Oxygen Monitor SGM7 4 Functional description
HB_SGM7_eng.docx 9
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.
NERNST
equation
U
F
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 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)
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