Trace Electrochemical Measurement; Fig. 2-7: Structure Of Electrochemical Trace Oxygen Sensor - Emerson ROSEMOUNT Analytical NGA 2000 Instruction Manual

Instruction Manual
90002929
07/2006
2.3.3 Trace Electrochemical Measurement (TEO
The MLT uses an electrochemical sensor technology to achieve the trace measurement of oxygen.
The principle structure of the oxygen sensor is shown in Fig. 2-7.
The sensor is a self contained disposable unit which requires no maintenance. The sensor utilizes
the principle of electrochemical reaction to generate a signal proportional to the oxygen
concentration in the sample.
The sensor consists of a cathode and anode which are in contact via a suitable electrolyte. The
sensor has a gas permeable membrane which covers the cathode allowing gas to pass into the
sensor while preventing liquid electrolyte from leaking out.
As the sample diffuses into the sensor, any oxygen present will dissolve in the electrolyte solution
and migrate to the surface of the cathode. The oxygen is reduced at the cathode. Simultaneously,
an oxidation reaction is occurring at the anode generating four electrons. These electrons flow to
the cathode to reduce the oxygen.
The representative half cell reactions are:
Cathode:
4e- + 2H
This flow of electrons constitutes an electric current which is directly proportional to the
concentration of oxygen present in the sample. In the absence of oxygen, no oxidation / reduction
reaction occurs and therefore no current is generated. This allows the sensor to have an absolute
zero.
Emerson Process Management GmbH & Co. OHG
→ 4OH-
O + O
2 2
The resultant overall cell reaction is:
2Pb + O

Fig. 2-7: Structure of electrochemical Trace Oxygen Sensor

)
2
Anode:
4OH- + 2Pb → 2PbO + 2H
→ 2PbO
2
NGA 2000 MLT Hardware
MEASURING PRINCIPLE
OXYGEN MEASUREMENT
O + 4e-
2
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