Teledyne 3000 Ultra Trace - PPB oxygen analyzer - Manual (Page 16)

2-4
2 Operational Theory Model Ultra Trace 3000
Teledyne Analytical Instruments
(These reactions are specific to oxygen as long as no gaseous components
capable of oxidizing lead—such as iodine, bromine, chlorine and fluorine—are
present in the sample.)
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 ppb or
parts-per-million 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 concentra-
tion 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 propor-
tional to the concentration of oxygen in the gaseous mixture. 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 of oxygen, the
characteristic curve has close to an absolute zero (less than ± 0.1 ppm oxygen).
Depending upon the application, zeroing may still be used to compensate for
the combined zero offsets of the cell and the electronics.
2.2.6 TEC Cooling System
Ultra Trace 3000 analyzers include an advance Thermal Electric
Cooler (TEC) system. This system enhances the performance of the Micro-fuel
Cell by cooling it and regulating its operating temperature. The TEC system
includes a TEC module, a temperature control PCB, a separate power supply,
a thermistor, and a special insulated cellblock. The system is used to regulate
the cell temperature at 11 degrees C. Operating the Micro-fuel Cell at a low