Instruction Manual
748441-D
April 2002
Rosemount Analytical Inc. A Division of Emerson Process Management
Description and Specifications 1-7
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
90
) of the sensor is dependent on the
thickness of the Teflon membrane.
The electric current between the elec-
trodes is proportional to the O
2
concentra-
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
2
cell requires a
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
2
) for less than
one hour (analyzer zeroing) will have
no effect on the sensitivity or stability.
Figure 1-5. Electrochemical Oxygen Sensor
Lead Wire (Anode)
Lead Wire (Cathode)
Anode (Lead)
O-Ring
Plastic Disc
Plastic Disk
Black
Red
Teflon Membrane
Cathode (Gold Film)
Sponge Disc
Acid Electrolyte
Thermistor
Resistor
