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W C Maskell

is the mean value of the observed EMF. The second B COS u t =(R T/4F)*(A1 / V ~ p oCOS ut

(49)

is the sinusoidal variation of the EMF about the mean. Thus

equation (47) may be written E=Eo-B coswt.

(50)

The two terms may be separated by appropriate signal processing, the amplitude B allowing determination of p o while Eo reveals p l / p oand hence p , . A simulation of the behaviour is shown in figure 15.

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w+

Time (arbitrary units)

Figure 15. Simulation of the gauge output of a pump-gauge device with a sinusoidal current applied to the pump.

6. Impedance-based devices Badwal and De Bruin (1979) have described a device consisting simply of an oxygen-ion conducting wafer coated on each side with a thin layer of Pd. This is operated at a temperature sufficiently elevated to promote substantial ionic conductivity in the ceramic. They found (see also De Bruin and Badwal 1980) that the electrode impedance increased by approximately two orders of magnitude when the Pd was oxidised to PdO, the behaviour being reversible. Measurements were readily made by applying an AC signal at low frequencies (5-100 Hz) as shown in figure 16.

The point at which the transition between Pd and PdO occurs is strictly defined by the thermodynamics of the reaction, the temperature depending upon the oxygen partial pressure. This technique for the determination of po2 is absolute and requires no reference gas.

A further refinement involves the addition of a third pseudo- reference electrode (Badwal and De Bruin 1982). This allows the working electrode to be biased permitting alteration of the oxidation-reduction point with consequent improved versatility.

The response time of these devices is likely to be slow because the oxidation-reduction effectively has a high pseudo- capacitance. However, in some applications this is not a critical factor.

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fl 700 800 Temperature I°C ) I

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900

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Figure 16. Impedance-temperature behaviour at 10 Hz for a Pd~zirconialP cell. Sample gas pressure (atm) (10’ Pa= 1 atm): e, I ; H, 0.208: V,0.049: A,0.0046. (Diagram courtesy Academic Press, Inc.)

Another impedance-based sensor has recently been discussed by Goge et a1 (1985). It consists of a cermet composed of Pt and zirconia. This is exposed to the sample gas and the DC resistance measured. The current traverses the cermet via oxygen ions in the ceramic and electrons in the metal. Consequently there is an interfacial impedance at each contact between the two solid phases due to the reaction (Maskell et a1 1987a)

Electrode impedance has been shown to vary as

(Verkerk

et a1 1983. Kaneko et a1 1986) and consequently with

calibration allows determination ofp,,.

The simplicity of this device is an attractive feature. A possible problem is that electrode impedance is normally sensitive to contaminants and the system may be poisoned by components resulting from the oxidation of fossil fuels.

Appendix The factors influencing the response of a potentiometric sensor are discussed below. It is not possible to quantify, in general terms, the relative magnitudes of each: these depend on the specific operating conditions pertaining such as temperature, gas concentration range, sampling or non-sampling system, etc. However, the list provides a basis for the systematic identification of the controlling processes in future studies.

A l . Double layer capacitance The following discussion is based upon considerations of Steele et a1 (1981). At the interface between an electronic conductor and an electrolyte there is, in general, a separation of charge in the latter balanced by an equal and opposite charge in the electronic conductor. This double layer behaves as a capacitor and can store charge. When a change in p i occurs at the electrode-electrolyte boundary the potential of the electrode changes and charge moves in or out of the double layer with a corresponding current on the electrode. This current is

dissipated in an electrochemical reaction to, + V O ’ +2e +0 6 .

(5 1)

Electrons are supplied from the electronic conductor. There is an electrical resistance to this reaction, related to the

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