Teledyne 3000ZA2G - Zirconium oxide analyzer User Manual
Page 20
Operational Theory
3000ZA2G
Teledyne
Analytical
Instruments
8
THE SENSOR ASSEMBLY REMAINS HOT EVEN IN
STANDBY MODE. REMOVE POWER FROM THE
INSTRUMENT AND ALLOW SENSOR TO COOL FOR
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:
Where T is the temperature (K) at the zirconia disc, R is the gas
constant, F is the Faraday constant and (P
O2
) Inside and (P
O2
) Outside
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:
Transposing this equation:
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.
Ouside
P
Inside
P
log
4
(mV)
O2
O2
e
F
RT
E
Ouside
P
0.21
log
10
154
.
2
(mV)
O2
2
e
T
x
E
T
E
421
.
46
exp
21
.
0
(Atm)
Outside
%O
2