GB2270164A

GB2270164A - Oxygen measuring system utilising solid electrolyte sensor and pump

Info

Publication number: GB2270164A
Application number: GB9218111A
Authority: GB
Inventors: Mohieddine Benamor Benammar
Original assignee: CERAMIC TECHNOLOGY CONSULTANTS
Current assignee: CERAMIC TECHNOLOGY CONSULTANTS
Priority date: 1992-08-26
Filing date: 1992-08-26
Publication date: 1994-03-02
Anticipated expiration: 2012-08-26
Also published as: GB2270164B; GB9218111D0

Abstract

The system comprises an enclosed volume bounded by a solid electrolyte oxygen pump and a solid electrolyte oxygen sensor, each of which has one electrode in the enclosed volume and the other in the sample gas. A heater maintains the temperature of the sensor at a desired value. The sensor EMF is compared with a separately generated periodically oscillating voltage (SGPOV) and the difference between them is maintained constant by a feedback loop controlling the current to the pump. The resulting periodically oscillating pump current is analysed to determine the oxygen partial pressure and/or concentration of the sample gas. The volume may be closed or a diffusion path comprising one or more small holes may be provided. The system provides a measurement of oxygen partial pressure which is independent of the degree of leakage between the enclosed volume and the sample gas: it can also quantify the degree of leakage. <IMAGE>

Description

GAS ANALYSIS APPARATUS This invention relates to the measurement of oxygen partial pressure and oxygen concentration.

The determination of the oxygen content of gases is important in many applications including the control of air-to-fuel ratio in fossil-fuelled combustion systems.

Electrochemical devices incorporating solid electrolytes are preferred to devices with liquid electrolytes because the former do not lose solvent by evaporation during extended periods of operation.

This invention describes an apparatus for oxygen partial pressure/concentration measurement which produces a linear output. The electrochemical sensor comprises a pump, a gauge and an internal volume. The device may be fully sealed or may include a pore or porous material to act as diffusion path between the internal volume and external atmosphere. Using appropriate electronics, the device is operated in a feedback loop in which the current applied to the pump is adjusted so as to cause the gauge EMF to follow a separately-generated periodically oscillating voltage (SGPOV). The feedback loop operates to minimise the difference voltage between the SGPOV and the gauge EMF or to maintain the difference constant. The SGPOV may be sinusoidal, triangular or of some other periodic form.The amplitude of the pumping current is normally proportional to the oxygen partial pressure in the sample gas. The system may be used for measuring the oxygen concentration and oxygen partial pressure simultaneously. Importantly the system measures oxygen partial pressure independently of the leak conductance: it also enables changes in leak conductance of the sensor to be quantified during measurement of the oxygen partial pressure of the sample gas.

According to a first aspect, the invention comprises an apparatus for determining the oxygen partial pressure/concentration in a sample gas including a sensor comprising a pump, a gauge and an internal volume, the pump and gauge each consisting of solid oxygen-ion conducting material and a pair of electronically conducting porous electrodes, one electrode being in contact with the gas in the internal volume and the other electrode being in contact with the sample gas of variable composition external to the sensor in each case, means for providing a separately-generated periodically oscillating voltage (SGPOV), means for sensing the difference between the gauge EMF and the SGPOV, means for applying a current to the pump electrodes to pump oxygen into or out of the internal volume so as to maintain the difference between the gauge EMF and the SGPOV constant, means for processing the current signal in the pump circuit so as to determine either the oxygen partial pressure or the oxygen concentration or both of the sample gas, a heater to hold the pump-gauge sensor at a temperature sufficient to result in adequate conductivity of the electrolyte of the pump and suitably rapid kinetics of the electrode reaction of the gauge.

According to a second aspect, the invention comprises a method for determining the oxygen partial pressure/concentration in a sample gas using a sensor comprising a pump, a gauge and an internal volume, the pump and gauge each consisting of solid oxygen-ion conducting material and a pair of electronically conducting porous electrodes, one electrode being in contact with the gas in the internal volume and the other electrode being in contact with the sample gas of variable composition external to the sensor in each case, means for providing a separately generated periodically oscillating voltage (SGPOV), means for sensing the difference between the gauge EMF and the SGPOV, means for applying a current to the pump electrodes to pump oxygen into or out of the internal volume so as to maintain the difference between the gauge EMF and the SGPOV constant, means for processing the current signal in the pump circuit so as to determine either the oxygen partial pressure or the oxygen concentration or both of the sample gas, a heater to hold the pump-gauge sensor at a temperature sufficient to result in adequate conductivity of the electrolyte of the pump and suitably rapid kinetics of the electrode reaction of the gauge.

In a preferred embodiment the internal volume is less than 5 mm3 and the frequency of operation is in the range 0.1 to 1OHz.

This invention also encompasses the inclusion of planar sensors made by thin and/or thick film techniques where the internal volume may result from porosity in one or more of the layers. Furthermore, in this case the inner two electrodes of the pumpgauge may be opposite faces of a single electrode layer.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings: Figure 1 shows a schematic of the pump-gauge sensor with sealed internal volume in cross-section.

Figure 2 shows a schematic of the pump-gauge sensor with a leak in cross section.

Figure 3 shows the cross-section of a sensor made from discrete components.

Figure 4 shows the perspective with part cut away of a sensor made from discrete components.

Figure 5 shows one version of a circuit diagram of the apparatus described in this invention. The pump current is determined by measuring the voltage drop across the known resistor Roo Figure 6 shows the pump current (lower traces) of an hermetically sealed device (with an internal volume v=lmm3) operated at T=1OOOK and P,=lkPa with a sinusoidal gauge EMF (upper trace; frequency vStl rad s-t) with adjustable amplitude (Vr).

Figure 7 shows the pump current (lower traces) of a device with an internal volume v=lmm3 and adjustable leak conductance (DS/RTL) operated at T=1OOOK and P,=lkPa with a sinusoidal gauge EMF (upper trace; Va30mV, omit rad s-l). Values for (DS/RTL) in [m kgl s mol].

Figure 8 shows the pump current of a device with an internal volume v=lmm3 and adjustable leak conductance (DS/RTL) operated at 1000K with a triangular gauge EMF (upper trace; 40mV peak-to-peak; v8s rad 6'). Values for (DS/RTL) in [m kg-l s mol].

A preferred embodiment of the sensor is shown in Fig.3 and Fig.4. The sensor consists of two discs of solid oxygen ion conductor 1 and 2 (e.g. stabilised zirconia) and a spacer component 3 which may be composed of a metal (e.g. Au or Pt), a glass or a ceramic. The spacer has a hole through it so that when the sensor is assembled an internal volume 4 is enclosed. The components may be fixed together by any of a number of means including the use of a glass, glass-ceramic, a metal-ceramic bond or combination of these where appropriate. On the plane surfaces of the discs 1 and 2 are porous electronically conducting electrodes 6-9. If the spacer 3 is not an electronic conductor then one or more electronically conducting connections 11 and 12 are made through the spacer between 7 and 9 and the outer space 5.The components 6, 1, 7 comprise the pump of the sensor while 9, 2, 8 comprise the gauge. The sensor may include a diffusion path 10 connecting the internal volume to the external atmosphere. The diffusion path may be a small hole or holes drilled through one or both of the two ceramic discs (1 or 2); alternatively, the seal 3 may include a leakage path or leakage paths.

The theory below has been developed for a pump-gauge device with an internal volume, v, and a diffusion path of length, L, and uniform cross-sectional area, S; the fullysealed device is a specific case where S/L=O. When a current, 1, is applied to the pump oxygen is electrochemically pumped into or out of the internal volume; the resulting flux is given by JCurr=Il4F (1) where F is the faraday. The convention adopted is that a positive current pumps oxygen out of the internal volume. Likewise, a positive oxygen diffusive flux represents oxygen transfer out of the internal volume.Assuming linearity of the gradient of oxygen concentration within the pore, the flux of oxygen leaking through the diffusion path is given by Jair(DSlRTL)(P2-P) (2) where D is the oxygen diffusion coefficient, R is the gas constant, T is the absolute temperature of operation and P1 and P2 are respectively the oxygen partial pressures inside the inner volume 4 and in the surrounding atmosphere 5. The total effective flux into or out of the internal volume is given by J=JJ,,=-dnldt (3) where n is the number of moles of oxygen transferred.The ideal gas equation applied to the internal gas gives dP21dt=(RTlv)dnidt (4) Eliminating dn/dt between eqn (1)-(4) leads to

The internal oxygen partial pressure may be written according to Nernst equation P2=P.eXp(-4FElRT) (6) where E is the gauge EMF. Using appropriate electronics, the device is operated in a feedback loop in which the current applied to the pump is adjusted so as to cause the gauge EMF to follow a separately-generated periodically oscillating voltage (SGPOV). The feedback loop operates to minimise the difference voltage between the SGPOV and the gauge EMF or to maintain the difference constant (Fig.5). The SGPOV may be sinusoidal, triangular or of some other periodic form.The following analysis is done for the special case of a purely sinusoidal EMF E=V, sinot (7) The expressions for P2 and dP2ldt determined (using eqn 6 and 7) and substituted into eqn(5) give

Equation (8) shows that the amplitude of the periodic current is proportional to the oxygen partial pressure in the atmosphere. Various methods may be used to convert the current into a measurement of P; for example using a microprocessor-based system it is possible to have an almost continuous reading of P,. Alternatively, using a sample and hold, the amplitude, 10, of the current measured at cot=2m7s (m=0,1,2,3..) lo=(16F2vV,lR2T2)P, (9) provides a measure of the oxygen partial pressure which is independent of the leak conductance.The amplitude of the current measured at (ss! /2+2ms (m=0,1,2,3...)

provides a measurement of the oxygen concentration. This arises because, for pure bulk diffusion, the oxygen diffusion coefficient (D) is inversely proportional to the barometric pressure. The ratio between IXt2 and provides a measure of the leak conductance (DS/RTL); this enables changes in leak conductance of the sensor to be quantified during measurement of the oxygen partial pressure and/or oxygen concentration of the sample gas.

The determination of oxygen partial pressure in the sample gas independent of leak conductance is achieved for periodic oscillating gauge EMFs of any form provided that the measurement of the current is made when the gauge EMF, E, crosses zero (i.e. P2=P1, see eqn 5 and 6). Also a measurement of the current when dE/dt=O (for example when the gauge EMF is at its peak) provides a measure of oxygen concentration. This arises because dP2/dt=O when dE/dt=O (see eqn 5 and 6).

Figure 6 shows the pump current for an hermetically sealed device (where S/L=O) and a sinusoidal gauge EMF. For low values of Vr the current may be considered sinusoidal; as Or is increased the current waveform deviates from a sinusoidal shape.

Figure 7 shows the pump current for a device with variable leak conductance operated with a sinusoidal gauge EMF. For an hermetically sealed device the current signal is symmetrical; the difference between the positive and negative half-cycles of the current increases as the leak conductance increases. This principle may be used to detect changes in leak conductance of the sensor such as might occur during progressive deterioration of the seal during measurement of the oxygen partial pressure and/or oxygen concentration of the sample gas.

Figure 8 shows the pump current for a device with variable leak conductance operated with a triangular gauge EMF. As with the operation involving a sinusoidal gauge EMF (or any other symmetrical oscillating voltage) the current signal is symmetrical for an hermetically sealed device; some dissymmetry appears if the device is leaking. This dissymmetry between the positive and negative half-cycles of the current increases with increasing leak conductance.

Claims

1. An apparatus for determining the oxygen partial pressure in a sample gas including a sensor comprising a pump, a gauge and an internal volume, the pump and gauge each consisting of solid oxygen-ion conducting material and a pair of electronically conducting porous electrodes, one electrode being in contact with the gas in the internal volume and the other electrode being in contact with the sample gas of variable composition external to the sensor in each case, means for providing a separately-generated periodically oscillating voltage (SGPOV), means for sensing the difference between the gauge EMF and the SGPOV, means for generating a current to the pump electrodes to pump oxygen into or out of the internal volume so as to maintain the difference between the gauge EMF and the SGPOV constant, means for processing the current signal in the pump circuit so as to determine either the oxygen partial pressure or the oxygen concentration or both of the sample gas, a heater to hold the pump-gauge sensor at a temperature sufficient to result in adequate conductivity of the electrolyte of the pump and suitably rapid kinetics of the electrode reaction of the gauge.

2. An apparatus as claimed in claim 1, wherein the sensor comprises two oxygen-ion conducting laminas and a spacer which when assembled enclose a volume of less than 5 mm3.

3. An apparatus as claimed in claims 1 or 2, wherein the frequency of operation is in the range 0.1 to 1OHz.

4. An apparatus as claimed in claims 1, 2 or 3, wherein the enclosed volume is hermetic ally sealed.

5. An apparatus as claimed in claims 1, 2 or 3, wherein the enclosed volume is connected to the sample gas via a path or paths through which gases may diffuse or flow.

6. An apparatus as claimed in claims 1, 2, 3, 4 or 5 wherein the periodically oscillating voltage is sinusoidal.

7. An apparatus as claimed in claims 1, 2, 3, 4 or 5 wherein the periodically oscillating voltage is triangular.

8. An apparatus as claimed in claims 1, 2, 3, 4, 5, 6 or 7 wherein the difference voltage between the SGPOV and the gauge EMF is maintained constant and equal to zero.

9. An apparatus as claimed in claims 1, 2, 3, 4, 5, 6, 7 or 8 wherein the measurement of the amplitude of the pump current is achieved using various AC to-DC converters such as sample and hold, phase sensitive detector, or RMS converter.

10. An apparatus as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8 or 9 wherein the processing of the pump current provides a measurement of the oxygen partial pressure independent of leak conductance.

11. An apparatus as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 wherein the processing of the pump current provides a measurement of the oxygen concentration

12. An apparatus as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11 wherein the processing of the pump current enables changes in leak conductance of the sensor to be quantified during measurement of the oxygen partial pressure and/or oxygen concentration of the sample gas.

13. An apparatus as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 wherein specific means are used for sensing the temperature of operation of the pump-gauge.

14. An apparatus as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13 wherein specific means are used for controlling the temperature of the pump-gauge.

15. An apparatus substantially as described herein with reference to Figures 1, 2, 3, 4, 5, 6, 7 and 8 of the accompanying drawings.

16. A method for determining the oxygen partial pressure/concentration in a sample gas using a sensor comprising a pump, a gauge and an internal volume, the pump and gauge each consisting of solid oxygen-ion conducting material and a pair of electronically conducting porous electrodes, one electrode being in contact with the gas in the internal volume and the other electrode being in contact with the sample gas of variable composition external to the sensor in each case, means for providing a separately generated periodically oscillating voltage (SGPOV), means for sensing the difference between the gauge EMF and the SGPOV, means for applying a current to the pump electrodes to pump oxygen into or out of the internal volume so as to maintain the difference between the gauge EMF and the SGPOV constant, means for processing the current signal in the pump circuit so as to determine either the oxygen partial pressure or the oxygen concentration or both of the sample gas, a heater to hold the pump-gauge sensor at a temperature sufficient to result in adequate conductivity of the electrolyte of the pump and suitably rapid kinetics of the electrode reaction of the gauge.

17. A method substantially as described herein with reference to Figures 1, 2, 3, 4, 5, 6, 7 and 8 of the accompanying drawings.