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Model CAT100
c.
Electrochemical Oxygen Method
The electrochemical method of determin-
ing oxygen concentration is based on the
galvanic cell principle shown in Figure 1-6
(page 1-8).
The electrochemical oxygen sensor
(Figure 1-5, page 1-7) incorporates a lead
and gold galvanic process with a lead an-
ode and a gold cathode, using an acid
electrolyte.
Oxygen molecules diffuse through a non-
porous Teflon membrane into the electro-
chemical cell and are reduced at the gold
cathode. Water is the byproduct of this
reaction.
On the anode, lead oxide is formed which
is transferred into the electrolyte. The lead
anode is continuously regenerated and,
therefore, the electrode potential remains
unchanged for a long time. The rate of dif-
fusion and corresponding response time
(t
) of the sensor is dependent on the
90
thickness of the Teflon membrane.
Lead Wire (Anode)
Lead Wire (Cathode)
Anode (Lead)
O-Ring
Plastic Disc
Plastic Disk
Rosemount Analytical Inc.
Figure 1-5. Electrochemical Oxygen Sensor
A Division of Emerson Process Management
Instruction Manual
The electric current between the elec-
trodes is proportional to the O
tion in the sample gas being measured.
The resultant signal is measured as a
voltage across the resistor and thermistor,
the latter of which is used for temperature
compensation. A change in the output
voltage (mV) represents oxygen concen-
tration.
NOTE
The electrochemical O
minimum internal consumption of
oxygen. Sample gases with an oxygen
concentration of less than 0.1 % could
result in a reversible detuning of sensi-
tivity and the output will become un-
stable. The recommended practice is
to purge the cell with conditioned am-
bient air between periods of measure-
ment. If the oxygen concentration is
below 0.1 % for several hours or days,
the cell must be regenerated for about
one day with ambient air. Temporary
flushing with nitrogen (N
one hour (analyzer zeroing) will have
no effect on the sensitivity or stability.
Black
Red
Resistor
Thermistor
Acid Electrolyte
Sponge Disc
Cathode (Gold Film)
Teflon Membrane
Description and Specifications
748441-D
April 2002
concentra-
2
cell requires a
2
) for less than
2
1-7
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