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Operational Theory
TWO HOURS BEFORE PERFORMING ANY
MAINTENANCE ON THE SENSOR.
The heater control is a time proportioning temperature controller
that maintains the thermocouple junction at the sensor operating
temperature of 575°C (1070°F).
When exposed to different oxygen partial pressures at the outside
and inside of the sensor, an EMF (E) is developed which obeys the
Nernst equation:
E
(mV)
Where T is the temperature (K) at the zirconia disc, R is the gas
constant, F is the Faraday constant and (P
are the oxygen partial pressures at the inner and outer electrodes,
respectively, with the higher oxygen partial pressure electrode being
positive.
When dry air at atmospheric pressure, (21% oxygen) is used as a
reference gas at the inner electrode, the following equations are
obtained:
E
(mV)
Transposing this equation:
%O
The analyzer solves this equation, which is valid above the
minimum operating temperature.
The oxygen sensor assembly provides a means of exposing the
zirconia sensor to the atmosphere to be measured on the inside of the
sensor while maintaining air as a reference gas at atmospheric pressure
on the outside of the sensor.
The sensor offset voltage changes with sample flowrate as shown in
Figure 2-1. Assuming that the environment remains constant and within
the specifications of the sensor, the difference in offset voltage between
Teledyne Analytical Instruments
P
Inside
RT
O2
log
e
4
F
P
Ouside
O2
O2
2
. 2
154
x
10
T
log
e
Outside
(Atm)
. 0
2
3000ZA2G
) Inside and (P
) Outside
O2
0.21
P
Ouside
O2
46
.
421
E
21
exp
T
8
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