Ametek Thermox Series 2000 WDG-IV User Manual page 133

If you do measure 15 volts at
terminals 2 (-) and 12 (+) of the
sensor board:

NOTE
Sensor Board
80418SE (prior to 1996)
CE Marked Systems
Sensor Boards
80471SE 80467SE
80485SE 80505SE
Measure across terminals D (+) and 6 (-) on the sensor
board. Twelve millivolts should be measured ( this signal
is a function of ambient temperature and may vary up to
2 millivolts at extreme ambient temperatures ). The circuit
compensates to absolute zero (0°K) so 273°C is 11 mV
and 25°c board temperature is 1 mV that results in the 12
mVs added from the compensation circuit.
If you don't measure this voltage, replace the sensor
board.
For a furnace temperature set point for a 615°C oxygen
cell temperature, the furnace thermocouple output would
be about 27 mVs (take memory location 88 which is the
T/C Ratio calculated during the primary - thermal calibra-
tion and divide the cell temperature by this value to come
up with the furnace temperature set point).
The cell thermocouple compensation circuit on the 80471SE,
80485SE, 80467SE and 80505SE sensor boards is different in
comparison to the 80418SE sensor board that was supplied with
Series 2000 based systems prior to 1996 (CE Marked Systems).
The 80418SE used the 40.2-ohm resistor on the display
module to sense the current sourced from the AD592. The
compensation current produces a voltage drop across the
resistor on the display module and then adds this voltage
to the raw T/C mVs sensed at the T/C + and - connections
at the wiring board and at the sensor board.
The change was implemented on later sensor board
designs (80471sE, 80485sE, 80467sE and 80505sE)
due to the confusion caused by seeing a display that was
approximately 12 mVs higher than the voltage measured
at the terminals on the wiring board.
Maintenance and Troubleshooting | 6-7