Teledyne 3000TA-EU - General purpose trace oxygen analyzer - Manual (Page 22)

Operational Theory 3000TA- EU
Teledyne Analytical Instruments
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measured and used to determine the oxygen concentration in the gas
mixture.
The overall reaction for the fuel cell is the SUM of the half
reactions above, or:
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 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 in
parts-per-million or a percentage 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 relatively 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 is directly proportional
to the rate at which oxygen molecules reach the cathode, and this rate is
directly proportional to the concentration of oxygen in the gaseous mix-
ture. In other words it has a linear characteristic curve, as shown in
Figure 2-3. Measuring circuits do not have to compensate for
nonlinearities.
In addition, since there is zero output in the absence oxygen, the
characteristic curve has close to an absolute zero (within ± 1 ppm
oxygen). In practical application, zeroing may still used to compensate
for the combined zero offsets of the cell and the electronics. (The