Teledyne 3220 - Multi-channel oxygen monitor system - Manual (Page 27)

Oxygen Monitor System 2 Operational Theory
Teledyne Analytical Instruments 13
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.
2.5.4 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 2-6. Measuring circuits do not have to compensate for
nonlinearity.
In addition, since there is zero output in the absence oxygen, the charac-
teristic curve has close to an absolute zero. In the percent ranges, the cell
itself does not need to be zeroed. In practical application zeroing is still used
to compensate for zero offsets in the electronics. (The electronics is zeroed
automatically when the instrument power is turned on.)
Figure 2-6: Characteristic Input/Output Curve for a Micro-fuel
Cell