ZIROX SGM7 - 10 Appendix (cont.); 10.2 Potentiometric ZrO2-Cells for H2 O;H2 Ratio Measurement

Oxygen Monitor SGM7 10 Appendix
HB_SGM7_eng.docx 35
10.2 Potentiometric ZrO2-cells for the measurement of the H2O/H2-
ratio
The (potentiometric) ZrO
2
-cell delivers a voltage in dependence on oxygen
concentration of the test gas. In inert gases like nitrogen or argon the free
oxygen
21
is measured. In reducing atmospheres like forming gas (nitrogen
with hydrogen, combustible components besides non-reactive gases) the
oxygen is measured as bound oxygen (oxygen in the chemical equilibrium).
The voltage of a potentiometric ZrO
2
-cell follows equation I (free oxygen) or
equation II (bound oxygen, reducing atmosphere):
I U/mV = -[0.03369 + 0.0496 lg ϕ(O
2
)] ⋅ T/K
II U/mV = 1290.6 - [0.3263 + 0,0992 lg ϕ(H
2
O)/ϕ(H
2
)] ⋅ T/K
For the oxygen concentration applies:
ϕ(O
2
)/vol% = 20.63 ⋅ exp[-46,42 ⋅ (U/mV)/(T/K)]
and for the ratio of H
2
O-vapor-/H
2
-concentration:
ϕ(H
2
O) / ϕ(H
2
) = exp[(1290.6 - U/mV)/(0.0431⋅T/K) – 7.5732].
2
Different conditions of oxygen in the measuring gas must be distinguished:
Free oxygen: Oxygen molecules in the gas are independent without a bond to other gas components
(inert gases such as N
2
or Ar). In combustion engines this is called “lean mixture”.
Bound oxygen: Free oxygen molecules do not exist in the gas, only in bound form e.g. as water
vapor. Higher temperatures cause a dissociation and oxygen molecules are available. Since the
dissociation degree increases with the temperature, the measurement result depends on the
temperature. In combustion engines this is called “rich mixture”. Possibly, free oxygen can react with
potential burnable gases at the hot platinum electrode. The result can be a reducing gas.
0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6
0
100
200
300
400
500
600
700
800
900
1000
1100
unburned part of the used CH
4
/ %
5
10
20
30
4050
6070
750 °C
Fuel gas : CH
4
10
-18
10
-17
10
-16
10
-14
vol.-% O
2
765
432
1
Excess
oxygen
Excess
fuel
- U
eq
/ mV
λ