11
A1
Principles of Operation
The EXGP probe comprises a ceramic detector cell housed in
a protective sheath. A thermocouple is fitted within the probe to
enable the process temperature to be monitored or for
automatic temperature compensation to be provided.
The sheath material may be either aluminous porcelain
(recommended for oxidizing atmospheres) or Incoloy 800
(recommended for reducing atmospheres) for use at
maximum temperatures of 1250
°
C and 1000
°
C respectively.
Special sheaths are available for use up to 1400
°
C.
Connections for the cell output, thermocouple and reference
air are all made at the probe connector head.
The probe contains a high temperature oxygen concentration
cell using zirconium oxide as a solid electrolyte. Inner and
outer electrodes are attached to the cell which is specific to
oxygen.
Air is supplied to the inner reference electrode to provide a
constant partial pressure of oxygen while the process gas to
be measured is in contact with the outer electrode.
A voltage is generated across the electrodes which is a
function of the ratio of the oxygen partial pressures at the two
electrodes. This voltage output represents 'oxygen potential'
and can be used to control the atmosphere of annealing
processes etc.
For the measurement of 'oxygen concentration' it is necessary
to correct for absolute temperature of the probe. The
temperature term in the Nernst equation, on which the voltage
output depends, illustrates this point:
Nernst Equation
E
=
0.0496 T (log
10
P
0
/P
1
) + C mV
Where:
T
=
Absolute Temperature (
°
K)
P
0
=
Partial Pressure Reference Gas
P
1
=
Partial Pressure Sample Gas
C
=
Cell Constant
A2
Range of Operation – Fig. A1
The internal resistance of the cell, i.e. the resistance of the
electrolyte between the electrodes, decreases approximately
exponentially with increasing temperature and for this reason
it is recommended that the probe is used at temperatures
greater than 600
°
C.
The maximum operating temperature is limited by two factors:
a) The onset of electronic conduction through the electrolyte
which reduces the measured output below its theoretical
value.
b) Evaporation of the outer electrode.
Electronic conduction is a function of both temperature,
oxygen partial pressure and, for stabilized zirconia, it occurs at
low levels of oxygen and high temperature. For the above
reasons it is recommended that the probe is used at
temperatures between 600
°
C and 1250
°
C – see Fig. A1.
Electronic Conduction
Errors
High
Resistance
Normal Operation
Non-carburising
Atmospheres only
400
600
800
1000
1200
1250
1400
150
100
50
Kilocalories
Probe
Output
mV
1500
500
1000
Degrees Celsius
Pt/Pt 13% Rh Thermocouple
Probe
Life
Reduced
APPENDIX
Fig. A1 Range of Operation
