Oxygen Monitor SGM7.2.4/7.2.6 4 Functional description
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4 Functional description
4.1
Measuring principle
In industry and laboratories, the measurement of the oxygen concentration
in gases is often required. Mostly, it is measured in gases, which have a
considerable, temperature-independent oxygen concentration.
The NERNST equation is used as a basis for determining the oxygen
concentration in gases with the oxygen monitor SGM7.2.4.
NERNST
equation
U
RT
F
p
p
O
air
O
meas gas
=
4
2
2
ln
,
,
.
(I)
U
– cell voltage in V
R
– molar gas constant, R = 8.314 J/(mol · K)
T
– measuring temperature in K
F
– Faraday-constant, F = 9.648 · 10
4
C/mol
p
O2, air
– partial pressure of the oxygen at the reference electrode
in dry air in Pa
p
O2, meas.gas
– partial pressure of the oxygen at the measuring electrode
in the measuring gas in Pa.
The sensor of the SGM7.2.4 is based on the conductivity of oxide ions in a special
ceramic substance (zirconium dioxide) with stabilizing additions. The conductivity
of these oxide ions increases exponentially with the temperature and reaches a
sufficiently high temperature above 600°C.
The gas to be measured passes through the ceramic oxide ion conductor, which
is a gas-tight tube. The ceramic tube is situated axially symmetrically in a thermally
well-insulated heater. The electrodes of the galvanic sensor are made from
platinum. The electrode on the outside of the tube, surrounded by dry air, is used
as a reference electrode with a constant, known electrode potential. (composition
see chapter 6.1.2)
