The electrochemical sensor is build
up of a chamber consisting of two or three electrodes and an electrolyte.
A membrane, like a porous PTFE-sheet, locks up the chamber for the
electrolyte to come out. Gas, however, can pass through the membrane
and reach the electrodes. The electrodes are usually made of platinum
or gold. On the measuring electrode an electro-chemical reaction with
the gas will take place where electrons will get free and diffuse
to the other electrode, the counter electrode. The current of electrodes
that diffuse from one electrode to the other is proportional to the
gas concentration.
Electrochemical sensor
The reference electrode is necessary
to build up a constant voltage between the measuring electrode and
the counter electrode. Most gases react only within a small range
of the reference voltage. Electrochemical sensors are available for
many gases like H2S, HCN, CO, Cl2, SO2, H2, NO and NO2.
Some typical reactions of the gas and the electrolyte:
Gas
Measuring electrode
Counter electrode
H2S
H2S +
4 H2O —> H2SO4 + 7 H+ + 8e-
O2 + 4
H + 4e- —> 2 H2O
HCN
2 HCN —>2
H + 2 CN-
O2 + 2
H + 2e- —> H2O
CO
CO + H2O
—> CO2 + 2 H+ + 2e-
O2 + 4
H + 4e- —> 2 H2O
Cl2
2 H2O
—> O2 + 4 H+ + 4e-
2 H+ + Cl2
+ 2e- —> 2 HCl
SO2
SO2 +
2 H2O —> H2SO4 + 2 H+ + 2e-
O2 + 4
H + 4e- —> 2 H2O
H2
H2 —>
2 H+ + 2e-
O2 + 4
H + 2e- —> H2O
NO
NO+ 2 H2O
—> HNO3+ 3 H+ + 3e-
O2 + 4
H + 4e- —> 2 H2O
NO2
2 H2O
—> O2 + 4 H+ + 4e-
NO2 +
2 H + 2e- —> NO + H2O
Depending on the measuring gas, different
catalysts, electrodes, electrolytes and reference voltages are used
to get the best selectivity and avoid cross sensitivity.
Electrochemical sensors are mainly used to control the ambient air
on one substance. The cross sensitivity is, in most cases, to low
to use the electrochemical sensor to measure several compounds at
the same time.
Electrochemical sensors for oxygen
Electrochemical sensors are often used to measure oxygen. These sensors
are built up similar to galvanic cells. They have 2 electrodes from
different material and an electrolyte. Usually a gold and led electrode
is found in the oxygen sensor. The oxygen diffuses through a PTFE-membrane
and reaches the electrodes.
Oxygen sensor
The voltage between the measuring electrode
and the counter electrode is proportional to the measured gas concentration.
There are oxygen sensors that measure the partial pressure and those
that measure directly the Vol%. The GfG-sensors for oxygen measure
directly the Vol%.
Vol% sensors measure the percentage of the gas in the atmosphere,
which is in clean dry air 20.93 vol% oxygen. The partial pressure
is defined as the pressure the gas will have if no other gas is in
the mixture. The partial pressure of oxygen in clean dry air is eg
at a presure of 1 atmosphere 0,2093 atm.
The concentration of the gas is independent from the absolute pressure.
The partial pressure changes proportional to the absolute pressure.
For clean dry air at an absolute pressure of 2 atm , the following
counts:
Volume
in % O2
Partial pressure of
O2
20,93
Vol%
0,419 atm.
The output signal of a sensor using the partial
pressure changes with the absolute pressure. The sensor has to be
calibrated, if it is brought in higher or lower regions (i.e. mountains
or mines). On the other hand the signal of a volume % measuring sensor
is uneffected by the above named changes. The oxygen volume in % did
not change.
