EP0066583A1

EP0066583A1 - Method for measuring the partial pressure of a gas and probe for implementing such method

Info

Publication number: EP0066583A1
Application number: EP81903251A
Authority: EP
Inventors: Erik Aslaksen
Original assignee: PROGRAMMELECTRONIC ENGINEERING AG
Current assignee: PROGRAMMELECTRONIC ENGINEERING AG
Priority date: 1980-12-12
Filing date: 1981-12-08
Publication date: 1982-12-15
Also published as: ZA818612B; BE891476A; ES8302303A1; IT8112706A0; ES507912A0; WO1982002093A1; IT1145999B

Abstract

Dans un procede pour la mesure de la pression partielle d'un gaz dans un milieu qui ne soit par critique quant au choix de ce milieu, on envoie une quantite de charge sur une matiere ayant une selectivite par rapport aux ions. On mesure avant et apres la difference de potentiel entre les milieux au moyen d'electrodes de derivation et on determine la pression partielle. La condition essentielle pour ce procede est que le volume d'un des milieux soit sensiblement plus petit que le volume de l'autre. Une sonde pour effectuer ce procede presente un corps creux (25) avec une chambre (27) de preference pour un milieu de reference. Les parois de la chambre la separent du milieu exterieur. On prevoit dans la paroi une zone permeable pour les ions gazeux et une electrode est prevue de chaque cote de la zone (25). La chambre (27) est entierement separee de l'atmosphere ambiante. Pour commander les conditions de temperature dans la zone selective pour les ions (25) on a prevu une disposition de mesure de temperature (21, 22) et/ou des organes de refroidissement (14) ou de chauffage sur la sonde.In a method for measuring the partial pressure of a gas in a medium which is not critical as to the choice of this medium, a quantity of charge is sent onto a material having selectivity with respect to the ions. The potential difference between the media is measured before and after by means of bypass electrodes and the partial pressure is determined. The essential condition for this process is that the volume of one of the media is appreciably smaller than the volume of the other. A probe for carrying out this process has a hollow body (25) with a chamber (27) preferably for a reference medium. The walls of the chamber separate it from the external environment. There is provided in the wall a permeable zone for gas ions and an electrode is provided on each side of the zone (25). The room (27) is completely separated from the ambient atmosphere. To control the temperature conditions in the selective zone for the ions (25), provision has been made for temperature measurement (21, 22) and / or cooling (14) or heating members on the probe.

Description

Method for determining the partial pressure of a gas and probe for carrying out the method

The present invention relates to a method for determining the partial pressure of a gas in a medium, in which a phase of matter which is ion-selective with respect to the gas ions is introduced between the medium to be measured and a second medium, and a discharge electrode is introduced into both media for measuring the electrical potential difference therebetween and a probe for Execution of the method with a hollow body which delimits a space for a first medium with its wall against a second medium, the wall having at least one ion-selective wall section which is permeable to ions of the gas, one electrode being arranged on each side of the wall and wherein further the first or second medium is a reference medium, corresponding to the second or first one to be measured.

State of the art

Methods for determining the partial pressure of a gas are known, in which an ion-selective material phase is inserted between the medium to be measured and a reference medium, and in which the electrical potential difference between the two media is measured by means of lead electrodes to determine the partial pressure. The electrical measuring circuit is closed via the ion-selective matter phase. With these methods, it is disadvantageous that the reference medium must be known and stable with regard to its absolute partial pressure. The following is known regarding the probe of the type mentioned at the beginning:

Such probes are known for determining the oxygen partial pressure in a medium, as an ion-selective membrane, with a tube made of stabilized ZrO ₂ , for example described in "Chemical plants and processes", issue 6/1973, pages 167 to 169 and 198 and 199. In the probe shown there, the medium, the oxygen content of which is to be determined, is drawn through the pipe, the outside of the pipe being surrounded by a reference gas. With this device, it is disadvantageous that it takes up space in use and requires an inlet and an outlet for the medium whose oxygen content is to be determined, and that a measurement error, due to the relative movement, between the medium to be measured and the ion-selective wall section may is taken into account.

Presentation of the invention

The purpose of the present invention is to propose a method of the type mentioned at the outset, in which the knowledge and the maintenance of a reference medium is considerably less critical than in the known methods.

According to the invention, this object is achieved in that the volume of the one medium is selected to be substantially smaller than that of the other medium, at least once a certain amount of electrical charge is passed through the ion-selective material phase and determined beforehand and then the potential difference between the lead electrodes and from the potential differences found and the determined amount of charge determines the partial pressure in one and / or in the other medium.

Basically, the forced flow of charge only changes the partial pressure of the medium whose volume is significantly smaller than that of the other. This does not change the partial pressure in the "medium sea" of the latter medium.

If the method is carried out with a probe for determining the oxygen partial pressure with a matter phase made of stabilized ZrO ₂ , the matter phase is previously heated to a temperature of at least 873 Kelvin. The heating can be carried out by the medium to be measured itself, or by a specially provided organ.

It goes without saying that, in order to increase the measurement accuracy, certain charge quantities can be sent through the ion-selective wall section several times, and that the desired partial pressure can be determined as a result of the evaluation of different measurement points by appropriately pairing measured potential differences.

The reproducibility of the measured values is also increased in that a compensation pause is made in which the system stabilizes between the passage of the determined amount of charge and the measurement of the potential difference. It is also proposed to measure the potential difference in each case over a certain period of time and to center it over this period In principle, it is relatively insignificant how closely the two media are separated from one another. It is only essential that the diffusion time constant of the gas molecules of interest from one medium to another is significantly greater than the measuring times. However, in order to keep the influence of diffusion as low as possible, it is proposed that the size and polarity of the amount of charge be selected so that the partial pressure in one medium - the small volume - approximates the partial pressure in the second - large volume.

A probe of the type mentioned at the outset, which is particularly suitable for carrying out the method mentioned, is characterized in that the space against the ambient atmosphere is completely closed after the introduction of the first medium.

This creates an extremely space-saving probe that can be placed directly into one of the two media, the medium whose partial pressure is to be determined or a reference medium. By enclosing the one medium in the space mentioned, dimensioned accordingly, the volume ratios that are essential for carrying out the above-mentioned method also arise, based on the surrounding space.

The first medium is preferably the reference medium, enclosed in the room, in particular if the probe is used to detect changing environments, such as in industrial plants. On the other hand, it is then appropriate to use the second medium as a reference medium, if measured by only small volumes are available as samples for the medium. In order to detect changing environments, the probe according to the invention enables it to be introduced directly into this medium, with voltage measuring devices and further units, possibly assigned, for example also a computer, being able to be set up at a distance therefrom, for example in a measuring and monitoring center.

In order to take into account the possibly changing ambient temperature, through which the ion-selective wall section in particular can be influenced with regard to its ionic conductivity, it is proposed that means for temperature detection are arranged at least in the immediate area of this ion-selective wall section. These means are preferably designed as a thermocouple for generating a thermal voltage, which enables the temperature to be detected directly by measuring an electrical voltage.

If the probe is used to determine the oxygen partial pressure, it is proposed that at least the wall section consists of stabilized ZrO ₂ and that preferably the first medium is the reference medium.

In order to make the probe as simple as possible with the means provided for temperature detection, it is further proposed that an electrical conductor of an electrode, preferably that in the reference medium, is at the same time an electrical conductor of the means for temperature detection. A further simplification of the structure results from the fact that at least part of an electrical conductor of an electrode, preferably that in the reference medium or the electrode itself, is part of a thermocouple.

If the reference medium is provided in the room, the electrode arranged in the room can be connected to a rhodium conductor within the room by means of a platinum conductor to form the means for temperature detection. This connection then acts as a thermocouple.

Depending on the areas of application, it may be desirable to influence the temperature of at least the ion-selective wall section.

Appropriate organs are provided for this purpose. When the probe is used as a measuring probe for oxygen partial pressure, the ion-selective wall section consisting of ZrO ₂ , these organs are designed such that the wall section can be heated to at least 873 ° Kelvin, to a temperature at which the material mentioned only becomes permeable to oxygen ions .

If the wall of the hollow body delimiting the space, at least partially, is surrounded by a chamber open to the ambient atmosphere, mechanical protection of the wall with the ion-selective part is achieved on the one hand, and on the other hand there is an improved, dynamic, thermal behavior of the arrangement, which in particular is essential if the temperature of the ion-selective wall section is to be influenced. Brief description of the figure

Then using the enclosed schematic

Drawing explains the invention for example.

In particular due to the ion-selective wall section made of ZrO ₂ used in the exemplary embodiment shown, this probe is intended for measuring the oxygen partial pressure. The use of other ion-selective materials as a wall section to determine other gas partial pressures, however, results from this without further ado.

Description of the single figure

The measuring probe shown has a base 1 made of heat-resistant material with parallel bores 2, 3 and 4 '. A tubular piece 6 made of alumina is placed on a cylindrical extension 5 of the base 1 and fastened thereon by means of a sealing compound 7. The pipe section 6 encases a thermal insulation insert 8 made of asbestos cement, earthenware or the like, which forms a cylindrical chamber 9. The insulating insert 8 is further provided with bores 10, 11 and 12, which connect coaxially to the bores 2 to 4 and open into the chamber 9. An opening 13 in the insulating insert 8 connects the chamber 9 to the ambient atmosphere. An annular cylindrical heating resistor 14 is inserted coaxially into the chamber 9, the connecting lines 15 of which are guided outwards through the bores 2, 3, 10 and 11 and are held therein by a sealing compound 16.

In the bore 12, a cylindrical support 17 is mounted, which consists of an electrically insulating, ceramic material. This is of two parallel ones Longitudinal bores 18, 19 penetrated. A platinum wire is guided through the bore 19 and a platinum-rhodium wire is passed through the bore 18, the ends of which are connected to one another at 22 to form a thermocouple. The exit of wires 20 and 21 from the holes

18 and 19 is sealed gas-tight by an electrically insulating mass 23. A tube 25 made of stabilized ZrO ₂ and closed on one side is attached to an annular rib 24 of the carrier 17 as an ion-selective wall. The interior 27 of the tube 25 containing a reference gas as a reference medium is sealed gas-tight by the carrier 17 and by the insulating materials 26 and 23. On the outside of the tube 25, a platinum conductor 28 is connected, which is formed at 29 as a porous platinum electrode. Accordingly, the platinum wire 20 contacts the tube 25 on the inside by likewise forming a porous platinum electrode at 30.

To determine the oxygen content by means of the probe described in a medium, such as a gas atmosphere, for example, it is brought into this gas atmosphere and, if necessary, brought to a temperature of at least 873 Kelvin with the heating resistor 14 and kept at this temperature, which is done with a corresponding control circuit can. A temperature equalization occurs between the chamber 27, the tube 25 and the ambient atmosphere of the tube 25, which is connected to the gas atmosphere through the opening 13. The difference in the oxygen partial pressures between the interior 27 and the ambient atmosphere creates a measurable potential difference (U) between the electrodes 29 and 30. Now after measuring the potential difference (U) by means of the conductors 20 and 28, a positive electrical charge of Q (As) is sent into the interior 27 through the wall of the ZrO ₂ tube 25, a corresponding number of oxygen ions from the ambient atmosphere are simultaneously transported through the wall of the tube 25 and the oxygen partial pressure in the interior 27 increased by a factor α. If the potential difference (U) is now measured again, the oxygen partial pressure can be calculated according to the equations below and in analogy to this, the procedure for probes for determining the partial pressure of other gases.

In the volume V (cm3) of the interior 27, the number n of the oxygen molecules at a partial pressure P (mbar) and a temperature T (K) is:

or generally a gas

with k as the known proportionality constant.

If an electrical charge Q (As) is sent through the ion-selective wall, here that of the ZrO ₂ tube 25 - with the gas ions as charge carriers - and it is assumed that this charge has a positive sign if the current flows from the outside to the inside, the increase Δn of the molecules in the interior is 27:

Δn = 1.56 10 ¹⁸ Q, (2) or in general

.DELTA.n = k · _Δ Q,

where k _{Δ is} again a gas-specific proportionality constant.

The oxygen partial pressure resp. generally the partial pressure of the gas from P to αP, where α can be calculated using the following equation:

the latter for oxygen.

If the oxygen partial pressure, generally the gas partial pressure, outside the tube 25 is designated P _x , the potential difference between the electrodes 29 and 30 is when no current is flowing:

or, for oxygen

where R is the general gas constant and F is the Faraday constant and Z is the valence of the gas molecules, for oxygen molecules Z = 4. The unit of measurement for the potential difference U is volt. The voltage change due to the electrical charge passage for oxygen is therefore:

ΔU = 0.4964 10 ^-4 T log α. (5)

A combination of equations (3) and (4a) gives the following equation for oxygen and P:

Such a calculation will then be presented in general:

If, with unknown partial pressure P in the reference medium, assumed here in space 27, and equally unknown partial pressure P _x in the medium to be measured, here assumed outside space 27, the potential difference is measured at the electrodes, this first measurement results from (4) , at a known temperature T:

If a certain amount of charge Q _o is now sent through the ion-selective wall, then according to (2)

.DELTA.n _o = k · _Δ Q _o (2 _a)

The second potential difference, measured according to the charge flow, again results from (4)

where α _o · P corresponds to the partial pressure changed by the charge quantity Q _o .

From (4 ₁ ) (4 ₂ ) we get

According to (3)

OMP Out

and (2 _a ), (1), (7) results

which turns P into

results. Similarly, by inserting n according to (8) in (3), α _o can be determined. P, at most α _o P, is used in (4 ₁ ) and / or (4 ₂ ), which results in P _x .

At this point it must be emphasized that this procedure is independent of whether the reference medium is now enclosed in the small-volume space and the medium to be measured is outside or vice versa. As shown, the partial pressures in the reference medium and the medium to be measured are calculated anyway. All that needs to be taken into account is that when the charge flow is applied, only the molecular concentration in the medium that is provided in the interior 27 is changed. It is therefore entirely possible that, in the case of rare medium samples, it is preferable to enclose the latter in the interior 27 and to provide the reference medium outside, for example when taking a sample and then transporting it to a laboratory. BEST MODE FOR CARRYING OUT THE INVENTION The probe implemented today has been described. It is therefore this, at most by replacing the ZrO ₂ wall with another ion-selective wall, the version known to us today as the best way. A preferred measurement process, however, applied to the specifically described probe, runs as follows:

a) A constant current (e.g. 10 mA) with positive polarity is sent through lines 25 and 28 over lines 20 and 28 for five or ten seconds.

b) The tube 25 is kept in a currentless state for five or ten seconds so that an equilibrium can be established.

c) In the de-energized state, the potential difference U between electrodes 29 and 30 is measured for 2.5 seconds and averaged over this period.

d) A constant current of the same size but with reversed polarity is sent through the conductors 20 and 28 for five to ten seconds.

e) The tube 25 is kept in the de-energized state for 5 seconds so that an equilibrium can be established.

f) The potential difference between electrodes 29 and 30 is measured and averaged over 2.5 seconds. The duration of the current flow in method step a is preferably determined by a potential difference previously determined. If the voltage is negative (P <P _x ), the duration is 5 seconds. The duration of the current flow in method step d is preferably complementary to that in method step a, so that a complete period always has the same length, preferably 30 seconds. The time between measurements is therefore 15 seconds. In general, the amount of charge transported should preferably be dimensioned in such a way that the partial pressure of the medium in space 27 approaches that of the medium outside as far as possible in order to minimize the effects of any leaks in space 27.

In the procedure described above, the oxygen partial pressures P in the interior 27 and the oxygen partial pressures P _{x are} calculated twice from equations (4) and (6) and the mean value of these calculated values is considered as the measured value. The value for the internal pressure P is calculated according to equation (6) and that for the external pressure P _x can be calculated according to the equation

(4a) can be calculated.

Commercial usability

The probe described and the measuring method are suitable on the one hand for use in industrial systems for monitoring the air in terms of environmental protection, etc., but also for the analysis of medium samples that are taken far from the laboratory, such as in the context of space research, and then examined in the laboratory to become. Of course they are on given areas of application, which necessarily result in commercial usability, are not exhaustively listed.

Claims

Claims:

1. A method for determining the partial pressure of a gas in a medium, in which a phase of matter which is ion-selective with respect to the gas ions is introduced between the medium to be measured and a second medium, and a discharge electrode is introduced in both media for measuring the electrical potential difference therebetween, characterized in that the Volume of one medium is chosen to be significantly smaller than that of the other medium, sends a certain amount of electrical charge (Q) through the ion-selective matter phase (25) at least once and then and then the potential difference (U; U ₁ , U ₂ ) between the lead electrodes (29 , 30) and determines the partial pressure (P _x ) in one and / or (P) in the other medium from the potential differences found and the determined amount of charge.

2. The method according to claim 1 with a probe for determining the oxygen partial pressure with a material phase made of stabilized ZrO ₂ , characterized in that the phase is previously heated to a temperature of at least 873 ° Kelvin.

3. The method according to one or both of claims 1 or 2, characterized in that a compensation pause is made between the passage of the determined amount of charge (Q) and the measurement of the potential difference (U).

4. The method according to one or more of claims 1 to 3, characterized in that. The potential difference (U) is measured in each case during a specific period of time and averaged over this period.

5. The method according to one or more of claims 1 to 4, characterized in that the size and polarity of the amount of charge (Q) are selected so that the partial pressure (P) in a medium approximates to the partial pressure

(P _x ) in the other.

6. Probe for performing the method according to claim 1, with a hollow body (25) with its wall

(24, 25) delimits a space (27) for a first medium against a second medium, the wall having at least one ion-selective wall section which is permeable to ions of the gas, one electrode (29, 30) on each side of the wall (24, 25) is arranged, and wherein further the first or second medium is a reference medium, corresponding to the second or first one to be measured, characterized in that the space (27) is completely sealed off from the ambient atmosphere after introduction of the first medium .

7. Probe according to claim 6, characterized in that the first medium is the reference medium.

8. Probe according to one or both of claims 6 and 7, characterized in that means (21, 22) for temperature detection are arranged at least in the immediate region of the ion-selective wall section.

9. A probe according to claim 8, characterized in that the means (21, 22) are designed to generate a thermal voltage.

10. Probe according to one or more of claims 6 to 9 for determining the oxygen partial pressure, characterized in that at least the wall section consists of stabilized ZrO ₂ , and preferably the first medium is the reference medium.

11. Probe according to one or more of claims 6 to 10, characterized in that the reference medium is a reference gas.

12. A probe according to one or more of claims 6 to 11, characterized in that an electrical conductor (20) of an electrode (30), preferably that in the reference medium, is at the same time an electrical conductor of the means (21, 22) for temperature detection.

13. Probe according to one or more of claims 6 to 12, characterized in that at least part of an electrical conductor of an electrode (30), preferably that in the reference medium or the electrode itself, is part of a thermocouple.

14. A probe according to claims 7 and 8, characterized in that to form the means (21, 22) the electrode (30) arranged in the space (27) is connected to a rhodium conductor by means of a platinum conductor inside the space (27).

15. A probe according to one or more of claims 6 to 14, characterized in that organs (14, 15) are provided in order to influence at least the temperature of the ion-selective wall section (25).

16. A probe according to claims 10 and 15, characterized in that the organs (14, 15) are designed to heat the wall section to at least 873 ° Kelvin.

17. A probe according to one or more of claims 6 to 16, characterized in that the wall (24, 25) of the hollow body (25) delimiting the space (27) is at least partially surrounded by a chamber (9) which is open to the ambient atmosphere.

18. Probe according to claims 15 and 17, characterized in that the organs (14, 15) are arranged within the chamber (9).

19. A method for determining the partial pressure of a gas in a medium which is separated from a second by an ion-selective material phase, characterized in that on the one hand the electrical potential difference between the media is measured and on the other hand a certain charge flow between the media is forced.