Teledyne 3190 manual Effect of Pressure, Calibration Characteristics

2 Operational Theory Model 3190

2Pb + O2 → 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 only likely components are the halogens.)

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 as a specific portion 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 rela- tively 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 of constant impedance is directly proportional to the rate at which oxygen molecules reach the cathode, and this rate is directly proportional to the concentration of oxy- gen 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 compen- sate for nonlinearities.

Also, since there is zero output in the absence oxygen, the characteris- tic curve has an absolute zero. The cell itself does not need to be zeroed.

As the cell reaches the end of its useful life, the slope seen in Figure 2-2 decreases. In the Model 3190, the slope is monitored. If the inverse of the slope:

Span Value (ppm) / Cell Output (nA)

is over 4.447 ppm/nA, a sensor failure alarm is triggered, indicating that the cell should be replaced.

2-4 Teledyne Analytical Instruments