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Trace Oxygen Analyzer
and therefore the external current, will increase, even though the
proportion of oxygen has not changed.
For a sensor vented to the atmosphere, the displayed oxygen value
will vary in direct proportion to the barometric pressure (absolute
atmospheric pressure). For example, if the analyzer is calibrated with
air at 20.9% oxygen at an ambient atmospheric pressure of 14.3 psia
(0.986 bar), and then the atmospheric pressure increases to 14.5 psia
(1.000 bar), the displayed value for air will be 21.2% oxygen. Ideally,
the analyzer should be calibrated when the atmospheric pressure is in the
middle of the normal barometric pressure range for the location and the
temperature is also in the middle of the normal operating ambient
temperature range.
Fortunately, Dalton's Law confirms that every gas in a mixture
contributes the same pressure to the mixture that it would exert if it were
alone in the same amount in that same volume. This means that as long
as the total pressure of the sample remains constant, the mixture can
change, but the diffusion of the oxygen will be affected only by the
concentration of the oxygen.
For this reason, the sample system supplying sample gas to the cell
should be designed to keep the pressure on the diffusion membrane
constant.
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 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
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 non-linearities.
Also, since there is zero output in the absence oxygen, the
characteristic 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 2-3 decreases. In the Model 3190, the slope is monitored. If the
inverse of the slope:
Span Value (ppm) / Cell Output in nano amps (nA, 1x10
Teledyne Analytical Instruments
Operational Theory
-9
A)
11
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