THERMOCOUPLE VOLTAGE MEASUREMENT PROCEDURE

StepExpected Result/Action
1, Shut off power and gas to unit.
If testing the thermocouple under no-load conditions, disconnect the
thermocouple from the gas valve by unscrewing the fitting.
If testing the thermocouple under load conditions, disconnect the
thermocouple from the gas valve by unscrewing the fitting. Screw the
thermocouple test adapter into the gas valve, then screw the thermocouple
into the adapter.
Power and gas to unit is turned off and thermocouple is disconnected from gas
valve (no-load testing) or connected to the test adapter (loaded testing).
2. Set up the VOM/DMM to measure DC voltage on a millivolt range. Note that
the voltage level expected can be 18 millivolts or higher.
If testing the thermocouple under no-load conditions, connect the one VOM/
DMM meter lead to the end contact (center) of the thermocouple and the
other meter lead to the thermocouple copper tubing as shown in Figure
SP-11-4, View A.
If testing the thermocouple under load conditions, connect one VOM/DMM
meter lead to the test adapter and the other meter lead to the thermocouple
copper tubing as shown in Figure SP-11 -4, View B.
Thermocouple and VOM/DMM prepared for measurement.
3. Turn on gas, then turn gas valve control to the pilot position. While holding
down the gas valve knob/reset button, light the pilot.
Continue to hold down the gas valve knob/reset button while observing the
VOM/DMM reading. As the thermocouple is heated by the pilot flame, the
VOM/DMM reading should begin increasing.
Continue holding down the gas valve knob or reset button for 5 minutes.
After 5 minutes, if VOM/DMM reads at least 18 millivolts (no-load test) or 9
millivolts (loaded test), this indicates the thermocouple is probably good.
If VOM/DMM reads less than 18 millivolts or 9 millivolts, check for one or more
of the following conditions:
• Yellow pilot flame • Pilot flame too small
• Pilot flame misdirected • Pilot flame flickering
• Pilot flame floating • Pilot flame lifting
• Thermocouple bent • Thermocouple lead kinked
• Thermocouple insulator damaged • Thermocouple end contact dirty
If VOM/DMM reads less than 18 millivolts or 9 millivolts, correct the problem
(listed above), or replace the thermocouple.

pic1 6 THERMOCOUPLE VOLTAGE MEASUREMENT PROCEDURE


Categories: Service Procedures | Leave a comment