How to Measure Microamps With a Fluke Meter

Testing electronic thermocouples for misfiring gas burners

To light its burner(s) automatically, a gas furnace sprays controlled jets of natural gas over a special red-hot metal component, causing the former to spontaneously combust. In order to determine when the component is hot enough to induce combustion, the furnace uses a thermocouple sensor. When the temperature of one end of a thermocouple (e.g., the tip of the flame sensor probe) becomes significantly higher than the other, voltage is generated. This voltage then triggers the furnace to release the jets of gas.

In HVAC maintenance and installation, the purpose of a microamp sensor is to test the efficacy of thermocouple-based flame sensors.

1. Turn the multimeter's dial until the end with the white dot is pointing at the microamperage setting, which is denoted by a yellow sine wave next to the letters "uA" printed in white. If done correctly, this will cause the letters "uA" to appear on the right-hand side of the LCD screen.

2. Check under the "uA" symbol on the LCD screen to make sure that the multimeter is in DC mode. If you're in AC mode, push the yellow button to switch to DC.

3. Unscrew the flame rod from its housing adjacent to the burner. Note: do not do this step until the furnace has been turned off for at least twenty minutes.

4. Slide the tip of the flame rod out of the housing and sand its entire surface thoroughly with the Emery paper.

5. Trace the flame rod's wire all the way back the controller, into which the wire plugs.

6. Remove the wire's plug from the controller.

7. Attach the Fluke meter's red probe to the wire's plug.

8. Attach the Fluke meter's black probe to the controller's outlet.

9. Screw the flame rod back into its housing.

10. Start the furnace and note the microamperage readings from the flame rod's thermocouple as they appear on the Fluke 116's LCD screen.