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OX-TRAN Model 2/28 Operator's Manual
Appendix D: Theory of Operation
How the Oxygen Sensor Works
The Coulox oxygen sensor is a fuel cell that performs in accordance with Faraday's Law. When exposed to
oxygen, the sensor generates an electrical current that is proportional to the amount of oxygen entering the
sensor.
The sensor has a cadmium anode and a graphite cathode. The cathodic and anodic reactions respectively:
1/2O
+ H
2
Cd + 2OH
The electrons create a current which can be used to calculate the amount of oxygen entering the Coulox
sensor. As noted, each oxygen molecule entering the sensor results in four free electrons creating an
electrical current. One mole of oxygen (22.4 liters at 0 °C and 760 mmHg) would produce four Faradays of
current.
Because one Faraday = 96,500 Ampere-seconds, each mole of oxygen will produce 4 x 96,500 = 3.86 x 10
Ampere-seconds. In more practical terms one cc of oxygen in 24 hours = 0.000199 Amperes of current.
How the Transmission Rate is measured
The OX-TRAN Model 2/28 measures transmission rate by directly measuring the current produced by the
sensor. The vertical axis on the Permeant Sensor (Raw Counts) graph is current (1 count = ~ 10 pA).
The measured current is then corrected and scaled using the gain, barometric pressure and area as is
appropriate for the Test Method and Permeant Sensor calibration specified for the measurement.
How the ReZero Cell Works
The transmission rate measured for samples mounted in the Test Cells is the sum of the transmission rate
due to permeation through the barrier material and all other sources of oxygen ingress. This includes the
system baseline. If the system baseline can be measured, subtracting it from the apparent transmission rate
of the Test Cells will give a more accurate representation for the transmission rate of the barrier material.
During a ReZero State the instrument routes carrier gas (which is at the same temperature as used in the
Test Cells) to the oxygen sensor. The instrument components used to perform this function are referred to
as the ReZero Cell. The ReZero Cell is different from the Test Cells in that it does not contain a barrier
material that is exposed to a Test Gas. This means that any residual oxygen in the carrier gas is not due to
permeation through the barrier materials mounted in the Test Cells. The residual oxygen measured in the
ReZero State is therefore considered to be a good representation of the system baseline.
Periodically the instrument baseline is measured using the ReZero Cell. The ReZero Interval and
Examination Time is determined the by currently active Test Method. The resulting data is used to correct
the transmission rate data for any active tests. This ensures that changes in the baseline do not affect the
accuracy of the transmission rate data.
MOCON, Inc.
O +2e→ 2OH
-
2
-
- 2e→ Cd (OH)
2
Revision J
Theory of Operation
5
D-1
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