WO1996006345A1

WO1996006345A1 - Infrared gas detector

Info

Publication number: WO1996006345A1
Application number: PCT/GB1995/001963
Authority: WO
Inventors: John Patterson Sinclair
Original assignee: Msa (Britain) Limited
Priority date: 1994-08-18
Filing date: 1995-08-18
Publication date: 1996-02-29
Also published as: GB9416711D0

Abstract

An infrared gas detector (40) for detecting dangerous or harmful gases (e.g. methane). The detector (40) comprises a first chamber (3) housing an electrical control arrangement (9) connected to an infrared energy source (15) and an infrared energy detector (17), and the second gas-permeable chamber (5) which is gas permeable and has a mirror (23) for reflecting infrared energy from the infrared energy source (15) to the infrared energy detector (17). The detector (40) further comprises a hollow cavity (52) formed of infrared transmissive material, positioned to intercept the infrared energy path between the infrared energy source (15) and the infrared energy detector (17), the cavity (52) being connected to a passageway (53) for allowing a calibrating gas to be supplied to said hollow cavity (52) for calibrating the detector (40).

Description

INFRA-RED GAS DETECTOR

The present invention relates to an infra-red gas detector.

Infra-red gas detectors are frequently used for the detection of dangerous or harmful gases. This is particularly the case on oil rigs or in mines, where combustible gases such as methane need to be detected for safety reasons.

A known form of infra-red gas detector generally comprises a first chamber containing control electronics and an infra-red energy source and detector, and a second, perforated, chamber which provides an air space into which the gas to be detected may permeate. Infra¬ red energy is emitted from the energy source through a window in a wall of the first chamber. This energy passes through the air space of the perforated chamber to a mirror which reflects the energy back to the detector in the sealed chamber. If any gas other than air is present in the perforated chamber it is detected, because it absorbs some of the infra-red energy. The decrease in energy received by the energy detector with respect to a reference is a measure of the amount of gas present in the perforated chamber.

To calibrate the detector or indeed to check the accuracy of the detector, it is necessary to fill the air space of the perforated chamber, between the window and the mirror with a known concentration of the gas of interest, in air. This is difficult to achieve because of air movements. It will, of course be understood that for normal operation of the detector, air must be able to move in and out of this space freely, so that any gas in the surrounding air can be detected.

Normally calibration gas is blown into the air space via a gas inlet at rates typically around 5 litres/min in an attempt to achieve the required known concentration in the air space. In still air this can be done, but in normal outdoor conditions, still air is unusual. Even very low wind speeds prevent proper calibration, despite the use of calibration gas at flow rates around 201itres/min for 4-5 minutes. This provides a reasonable but not accurate calibration.

One method used to improve the situation is to fit a calibration cap over the perforated chamber during calibration, thus providing an enclosed space to contain the calibration gas. This method is usually adequate provided easy access to the position in which the detector is installed can be obtained. However, the detectors are often in poorly accessible positions, and frequently this is not possible.

Often the detectors are used outdoors, or in other non-controlled environments. A problem that arises from this is that there is a tendency for condensation to occur on the mirror during damp conditions. A solution to this is to provide heaters which heat the air surrounding the mirror and prevent condensation from forming on the mirror. However, the heaters require power and in situations where battery power is required (such as mains power failure) , the heaters drain the power which is necessary to operate the control electronics.

It is an object of the present invention to provide an infra-red gas detector which obviates and/or mitigates some of the aforementioned problems.

The present invention provides an infra-red gas detector comprising a first chamber housing an electrical control arrangement connected to an infra-red energy source and an infra-red energy detector and a second chamber which is gas permeable and has a mirror for reflecting infra-red energy, the arrangement being such that, in operation, infra-red energy is directionally emitted along a path from the source towards the mirror in the second chamber and is reflected therefrom towards the detector in the first chamber, the detected level of infra-red energy providing a measure of IR absorbing gas within the second chamber the improvement being that the detector further comprises a hollow cavity formed of infra-red transmissive material and which is positioned to intercept the infra-red energy path, the cavity being accessible by way of a passageway such as to allow a calibrating gas to be supplied to said hollow cavity for calibrating the infra-red detector.

Preferably the first and second chambers are mutually isolated by a wall means which comprises an infra-red transmitting window intercepting the energy path and carried within a non-transmitting support wall, and the cavity is formed within the thickness of said window.

The cavity and the associated passageway should preferably be narrow so as to limit the ingress of any gas into the cavity under normal atmospheric conditions. For example the cavity may be 5mm, i.e. the infra-red energy path extends a distance of 5mm through the cavity, and the passageway may be 2mm in diameter.

Conventional infra-red transmitting materials, such as sapphire are expensive, so preferably the path of the infra-red energy between the first and second chambers passes through an infra-red transmitting window formed within a non-transmitting support wall, the window being adjacent the infra-red energy source and infra-red energy detector. Optionally, two windows may be provided one for the infra-red energy source and one for the infra-red energy detector. The infra-red energy source and the infra-red energy detector may be selected from those known in the art.

In the embodiment where the cavity is within the wall means, the window may be formed of a single piece, through which the cavity extends, or alternatively the window may be formed of two pieces, between which the cavity extends. The cavity is positioned between the infra-red energy source and infra-red energy detector and the mirror means so as to intercept the infra-red energy path between the infra-red energy source and the infra-red energy detector.

Preferably the non-transmitting support wall is formed of a material which can conduct heat, such as a metal or carbon fibre. A suitable metal is aluminium. Waste heat produced by the electrical control arrangement in the first chamber can thus be conducted away. The waste heat also serves to keep the infra-red- transmitting window above ambient temperature and stops condensation from occurring thereon.

The second gas-permeable chamber should allow free movement of gas and air into and out of the chamber, but preferably be constructed so that dirt particles which may affect the functioning of the gas detector, do not enter. This may be achieved by encasing the permeable chamber with a perforated cover.

It is a further preferred feature that the second gas-permeable chamber has a structure which comprises a heat conductive material in communication with the support wall and the mirror means so that waste heat is conducted from the support wall, along the conductive material to the mirror means so as to keep the mirror means free from condensation. The heat conductive material may be of any structural form and may in particular be a series of pillars which support the mirror means, or more preferably a rigid perforated tube, at one end of which the mirror means is connectively attached. The heat conductive material may be a metal or carbon fibre arrangement, in the latter case with the carbon fibres directionally aligned between the support wall and the mirror means.

Optionally, lengths of pipe may be attached to the openings of the passageway; One pipe to supply the calibrating gas and the other to exhaust the gas. This is to facilitate supply of a calibration gas from a remote location.

In order to calibrate the gas detector, a known concentration of a calibrating gas in air is supplied at a fixed rate to the cavity, which is of known dimension, whilst the gas-permeable chamber is open to an atmosphere which is substantially free of the gas of interest.

Typically the calibrating gas is provided at rates less than 1 litre/min.

Optionally the permeable chamber may be flushed with air during calibration so as to minimise any gas in the permeable chamber which may affect calibration of the gas detector.

Preferably after calibration, the hollow cavity is flushed with air to remove any calibrating gas remaining, which could affect the normal functioning of the gas detector.

Embodiments of the present invention will now be described by way of example with reference to the following drawings presented herein. A description of a prior art device is included for comparison.

Figure 1 shows a cross-sectional view of a generally cylindrical prior art gas detector;

Figure 2 shows a corresponding cross-sectional view of an embodiment of a gas detector of the present invention;

Figure 3 shows a schematical representation of a further embodiment of a gas detector of the present invention;

The infra-red gas detector 1 shown in Figure 1 comprises a first chamber 3 and a second gas-permeable chamber 5. The chambers 3,5 are separated by a wall 7. Chamber 3 may be hermetically sealed if so desired.

The first chamber 3 houses an electrical control device 9 which is electrically connected by leads 11 and 13 to an infra-red energy source 15 and an infra-red energy detector 17 respectively.

The wall 7 has an infra-red transmitting sapphire window 19 disposed therein to allow infra-red energy to pass therethrough. The infra-red energy source 15 and the infra-red energy detector 17 are positioned adjacent the window 19 so that infra-red energy may be transmitted from the energy source 15 through the window 19 and received by the energy detector 17 through the window 19.

The second chamber 5 comprises a peripheral structure formed by a series of plastic pillars 21 on which a mirror 23 is positioned to reflect the energy transmitted from the energy source 15 back to the energy detector 17. The pillars 21 have electrical heaters 25 disposed therein and by heat convection prevent the mirror 23 from misting up due to condensation. The pillars 21 and the mirror 23 are encased by a perforated cover 27.

The gas detector 1 is used in the following manner; infra-red energy is emitted from the energy source 15 through the window 19 and through the air space 29 of the second chamber to the mirror 23. Infra-red energy is then reflected by the mirror 23 back through the air space 29 and the window 19 to the energy detector 17. Typically the energy includes two separable wavelengths one of which (the sample energy wavelength) is attenuated by the detectable gas whereas the other (the reference energy wavelength) is not. The control device 9 determines the difference in quantum between the sample and reference energy, allowing determination of whether a detectable gas is present or not. Thus, the difference quantum is used as a measure of the amount of detectable gas present in the air space 29.

In order to calibrate the gas detector 1, a calibration gas 31 (which should be the same as the gas to be detected) , is blown through a port 33 and into the air space 29 of the second chamber 5. The calibration gas 31 is blown at a specific rate for a number of minutes (typically greater than 5 litres/min) to achieve a required concentration of gas in the air space 29. The reading from the control device 9 is then adjusted so that the gas detector 1 determines that the measured amount of gas detected is in agreement with the known concentration of calibration gas 31 in the air space 29.

The infra-red gas detector 40 shown in Figure 2 is essentially the same as the detector 1 shown in Figure 1 and like parts are correspondingly numbered. However, the wall 7 in detector 40 is modified in accordance with one example of the present invention. In detector 40, the wall 7 is of increased thickness to accommodate a hollow cavity 52 which intercepts the path of infra-red energy between the source 15 and the detector 17. The cavity 52 is located within the thickness of the wall 7 and is connected to a passageway 53 also within the thickness of the wall 7. The passageway 53 extends diametrically through the wall 7 to emerge at ports 54,55 in the side wall of the first chamber 3. The wall 7 is made of heat conductive aluminium and the window 19 is formed by two window elements 56,57 spaced on either side of the cavity 52. The hollow cavity 52 and the passageway 53 are of narrow bore to prevent ingress of gas or moisture occurring under normal atmospheric operating conditions.

The peripheral structure of the second chamber 5 is also modified in the detector 40 and comprises a perforated carbon fibre tube 62 which has the fibres lying in a longitudinal or axial direction. A first end 64 of the tube 62 is in thermal conductive communication with the wall 7 and a second end 66 of the tube 62 is in thermal conductive communication with the mirror 23. The tube 62 and the mirror 23 are encased by a perforated cover 27. The waste heat generated in the first chamber 3 by the electrical control device 9 is conducted away by the thermally conductive wall 7 in part to heat the window elements 56,57 and in part to be conducted by the carbon fibre tube 62 to heat the mirror 23. The conducted heat is normally sufficient to prevent the elements 56, 57 and the mirror 23 from misting up due to condensation.

In order to calibrate the gas detector 40, a calibration gas 72 is blown at a specific rate (typically less than llitre/ in) into the port 54 so as to pass into and fill the hollow cavity 52. Calibration gas is exhausted from the passageway 53 at port 55. A required concentration of gas is achieved in the hollow cavity 52 and the detector 40 is calibrated to agree with this. At the same time air may be blown into the second chamber 5 by way of a port 78, so as to reduce any gas present in the air space 29 (which may alter the calibration of the detector) to negligible levels.

The hollow cavity 52 is a known fraction of the size of the air space 29 of the second chamber 5 and this is taken into account when calibrating the detector 40. That is to say, if the energy path length through the hollow cavity 52 is a tenth of the energy path length through the air space 29, the required concentration of the calibrating gas 72 in the hollow cavity 52, will be ten times greater than that previously required for calibration in the air space 29.

Before using the detector 40, after calibration, the hollow cavity is flushed with air to remove any calibrating gas 72, which would effect the detector 40 in use.

If the detector 40 is positioned in a remote or inaccessible location, as schematically represented in Figure 3, pipes 82,84 and 86 may be connected to the ports 54,55 and 78 respectively. This allows the detector to be calibrated in situ while supplying calibrating gas 72, or air, remotely.

The detector of the present invention provides a number of advantages over prior art detectors.

By providing the calibrating gas through the hollow cavity, the calibrating gas may be isolated from the external atmosphere. This allows a rapid and accurate response, even in windy conditions, to be achieved in less than a minute. Furthermore, less calibration gas is used and the time spent calibrating the device is shorter, thus reducing maintenance costs.

Also, by attaching pipes 82 and 86 (and optionally pipe 84) to the device, it is possible to calibrate the detector in areas where it may not easily be removed, or even got at, such as narrow mine shafts. It is particularly desirable of course to connect a pipe to the exhaust port 55, so that the calibrating gas can be removed from the air surrounding the detector.

By constructing the wall 7 and the perforated tube 62 out of heat conductive materials, heaters are not required, which results in an energy saving in running the gas detector.

Claims

1. An infra-red gas detector comprising a first chamber housing an electrical control arrangement connected to an infra-red energy source and an infra-red energy detector and a second chamber which is gas permeable and has a mirror for reflecting infra-red energy, the arrangement being such that, in operation, infra-red energy is directionally emitted along a path from the source towards the mirror in the second chamber and is reflected therefrom towards the detector in the first chamber, the detected level of infra-red energy providing a measure of IR absorbing gas within the second chamber the improvement being that the detector further comprises a hollow cavity formed of infra-red transmissive material and, which is positioned to intercept the infra-red energy path, the cavity being accessible by way of a passageway such as to allow a calibrating gas to be supplied to said hollow cavity for calibrating the infra-red gas detector.

2. An infra-red gas detector according to claim 1 wherein the first and second chambers are mutually isolated by a wall means which comprises an infra¬ red transmitting window intercepting the energy path and carried within a non-transmitting support wall, and the cavity is formed within the thickness of said window.

3. An infra-red gas detector according to claim 2 wherein the window is formed of a single piece, through which the cavity extends.

4. An infra-red gas detector according to claim 2 wherein the window is formed of two pieces between which the cavity extends.

5. An infra-red gas detector according to any preceding claim, wherein the cavity has a width of 5mm, and the associated passageway has a width of 2mm.

6. An infra-red gas detector according to any one of claims 2-5 wherein the non-transmitting support wall is formed of a heat conducting material.

7. An infra-red gas detector according to any preceding claim wherein the second-gas permeable chamber has a structure in communication with the first chamber and the mirror, and wherein said structure comprises a heat conductive material.

8. An infra-red gas detector according to claim 7 wherein the structure is formed of a series of pillars which support the mirror.

9. An infra-red gas detector according to claim 7 wherein the structure is a rigid perforated tube at one end of which the mirror is attached

10. An infra-red gas detector according to any preceding claim wherein the second gas-permeable chamber is encased with a perforated cover designed to minimise ingress of dirt particles to the second chamber.

11. An infra-red gas detector according to any preceding claim wherein lengths of pipe are attached to the openings of the passageway to faciliate supply of a calibration gas from a remote location.

12. An infra-red gas detector substantially as hereinbefore described with reference to any one of the embodiments illustrated in the accompanying drawings.