OX-TRAN Model 2/22 Operator’s Manual
Theory of Operation
MOCON, Inc.
Revision R
D-1
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
2
+ H
2
O +2e→ 2OH
-
Cd + 2OH
-
-
2e→ Cd (OH)
2
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
5
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 2/22 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.