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10. DETECTION PRINCIPLE
By connecting a noble metal and lead immersed in an electrolyte with a lead wire, an
electrochemical cell (galvanic cell) is made. A membrane covers the cell and allows oxygen
to diffuse into the electrolyte where reduction on the metal electrode and oxidation on the
lead electrode occur respectively as shown in the following chemical equation ;
Cathode (noble metal) :
O
2
+ H
2
O + 4e
-
4OH
-
Anode (lead) :
2Pb 2Pb
2+
+ 4e
-
The oxygen reacts in the cell as above equation and produce a current proportional to the
oxygen concentration. The current develops a voltage across a thermistor/resistor network.
If oxygen concentration decreases, reduction on noble metal electrode decreases and
voltage on both sides of thermistor also decreases.
[Structure]
Membrane
Cathode
Electrolyte
Thermistor
Anode
Output terminal
