WO2006006006A1

WO2006006006A1 - Method for calibrating a gas analyser

Info

Publication number: WO2006006006A1
Application number: PCT/IB2005/001797
Authority: WO
Inventors: Gérard STEHLE; Arnaud Aguzzi
Original assignee: Hach Ultra Analytics
Priority date: 2004-06-29
Filing date: 2005-05-30
Publication date: 2006-01-19
Also published as: CH697107A5

Abstract

Method for calibrating a device for measuring the concentration of a gas dissolved in a liquid consists in analysing a standard solution by a measuring device, wherein said standard solution is obtainable by mixing the determined amounts of one or several reagents with a first liquid in a closed recipient having a determined volume in such a way that a gas and a second liquid different from the first liquid are produced by reaction until an equilibrium is obtained, wherein at least one part of produced gas is dissolved in the second liquid, and in calibrating the measuring device according to the gas concentration measured at the analysis stage and the gas reference concentration associated with a standard solution, wherein said reference concentration takes into account the gas solubility in the second liquid.

Description

Method of calibrating a gas analyzer

The present invention relates to a method of calibrating a gas analyzer, more particularly to an apparatus for measuring the concentration of a gas dissolved in a liquid.

Such gas analyzers are known, allowing for example to determine the concentration of carbon dioxide (CO ₂ ) in a carbonated beverage such as water, beer or soda. Some of these analyzers operate by directly measuring infrared concentration of the gas in the liquid, others by measuring the total pressure of the gas mixture in equilibrium with the liquid and deducing from this total pressure the concentration of the gas in the liquid by a calculation, and still others by measuring the partial pressure of the gas in equilibrium with the liquid and using previously stored solubility data to deduce the concentration of the gas in the liquid. Like most measuring devices, these gas analyzers must be calibrated before their first use and their calibration must be checked regularly. Different methods exist to calibrate or verify these analyzers. One of them, in the case of analyzers measuring total or partial pressure, to calibrate the analyzer pressure and temperature in a manner comparable to a conventional pressure sensor. Another method is to prepare a standard solution formed by the gas dissolved in a liquid, to have this standard solution analyzed by the analyzer, to compare the concentration of the gas indicated by the analyzer with a reference concentration of this gas in the liquid. standard solution, and to calibrate the analyzer, that is to say make him apply a corrective factor, depending on the difference between these two concentrations if they are indeed different. This second method appears preferable to the first because it takes into account any physical effects due to the presence of the liquid. To prepare the standard solution, a known amount of the gas to be analyzed can be introduced into a container of known volume containing a known volume of liquid (typically water) and the liquid and the gas agitated to dissolve the latter. These operations are, however, in practice quite complicated to implement. A simpler alternative to achieve is to mix a known amount of one or more reagents with a liquid (typically water) to generate a reaction producing the above gas until an equilibrium is achieved where at least a portion of the gas produced is dissolved in the liquid resulting from the reaction. By way of example, in the case of CO ₂ , commonly used reagents are sodium bicarbonate and citric acid.

The present inventors have found that the above calibration method using a standard solution obtained using reagents, as currently applied, is inaccurate and leads to measurement errors that may be significant. The present invention aims to remedy this drawback and proposes for this purpose a calibration method according to appended claim 1, a method for preparing a calibration kit according to claim 9 and a calibration kit according to claim 14 particular embodiments of the invention being defined in the dependent claims. The present invention is based on the finding that the reagents used to produce the standard solution have a significant influence on the solubility of the gas and that this influence must be taken into account in determining the reference concentration of the gas in the standard solution. In other words, the solubility of the gas in the final liquid (after reaction) of the standard solution is not the same as that in the initial liquid (typically water) to which the reactants have been mixed. However, to the knowledge of the inventors, this reference concentration was, up to the present invention, determined by calculating the mass of gas produced on the basis of the amount of reagents used and taking into account the solubility of the gas in the liquid. initial, and not in the final liquid, to deduce the concentration of dissolved gas. Sometimes even this reference concentration was determined simply by calculating the mass of gas produced and dividing that mass by the mass of liquid. In addition, the standard solution was often analyzed by a gas analyzer designed or configured for a particular liquid, such as wine, beer, soda, etc., without taking into account the difference in solubility of the gas in this particular liquid. relative to the solubility of the same gas in the standard solution. The calibration error made with these methods could reach 10%. Taking into account the solubility of the gas in the liquid mixture resulting from the reaction, the present invention makes it possible to obtain a much better calibration accuracy, typically less than 1%.

Several embodiments of the calibration method of the invention will now be described in detail. As already indicated, this calibration method is intended to be applied to a gas analyzer, more specifically to an apparatus capable of measuring the concentration of a gas dissolved in a liquid. The gas analyzers of particular interest here are analyzers designed to measure a concentration of gas in a liquid by detecting the partial pressure of the gas in equilibrium with the liquid. Such gas analyzers are currently marketed under the trademark "Orbisphere" by the present applicant. They are designed, or can be configured, for the analysis of a specific gas, such as carbon dioxide

(CO ₂ ), nitrogen (N ₂ ) or hydrogen (H ₂ ), in a specific liquid, such as water, wine, beer or soda. For this purpose, they contain in a memory data of solubility of the determined gas in the liquid determined as a function of the temperature, data presented for example in the form of a table or a formula, and comprise a calculator (microprocessor ) programmed to calculate determined gas concentration values in the liquid determined as a function of measured partial pressure values and solubility data. It is recalled that the solubility of a gas in a liquid is the distribution of the gas between the liquid phase and the gas phase at a given temperature when the system is in equilibrium (equality of the partial pressures of the gas in the liquid phase and in the gaseous phase). Solubility is related to the partial pressure of the gas and the concentration of the gas dissolved in the liquid by the law of

Henry: Solubility = Dissolved Gas Concentration / Partial Gas Pressure

Tables or solubility formulas exist in the literature for the examples of liquids mentioned above.

The calibration method according to the invention comprises preliminary steps for preparing a calibration kit, made for example by the manufacturer or the seller of the gas analyzer or by an independent supplier of the manufacturer and the seller, and calibration steps themselves, performed by the gas analyzer using the calibration kit and under the control of the user.

The calibration kit comprises: - a container of determined volume comprising a stopper,

respective predetermined quantities of one or more reagents and a first liquid, the reagent (s) and the first liquid being chosen so that, mixed together in the aforesaid container, they generate a reaction producing the determined gas and a second liquid, or reference liquid, different from the first liquid, at least a portion of the product gas being, in equilibrium, dissolved in this reference liquid and forming with it a standard solution,

a table indicating, as a function of temperature, concentration values of the determined gas in the reference liquid contained in the container after the above-mentioned reaction and values of equivalent concentration of the determined gas in other liquids such as water; , wine, beer, soda, etc.

The reagent (s) and the first liquid are supplied in suitable closed containers, one of which may be the aforesaid container in which the mixed. The mixture of the reagent (s) and the first liquid will be made by the user just prior to calibration to minimize gas losses.

The values of concentration and equivalent concentration indicated in the table are expressed for example in V / V or g / kg. For each temperature, a concentration value of the determined gas in the reference liquid (standard solution) and respective values of equivalent concentration of the same gas in the other liquids are indicated. The expression "equivalent concentration" is understood in the context of the invention of a concentration of the gas dissolved in a liquid other than the reference liquid which would give, at a given temperature, a partial pressure of the gas in equilibrium with this other liquid identical to the partial pressure of the gas in equilibrium with the reference liquid in the standard solution at the same temperature. Thus, for each temperature, the table indicates a concentration value and equivalent concentration values corresponding to the same partial pressure of the determined gas.

As an example, a table of CO ₂ concentrations in the reference liquid (standard solution) and equivalent concentrations of CO ₂ in water, beer and cola is given below:

The standard solution, in this example, is obtained by diluting 6.81 g of sodium bicarbonate and 20 g of citric acid in 555 g of water contained in a 597 ml bottle. The liquid resulting from the reaction (reference liquid) is a mixture of water and acidic sodium citrate. As can be seen, the CO ₂ concentration of this standard solution is 3.055 V / V at 20 ^° C. The partial pressure of CO ₂ in the neck of the bottle is 3.715 bar. To obtain the same equilibrium partial pressure with a solution of CO ₂ dissolved in water (respectively beer or cola), it would have been necessary to dissolve, at the same temperature, 3,207 V / V (respectively 3,096 V / V or 3,318 V / V) of CO ₂ .

According to the invention, the values of concentration and equivalent concentration indicated in the table take into account the solubility of the gas determined in the reference liquid. More particularly, the concentration values indicated in the table are obtained in the following manner, at a given temperature: the quantity (mass or number of moles) of reagents In order to form the standard solution, the total amount (mass or number of moles) of the determined gas that will be produced by the reaction when these reagents are diluted in the first liquid inside the container provided for this purpose is deduced. From the solubility of the determined gas in the reference liquid (obtained according to a method which will be described later) and from the total quantity of the determined gas produced by the reaction, the quantity (mass or number of moles) of the determined gas present is deduced. in the gaseous phase and that present in the liquid phase (reference liquid). This quantity of the determined gas in the reference liquid makes it possible to determine the concentration of the determined gas dissolved in the reference liquid, the volume of the reference liquid being known. From the quantity of the gas determined in the gas phase and the volume of the gas phase (equal to the difference between the known volume of the vessel and the volume of the liquid phase), the ideal gases (PV≈) are deduced by the law. nRT) the partial pressure of the determined gas in equilibrium with the reference liquid. Finally, for each of the liquids other than the reference liquid, namely for example water, wine, beer, cola, etc., it is deduced from the partial pressure of the determined gas, using the law of Henry and a table or known formula of solubility of the gas determined in the liquid as a function of temperature, the equivalent concentration of the gas determined. Prior to these steps, the following method is applied to measure the solubility of the determined gas in the reference liquid: a first container of known volume is prepared containing a known volume of the first liquid (water) in which the reagent (s) is mixed. in proportions corresponding to those of the calibration kit. The reaction that follows produces the determined gas and the reference liquid. After this reaction, the first can is evacuated so as to evacuate the air and the determined gas and to keep in this first can only the reference liquid. A second can of known volume is prepared containing a known pressure of the determined gas. The two cans are put in communication then are agitated until reaching a balance where a part of the determined gas is dissolved in the reference liquid and another part remains in gaseous form, above the liquid. Using a pressure sensor, the equilibrium pressure of the determined gas is measured at different system temperatures. Using the ideal gas law (PV≈nRT), then, for each temperature value, the number of moles of the determined gas present in the gas phase is deduced, then, on the basis of the initial known quantity of reagents and the first liquid, the number of moles and the concentration of the determined gas dissolved in the reference liquid. Using Henry's law, solubility values of the determined gas in the reference liquid are then determined as a function of temperature. A solubility curve is then plotted and a curve fit is made on this curve, to obtain an equation or solubility formula.

To calibrate the gas analyzer, the user first prepares the standard solution based on the calibration kit available. To do this, it introduces into the container of the kit for mixing that or the reagents or reagents and the first liquid which are contained in a separate container, then closes the container with the cap. The user then agitates the container so that the reagent (s) and the first liquid mix to reach an equilibrium where a part of the gas produced by the reaction is dissolved in the liquid mixture resulting from the reaction and another part of this gas is in the gaseous phase, above the liquid.

In the particular application of the invention in which the determined gas is CO ₂ , the first liquid (water) and one of the reagents (citric acid) are already contained in the container intended for mixing. The other reagent (sodium bicarbonate) is supplied in a test tube. To prepare the standard solution, the user introduces the test tube into the container, closes it and then turns it over and shakes it to dilute the sodium bicarbonate in the water-citric acid mixture. The volume occupied by the test tube in the container is taken into account during the preparation of the concentration table to determine the volume of the gas phase.

The actual calibration of the gas analyzer is performed by having the standard solution analyzed by the gas analyzer. The procedure for introducing the standard solution into the analyzer is of course specific to each analyzer. In the case, for example, selective analyzers of the brand Orbisphere, operating by partial pressure measurements, it comes to pierce the cap of the container in a sealed manner with a hollow knife connected to a pipe. The standard solution then passes into the pipe to flow into a chamber inside the analyzer and come into contact with a gas-liquid separation membrane. The analyzer measures the partial pressure and the temperature of the determined gas (eg CO ₂ ) passed through the membrane as the liquid continues to flow. Then, using Henry's Law and a table or solubility formula that it contains in memory, the analyzer determines a corresponding concentration value of the determined gas in a particular liquid, as explained below.

In a first embodiment of the invention, the analyzer is designed or configured to analyze the determined gas in a liquid other than the reference liquid, that is to say in a liquid (water, wine, beer , soda, etc.) for the analysis of which the user normally uses his analyzer. In this case, the table or solubility formula stored in the analyzer corresponds to this other liquid and the concentration value indicated by the analyzer is in fact an equivalent concentration value of the determined gas in this other liquid. To calibrate the analyzer, the user therefore indicates to the analyzer the equivalent concentration value entered in the table for this other liquid and for the temperature at which the analysis was made. If these two equivalent concentration values are different, the analyzer determines a corrective factor according to a procedure known per se, a corrective factor that will be applied to the concentration values measured during the next analyzes. If the two aforementioned equivalent concentration values are identical, the analyzer concludes that it is correctly calibrated and therefore there is no need for a corrective factor. Note that in this first embodiment, the table could contain only the equivalent concentration values in the liquid for which the analyzer is designed or configured. In a second embodiment of the invention, the analyzer can be set in at least two different configurations, namely a normal use configuration of the analyzer, for measuring concentrations of the determined gas in a liquid ( water, wine, beer, soda, etc.) other than the reference liquid, and a calibration configuration. The analyzer contains in memory a table or formula for solubility of the determined gas in the other liquid as a function of temperature, used in the normal use configuration, and a table or formula for solubility of the determined gas in the reference liquid. depending on the temperature, used in the calibration setup. To calibrate the analyzer, the user sets the analyzer in the calibration configuration, the standard solution is analyzed and the analyzer is informed of the concentration value of the determined gas in the reference liquid in the table for the temperature. to which the analysis was made. Note that in this second embodiment, the table could contain only the concentration values in the reference liquid. In a third embodiment of the invention, the analyzer is designed or configured to analyze the determined gas in a liquid other than the reference liquid, and the composition of the reference liquid is adjusted so that the solubility curve of the The gas determined in this reference liquid as a function of temperature is substantially the same as that in the liquid for which the analyzer is designed or configured. To do this, ingredients are added to the first liquid and reagents before or during the preparation of the standard solution, which ingredients will modify the solubility of the determined gas in the reference liquid. These ingredients are for example sugar and / or alcohol. By way of example, in the particular application of the invention in which the determined gas is CO ₂ obtained by mixing sodium bicarbonate and citric acid in water, a sugar concentration of between 20 g / l and 100 g / l and / or an alcohol concentration in the range of 2 to 10% makes it possible to approach the solubility of CO ₂ in beer (the precise concentrations of sugar and / or alcohol to be chosen depending, of course, on the type of beer). To calibrate the analyzer, the user has the standard solution analyzed and then indicates to the analyzer the concentration value entered in the table in the column corresponding to the liquid for which the analyzer is designed or configured and for the temperature measured by the analyzer. analyzer. Note that in this third embodiment, the table could contain only the concentration values in the liquid for which the analyzer is designed or configured.

Taking into account the solubility of the determined gas in the reference liquid, as described above, the present invention makes it possible to significantly increase the accuracy of the reference concentration values used for calibration. It will be noted in particular that, in the example given above, if the standard solution is assimilated to water containing dissolved CO ₂ , as is often the case in the prior art, the error made at a temperature of 20 ° C. ⁰ C is about 4.7%, and if we equate the standard solution with another liquid for which the analyzer is designed or configured, as is also sometimes done, the error can be even more important (About 8% in the case of cola at 20 ⁰ C). The present invention has been described above by way of example only. It goes without saying that modifications can be made without departing from the scope of the invention. In particular, the invention can be applied to gases other than CO ₂ , for example SO ₂ (sulfur dioxide), which can be obtained by reaction between sodium sulphite and citric acid, or H ₂ (hydrogen), which can be obtained by reaction between an acid and a metal (zinc, iron, aluminum, etc.).

Claims

1. A method of calibrating a device for measuring the concentration of a gas dissolved in a liquid, characterized in that it comprises the following steps:

a standard solution obtained by mixing determined quantities of one or more reagents and a first liquid in a closed container of a determined volume so as to produce by reaction said gas and a second liquid different by the measuring device. from the first liquid to an equilibrium where at least a portion of the product gas is dissolved in the second liquid, and

- calibrate the measuring apparatus according to a concentration of the gas measured by the measuring apparatus during the analysis step and a reference concentration of the gas associated with the standard solution, this reference concentration holding account of the solubility of the gas in the second liquid.

2. Method according to claim 1, characterized in that the concentration measured by the measuring apparatus and the reference concentration are concentrations of the gas in the second liquid.

3. Method according to claim 1 or 2, characterized in that the measuring apparatus is an apparatus which, for measuring a concentration of the gas in a given liquid, operates by measuring the partial pressure of the gas in equilibrium with the given liquid. and then determining the concentration as a function of the measured partial pressure and gas solubility data in the given liquid.

4. Method according to claim 3, characterized in that the concentration measured by the measuring apparatus during the analysis step and the reference concentration are equivalent concentrations of the gas in a liquid other than the second liquid, c that is to say concentrations which would give, at a given temperature, a partial pressure of the gas in equilibrium with this other liquid identical to the partial pressure of the gas in equilibrium with the second liquid in the standard solution at the same temperature.

5. Method according to claim 4, characterized in that the other liquid is a liquid for which the measuring apparatus is designed or configured.

6. Method according to claim 3, characterized in that the measuring apparatus can be adjusted according to at least one normal use configuration, for the measurement of concentrations of the gas in a liquid other than the reference liquid, and a configuration calibration, and in that the measuring apparatus contains in memory gas solubility data in the other liquid, used in the normal use configuration, and gas solubility data in the second liquid, used in the calibration configuration.

Method according to claim 3, characterized in that the measuring apparatus is designed or configured to measure concentration values of the gas in a liquid other than the second liquid, and in that the standard solution contains at least one ingredient altering the solubility of the gas in the second liquid so that this solubility is substantially the same as the solubility of the gas in the other liquid.

8. Method according to any one of claims 1 to 7, characterized in that it comprises a preliminary step of preparing the standard solution.

9. A method of preparing a calibration kit for calibrating an apparatus for measuring the concentration of a gas dissolved in a liquid according to the method defined in any one of claims 1 to 8, characterized in that it comprises the following steps: - preparing a container of determined volume comprising a stopper, - preparing determined quantities of one or more reagents and a first liquid chosen so that, mixed together in the container, they produce by reaction, a standard solution comprising said gas dissolved in a second liquid different from the first liquid, and - preparing reference gas concentration data associated with the standard solution, taking into account the solubility of the gas in the second liquid.

10. Process according to claim 9, characterized in that the step of preparing said data consists in preparing a table containing equivalent gas concentration values in at least one liquid other than the second liquid as a function of the temperature, each value of equivalent concentration corresponding to a pressure of the gas in equilibrium with the other liquid identical to the pressure of the gas in equilibrium with the second liquid in the standard solution at the same temperature.

11. The method of claim 9 or 10, characterized in that to determine the solubility of the gas in the second liquid, the following steps are implemented:

mixing, in a first container of known volume, known quantities of the reagent (s) and the first liquid in proportions corresponding to those of the calibration kit to generate a reaction producing said gas and the second liquid,

evacuating the first container so as to keep in this first container only the second liquid, preparing a second container of known volume, containing a known pressure of said gas,

placing the first and second containers in communication with one another and then stirring them until an equilibrium is reached where part of said gas is dissolved in the second liquid and another part remains in gaseous form;

measure the equilibrium pressure of said gas at different temperatures,

deducing from this equilibrium pressure and from the quantity of reagents and from the first liquid the concentration of said dissolved gas in the second liquid and then, using Henry's law, solubility values of said gas in the second liquid as a function of the temperature.

12. Method according to any one of claims 1 to 11, characterized in that said gas is carbon dioxide, the first liquid is water and the reactants are an acid and a bicarbonate.

13. Method according to any one of claims 1 to 11, characterized in that said gas is hydrogen, the first liquid is water and the reactants are an acid and a metal.

14. Calibration kit for the calibration of an apparatus for measuring the concentration of a gas dissolved in a liquid according to the process defined in any one of Claims 1 to 8, characterized in that it comprises: a container of determined volume comprising a stopper,

determined quantities of one or more reagents and a first liquid chosen so that, mixed together in the container, they produce by reaction a standard solution comprising said gas dissolved in a second liquid different from the first liquid, and reference gas concentration data associated with the standard solution, these data taking into account the solubility of the gas in the second liquid.

Calibration kit according to claim 14, characterized in that said data are in the form of a table containing values of equivalent concentration of the gas in at least one liquid other than the second liquid as a function of the temperature, each equivalent concentration value corresponding to a pressure of the gas in equilibrium with the other liquid identical to the pressure of the gas in equilibrium with the second liquid in the standard solution at the same temperature.