Teledyne OT-3 operating instructions Effect of Pressure, Calibration Characteristics

Trace Oxygen Analyzer Installation

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.

Teledyne Analytical Instruments 9