Oxygen Monitor SGM7.2 10 Attachment
U
eq
/mV = 0.049606
⋅
T/K
⋅
lg [p(O
2
) / (0.2093
⋅
1013.25 mbar)] .
In practice often the oxygen concentration
ϕ
(O
2
) in vol.-% is used for calculations,
which relates to the partial pressure as p(O
2
) =
ϕ
(O
2
)
⋅
p / 100. If the total
pressure does not vary much from the normal pressure (1013.25 mbar), the
following equations are used for calculations :
U
eq
/mV = 0.049606
⋅
T/K
⋅
lg [
ϕ
(O
2
) / 20.93 vol.-%] ,
ϕ
(O
2
)/vol.-% = 20.93
⋅
10
(U
eq
/mV)/(0.049606
⋅
T/K)
.
If a gas is present at the measuring electrode that mainly consists of reducing
components, the chemical standard potentials do not drop out in deducing the
cell voltage equation. In this case one obtains a NERNST-equation for reaction
cells with concentration-independent terms, for example for cells with hydrogen
and water vapour at one of the electrodes and air at the other electrode the
equation, valid in the range 400...1000 °C,
U
eq
(H
2
,H
2
O-air)/mV = -1280.6 + { 0.3165 + 0.0992
⋅
lg [
ϕ
(H
2
O)/
ϕ
(H
2
)] }
⋅
T/K +/-1.
In several technical processes the quotient Q =
ϕ
(H
2
O)/
ϕ
(H
2
) has to be known,
which can be calculated from this equation. If the measuring electrode is the
negative pole of the cell, the cell voltage is given a negative sign.
In the diagram plotted below the dependence of the equilibrium cell voltage on
λ
,
the oxygen concentration, and an excess CH
4
in methane combustion is shown.
The cell voltages calculated with the above-mentioned equations and presented
in the diagram are only valid for isotherm cells, with equal temperature at both
electrodes. Such isotherm cells have been realized very carefully in the ZIROX
products. Unlike these, the well-known lambda probes used in cars with catalysts
in the exhaust pipe, are not isotherm. Their main purpose is indicating
λ
> or < 1,
and they are less suited for precise gaspotentiometric analysis.
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
40
50
60
70
750 °C
Fuel gas : CH
4
10
-18
10
-17
10
-16
10
-14
vol.-% O
2
7
6
5
4
3
2
1
Excess
oxygen
Excess
fuel
- U
eq
/
mV
λ
HB_SGM72_Rehm_eng.DOC
34
