Teledyne 3000TA - General purpose trace oxygen analyzer User Manual
Page 16
2-4
2 Operational Theory
Model 3000TA
Teledyne Analytical Instruments
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 electronics is
zeroed automatically when the instrument power is turned on.)