Electrochemical oxygen gas sensors
Figure 12. Circuit for operating the pump-gauge at a constant ratio of p i / p z . The gauge EMF is automatically maintained equal to the reference voltage E , by an appropriate pumping current.
it is essential that the A/F ratio is always maintained with excess air to minimise CO formation. This problem is solved by the devices discussed in the next section.
5.4. Amperometric withfixed reference Dietz (1982) pointed out the advantage to be gained by providing a gas of fixed composition adjacent to the anode on a single two-electrode amperometric sensor (figure 7 ) :traversal of stoichiometry then results in reversal of current fiow so that the regions k < 1 and A > 1 are clearly identified.
With only a small increase in complexity, application of a fixed gaseous reference to the gauge of the pump-gauge provides a stable reference as negligible current need be carried by it. A schematic representation of this argument is shown in figure 13
Reference gas of fixed composition
I I I I 1
r - I
Figure 13. Amperometric pump-gauge with reference gas of fixed composition.
and has been translated into a practical device by Soejima and Mase (1985) with air as the reference gas. Some of their data are presented in figure 14 showing that the device can be used on both sides of stoichiometry by suitable choice of the working gauge EMF (i.e. 0.3-0.6 V).
A disadvantage of this arrangement is that a connection and suitable seal to the device must be provided to supply the reference gas.
0.4 0.6 Gauge EMF [,(VI
Figure 14. Characteristics of a cell of the type in figure 13 for various airifuel ratios in a propane burner test. 2, A/F values: A, 1.6, 24; B, 1.23, 18; C, 1.0, 14.8; D, 0.89, 13; E, 0.82, 12 (Soejima and Mase 1985). (Reprinted with permission 'C 1985 Society of Automotive Engineers, Inc.)
5.5. Potentiometric with monitored internal reference Maskell et a1 (1986) have described a novel mode of operation of a device of the type shown in figure ll(a). A sinusoidal current is applied to the pump and results in a sinusoidal variation of the internal reference gas pressure. This, in turn, leads to a pseudo-sinusoidal variation of the gauge EMF superimposed upon the potentiometric EMF resulting from the ratio of the external and mean internal oxygen partial pressures.
For a net applied current, I , given by Z=Az sinojt
it has been shown (Maskell et a1 1987b) that the gauge EMF is
where p o is the mean internal (reference) pressure. For small amplitude (<2.3 mV at 800 "c)of the EMF variation, equation (46) simplifies to
Equation (47) consists of two terms. The first Eo =(RT/4F) In ( P1/Po)