GB2299864A

GB2299864A - Determining type and amount of an electrochemically reactive sample in a gas

Info

Publication number: GB2299864A
Application number: GB9607462A
Authority: GB
Inventors: Herbert Kiesele; Uwe Kuehn
Original assignee: Draegerwerk AG and Co KGaA
Current assignee: Draegerwerk AG and Co KGaA
Priority date: 1995-04-15
Filing date: 1996-04-10
Publication date: 1996-10-16
Anticipated expiration: 2016-04-10
Also published as: AU675765B2; DE19514215C2; CA2173904C; GB9607462D0; FR2733054B1; DE19514215A1; AU5061596A; GB2299864B; CA2173904A1; FR2733054A1

Description

2299864 METHOD FOR DETERMINING THE TYPE AND/OR AMOUNT OF AN

ELECTROCHEMICALLY CONVERTIBLE SUBSTANCE IN A GAS SAMPLE This invention relates to a method for determining the type and/or amount of an electrochemically convertible substance in a gas sample, which substance in a measuring cell produces a measured physical variable which changes over time, rises from a reference value to a maximum value and falls back to the reference value again.

A device for measuring the concentration of alcohol as a substance to be detected in respiratory gas is known from US-A-4770026. In this known device, a fuel cell is gassed with a gas sample having a portion of alcohol vapour, and the measured physical variable i(t) which is obtained by electrochemical conversion is supplied to an evaluating circuit which establishes a value which is proportional to the alcohol vapour concentration, by integrating the signal of the measured physical variable over the time. When alcohol vapour is admitted to the measuring cell, the measurement signal first of all rises, starting from a reference value, passes through a maximum value i and, after the complete electrochemical conversion of the alcohol, falls back to a minimum value in the vicinity of the reference value. The region which is enclosed between the operational value of the measurement signal and the reference value represents the electrical charge that has flowed and is proportional to the amount and concentration of the alcohol vapour in the gas sample.

In this known measuring cell, the curve of the measurement signal changes with increasing ageing of the measuring cell. Thus, the curve becomes flatter and wider with the passage of time. A similar change in the curve of the measurement signal can result from a plurality of measuring cycles following each other -) in quick succession, in which case the change in the signal can be at least partially reversed by a comparatively long recovery phase. Because of the fact that, in order to determine the concentration of the alcohol vapour in the gas sample integration is carried out over almost the whole course of the measurement signal, the changes in the curve affect the area and thus also the accuracy of measurement, so that calibrating cycles with a gas sample having a known alcohol concentration have to be carried out repeatedly. Such calibrating cycles make the apparatus more difficult to operate, in particular if the measuring cell is to be gassed in short time intervals. Apart from this, in the case of the known evaluating method, the composition of the substance to be detected has to be known. Accompanying substances such as methanol or acetone are not recognised in this type of evaluation.

DE-A-4344196 relates to a determination method of generic type, in which method quantitative conclusions about the amount and type of the substance to be detected are made by integration over sections of the curve of the measured variable.

All known evaluating methods have disadvantages, in particular the high calculation cost in the case of integration methods as well as the dependency of the shape of the curve on the age of the sensor, on temperature effects and boundary effects. Accompanying substances are not recognised, nor are deviating curve shapes, which are thus attributed to faulty sensors or to apparatus defects.

The object of the present invention is to improve an evaluating method for electrochemical measuring cells, in such a way that the type and/or amount of an electrochemically convertible substance in a gas sample c n be established in a fast and reproducible manner.

is According to the present invention, there is provided a method for determining the type and/or amount of an electrochemically convertible substance in a gas sample, which method comprises subjecting the substance in the sample to electrochemical conversion in a measuring cell to produce a current whose intensity changes over time in that it rises from a reference value to a maximum value and then falls back to the reference value again, wherein the type and/or amount of the substance is established from the linear relationship between the current intensity and the charge that has flowed in the time period after the maximum value of the current intensity has been reached.

Preferably, the total amount of charge that has flowed, and thus the amount of the substance (which is directly proportional thereto), are established from the linear relationship between the current intensity and the charge that has flowed as a value corresponding 20 to a current intensity of zero.

Preferably, from the linear relationship between the current intensity and the charge that has flowed, a proportionality constant is used to determine the type of the substance.

Preferably, the relationship between the current intensity and the charge that has flowed is effected with the aid of a microprocessor.

Preferably, the electrochemically convertible substance is ethanol, methanol or a mixture of ethanol 30 and methanol.

According to the invention, the physical variable which is measured in the current intensity. One substantial advantage of the invention arises as a result of the fact that the linear relationship between 35 the current intensity and the charge that has flowed "I forms very easily a determination of the type and/or amount of the substance to be detected. Furthermore, temperature and ageing effects during the determination can be reduced, because only the linear range of the measurement curve in the time period directly after the maximum value, is taken as a basis for the current intensity.

Surprisingly, it was now discovered that coulometric analysing methods (which were used hitherto in the case of liquid samples) and the underlying theory could also be used in the gas phase. This means in particular that the current intensity and the charge that has flowed, have a linear relationship, that the total amount of charge that has flowed, from the state where the current intensity is zero, can be established from the corresponding linear equation/straight line, and that, ultimately, from the total charge that has flowed, the amount/concentration of the substance in a gas sample can be determined.

A description of the known theory is found, for example, in the book "Instrumental Methods in Electrochemistry" by R. Greef et al, 1985, pages 44-47 The determination of ethanol in particular, in gas samples was tested for the present invention.

Reference will now be made, by way of example, to the drawings in which Figures 1 and 2 are graphs that will be explained hereinafter.

Figure 1 shows a typical measurement curve of current intensity I (in kLA) ("sensor current") as a function of time t (in seconds) in the determination of ethanol using an electrochemical sensor.

If the current intensity I (in LA) is instead measured as a function of the charge that has flowed (in IiC), as is represented graphically in Figure 2 for a two-electrode operation and a three-electrode operation of an electrochemical measuring cell (reference numeral 2 relating to the two-electrode operation and reference numeral 1 relating to the three-electrode operation), then, if the section of the curve after the maximum is linear, the total amount of charge that has flowed, as a consequence of the electrochemical conversion of the ethanol, is obtained. This results from theoretical considerations for potential-controlled coulometry, namely that in the case of a linear relationship between current intensity and charge, the total charge that has flowed results from the condition that the current intensity is equal to zero on the charge axis. This likewise applies to a defined measuring cell.

In the example shown in Figure 2, the gradient of the linear section after the maximum is the same for both curves, this obviously being an "indicator" that the material measured is the same. Numeral 3 represents the regression straight lines. In the further course of the curves, the gradient and appearance of the curves changes, which could be an indication that continuing electrochemical reactions of the ethanol (i.e. electrochemical reactions of the ethanol that are subsequent with respect to time), are characteristic of the three-electrode sensor (curve 1).

In all, the fact that the linear portion of the curve after the maximum is characteristic of the material (i.e. the fact that the presence of additional substances which influence the measuring method, e.g. methanol, leads to a changed gradient of the straight line and can consequently be recognised) was supported by measurements. For various measuring cells, parallel straight lines are obtained for a given substance, e.g. ethanol. The total amount of charge that has flowed is cons equently different, as can be seen in Figure 2. The correlation coefficient for the regression straight lines 3 permits conclusions to be drawn regarding the quality of the sensor and can consequently be used for -I operational control.

In comparison with the prior art, the present invention permits a considerably faster and more reliable evaluation of the measurement, because the straight line can be plotted with few measurement points and it is no longer necessary to carry out complete evaluation.

In practical usage, the evaluation of the measurement and the calculation of the ultimately desired concentration of the substance which is tested is effected with the aid of suitable electronic components, in particular using a microprocessor.

X

Claims

1. A method for determining the type and/or amount of an electrochemically convertible substance in a gas sample, which method comprises subjecting the substance in the sample to electrochemical conversion in a measuring cell to produce a current whose intensity changes over time in that it rises from a reference value to a maximum value and then falls back to the reference value again, wherein the type and/or amount of the substance is established from the linear relationship between the current intensity and the charge that has flowed in the time period after the maximum value of the current intensity has been reached.

2. A method according to claim 1, wherein the total amount of charge that has flowed, and thus the amount of the substance (which is directly proportional thereto), are established from the linear relationship between the current intensity and the charge that has flowed as a value corresponding to a current intensity of zero.

3. A method according to claim 1 or 2, wherein, from the linear relationship between the current intensity and the charge that has flowed, a proportionality constant is used to determine the type of the substance.

4. A method according to any of claims 1 to 3, wherein the relationship between the current intensity and the charge that has flowed is effected with the aid of a microprocessor.

5. A method according to any of claims 1 to 4, wherein the electrochemically convertible substance is ethanol, methanol or a mixture of ethanol and methanol.

6. A method according to claim 1, substantially as hereinbefore described with reference to the N drawings.

-9.

6. A method according to claim 1, substantially as hereinbefore described with reference to the drawings.

7. Method for determining the type and amount of an electrochemically convertible substance in a gas sample, which substance in a measuring cell produces a physical measured variable which changes over time, rises from a reference line to a maximum value and falls back to the reference line again, characterised in that the measured variable which changes over time is the current intensity, and in that the type and amount of the electrochemically convertible substance is established from the linearly adapted connection between current intensity and flowed charge in the time period after the maximum value of the current intensity has been reached.

AMENDMENTS TO THE CLAIMS 1. A method for determining the type and/or amount of an electrochemically convertible substance in a gas sample, which method comprises subjecting the substance in the sample to electrochemical conversion in a measuring cell to produce a current whose intensity changes over time in that it rises from a reference value to a maximum value and then falls back to the reference value again, wherein the type and/or amount of the substance is established from the linear relationship between the current intensity and the charge that has flowed in the time period after the maximum value of the current intensity has been reached.