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1.3
THE ZIRCONIA SENSOR
The analyser input is provided from one or two (two where an RGS-01 is required) zirconia oxygen sensors and
thermocouples. The probe is designed to be inserted into the dryer or oven. A reference air supply is required for the
probe. A reference air pump is integral with the 1635 analyser. The RGS-01 withdraws a sample from the dryer of
oven. It uses air as a reference but a pump is not required. The sensor construction is shown below.
Thermocouple
Wires
External Wire
Contact
Heater
(required only
with heated
probes)
Electrode Material
The heater control is a time proportioning temperature controller and triac so that the thermocouple junction is
controlled to approximately 720 °C. 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:
(
)
RT
=
E
millivolts
ln
4
F
Where T is the temperature (° K) at the sensor (> 650°C), R is the gas constant, F is the Faraday constant and (PO2)
INSIDE and (PO2) OUTSIDE are the oxygen partial pressures at the inner and outer electrodes, respectively, with the
higher oxygen partial pressure electrode being positive. If dry air at atmospheric pressure, (21% oxygen) is used as a
reference gas at the inner electrode, the following equations are obtained:
(
)
=
×
E
millivolts
. 2
154
Transposing this equation
(
)
(
%
O
OUTSIDE
ATM
2
The 1635 analyser solves this equation which is valid above 650°C.
A 1231 probe can measure the oxygen in the dryer or oven, and in direct fired ovens and dryers the RGS-01 can
measures the oxygen in the dried oven atmosphere. The 1635 uses both these readings to calculate the moisture content
in the oven or dryer. The 1635 can then use the moisture content to calculate the dew point, g/Kg and relative humidity.
August 2009
1635 Water Vapour Analyser
Schematic View of a Sensor Assembly
(
)
⎛
⎞
%
O
INSIDE
⎜ ⎜
⎟ ⎟
2
(
)
⎝
⎠
%
O
OUTSIDE
2
. 0
21
−
2
10
T
ln
(
)
%
O
OUTSIDE
2
−
)
46
.
421
=
. 0
21
EXP
T
Ziconia Disc
E
Internal electrode wire
Four-Bore Thermocouple
Insulating Tubing
Alumina Tube
Thermocouple
Junction
Internal Electrode
9
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