Model MicroCEM
Instruction Manual
c.Electrochemical Oxygen Method
The electrochemical method of determin- ing oxygen concentration is based on the galvanic cell principle shown in Figure
The electrochemical oxygen sensor (Fig- ure
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 diffusion and corresponding response time (t90) of the sensor is dependent on the thickness of the Teflon membrane.
The electric current between the electrodes is proportional to the O2 concentration 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 O2 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 un- stable. The recommended practice is to purge the cell with conditioned ambient air between periods of measure- ment. 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 (N2) for less than one hour (analyzer zeroing) will have no effect on the sensitivity or stability.
(Red) | V out | (Black) |
Thermistor (5) |
| Resistor (6) |
(+) | |
Gold | Lead |
Cathode (2) | Anode (1) |
O2 + 4 H + 4 e → 2 H2O | 2 Pb + 2 H2O → 2PbO + 4 H + 4 e |
Electrolyte (3)
(ph 6)
Summary reaction O2 + 2 Pb → 2 PbO
Figure
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