The Effect Of Pressure - Teledyne 3290 Operating Instructions Manual

Operational Theory

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.
The sensing technology deployed in the Model 3290 is an
electrochemical oxygen sensor which is a partial pressure device. Any
changes in the vent pressure of the unit will affect the pressure of the
sample the sensor sees. This will influence the output of the sensor
according to Dalton's Law of partial pressures. Therefore changes in
barometric pressure should be considered in critical applications.
As the pressure changes, 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
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.
Teledyne Analytical Instruments
Model 3290
10