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2-4
2 Operational Theory Model 3190
Teledyne Analytical Instruments
2Pb + O
2
→ 2PbO
(These reactions will hold as long as no gaseous components capable
of oxidizing lead—such as iodine, bromine, chlorine and fluorine—are
present in the sample. The only likely components are the halogens.)
The output of the fuel cell is limited by (1) the amount of oxygen in
the cell at the time and (2) the amount of stored anode material. In the
absence of oxygen, no current is generated.
2.2.4 The Effect of Pressure
In order to state the amount of oxygen present in the sample as a
specific portion of the gas mixture, it is necessary that the sample diffuse
into the cell under constant pressure.
If the total pressure increases, the rate that oxygen reaches the cathode
through the diffusing membrane will also increase. The electron transfer,
and therefore the external current, will increase, even though the oxygen
concentration of the sample has not changed. It is therefore important that
the sample pressure at the fuel cell (Usually vent pressure) remain rela-
tively constant between calibrations.
2.2.5 Calibration Characteristics
Given that the total pressure of the sample gas on the surface of the
Micro-Fuel Cell input is constant, a convenient characteristic of the cell is
that the current produced in an external circuit of constant impedance is
directly proportional to the rate at which oxygen molecules reach the
cathode, and this rate is directly proportional to the concentration of oxy-
gen in the gaseous mixture. In other words it has a linear characteristic
curve, as shown in Figure 2-2. Measuring circuits do not have to compen-
sate for nonlinearities.
Also, since there is zero output in the absence oxygen, the characteris-
tic curve has an absolute zero. The cell itself does not need to be zeroed.
As the cell reaches the end of its useful life, the slope seen in Figure
2-2 decreases. In the Model 3190, the slope is monitored. If the inverse of
the slope:
Span Value (ppm) / Cell Output (nA)
is over 4.447 ppm/nA, a sensor failure alarm is triggered, indicating that
the cell should be replaced.