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Thermocouples provide a safety control in gas-fed appliances (such as furnaces), controlling the main gas valve to prevent the buildup of gas should the device's pilot flame go out. Composed of two dissimilar metals that are joined on one end, the device reads the temperature and produces a small voltage that's measured by a thermometer built into the thermocouple. If the temperature is too low, the gas valve closes.

Types Of Thermocouples

Thermocouples are available in a wide variety of different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. For example, a J component at its peak usable wire diameter--termed American Wire Gauge 8, or .125 inches --can withstand a temperature limit of 760 degrees Celsius. Different calibrations can withstand temperatures of more than 1,200 degrees Celsius. Thermocouple structures consist not only of the wiring itself but a wrapping of magnesium oxide insulation, which is then covered by a metal sheath. Junctions of thermocouple fittings can be either grounded, exposed or ungrounded.

Replacing Or Repairing A Thermocouple

Because a thermocouple runs consistently at high temperatures, the component is one that wears out quickly. As such, the higher the operating temperature, the more quickly a thermocouple will lose its functionality. Generally, a pilot flame's failure to stay lit is a defining symptom of a faulty thermocouple. Type K components will need to be replaced every year or two. A thermocouple running at a temperature beyond K level--uncommon in a gas furnace--may need replacing as often as every three months.

The actual replacement procedure is fairly simple. Inside the threaded connection to the gas line, unscrew both the copper lead and connection nut. Then, under the mounting bracket at the thermocouple tube, unscrew the bracket nut that holds the tube in place. Place the new thermocouple into the bracket hole, making sure the steel tube is pointed up while the copper lead is aimed down. Screw the bracket nut back over the tube and tighten the connection nut to the threaded connection, but only to a point slightly tighter than if twisted by hand.

Who Makes Replacement Thermocouples?

Thermocouples can be found at any major home improvement store or plumbing supply store, or similarly focused website. While many large companies, such as Honeywell or Omega Engineering, make the component, there are a large number of vendors who make the product as well, including Brearley, Conax Buffalo Technologies, GEC Instruments, Hewitt Industries, JMS Southeast, Nanmac, Pyro Electric and Versa Gauge USA.

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