19
１０. DETECTION PRINCIPLE
(1) Structure
In a resin container, there are noble metal electrode as cathode and lead electrode as anode
immersed in an electrolyte.
A part of container is opened and put a membrane over there. A lead wire from each electrode
(anode and cathode) is led to take out an electrical output across a temperature compensation
thermistor net work.
Structure of galvanic cell type oxygen sensor is shown below.
Structure of galvanic cell type oxygen sensor
(2) Working principle
By connecting a noble metal and base metal electrodes immersed in an electrolyte with a lead wire,
an electrochemical cell is formed. A membrane covers the cell and allows oxygen to diffuse into
the electrolyte at a rate proportional to the partial pressure to oxygen where reduction on the
noble electrode and oxidation on the lead electrode occur respectively as shown in the following
chemical equations and produce a current between electrodes.
Oxygen concentration can be determined by measuring this current, since it is proportional to the
oxygen content diffusing the membrane.
［Cathode］
O2
＋ 2H2O ＋ 4e− → 4OH−
・・・・・・ ①
［Anode］
2Pb
→ 2Pb2＋
＋ 4e−
・・・・・・ ②
［Whole reaction］ O2
＋ 2Pb
＋ 2H2O → 2Pb(OH)2
・・・・・・ ③
Membran
Lead electrode
(Anode)
Gold electrode
(Cathode)
Electrolyte
Thermistor
Output terminal
