How to Read a Thermocouple With a Voltmeter


A thermocouple is a small electrical device that is basically the junction of two wires that are composed of different metals. Because different metals react differently to temperature, an electric potential develops at the juncture. This electrical potential varies with temperature, and therefore the temperature can be calculated when the voltage is known. A voltmeter can measure the voltage that develops by the thermocouple, and a table of calculated values can be looked at to determine the corresponding temperature. This can be useful for troubleshooting thermocouples used in the home, such as those found in clothes dryers.

Things You'll Need

  • Screwdriver
  • Access the thermocouple terminal wires. These are the ends that are not welded together to form a junction. They can be found at a terminal block or in a two-pin connector. If the thermocouple is in an electrical device, make sure power is removed from the device before beginning.

  • Disconnect the thermocouple wires. If they're connected to a terminal block, unscrew the terminal screws. If they're in a connector, unplug the connector. It's important that the thermocouple is disconnected from the circuit, which could be providing a voltage that would be misleading.

  • Turn the voltmeter on. Take one voltmeter probe and touch the end of one of the wires. Make sure it's touching metal. Touch the other voltmeter probe to the other wire. Read the voltage on the voltmeter display. Most voltmeters will automatically select the range, but if yours does not, find the correct range to get a reading. It will usually be the lowest possible range because thermocouple put out voltage in the millivolt range.

  • Turn the voltmeter off after obtaining your reading.

  • Look up the voltage in a temperature conversion table. This will tell you what the temperature is for the measured voltage. It should be giving you a temperature that is close to the temperature of the room if the electrical device hasn't been operated recently. You'll need to know the type of thermocouple, which is associated with a letter such as J or K. Consult your owner's manual. Many household devices use a K-type thermocouple.

  • Reattach your thermocouple by securing the wires to the terminal block or plugging it back in. Reapply power to your device.

Tips & Warnings

  • Write down your measurements so you can refer to them for future measurements.
  • Thermocouples can be located near electrical contacts. Electricity is dangerous and can cause electrical shock.

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  • Photo Credit morguefile
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