Ametek Thermox 2000 ATEX Series User Manual page 137

Thermocouple Checks
Open thermocouple
Remove power to the control unit and the sensor. Measure across termi-
nals "C" and "D" on the sensor board with an ohm meter. If an open is
measured, replace the thermocouple.
Shorted/Failed thermocouple
Check that the thermocouple leads are not shorted to chassis ground. To
do this, use an ohm meter to measure between terminal C on the sensor
board and chassis ground, and between terminal D on the sensor board
and chassis ground. If shorted, replace the thermocouple.
To verify that the interconnecting cable for the thermocouple is not shorted
together, remove the thermocouple wires from terminals C and D on the
sensor board and measure across terminals 5 and 6 on the sensor board. If
shorted, correct the interconnecting problem.
To verify the operation of the thermocouple itself, remove the thermo-
couple from the sensor and heat its ceramic tip to a known temperature.
Measure the millivolt output with a proper temperature indicating meter
(type K thermocouple). If the thermocouple reads inaccurately, replace it.
Reversed thermocouple wires
First, have one of the control unit display lines show the sensor cell tem-
perature. If the thermocouple leads are reversed, the displayed tempera-
ture will be decreasing as the sensor warms up (this will usually happen at
start-up or after you replace a thermocouple). This indicates that the ther-
mocouple wiring is reversed. If you just replaced a thermocouple, check
the leads from the thermocouple to the sensor board - yellow wire con-
nects to terminal C (+), red wire connects to terminal D (-).
Thermocouple compensation circuit check
With a volt meter, measure across terminals 2 (-) and 12 (+) on the sensor
board. If approximately 15 volts is not present, check that interconnecting
wiring is correct. If interconnecting wiring is correct, check across 15V
Sup and 15V Com on the control unit for approximately 15 volts. If 15 volts
is not present, check fuse F1 on the wiring card. If this fuse is OK, replace
Maintenance and Troubleshooting | 6-9