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There are several different types of thermocouples, using different materials for different temperature ranges, but they all operate by the same basic means.

A thermocouple consists of two wires, of different materials, welded or fused together. For the temperature range we are most interested in, the type K thermocouple is most suitable with a maximum temperature of 1900 degrees Fahrenheit.



In a type K device one wire is an alloy called CHROMEL®*, and the other an alloy called ALUMEL®*. A small portion of each wire is exposed and the two are welded or fused together. That assembly is encased in an electrically insulated sheath and the other ends of the wires are connected to a very sensitive voltmeter.

When the fused end of the thermocouple wire is heated, it generates its own current. It's only a matter of millivolts (that's one one-thousandth of a volt), but the voltage generated is an accurate indicator of the temperature of the end of the thermocouple.

These thermocouples are remarkably sturdy and reliable. With no delicate parts to break, unless you exceed their maximum temperature, they're pretty hard to damage. In fact, every gas-fired furnace and water heater uses one to tell the gas valve that the pilot flame is lit.

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Do not confuse thermocouples with the resistance type devices called RTD (Resistance Temperature Detection) devices.

This type of device is basically a very fine wire encased in a container, or bulb. As the temperature of the bulb changes, the electrical resistance of the wire changes. By passing a small current through this wire and measuring the resistance, the temperature can be determined. These sensors, while accurate, are relatively delicate. Rough service is not generally recommended. They also have a temperature limit that makes them unsuitable for use measuring EGT.
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Other types of thermocouples include Type E which were used on the earlier EGT Systems. They have red (-) and brown or tan (+) color codes and are composed of chromel and constantan wires and elements and produce twice the voltage of K type. Many CHT (Cylinder Head Temperature) systems use a type J system or iron and constantan metals. They have either black (+) and yellow (-), or red (-) and white (+) color codes.


If your gauge reads thermocouple voltage directly, as the SPA does, accuracy is not affected by thermocouple wire length or gauge.
Do not cut the portion connected to the probe itself. Before modifying YOUR thermocouple wire, be sure to check with the gauge/thermocouple manufacturer for their specific instructions. Often an extension wire is available that is modifiable.

Typically each connector has a male and female end. For consistancy install all female ends on the EGT probe wires. EGT polarity is very critical and it must be noted the red wire is negative.

When routing EGT cables, the cables must be a minimum of 6 inches from all other ignition wiring. If
wired to close to other wires, erratic EGT readings could occur.

Typical "K" type EGT cable plug:

Edited by: hoot
 
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