Teledyne OT-3 - Manual (Page 17)

Trace Oxygen Analyzer Installation
Teledyne Analytical Instruments 9
2.2.4 The Effect of Pressure
In order to state the amount of oxygen present in the sample as 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 constant between calibrations.
2.2.5 Calibration Characteristics
Given that the total pressure of the sample gas at 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. This rate is
directly proportional to the concentration of oxygen 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 compensate for
nonlinearity.
In addition, since there is zero output in the absence oxygen, the
characteristic curve is close to an absolute zero. The cell itself does not
need to be zeroed on ranges as sensitive as 0-10 ppm. The electronics
are zeroed to compensate for zero offsets in the electronics. (The
electronics is zeroed automatically when the instrument power is turned
on during the Self-Test.)
As the cell reaches the end of its useful life, the slope seen in
Figure 2-2 decreases. As this occurs, the span adjustments will become
larger.