronic circuitry to convert the resistance of the RTD into an electrical signal which varies as a function of temperature and it enables the use of linear curve fitting techniques. Further, a repeatable TCR is important for the precise measurement of temperature.

A second desirable feature of RTDs is that their RT curves be repeatable. This primarily means that the RT curve for a particular RTD be the same after every temperature cycle excluding burn-in cycles if any (or that the TCR of the RTD be substantially constant over time) . For example, most RTD's physically expand or contract as the temperature changes. This expansion and contraction often causes the RT curve (i.e., the TCR) of a particular device to change over time. Thus, the RT curve of one specific RTD subject to several temperature fluctuations may be substantially different than it originally was. The RTD industry has established certain standards es¬ tablishing the acceptable percent variability of the TCR for commercial RTDs; A change in the RT curve is undesirable because, among other things, the electrical circuitry used with the RTD is generally designed to operate with only one RT curve (i.e., one TCR) . Third, because of the wide range of applications in which RTDs are used, it is desirable that the RTD be durable and rugged. Temperature measurements must be made in many harsh environments, e.g., environments having harsh chemicals or strong vibrations, and it is desirable to have an RTD insensitive to such vibrations, chemicals and the like.

Fourth, it is generally desirable that the RTD respond rapidly to changes in temperature. In certain applications (e.g., fire detection, heat detection) it is important to detect the change in temperature almost immediately. Thus, a desired attribute of an RTD is rapid response to temperature changes.

Finally, it is generally desirable to have an RTD that can be easily and economically produced.

B. Prior RTD Technology Prior art devices have been able to combine only some of the above describe attributes in any one RTD. For example, the platinum wire RTD, developed by CH. Myers in 1932, has a substantially constant TCR, is able to accurately measure temperature changes, but is fragile and often uneconomical. This device is produced by winding a helical coil of platinum wire on a crossed mica web, and mounting this assembly inside a glass tube. The winding of the wire on the web, the uniformity of wire thickness and the purity of the metal, are necessary for a substan- tially constant and repeatable RT curve (i.e., the TCR is constant) .

This particular arrangement, of an unsupported wire structure in a glass tube, is not very durable and is susceptible to strong vibrations. As such, the platinum wire RTD is not practicable for many applications. Further, the high cost of platinum renders the platinum wire RTD relatively expensive. Recent attempts have been made to increase the durability of platinum RTD's. According to the most recent construction techniques, a platinum or metal glass slurry is deposited onto a small flat ceramic substrate and sealed. One example of such a technique is proposed in United States patent No. 4,139,833 to Kirsch. These ceramic based RTD's are more resistant to harsh environments than the platinum wire RTDs, but are often brittle and rigid. Further, if the coefficient of thermal expansion of the ceramic substrate differs from that of the deposited metal or slurry, which it usually does, substantial changes in the RT curve may occur. These changes are a result of the stress and strain placed on the deposited material when the substrate and the material attempt to expand at different rates. In an attempt to avoid such a change in the RT curve, the substrates used in many prior art devices are selected (e.g., ceramics) such that they substantially do not expand or contract at operating temperatures. In dealing with such prior art devices care must be taken to evaluate problems caused by expansion and contraction effects. Further, differential expansion, vibrations, and surface irregularities all affect the theoretical and actual TCRs of ceramic based RTDs.

In addition to ceramic based RTDs, prior art RTDs have used thin film devices as alternatives to the platinum wire RTD. U.S. Patent No. 4,375,056 to Baxter e al. proposes a thin film RTD comprising a serpentine meta pattern deposited upon an insulating substrate. The deposited platinum is described as being between the rang of 0.05 - 0.8 microns, with depositions below 0.05 micron described as being too thin to be practically handled. A with the ceramic based RTDs discussed above, devices of the type disclosed in Baxter are susceptible to differen- ces between the coefficient of thermal expansion of the deposition layer and the insulating substrate.

Each of these proposed prior art devices appears to utilize a thin (or thick) metal film deposited upon a rigid substrate. In order to prevent strain on the metal deposition, the substrate is generally selected such that its expansion is either minimal or matched to that of the metal layer at operating temperatures.

As discussed above, RTDs are based on the principle that certain materials exhibit a change in resistance for a change in temperature. In order to allow for accurate measurements of the resistance change, it is generally desirable for the RTD to exhibit a measurable degree of resistance. In the prior art, metal is frequently the material of choice for RTD's. Because most metals are fairly conductive, prior art devices generally follow one of two alternatives: First, prior art devices often utilize metals having a relatively high resistivity for a conductor (e.g., platinum). Metals having low to moderate resistivity (i.e., metals that are highly conductive) such as gold, silver, and aluminum are rarely used.

Second, some prior art devices use a metal with low to moderate resistivity (e.g., copper). To compensate for the low to moderate resistivity of copper, typical prior art devices merely use more copper. Thus, while copper RTDs often have nominal ice points similar to those of platinum, the length of the resistance element is sub¬ stantially greater.

This focus on metals with low conductivity results in a dilemma for the prior art. The devices can be made out of a metal having relatively low to moderate conductivity and comparatively high cost (e.g., platinum); or the devices can be made out of a metal having relatively moderate conductivity and low resistivity (e.g., copper), requiring the use of a longer resistive element and thus the often costly and complex manufacturing processes associated with such lengthy elements. The use of metals of comparatively high conductivity, i.e., of low to moderate resistivity (e.g., gold, silver, aluminum) in RTDs was commonly avoided in the prior art because of the size of the resistive elements that were believed to be necessary.

In summary, many prior art devices can suffer from at least four limitations: (1) The need for protective insulation and encapsulation; (2) vulnerability to vibra- tion, impact and thermal cycling; (3) moderate to slow response time due to the required protective sheathing and indirect sensor element contact; and (4) size constraints due to the resistive properties of the conductive metals commonly used.

c. improved Thin-Film Flexible RTD

The present invention realizes substantially all of the above-described attributes desirable for a RTD and avoids many prior art difficulties. Through the practice of this invention a polymer thin film RTD can be constructed to be flexible, vibration and impact resis¬ tant, to require reduced protective insulation or sheath¬ ing, and to have extremely fast response time. Further, the present invention provides an RTD that may be adapted to most all possible RTD applications within operating- temperature limitations.

The present invention provides an RTD that has a substantially linear RT curve, has a substantially cons- tant TCR, and is adaptable to operation in multiple envi¬ ronments. The invention further provides an RTD with a fast response time which may be easily and economically produced using low cost, highly conductive metals uncommo to RTDs, such as aluminum, silver, and gold.

The invention further provides an improved RTD for use in measuring the true average air duct temperatures i heating, ventilation and air conditioning systems.

A flexible, low cost, durable, chemical resistant, vibration resistant RTD is disclosed. This RTD includes an polymer-layer/metal-layer composite which exhibits a highly accurate, rapid, and repeatable positive response to temperature changes.

Generally the invention involves a thin film RTD and its method of manufacture. A thin film of a conductive material such as metal, especially a gold-chrome composite, is bonded to a substrate such as a sheet comprising a thermoplastic polymer, and a protective barrier such as second sheet of polymer is adhered to the thin film. The composite "sandwich" may then be trimmed or etched into a pattern having terminal ends.

Connectors are then attached to the terminal ends of the pattern. This thin film conductor and connector system is calibrated to yield a particular resistance at a known temperature. This calibrated device is a highly accurate, flexible RTD which has a TCR that is substantially constant over changes in time and temperature.

Not fully understood at this time, the process described herein results in a temperature coefficient of resistance believed to be unique to the polymer/metal composite that is both linear and repeatable over the operating range. In the disclosed RTD, the polymer sub- strate is not perfectly smooth and the thin film of con¬ ductive material is believed to be affected by the substrate surface characteristics. Although not fully understood, characteristics (i.e., current and resistance) defined in conjunction with an electrical potential across the RTD of the present invention show evidence of properties that are affected by the polymer surface in a manner analogous to the effect of grain in a field on the flow of wind across the field.

D. Improved HVAC Air Duct Temperature Sensor using RTD

One particular application for the RTD of the present invention is in heating ventilation and air conditioning (HVAC) . HVAC is a multi-billion dollar industry encomp- assing many segments of industrial, commercial and residential environment controls. Of primary importance in this regard in the utilization of controls to manage the expenditure of heating and cooling energy. With the introduction of microprocessor based control systems, there has been a growing need for sensors to provide fast, accurate temperature readings. Traditionally, bimetallic thermostats and low-end inexpensive electronics are used for this purpose. RTDs are seldom employed because of their price and difficulties in applying the technology.

Fast, accurate air duct temperature sensors can result in beneficial energy savings. For optimum performance it is desirable to measure the true average temperature throughout the duct. Traditional temperature sensors are unable adequately to measure the average duct temperature because of their slow response times. Many prior art RTDs can only provide spot sensing over a small area. Furthermore," because flowing air in the air ducts often creates strong vibrations the use of such prior art RTDs to measure air duct temperatures often results in poor long-term performance. In contrast to such prior art RTJs, the RTD of the present invention can be manufactured in continuous lengths of up to 60 feet. As such, the true average temperature of an air duct may be measured. Further, the device of the present invention is flexible and significantly less subject to damage resulting from vibration, and can be suspended in such a manner as to traverse the entire duct. The savings in energy resulting from the improved temperature readings can be enormous.

Figure 1 illustrates a resistive temperature device of the present invention;

Figures 2A-2B illustrates two methods for applying metal film to a thermoplastic polymer;

Figure 3 is a chart illustrating a general relationship between resistivity, percent transmission, and optical density for thin film metals in certain circumstances;

Figure 4 illustrates the application of a protective film to a metallized roll of thermoplastic;

Figures 5A - 5B illustrate alternate methods of applying the protective film to the metallized roll;

Figures 6A - 6B illustrate geometric resistive patterns;

Figure 7, illustrates the heating of a resistive pattern;

Figure 8A - 8H illustrate various connectors between a thin metal film and an electrical lead; Figures 9A - 9C illustrate one method for calibrating the RTD of the present invention;

Figures 10A - IOC illustrate novel devices for temperature measurement;

Figures 11A - 11C illustrate a novel device for measuring fluid level and fluid temperature;

Figures 12-14 illustrate various embodiments of the present invention;

Figures 15A-15D illustrate the application of the present invention to heating, ventilation and air conditioning systems.

Similar reference numerals refer to the same components, subsystems, or the like in all figures of the drawings.

E. Overview of RTD Design

Figure 1 illustrates a basic embodiment of a resistance temperature detector in accordance with the present invention. An RTD 10 is illustrated as comprising a composite or "sandwich" of three layers. A thin film of a conductive material such as a metal 12 is bonded between a substrate and a barrier such as thermoplastic films 16a and 16b. Electrical leads 14 are connected to the metal film 12 according to one of the methods discussed below.

The metal layer 12 may be vacuum metallized aluminum, although silver, gold, lead, copper, platinum, titanium, nickel, molybdenum, tungsten, rhodium, iridium, palladium, doped silicon, tin, zirconium, columbium, alloys including the foregoing, conductive pigments, and foils are believed acceptable. The thickness of the metal layer 12 depends in part on the overall size and characteristics desired for the RTD, but is generally within the range between a mono-atomic layer and about 3000 angstroms (A; lA - 1 X 10^"10 Meters) and frequently between about 25θA and about IOOOA.

Metal layer 12 may be attached to the thermoplastic sheet 16a via thermal vacuum deposition, although other methods of attachment are envisioned. Currently, sputtering, chemical vapor deposition and the like are believed to be acceptable alternatives for attaching metal layer 12 to thermoplastic sheet 16a. Thermoplastic sheet 16b may be attached to metal layer 12 through the use of known adhesives.

A substrate such as thermoplastic sheets 16a and 16b preferably comprise a thermoplastic polymer which is operable in the temperature range to be measured. For example, depending on the application, polyester has been found to be an acceptable polymer for temperatures between -70°C and 150^βC. Such thin sheets of polyester are t jmmercially available under the trade name MYLAR available from DuPont, or other commercially available equivalents. For temperatures over 200°C commercially available polymers such as KAPTON, UPILEX, TEFLON, or fluorocarbons or the like are believed to be acceptable, KAPTON and TEFLON are trademarks of the E.I. duPont deNemours company of Wilmington, Delaware; UPILEX is a trademark of ICI Ltd. of England.) At the present time polymers such as polyethylene, polypropylene, nylon, polycarbonate, poly(4- methyl-1-pentene) , polybutene-1, blended plastics and the like are believed to be acceptable substitutes for MYLAR and KAPTON. The polymers are believed to be acceptable substitutes for MYLAR for lower temperature applications, as well as some higher temperature applications. Generally, when selecting the materials of thermoplastic sheets 16a and 16b, it is important to consider the temperature range to be measured, and the polymer's ability to accept metallization. The ability o a thermoplastic polymer to accept metallization is believed to be related to the surface characteristics of the particular polymer. Type A MYLAR has been found to be an acceptable thermoplastic for most applications. (MYLAR is a trademark of the E. I. duPont deNemours company.)

The thickness of the thermoplastic sheets 16a and 16 may be determined by the response time and the handling characteristics desired for the RTD. Thicker sheets are envisioned when a slower response time is permissible or desired (i.e., the time required to detect a given absolute temperature rise) , or if increased environmental protection is desired.

Electrical leads 14 are connected to metal layer 12 and may be connected to known circuitry to measure the resistance between the leads. By measuring the resistanc between the leads 14 for several temperatures, a characteristic RT curve for the RTD 10 may be obtained. Once the RT curve is obtained, the temperature of the device may be determined by measuring the resistance across leads 14.

F. Selection of Materials for the RTD

The first step in making the RTD of the present invention is to determine the composition of the metal layer 12, and of the thermoplastic sheets 16a and 16b. A discussed above, aluminum has been found to be an acceptable metal for layer 12 although other possibilitie are envisioned. 1. Selection of the Thermoplastic Polymer. The selection of sheets 16a and 16b involves the consideratio of several factors. First, the thermoplastic film selected should be able to accept metallized films.

Second, the thermoplastic film selected should be able to operate at the temperatures which the device will be measuring; this includes temperature spikes (temp¬ erature "spikes" are short duration rises (or declines) in the temperature) . For example, if the RTD is to measure temperatures in an environment that averages approximately 150°C but is subject to temperature spikes of over 200°C, a film capable of withstanding these temperature spikes should be selected. If the highest possible temperature is not considered in selecting the sheets, the RTD may be destroyed if a temperature spike occurs. As discussed above, MYLAR type A has been found acceptable for most applications at or below 150°C.

Third, the particular thickness of the thermoplastic should be selected with the desired response time in mind. The thickness of the thermoplastic sheet is inversely related to the response time of the RTD. Generally, the thicker the thermoplastic sheet, the slower the response time. The needed thickness can be calculated given the desired response time and the thermal conductivity of a given thermoplastic material. This calculation can be accomplished through simple engineering techniques well known in the art. Thicknesses of between 0.5 and 10.0 mils have been found to be acceptable.

For many applications in the lower temperature range requiring comparatively rapid response times. Type A, 300- gauge MYLAR has been found acceptable. Certain polymers are known to emit toxic vapors at high temperatures. Consequently, appropriate precautions may need to be made when selecting the thermoplastic material 16.

2. Selection of the RTD Metal Film. As with the thermoplastic layers 16a and 16b, the selection of the metal material 12 involves many factors. First, as discussed above, it is generally desirable to have a RTD with a linear RT curve. As such, it is generally desir¬ able to select a metal having a substantially constant TCR over the expected operating temperatures. Second, for economy reasons, the cost of the material and its strategic availability may be considered. The coef- ficients of thermal expansion (TCE) for the metal and the thermoplastic material should also be considered. Gen¬ erally it is desirable to have the coefficients of thermal expansion of the metal and the thermoplastic sufficiently close to avoid problems from strain such as altered temperature coefficients of resistance. For example, a film of aluminum (having a TCE of about 26 microinches per inches per degree Celsius, 26 μin/in/°C) on a substrate of MYLAR (having a TCE of about 17 μin/in/°C) has been found to be acceptable. Other considerations should be: (1) the thickness of the conductive film and the current load, (2) possible Seebeck effects arising from use of dissimilar metals at an interconnect, (3) the temperature range at which the device will operate, and (4) the strain gage factor of the metal — it is believed advantageous to select metals that have a strain gauge factor close to zero.

Using Type A MYLAR, aluminum has been found to be an acceptable metal for most applications, although it is believed that any conductive metal meeting the above criteria will suffice. G. Attachment of the Metal Film to the Thermoplastic Layer

1. Vacuum Deposition. When the metal 12 and the thermoplastic material 16 have been selected, metal 12 ma be vacuum deposited on a first roll or individual sheet o thermoplastic material 16. Vacuum deposition procedures ar. well known in the art and will not be discussed herei in great detail. Figure 2 show a vacuum deposition chamber 20. A roll of the selected thermoplastic polymer 26 is positioned within the chamber. The selected metal 28 is then deposited onto the sheet 26.

The size of roll 26 depends on the application for which the RTD is being manufactured. For example, for small RTD's a roll width of 1 inch will suffice; for larger RTDs, sizes up to the standard 60-inch roll may be necessary. Generally any roll size may be used because the size of the roll is not believed to affect the performance of this invention except for variability related to the uniformity and consistency of the deposition.

The deposition thickness of the metal is an important feature of this invention. For aluminum, the metal 28 should be deposited on the sheet until its depth is in the desired range.

Although not fully understood at this time, the depth of the deposition plays an important role in allowing the RTD of the present invention to yield a repeatable and substantially linear RT curve.

As discussed above, the RTD of the present invention exhibits a substantially linear RTD curve that is not believed to be significantly affected or degraded by numerous temperature cycles over an expected operating range after an appropriate burn-in period. This is believed to be a result of a special interaction between the deposited metal and the resin surface of the thermoplastic polymer. This special interaction between the metal and the polymer is believed to be closely re¬ lated to the deposition thickness of the metal. For example, when aluminum deposition depths of greater than about 300θA are used, it is believed that the effects of the metal layer 12 dominate those of the thermoplastic sheets 16a and 16b. As such, any RTD produced in such a process would exhibit substantially the same qualities as a metal film on a substrate other than the polymer.

At aluminum deposition depths of lower than about 25θA, the effect of the metal on the RTD begin to diminish and the RTD begins to operate as would unmetallized sheets of thermoplastic material.

As disclosed in this application, for aluminum, the region approximately between 25θA - lOOoA allows the RTD to exhibit several unique characteristics, i.e., substantial linearity and repeatability. The mechanisms causing the linear RT curve and the high repeatability for this region are not yet fully understood but are believed to involve the interaction of the thin metal layer and the relatively rough surface of the thermoplastic sheet. Generally, it is desirable to deposit the metal on the substrate such that a resistance of 0.5 to 1.5 ohms/squar is obtained. At this ohms/square level temperature coefficients of resistance between .002530 and .003100 ohms/ohms/°C are observed.

The resistances of thin films on thermoplastic sheet has been found by the inventors to be slightly higher tha would be expected. This is believed to be result from th coarseness of the thermoplastic film, having small, micro scopic peaks and valleys, imparting a uniform reduction in the electron flow of the thin film deposited metal. This reduction on electron flow is believed to result from the interferences of the non-conducting resin of which the thermoplastic is made with the continuity of the metallic deposit. The effect of this interaction may be labeled the "cornfield effect" as the resistance provided by the roughness of the resin surface may be likened to the resistance to air flow that exists over a cornfield as opposed to a flat open area.

The following disclosure discusses the deposition of aluminum on Type A MYLAR. For this metal/thermoplastic combination a deposition thickness of 25θA - lOOoA has been found to exhibit this "cornfield effect" and render the improved RTD of this invention. It will be understood that those of ordinary skill having the benefit of this disclosure will be able to find other metal/thermoplastic combinations which are acceptable for use in the present invention. The depth of the metal deposition for these combinations may change depending on the surface characteristics of the selected thermoplastic. It is to be understood that these combinations are within the scope of the present invention.

2. Variables in the Deposition Process. In order to ensure that several RTD's mav be made having similar qualities, it is desirable to have some way of measuring the level of deposition. This may be done using any number of known methods. For example. Figure 3 sets forth a chart illustrating three variables that may be assigned to a sheet that has received a metal deposition (a metallized film) . The chart shows that for aluminum, the optical density, percent transmission and sheet resistance (ohms per square unit of area) may be used to describe any given metallized film. This chart is merely representative of several available in the prior art.

It is important to note that the relationship between resistivity, percent transmission, and optical density is not always as shown in Figure 3. As such, it is generally desirable to use only one variable when defining metallized sheets for the present invention. The sheet resistance (ohms/square) has been found acceptable, although the other variables may be used. Depositions to yield resistivity between about 0.5 ohms/square to about 1.5 ohms/square are believed to be satisfactory; with a sheet resistance of 1 ohm/square being preferred. Aluminum deposition thicknesses between approximately 250A - 300θA are also believed to be satisfactory. Techniques for depositing a metal on a thermoplastic film given a desired sheet resistance (ohms/square) are well known in the art and will not be discussed here.

3. Alternatives to vacuum Deposition. Vacuum deposition is not the only way that the metal can be attached to the thermoplastic material. Other techniques, such as metallization, lamination, pressure sensitization, thermal curing, printing, electrodeposition techniques, chemical vapor deposition, vacuum deposition, sputtering, glow discharge, radiation curing (e.g., ultraviolet curing) , printing techniques, plasma deposition, thermal evaporation, E-beam evaporation, and the like are believe to be viable. What is important when using alternate techniques is that, for aluminum, the layer of metal have a thickness generally within the above described limit, i.e., approximately 25θA - lOOoA. H. Gold/Chrome Embodiments of the RTD

Prior to the application of the metal to the thermoplastic material, a coating or coatings to reduce the effects of heat aging may be applied to the thermoplastic. Additionally, it has been found beneficial to deposit more than one type of metal 12 on the thermoplastic polymer.

Specifically, it has been found beneficial to deposit a combination of both gold and chrome. Figure 2B illustrates such a device. Referring to Figure 2B, a thin layer of chrome 12a (approximately lOoA to 15θA) is deposited on the polymer sheet 26 using known methods. Preferably, a glow discharge is used prior to the chrome deposition to both improve the adhesive qualities of the polymer sheet and remove contaminates.

A slightly thicker layer of gold 12b (approximately lOOoA) is deposited on the chrome. After depositing the gold, the gold/chrome device may be processed according to the methods discussed below in regard to aluminum devices. This embodiment has been found to have several advantages over the aluminum device (e.g., the gold/chrome device is better adapted for harsh environments) .

In the embodiment discussed above, the chrome layer serves primarily to promote adherence of the gold film. Because the sheet resistance of chrome in this thickness is substantially greater than that of thin film gold, the sheet resistance of the gold/chrome layer RTD is substantially the same as that of a gold layer RTD.

In the gold/chrome embodiment it has been found desirable to apply a silicon dioxide overcoat of at least 150θA to the gold/chrome layer. This overcoat layer prevents scratching of the gold/chrome surface. I. Application of a Protective Coating

After the metal layer is deposited on the first roll 26 of thermoplastic, a second roll 46 is applied as a protective cover. This process is illustrated in Figure 4. This top laminate is for the protection of the conductive surface and is not believed to enter into the electrical process.

As Figure 4 shows, a second roll 46 is provided. This second roll 46 may be made of the same thermoplastic material as the first roll 26. Further, the second roll 46 may have an adhesive precoated on one side. For operating ranges at or below 125°C, the V-95 adhesive from the Flexcon company (near Boston) has been found to be acceptable, although with sustained use at higher temperatures, potential problems may arise due to heat aging and possible interactions between the adhesive and the film. When the adhesive is precoated on the second roll 46, it may be attached to the first via pressure from rollers 40 and 42. This second roll of thermoplastic is useful to protect the metal layer from scratching, erosion and corrosion.

1. Attachment of Protective Thermoplastic Polymer. In alternate embodiments the second roll of thermoplastic does not cover all of the metallized surface of the first roll. These embodiments are illustrated in Figure 5A - 5C As Figure 5A shows, the second roll 46 may cover all but the edges of the metallized surface of the first roll 26. The exposed region 50 may be used to make connections to the metal layer to allow for the attachments of electrical leads 14 as illustrated in Figure 1.

Referring to Figure 5B an embodiment is illustrated where the second layer of thermoplastic 46 covers all but two strips of the metallized surface of the first roll 26. The two exposed surfaces are labeled as 52. As with the embodiment in Figure 5A, the exposed surfaces may be used to make connections to the metallized surface of roll 26.

While the geometry of the second, protective layer may be of any shape, it is generally beneficial to leave regions close to the center of the roll exposed for connections. The reason for this is that vacuum deposition, while creating an essentially uniform surface, tends to deposit a more uniform layer of metal at the center of the roll. Further, the thickness of the metal at the center is sometimes greater than that at the edges. This slight difference in thickness and uniformity becomes significant when deposition thicknesses in the region of this invention are used.

2. Other Protective Barriers. Alternate embodiments are envisioned where the second roll of ther¬ moplastic is not applied to the metallized first sheet roll. In these embodiments the metal may be left unprotected, or a protective coating or coatings, such as silicon monoxide or other standard barrier coatings, may be adhered to the surface. The well-known ADCOTE 554 from the Morton-Thiokol company has been found to be an ade- quate barrier coat. It has been found advantageous to thin the ADCOTE with methyl ethyl ketone (MEK) for application to exposed metal surfaces. Methods for coating such metal layers, e.g., painting, spraying, are known in the art and will not be discussed herein. As discussed above, it may be desirable to leave regions close to the center of the roll uncoated for the purposes of making electrical connections.

In some situations it may be beneficial to cure the applied barrier coating through a heat treatment. For ADCOTE, a two step heat treatment has been found to be adequate; e.g., heating the coated RTD to 35 °C for fifteen minutes, and then heating the RTD to 50 °C for an additional fifteen minutes.

J. Cutting and Adjustment of the RTD Resistive Patterns

The metal/thermoplastic composite "sandwich" is cut into a desired resistance pattern. First, a portion of the composite (e.g., the outer portions of the rolled thermoplastic sheet) may be cut away to increase the uni- formity of the deposition on the remaining portions.

Second, the remaining composite is rough-cut into desired shapes. Finally, each such shape is physically trimmed o otherwise treated to increase its resistance as desired t meet a specified standard.

In an alternate embodiment, photoprocessing (e.g., a photo-etch) is employed to obtained the desired resistor pattern before the second, protective roll is affixed to the metallized first roll.

l. Methods for Rough-Cutting the Resistive Patterns. Methods for cutting a thermoplastic film are generally known in the art. For example, for large RTDs (e.g., a 60" or 12" roll) die cutting or other blade cutting may be used; for smaller rolls (e.g., 1") laser cutting or water jet cutting may be used. Such technique are known and will not be discussed herein. Generally, any method of cutting that does not destroy the main body of the thermoplastic (e.g., by elevating the temperature of the sandwich to greater than that allowed for the thermoplastic film) may be used.

As discussed above, when cutting the resistive patterns, it is possible to have the terminal points of the pattern located in an uncovered region of the metallized film. However, it is generally desirable to environmentally isolate the conductive film and the terminal points.

It has been found that the variances in the TCRs of resistive elements cut from the same sheet of polymer are lower for devices where laser cutting is used. In particular, for MYLAR it is envisioned that substantial benefits may be derived by cutting the resistive pattern in the conductive layer using YAG lasers. When such lasers are used, only the deposited metal is affected and the thermoplastic polymer remains substantially intact.

Additionally, it has been found that the use of C0₂ lasers on KAPTON may result in an incomplete vaporization of the substrate material. This incomplete vaporization may result in excess material clinging to the RTD and can pose difficulties in cleaning the sensor.

2. Geometry of the Resistive Patterns. Resistive patterns for thin and thick films are known in the art and will not be discussed in detail. These patterns may include rectangles, circles, other basic geometric shapes, and serpentine patterns. Examples of such patterns are the well known bar, top-hat, loop, and ladder patterns. Figures 6A and 6b are provided as examples of such patterns. In Figure 6A a square spiral pattern 60 is shown, while a rectangular pattern 62 is illustrated in Figure 6B.

Generally, when using a material whose resistivity is a function of its depth, the resistance of the RTD is proportional to the aspect ratio, or number of squares, between the two terminal points. The number of squares in a resistor is calculated using the length of the conduc- tive path divided by its width factoring in corrections for contacts, bends, and changes in shape. Methods for computing the resistivity of a pattern resister, given th ohms/square value for the material, and the geometry of the pattern, are known and will not be discussed herein.

As illustrated in both Figures 6A and 6B, a calibration strip 64 may be provided. As discussed below this calibration strip may be used to "fine tune" or calibrate the RTD to a desired resistance at a certain temperature. One desirable resistance is 1000 ± .5, 1, o 2 ohms at 0°C

One advantage of this invention is that the material comprising the RTD is flexible. This allows the resistiv patterns to be cut to virtually any geometry and to be customized to specific applications in a cost effective manner. For example, a spiral-square or other pattern ma allow the resistive element to conform to the shape of a particular object, e.g. , an air duct. Resistive patterns may be designed and customized to meet the requirements o any number of specific applications.

Further, the present invention allows for the manufacture of sensors having identical base resistances (e.g., 100 ohms at 0°C) but very different dimensions. For example, using the methods of the present invention, RTD of dimension 5/8" X 2-1/4" can have the same base resistance (e.g., 1000 ohms at 0°C) as RTDs of dimension 1/2" X 24 feet, 1/4" X 48 feet, and 12" X 5/8". This attribute while not unique to the present invention, is considerably easier than with either current ceramic base RTDs or wire-wound technology.

. Heat Treatment of the RTD

After being cut from the thermoplastic/metal sandwich, the resistive patterns are then heated to a temperature slightly above the expected operating temp- erature (e.g., approximately 155°C for MYLAR) and left at this temperature for a short period of time (approximatel 30 ± 5 minutes) . Vendor process recommendations for heat- stabilizing MYLAR indicate that a temperature 30°C above the expected maximum operating temperature is beneficial. This process is illustrated in Figure 7. As Figure 7 shows, two metal plates 70, 72 may be used during this heating, or annealing, to compress the resistive patterns and prevent these patterns from distorting. This annealing step is sometimes necessary to improve the dimensional stability of the RTD over temperature cycles.

Generally, the devices should not be calibrated until after the completion of the heat treatment step. It may be possible to omit this annealing step if heat-stabilized materials are used.

The temperature and length for the heat treatment step may be adjusted depending on the prior or subsequent heat treatment to which the RTD has been or will be subject to (e.g., overcoat curing steps). Care should be taken to ensure that the temperature and time of the heat treatment step are acceptable for the selected laminate adhesive.

Additionally, it has been found that the temperature at which the heat treatment step is performed may affect the TCR of the resulting RTD. Basically, the higher the temperature during the heat treatment step, the higher the TCR.

L. Attachment of Electrical Connectors

After the resistive pattern has been heat treated or annealed, electrical connectors are attached. Alternate methods for affixing the connectors to the resistive patterns are shown in Figures 8A - 8E. 1. Conventional Connector Attachment. As Figure 8 illustrates, connections to the resistive pattern may be made by a conventional crimp connector 80, such as those currently available from the AMP company of Harrisburg, Pennsylvania. These connectors, much like staples, may b affixed to the terminal ends of the resistive pattern as shown.

Because the metal layer is extremely thin, it is often difficult to make a good electrical connection to the connector. Further, the combined metal and therm¬ oplastic layer thickness is so thin that the connector ma not be able to "grab" enough of the resistive element to hold the device. One possible method of overcoming this problem is to add a backing 82 to the terminal ends of th resistive elements 60. An additional backing of approximately 5 to 7 mils has been found sufficient for purposes of making the connector attachment. In addition to the backing, a layer of conductive epoxy 84 may be applied to the exposed metal surface of the resistive element. This exposed surface provides a larger area ove which electrical contact may be made, and addresses the additional concern, related to a finely divided reactive thin film, that when the thin film is disintegrated, it oxidizes very readily because of the high surface area. These alternatives are illustrated in Figure 8b with the conductive epoxy illustrated on top of the exposed region of sheet, and the backup illustrated directly underneath.

As Figure 8F illustrates, an electrical connector ma be attached to the resistive element without the use of a backing element. When a backing element is not used, a layer of conductive^" epoxy 84 is applied directly to a portion of the resistive element. A silver-based or nickel-based epoxy has been found adequate for this pur¬ pose. Alternately, a plate of copper or nickel may be attached to the resistive element. In the gold/chrome embodiment it is helpful to use only a nickel plate or to have a thin nickel plate separating the gold layer and the copper plate. The precise thickness of the epoxy layer 84 or the plate is believed not to be critical so long as the material thickness specifications for use of the AMP connectors are met, but it is beneficial if the epoxy or plate covers the entire underside of the AMP connector. A connector of the type illustrated in Figure 8A may be attached using known methods.

Following attachment of the connector, a protective barrier coat may be applied to the resistive element/con¬ nector combination or a protective resin may be applied to the exposed surfaces and edges.

If epoxy is used, it may be necessary to cure the epoxy material via heat treatment. For the Cotronics DURALCO 122 nickel-based epoxy, a 60 ± 5 minutes heat treatment at 121°C has been found acceptable per vendor use recommendations.

2. Alternative Electrical-Connector Embodiments. A second method for attaching a connector to the resistive pattern is illustrated in Figures 8C - 8E. In order to effect this alternate connection, an interconnect board should first be selected that can meet the temperature extremes established for the RTD (e.g., a board comprising an injected molded or molded polymer resin) . The interconnect board may be matched to the thermoplastic material used in the RTD (i.e., may be made of the same material) . Such a board is illustrated as 86 in Figure 8C. This board is then patterned, or etched to a pattern, of approximately 4 mils. Several conductive traces 88 are then attached to the board via conventional plating techniques or adhesion. The number of traces depends on the number of connections to be made to the RTD. Electrical leads 90 are attached to the conductive traces or strips.

One alternative to the aforementioned interconnect board is to use a molded 3-D interconnect, as illustrated in Figure 8B. These devices are currently known in the art and include elements which correspond to those discussed above with reference to the PC board. Like elements have been given like reference numbers. The molded 3-D interconnects generally include one or more conductive strips 88', usually copper with a top plate of nickel, an insulating base 86', and electrical leads 90'. The following discussion applies to both the prepared PC board and the 3-D molded interconnect.

An illustration of the complete connector, as well a the steps followed in constructing the connector, is show in Figure 8E. A first sheet of thermoplastic polymer 92 is attached to the base of the connector via adhesive or mechanical rivets. This sheet should be attached to the base 86 such that the conductive strip 88 and the thermo¬ plastic sheet 92 form a substantially planer interconnect surface 94. The thermoplastic sheet 92 with the connecto attached is then placed into a vacuum deposition chamber or the like and a highly conductive film is deposited at the junction between the copper/nickel conductor 88 and the thermoplastic sheet 92. The thickness of the conductive film is not critical so long as the conductivity is high compared to the sheet resistance of the RTD material and it meets the mechanical requirements of he device (e.g., no cracking over the edges). Shadow mask tooling or photolithography may be used to ensure that the conductive material is applied to or remaining o only the junction region. Because different materials all come together at the junction region, and that region may be thermally cycled, it is useful to ensure that a conductive path always exists across the junction region. The highly conductive film 96 provides such a conductive path. The connector- thermoplastic sheet provides a planerized interconnect area with the leads already connected.

The shadow mask is removed (if shadow mask tooling is used) and a sputter etch may be performed to remove any oxides or other contaminates. The thermoplastic sheet with the connector may be placed into a vacuum deposition chamber, and a thin layer of metal 96 (approx. 25θA - lOOoA) may be deposited onto it as discussed above. A protective coat (e.g., silicon dioxide) may be applied to the thin metal layer 96 prior to removal from the vacuum deposition chamber. Such a protective coat may inhibit the oxidation of metal layer 96. The use of a protective coat may be especially helpful when the metal layer 96 comprises a reactive metal.

Following the above described procedures a second, protective sheet of thermoplastic 98 may be applied to the metallized sheet and the resistive patterns may be cut out. The only significant difference between this embodi¬ ment and the ones previously described is that in this embodiment the connectors are attached to the first ther¬ moplastic sheet throughout the process.

In several of the connection configurations discussed above, the exposed metal region is left exposed after the connector is attached. This may leave the exposed region vulnerable to corrosion, erosion, or scratching. To inhibit such damage, a barrier coat may be applied to the exposed surface after the connector is attached. A further method of attaching a connector to the resistive pattern is illustrated in Figure 8G. As discussed above, crimp connectors of the type illustrate in Figure 8A were originally designed to be attached to interconnects having thicknesses much greater than that of the resistive element of the present invention. As such, it is often difficult to make good electrical connections between the present resistive element and traditional crimp connectors. To improve the electrical connection, one may use a custom extruded substrate of thermoplastic polymer having a minimal thickness (approximately 3 mils thick) throughout most of the area, but gradually increa¬ sing to a thickness on one end 93 sufficient to accommod¬ ate most traditional crimp connectors (approximately 10 mils). Such a substrate is illustrated as 92' in Fig¬ ure 8G.

When the custom substrate is used, resistive elements are formed according to the steps disclosed above. Once the processing steps are completed a crimp connector may be attached to the thick end 93 of substrate 92' using known methods.

A still further method for attaching a connector to resistive patten is illustrated in Figure 8H. In this method a layer of flexible polymer 92 is attached to a molded interconnect 86" using an adhesive suitable for the temperature range to which the resistive element will be subjected to. Interconnect 86" is similar to that described above in regard to Figure 8D except the interconnect of this embodiment has a planer interconnect surface. It is generally desirable that the layer of polymer 92 fully cover the plated surface of the interconnect 86". Once the adhesive has cured, vias 95 are manufactured through the adhesive and the polymer layer 92 down to the surface of the plated leads of interconnect 86. The vias 92 can be produced using mechanical methods, e.g., drilling, or through chemical methods, e.g., photolithography techniques and wet or dry etching. After producing the vias, the conductive material 12 is deposited over the polymer and the resistive elements are produced as disclosed above.

One factor in ensuring good coverage of the vias is the slope of the vias down to the plated surface. The particular slope selected depends on the step height, the dimension of the deposition source, and the source to substrate distance. Calculations to determine the maximum permissible slope angle are known and will not be discussed herein. Depending on the temperature range to be encountered and the characteristics of the materials used — primarily the coefficient of thermal expansion — it may be necessary to deposit additional conductive material at the interconnect area and plate up the vias to the surface of the substrate.

3. Recoating and/or Overlaminating the RTD. Once an electrical connector has been affixed to the resistive pattern, it is often desirable to coat (or recoat) the re¬ sistive element wit__ a protective barrier coat to prevent drift and erosion of the metal layer. If such a coating (or recoating) is employed, both the surfaces and the edges of the resistive elements should be coated.

Alternately, additional layers of thermoplastic can be applied to the resistive patten to completely cover both sides of the patterns to prevent erosion. This process is known in the art as overlaminating and will not be described in detail. 4. Applying an Overmold to the RTD. After the overlaminate and/or recoat has been applied to the RTD, it may be desirable to provide an overmold to the portion of the RTD where the connector is affixed to the resistive pattern. This overmold may be either a small plastic or an ejected rubber shell. The overmold may be used to both isolate the exposed thin film and electrical connector from the environment and to provide strain relief on the connector/RTD interface by limiting the bend radius at the interface.

M. calibration of the RTD

Once the connectors are applied to the resistive pattern (or in the alternate embodiment, once the resistive patterns have been heat treated) the resistive elements may be calibrated. Calibration may be necessary because of the percent variation that is inherent is several of the previous processing steps.

Generally, it is desirable to have the resistive patterns calibrated such that the resistance is defined a 0°C (e.g., 100 ohms at 0°C, 250 ohms at 0°C, 500 ohms at 0°C, 1000 ohms at 0°C) . Because the RT curve of the RTD of this invention may be determined, it is possible to calibrate the devices at temperatures other than 0°C.

Known iterative techniques using a calibration bath may b employed (i.e., calibrate in a bath; trim; calibrate again; trim again; and so forth) .

Figure 9A - 9B illustrate another calibration technique. A table having a glass top 100 is provided with a connection strip 102 attached to a personal computer 104. Alternatively, a comparator board may be used in place of personal computer 104. The connection strip 102 contains several leads 103 that may be attached to the electrical leads of the RTD's to be calibrated. Within the connection strip 102 is provided a RTD 106 tha has already been calibrated. A heater 108 is provided to heat the glass top 100 to approximately 100°F. The glass top 100 table is heated so that the RTDs to be calibrated and the pre-calibrated standard are at substantially the same temperature.

A switch on the connection strip 110 allows the operator performing the calibration to select which of the uncalibrated RTDs is to be measured. The personal computer 104 is attached to conventional circuity which measures the resistive value of the uncalibrated RTD 112 and that of the calibrated RTD 106. A visual indication is provided to indicate the difference between the two devices.

As discussed above, a calibration strip 114 may be designed into the RTD. This calibration strip 114 is illustrated in Figure 9B. As discussed above, the resistive value of the RTD depends on the area (in number of squares) that is between the terminal points of the resistive element. By carefully cutting along the slit 116, the number of squares between point A and point B may be increased.

For example, in Figure 9B1 the area between point A and point B is about 1 square unit of area. After extending the slit to point X, there are four square units between the points. This is illustrated in Figure 9B2. By designing the original pattern of the resistive element such that its resistance will be slightly less than desired, the above describe calibration method may be followed to increase the resistive value to precisely the right amount. One method for extending the slit 116 is illustrated in Figure 9C. In this method a slotted cover 120 is provided that may be used to cover the resistive element being calibrated. A cutting razor 122 is then inserted through the slot. The slotted cover 120 is necessary to prevent the heat from the individual performing the calibration from affecting the temperature of the device being calibrated.

Once the RTD to be calibrated is covered and the razor positioned at calibration slit 116, the person performing the calibration begins to extend the slit by cutting with the razor 120, while watching the output fro personal computer 104. When the personal computer 104 indicates that the resistive value of the known RTD and that being calibrated are sufficiently close, the operato stops extending the slit, and removes the cover. As illustrated in Figure 9A, an ice bath 124 may be provided for use as a reference standard. The use of an ice bath in calibrating RTDs is discussed in ASTM Publication No. E-644.

Other methods of calibration, such as cutting off a portion of the resistive element to increase the number o squares, laser trimming, and controlled oxidation may be used.

If an ice bath is used, it has been found advantageous to place the calibrated RTD in a vacuum chamber for approximately one hour to remove excess moisture. A layer of a barrier coating (e.g., ADCOTE) may be applied to the area of the RTD trimmed during calibration for protection.

Once calibrated, the resistance of the RTD may be measured using known electronic devices. These electroni -35-

devices may be configured to produce an industry standard 4-20 milliamp output using the RTD of the present invention.

N. Temperature-Integration Apparatus

The RTD of the present invention has several applications. One application facilitated by the present invention is that of large scale temperature integration. Temperature integration involves the determination of the integrated temperature of a large single object (e.g., a large wall) , or of a particular region of material (e.g. , water flowing through a pipe) . The integrated temperature of such objects or regions is often useful for making engineering decisions, for process control, and for basic temperature detection. In the past, large scale temperature integration has often been done through the use of numerous temperature sensors whose outputs had to be averaged by other expensive and complex apparatus.

The RTD of the present invention allows temperature integration to be accomplished economically and effectively. Figure 10A illustrates one method of using the present invention to measure the integrated tempera¬ ture of a large wall. Basically, a large RTD 130 is manufactured and affixed to the wall 132. For purposes of illustration a square spiral pattern is used although oth¬ ers are possible. This large RTD 130 may be adhered to the wall (if the measurements are to be permanently taken) , or held in place by non-permanent connectors (e.g., tape, tacks, etc.). Because a single RTD (or multiple RTD's connected in series) covers the entire wall, and because of the quick response time of the present invention, the RTD will quickly yield an indication of the integrated wall temperature. Of special interest is the fact that the RTD 130 can, because it is flexible, be rolled into a compact package once the wall temperature is measured. Further, because of the flexible nature of the device, a "RTD shell" can be produced which could essentially conform to the shape of the object whose integrated temperature is desired to be determined.

It is envisioned that RTDs of the type shown in Figure 10a may be embedded within a wall, airplane wings, or other objects to constantly measure the average temperature of those devices. Figure 10B illustrates a portion of pipe that can measure the integrated temperature of the fluid flowing therethrough. A long, rectangular, RTD 134 is coiled and affixed to the inside of the pipe 136. As the fluid passes through the pipe, it causes a change in the resistance of the RTD. The new re- sistance can be measured, and the integrated temperature of the fluid may thereby be determined with reasonable accuracy and precision. In this manner the temperature of large fluid containers, such as the storage areas of shipping tankers, may be measured.

A still further embodiment of the present invention is illustrated in Figure IOC. In that figure a flexible RTD 140 of the present invention is designed and patterne so as to conform to the shape of a particular motor 142. Alternate embodiments are envisioned wherein the thermoplastic sheets used in manufacturing the RTD 140 comprise so-called shrink wrap material, and the RTD 140 is "shrink-wrapped" (e.g., thermally conformed) around th motor 142. This particular embodiment allows the heat of the motor to be effectively monitored during operation. quick rise in temperature or a gradual rise to a high temperature could provide a fast indication of improper motor operation. O. Combined Fluid-Level and Temperature Measurement

Figures 11A and 11B illustrate a novel fluid level and temperature measuring device that is made possible by the present invention. Figure lla illustrates one embodiment of this device.

A first RTD 150 is applied along the walls of a large fluid container, e.g. , the cargo area of a shipping vessel. As discussed above, this RTD may be used to measure the integrated temperature of the walls of the container. Although illustrated as covering only one wall, the RTD may be designed to cover substantially all of the container.

Several other RTDs 152 are positioned so as to run horizontally along the wall. These RTDs 152 are used to determine the level of the fluid in the container.

In most instances the thermal conductivity of a substance in fluid form is much greater than the thermal conductivity of that same substance in gaseous form. As such, when heat is added to a system having a substance that is both gaseous and fluid, the temperature of thermally conductive materials in contact with the fluid will change at a rate substantially different from materi¬ als in contact with the gas. The invention of Figure lla makes use of the difference in conductivity to determine the fluid level in the container.

As shown in Figure 11D, RTDs 152a and 152b consistently monitc- the temperature of the walls of the container. RTD 152a will provide an indication as to the general temperature^" of the gas/wall interface; RTD 152b provides an indication as to the temperature of the fluid/wall interface. Thus, from RTD 152a the gas/wall temperature Tg may be measured, while RTD 152b yields the fluid/wall temperature Tf.

RTD 155, of known length L, is positioned vertically in the container such that its length runs from the botto of the container to the top. From the resistance of RTD 155, the temperature measured by that device may be cal¬ culated. This advantageously permits rapid, integrated measurement of differential heat transfer rates between the liquid and gas.

Given the wall temperature in contact with the gas, Tg, the wall temperature in contact with the fluid, Tf, and the measured temperature, Trtd, the level of the flui may be determined by a ratio of the resistance values.

Alternate embodiments are envisioned where a heating element is provided to add heat to the system for brief periods of time. During the time the heating element is activated, the change in the temperature of each RTD can be measured and compared as discussed above.

Figure 11B illustrates an alternate embodiment of fluid level and temperature device. A single RTD 155 is positioned vertically in a container 160 which is capable of receiving fluid 162. In order to determine the level of a particular fluid 162, the RTD 155 should first be calibrated. This is done by measuring the RTD's 155 resistance when the container 160 is empty, i.e., R₀, and when the container 160 is filled with the fluid to be monitored at a known level KL, i.e., R₁. Given that the TCR of the RTD 155 is substantially constant, the propor¬ tional relationship"between resistance measured to fluid level can be calculated. A basic method for performing such a calculation is to compute the quantity L_? KL/(R_x

- RQ) . Once the RTD is calibrated, the level of the fluid may be measured by taking the resistance value that is measured, and multiplying it by the proportionality constant determined when the device was calibrated.

An alternate embodiment to the device of Figure 11B is illustrated in Figure lie. This device operates similar to that describe above in respect to Figure 11B, but the RTD 155 covers several sides of the container.

A still further embodiment of the present invention is shown in Figure 12. In that figure a web of RTDs 160 is shown. Web 160 comprises vertical strips 161, as well as horizontal strips 162. By monitoring the temperatures of all of the RTD's, it may be possible to pinpoint the location of a rise (or fall) in temperature. For example, if a temperature rise occurs at point X, both RTD 161' and 162' will show a rise in temperature. By comparing the temperatures of the RTDs comprising web 160, the precise location of a temperature disturbance may be located.

Such information may be particularity important when the location of a dangerous temperature disturbance needs to be known.

A still further embodiment of the invention is illustrated in Figure 13. There, a multilayer RTD 170 is shown. RTD 170 comprises several RTD's, 171, 172 of the type discussed above. A thin metallic film 173 is also provided. The RTDs 171 and 172 should be selected such that the thermoplastic material of the RTD 171 will break down at a lower temperature than that of the RTD 172.

By having RTDs"171, 172 of different temperature ranges, a sensor is created whereby the degree of a temperature disturbance may be monitored and determined. For example, a rise in temperature up to a certain level may cause RTD 171 to break (e.g., separate). Such a brea would yield an infinite resistance for that RTD. This could set off one alarm indicating that some action shoul be taken (e.g., RTD 172 should be closely monitored.)

Because the temperature range of RTD 172 is greater than that of RTD 171, the temperature of the system being monitored can still be measured. Then, by progressively stacking RTDs of different temperature ranges, a sensor may be produced whereby various alarm signals are provide depending on the degree of temperature rise.

Conductive film 173 is provided so that a complete break of the RTD may be measured. For example, a break o all the RTD's as well as film 173 may indicate that the sensor has been inadvertently severed as opposed to act¬ ually destroyed by a temperature disturbance.

P. Combined Temperature/Pressure Sensor A still further embodiment of the present invention is illustrated in Figure 14. In this embodiment the RTD of the present invention is combined with a pressure sensor to yield a unitary pressure/temperature sensor. First an RTD 180 of the present invention is affixed to a thermoplastic protective layer, or a base support layer 181. If YAG laser processing is employed (or its equivalent) , the base support layer 181 may be the substrate of RTD 180.

A partial grid is then constructed on the base support layer using parallel lines 183 of conductive ink. These lines may be applied to base 181 through known met ods. The base support layer 181, including the conducti lines, is then coated with a conductive elastomer 182 su as a polyvinylidene fluoride polymer (PVDF) , a piezo¬ electric material. A second group of conductive lines 18 is then applied to the piezomaterial coating 182. Electrical leads 187 may then be attached to the conductive lines.

Application of force to the piezomaterial 182 create an electrical current in lines 183 and 185 proportional t the change in force. By observing the current in lines 183, 185, the pressure being applied to RTD 180 may be monitored.

Alternate embodiments are envisioned where material 182 comprises a resistive conductor material. In this embodiment the grid lines 183, 185 may be used or the resistance of the coating 182 itself may be measured. A force applied to the coating of this embodiment increases the contact area between two lines. This results in a decrease of resistance between the lines. By applying a voltage to the lines 183, 185, the applied force can be determined.

. Air Duct Temperature Sensor

The RTD of the present invention may be constructed in such a manner to allow its suspension in an air duct so that it may be exposed to the total air flow through the duct. When used in this manner, the RTDs of the present invention function as temperature integrating sensors (TIS) . A single TIS is advantageous over numerous RTDs because it measures the true average (integrated) temperature rather than an approximate average obtained from a few discrete temperature measurements. Such an ap¬ plication of the present invention is illustrated in Figures 15A-C

As Figure 15A illustrates, a sensor 200 comprising a flexible RTD 202 is sandwiched between two polyester layers 204a and 204b. Although only one RTD 202 is illustrated, several different RTD's may be sandwiched between the polyester layers 204a and 204b.

In one embodiment, polyester tabs 206 are attached t the sensor 200 at six inch intervals. The tabs 206 preferably protrude 1" to 2" above the sensor and are use to suspend the sensor in the air duct. Figure 15B illustrates one way in which the sensor of 15A may be suspended in a air duct. An alternative to using tab 206 is illustrated in Figure 15C. In this embodiment a pressure sensitive adhesive is applied to the backside of sensor 200. The sensor 200 is then either attached to an interior wall 208 of the air duct or attached to a piece of interior tubing 210 used to traverse the duct.

Figure 15D illustrates one embodiment in which a rectangular base 212 is used to support the sensor in the air duct.

An advantage of the sensor of the present invention is its ability to bend, fold, or wrap inside the air duct This feature allows the sensor to measure the temperature of flowing air (which may stratify, form eddy currents) , dead zones, or areas of laminar flow, without being undul influenced by the temperature of ambient air and/or the wall of the air duct.

Due to the potential size and flexibility of the sensors of the present invention, an entire zone of air duct may be monitored. For example, using the methods described above an air duct temperature integrating senso of 60 feet in length may be produced.

The foregoing embodiments are presented to illustrat but not to limit the invention, which is particularly pointed out in the following claims.

Claims

CLAIMS:

1. A resistive temperature device (RTD) comprising:

(a) a substrate comprising a flexible thermoplastic material; and

(b) a thin film of conductive material bonded to the substrate.

2. The RTD of claim 1 wherein the thermoplastic material is selected from among a group comprising polyolefins, polyester, polyamides, polyimides, polyvinyls, polystyrenics, nylon, polycarbonates, and fluorocarbons.

3. The RTD of claim 1 wherein the thermoplastic material is a polyester.

4. The RTD of claim 1 wherein the thermoplastic material is a polyimide.

5. The RTD of claim 1 wherein the conductive material is selected from among a group comprising aluminum, silver, gold, copper, platinum, and nickel.

The RTD of claim 1 wherein the conductive material is aluminum.

7. The RTD of claim 1 wherein the conductive material is silver.

8. The RTD of claim 1 wherein the conductive material i gold.

9. The RTD of claim 1 wherein the conductive material i copper.

10. The RTD of claim 1 wherein the conductive material i platinum.

11. The RTD of claim 1 wherein the conductive material i nickel.

12. The RTD of claim 1 wherein the thin film has a thickness of less than about 2000 angstroms.

13. The RTD of claim 1 wherein the thin film has a thickness of at least one mono-atomic layer and less than about 2000 angstroms.

14. The RTD of claim 1 wherein the thin film has a thickness of at least one mono-atomic layer and less than about 1000 angstroms.

15. The RTD of claim 6 wherein the thin film has a thickness corresponding to a surface resistivity in a range of between about 0.5 ohms per square to about 1.5 ohms per square.

16. The RTD of claim 1 wherein the thin film is vacuum deposited on the substrate.

17. The RTD of claim 1 wherein the thin film is bonded to the substrate by metallization.

18. The RTD of claim 17 wherein the metallization is ac- complished by a process selected from among a group com¬ prising vacuum deposition, chemical vapor coating, glow discharge, electrodeposition, pressure sensitization, thermal curing, thermoplastic lamination, radiation curing, printing techniques, and plasma-enhanced deposi- tion.

19. The RTD of claim 1 further comprising a protective barrier affixed to the thin film.

20. The RTD of claim 19 wherein said protective barrier comprises a material selected from among a group comprising cross-linkable polymer resins, thermoplastic resins, thermosetting resins, silicon monoxide, silicon dioxide, silicon nitride.

21. The RTD of claim 19 wherein said protective barrier comprises a cross-linkable polymer resin.

22. The RTD of claim 19 wherein said protective barrier comprises a thermoplastic resin.

23. The RTD of claim 19 wherein said protective barrier comprises a thermosetting resin.

24. The RTD of claim 19 wherein said protective barrier comprises silicon monoxide.

25. The RTD of claim 19 wherein said protective barrier comprises silicon dioxide.

26. The RTD of claim 19 wherein said protective barrier comprises silicon nitride.

27. The RTD of claim 19 wherein said protective barrier is affixed to the thin film by a process selected from among a group comprising vacuum deposition, chemical vapo coating, glow discharge, electrodeposition, pressure sens- itization, thermal curing, thermoplastic lamination, radi ation curing, printing techniques, and plasma-enhanced deposition.

28. The RTD of claim 19 wherein said protective barrier is affixed to the thin film by thermoplastic lamination.

29. The RTD of claim 19 wherein said protective barrier is affixed to the thin film by vacuum deposition.

30. The RTD of claim 19 wherein said protective barrier is affixed to the thin film by pressure sensitization.

31. A method for manufacturing a resistive temperature device for operation in a known temperature range comprising:

(a) selecting a flexible substrate of thermoplastic material that retains its thermoplastic characteristics within the known temperature range;

(b) selecting a material that exhibits a known temperature coefficient of resistance (TCR) over the known temperature range, and that has a temperature coefficient of expansion (TCE) similar to the TCE of the substrate;

(c) bonding a thin film of the material to the substrate to form an RTD element.

32. The method of claim 31, further comprising a step (d) of covering the thin film with a protective barrier.

33. The method of claim 31, further comprising a step (e) of fabricating the RTD element to create a predetermined resistor range.

34. The method of claim 31 further comprising a step (e') of fabricating the RTD element to create a nominal resistor value.

35. The method of claim 31 wherein the material is selected from among a group comprising aluminum, silver, gold, copper, platinum, and nickel.

36. The method of claim 31 wherein said material is aluminum of a thickness of at least one mono-atomic layer and less than about 2000 angstroms.

37. The method of claim 31 wherein said material is aluminum of a thickness of at least one mono-atomic layer and less than about 1000 angstroms.

38. The method of claim 31 wherein said material is aluminum of a thickness corresponding to a surface resistivity in a range of between about 0.5 ohms per square to about 1.5 ohms per square.

39. The method of claim 31 wherein said bonding in step (c) is accomplished by a process selected from among a group comprising vacuum deposition, chemical vapor deposition, glow discharge, electrodeposition, pressure sensitization, thermal curing, thermoplastic lamination, radiation curing, printing techniques, and plasma deposition.

40. The method of claim 31 wherein said bonding in step (c) is accomplished by vacuum deposition.

41. The method of claim 40 wherein said bonding in step (c) is accomplished by thermal evaporation.

42. The method of claim 32 wherein step (d) comprises th step of applying a barrier coating to the thin film.

43. The method of claim 32 wherein step (d) comprises th step of affixing a flexible thermoplastic material to the thin film.

44. The method of claim 32 wherein step (d) comprises th step of affixing a thermosetting resin to the thin film.

45. The method of claim 32 wherein step (d) comprises the step of affixing a rigid protective barrier material to the thin film.

46. The method of claim 32 wherein step (d) comprises the step of bonding a silicon monoxide coating to the film.

47. The method of claim 32 wherein step (d) comprises the step of bonding a silicon dioxide coating to the film.

48. The method of claim 32 wherein step (d) comprises the step of bonding a silicon nitride coating to the film.

49. The method of claim 33 further comprising the steps, following step (e) , of:

(1) heat-stabilizing the RTD element; and

(2) affixing an electrical connector to the RTD element.

50. The method of claim 33 further comprising the steps, following step (e) , of:

(1) heat-stabilizing the RTD element at a temperature slightly above an expected operatin temperature range of the RTD element; and

(2) affixing an electrical connector to the RTD element.

51. The method of claim 33 further comprising a step (f) following step (e) of calibrating the RTD element to a specified nominal range by increasing the resistance of the RTD element.

52. The method of claim 33 further comprising a step (fl) following step (e) of calibrating the RTD element to a specified nominal range by physically trimming the RTD element to increase the resistance of the RTD element.

53. The method of claim 33 further comprising a step (f2 following step (e) of calibrating the RTD element to a specified nominal range by chemically increasing the resistance of the RTD element.

54. A device whose resistance changes in response to changes in its temperature, comprising:

(a) a substrate of flexible thermoplastic film; and

(b) a thin layer of conductive material bonded to the substrate such that the resistivity of the thin layer differs from the resistivity of the conductive material in bulk form.

55. The device of claim 54 wherein the conductive material is aluminum of a thickness of at least one mono- atomic layer and less than about 2000 angstroms.

56. The device of claim 54 wherein the conductive material is aluminum of a thickness of at least one mono- atomic layer and less than about 1000 angstroms.

57. The device of claim 54 wherein the conductive material is aluminum of a thickness corresponding to a surface resistivity in a range of between about 0.5 ohms per square to about 1.5 ohms per square.

58. The device of claim 54 wherein the substrate is fabricated to create a predetermined resistor range.

59. The device of claim 54 wherein the substrate is fabricated to create a nominal resistor value.

60. The device of claim 54 further comprising an electrical connector electrically coupling the thin layer of conductive material to an electrical lead.

61. The device of claim 60 wherein said electrical connector comprises a conductive adhesive.

62. The device of claim 60 wherein said electrical connector comprises a conductive foil.

63. The device of claim 60 wherein said electrical connector comprises an electroplated surface.

64. An electrical connector for electrically coupling a thin conductive film with an electrical lead, the connector comprising:

(a) an insulating base;

(b) an electrically conductive strip attached to th base, the conductive strip having an exposed surface and an attached surface, the attached surface being attached to the base;

(c) an electrical lead attached to the conductive strip;

(d) a flexible substrate of polymer attached to the base and abutting the conductive strip, the flexible sheet having an attached surface attached to the base and an exposed surface, such that the exposed surface of the conductive strip and the exposed surface of the substrate form a substantially planer surface; and

(e) a thin conductive film affixed to the substan¬ tially planer surface.

65. The connector of claim 65 further including a buildu of conductive material at the junction of the exposed surfaces of the conductive strip and the substrate.

66. A method for electrically connecting a thin conductive film to an electrical lead comprising the steps of:

(a) affixing a highly conductive strip to an insulating base, the conductive strips having a attached surface attached to the base, and an exposed surface;

(b) affixing an electrical lead to the highly conductible strip;

(c) affixing a flexible substrate of polymer to the base, the flexible sheet having an attached surface attached to the base and an exposed surface, such that the exposed surfaces of the conductive strip and the flexible substrate form a substantially planer surface;

(d) affixing a thin conductive film to the substantially planer surface.

67. The method of claim 66 further included a step, between steps (c) and (d) of affixing a buildup of conductive material at the junction of the exposed surfaces of the conductive strip and the substrate.

68. The method of claim 66 where the thin conductive fil is attached to the substantially planer surface using vacuum deposition.

69. A method for electrically coupling a thin conductive film with an electrical lead, given an electrical connector having an insulating base, a highly conductive strip having an upper surface, and an electrical lead attached to the strip, the method comprising the steps of:

(a) attaching one surface a thin substrate of flexible polymer to the insulating base such that the upper surface of the conductive strip and the unattached surface of the flexible substrate form a substantially planer surface;

(b) depositing a thin conductive film on the substantially planer surface.

70. The method of claim 69 where the thin conductive fil is deposited through vacuum deposition.

71. Apparatus for measuring an integrated temperature of an object, comprising:

(a) a large substrate of flexible polymer material formed in a pattern, the pattern having two terminal points, the large substrate being size so as to substantially cover the object;

(b) a thin film of conductive material affixed to the large substrate of flexible polymer; and -55-

(c) electrical leads attached the thin conductive film at the two terminal points of the pattern such that the resistance between the two points may be measured.

72. A resistive temperature device (RTD) comprising:

(a) a substrate comprising a flexible thermoplastic material;

(b) a first thin film of material deposited on the substrate; and

(c) a second thin film of conductive material bonded to the first thin film, said first thin film promoting adhesion of the second thin film.

73. The RTD of claim 72 wherein the first thin film has a thickness in the range of about lOoA to about 15θA inclusive.

74. The RTD of claim 72 wherein the second thin film has a thickness in the range of about 0.5 ohms/square to about 1.5 ohms/square inclusive.

75. The RTD of claim 72 wherein the material of the first thin film is chromium and the conductive material of the second thin film is gold.

76. The RTD of claim 75 wherein the thickness of the first thin film is in the range of about lOoA to about 15θA inclusive, and the thickness of the second thin film is in the range of about 0.5 ohms/square to about 1.5 ohms/square inclusive.

77. A resistive temperature device (RTD) comprising:

(a) a substrate comprising a flexible thermoplastic material;

(b) a first thin film of chrome deposited on the substrate; and

(c) a second thin film of gold deposited on the first thin film.

78. The RTD of claim 77 wherein the thickness of the first thin film is in the range of about IOOA to about 15θA inclusive, and the thickness of the second thin film is in the range of about 0.5 ohms/square to about 1.5 ohms/square inclusive.

79. A method for affixing a thin conductive film to a crimp-type electrical connector comprising the steps of:

(a) applying a layer of conductive epoxy to a selec portion of the electrical connector; and

(b) attaching the crimp connector to the select portion of the thin conductive film through the conductive epoxy.

80. The method of claim 79 wherein the conductive epoxy is a silver-based epoxy.

81. The method of claim 79 wherein the conductive epoxy is a nickel-based epoxy.

82. A method for electrically connecting a thin conductive film to an electrical lead comprising the step of:

(a) affixing a flexible substrate of thermoplastic polymer to an insulating base, the insulating base being attached to an electrical lead at a plated interconnect;

(b) manufacturing a via through the flexible substrate and the insulating base down to the plated interconnect; and

(c) depositing the thin conductive film on the flexible substrate such that a portion of the thin conductive film covers the via.

83. The method of claim 82 wherein the via is manufactured by mechanical means.

84. The method of claim 82 wherein the via is manufactured through a wet etch chemical process.

85. The method of claim 82 wherein the via is manufactured through a dry etch chemical process.

86. The method of claim 82 including a step (d) , performed after step (c) , of depositing additional conductive material over the via.

87. Apparatus for measuring both temperature and pressure comprising:

(a) a substrate comprising a flexible thermoplastic material;

(b) a thin film of conductive material bonded to the substrate;

(c) a base support layer attached to the flexible substrate;

(d) a conductive elastomer coating bonded to the base support layer.

88. The apparatus of claim 87 wherein the conductive elastomer coating comprises polyvinylidene.

89. The apparatus of claim 87 wherein the conductive elastomer coating is sandwiched between at least two lines of conductive ink.

90. A resistance temperature device (RTD) for detecting temperatures over a range of -200°C to 200°C comprising:

(a) a substrate comprising a flexible thermoplastic material; (b) a thin metal film deposited on the substrate having a temperature coefficient of resistance (TCR) that is different than the bulk TCR of th metal; and

(c) a protective film of thermoplastic material bonded to the metal film.

91. The RTD of claim 72 where the thin metal film is vapor deposited aluminum having a thickness of between about 20θA to about 200θA inclusive and a sheet resistance of between about 0.5 to about 1.5 ohms/square inclusive.

92. The RTD of claim 72 where the TCR of the metal film is within the range about .002530Ω/Ω/°C to about .003400Ω/Ω/°C inclusive.

93. A temperature integrating sensor for detecting the average temperature of a material flowing through a chamber over an expected operating range of temperatures, the sensor comprising:

(a) a thin-film, flexible resistive temperature device (RTD) ;

(b) an overlaminate surrounding the RTD; and

(c) support elements attached to the RTD for suspending the sensor in the chamber.

94. The sensor of claim 93 wherein the chamber is an air duct and the material is still air or moving air.

95. The sensor of claim 93 wherein the RTD comprises a thermoplastic substrate and a layer of vapor-deposited metal having a thickness in the range of about 25θA to about 200θA inclusive.

96. The sensor of claim 95 wherein the thin layer of metal has a sheet resistance in the range of about 0.5 ohms/square to about 1.5 ohms/square inclusive.

97. A temperature integrating sensor for detecting the average temperature of a material flowing through a chamber comprising:

(a) a thin film, flexible resistive temperature device (RTD) ;

(b) a thermoplastic overlaminate attached to the RTD; and

(c) an adhesive layer applied to the overlaminate.

98. The sensor of claim 97, having a length of between about 1 foot and about 60 feet inclusive.

99. The sensor of claim 97 having a length of between 1 inch and about 36 inches inclusive.

100. The sensor of claim 97 wherein the chamber is an air duct and the flowing fluid is still air or moving air.

101. A method for measuring the average temperature in an air duct using the sensor of claim 93, the method comprising the steps of:

(a) suspending the sensor in an air duct; and

(b) monitoring the electrical resistance of the RTD.

102. A method for measuring the average temperature of a fluid flowing through a chamber with walls using the sensor of claim 97, the method comprising the steps of:

(a) affixing an adhesive layer to a flexible resistance temperature device (RTD) ;

(b) affixing the adhesive layer to a wall of the chamber; and

(c) monitoring the resistance of the RTD.

103. The method of claim 101 wherein the chamber is an air duct and the fluid is still air or moving .

104. A method for measuring the average temperature of a flowing fluid, using the sensor of claim 97 and a pipe; the method compr: irig the steps of:

(a) affixing an adhesive layer to a flexible resistance temperature device (RTD) ; (b) affixing the adhesive layer to the pipe;

(c) placing the pipe in the stream of the flowing fluid; and

(d) measuring the electrical resistance of the RTD.

105. The method of claim 104 wherein the fluid is still air or moving air.

106. A temperature integrating sensor for detecting the average temperature of a material flowing through a chamber over an expected operating range of temperatures, the sensor comprising:

(a) a substrate comprising a flexible thermoplastic material;

(b) a first thin film of material bonded to the sub strate to form an adhesive surface;

(c) a second thin film of conductive material bonde to the first thin film; and

(d) support elements attached to the substrate for suspending the sensor in the chamber.

107. The sensor of claim 106 wherein the material of the first thin film is chromium and the conductive material o the second thin film is gold.

108. The sensor of claim 107 wherein the thickness of the first thin film is approximately 125A, and the thickness of the second thin film is approximately lOOoA.

109. The sensor of claim 106 wherein the support elements comprise a rectangular base.