Oxygen Monitor SGM7.2 4 Functional Description
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 remarkable 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.
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 measured gas in Pa.
The sensor of the SGM7.2 is based on the conductivity of
oxide ions in a special ceramic substance (zirconium
dioxide) with stabilising additives. The conductivity of these
oxide ions increases exponentially with the temperature and
it is sufficiently high for temperatures above 600°C.
The gas to be measured passes through the ceramic oxide
ion conductor which is a gas-tight pipe. The ceramic pipe is
situated axially symmetric in a thermally well-insulated
heater. The electrodes of the galvanic sensor are made from
platinum. The electrode on the outside of the pipe,
surrounded by dry air, is used as a reference electrode with
a constant, known electrode potential.
HB_SGM72_Rehm_eng.DOC
9
