Specifications

DETECTOR METHODOLOGIES
2.3 Electrochemical Oxygen Method
The electrochemical method of determining oxygen concentration is based on the
galvanic cell principle shown in Figure 2-4 below.
Figure 2-4. Electrochemical Oxygen Sensor
The electrochemical oxygen sensor incorporates a lead and gold galvanic process with
a lead anode (1) and a gold cathode (2), using an acid electrolyte (3).
Oxygen molecules diffuse through a non-porous Teflon membrane (4) into the
electrochemical 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 diffusion and corresponding response time (t
90
)
of the sensor is dependent on the thickness of the Teflon membrane.
The electric current between the electrodes is proportional to the O
2
concentration in
the sample gas being measured. The resultant signal is measured as a voltage across
the resistor (6) and thermistor (5), the latter of which is used for temperature
compensation. A change in the output voltage (mV) represents oxygen concentration.
NOTE: The electrochemical O
2
cell requires a minimum internal consumption of
oxygen. Sample gases with an oxygen concentration of less than 2% could result in a
reversible detuning of sensitivity and the output will become unstable. The
recommended practice is to purge the cell with conditioned ambient air between
periods of measurement. If the oxygen concentration is below 2% 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.
(Black)
(Red)
Resistor (6)
Thermistor (5)
Acid electrolyte (3)
Spong3 disc (7)
Cathode (2) (Gold film)
Teflon membrane (4)
Lead wire (Anode)
Lead wire (Cathode)
Anode (1) (Lead)
O-Ring
Plastic disc (9)
Plastic top (10)
Rosemount Analytical µCEM Continuous Analyzer Transmitter 2–6