Bühler technologies BA 2000 Bedienungs- Und Installationsanleitung Seite 47

T - Response time against Volume Flow and Pressure
90
Volume Flow instrument Air [l/h]
Volume Flow Sample Gas [l/h]
Measurement of O
Measurement of O concentration in gases using zirconia dioxide cells is based on the NERNST equation.
2
RT p(O ), air
2
U = ln
(I)
4F p(O ), sample gas
2
The electrical conductivity of zirconia dioxide rises exponentially with increasing temperature and reaches high
enough values above 1110°F.
The oxygen concentration may be derived from the volume concentration instead of the partial pressure,
presuming the total pressure of the gases are almost equal at the two electrodes. In this case, replacing the
constants with their values, equation (I) transforms to equation (II):
(-46,42
= 20,9 e
(II) φ(O )
2
The BA 2000 uses a potentiometric cell
tight ZrO tube in two volumes with different partial pressure of oxygen. At the electrodes, an e.m.f. (electromotive
2
force) is produced which is proportional to the difference of the partial pressures of oxygen. The NERNST-equation
is valid.
DA 550012
04/2008
Page 2/4
using ZrO -Cells
2
2
U
R
T
F
air
p(O ),
2
sample gas
p(O ),
2
φ(O )
2
U
U
)
T
T
20.9
. Reference electrode and sample gas electrode are separated by the gas-
Pressure Instrument Air [bar]
at a process pressure of 1 bar absolute
cell voltage in V
universal gas constant, R = 8,31447 J/(mol K)
measurement temperatue in K
Faraday-constant, F = 96485.34 C/mol
Partial pressure of oxygen at the reference electrode in
dry air in Pa
Partial pressure of oxygen at the reference electrode in
dry air in Pa
Oxygen concentration in sample gas in Vol.-%
Potential difference in mV
Measuring temperature in K
Oxygen concentration in dry air in Vol.-%
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