More about Thermocouples

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Thermocouples are used to measure temperature. They are made up of two dissimilar pieces of conductive material joined together at their ends. Most commercial thermocouples can measure temperatures ranging from -270 to 1,800 degrees Celsius. Some devices can record measurements above 2,300 degrees Celsius. Standard error ranges are between one and three degrees. They are divided into various calibrations, common varieties consisting of J, K, T and E, with high temperature calibrations classified as R, S, C and GB. A tremendous range of industries utilize thermocouples for numerous applications, from appliance and boiler safety to chemical temperature measurement.

When considering whether to use a thermocouple, it is important to understand how they operate. Certain factors in their design, as well as their general error range, can lead to problems in a situation unsuited to their use. As mentioned, a thermocouple consists of two dissimilar conductors. These conductors are usually different types of metal wires. The wires are connected at two junctions. One of the junctions is kept at a standard, known temperature and the other is subjected to the material being measured. The “known” junction is also referred to as a cold junction.

When two different conductive metals are joined, the junction between them generates a small electric voltage. This voltage is also a function of the temperature, meaning it changes as the temperature changes. In a basic thermocouple, the temperature of “known” junction of the two conductors in a thermocouple produces one signal. This signal is usually measured by a semiconductor temperature sensor. The cold junction signal, or compensation as it is often called, is combined with the signal from the input signal at the measuring end of the thermocouple. The resulting reading is then translated to a temperature by a simple set of calculations. Most commercial thermocouples perform these calculations and provide a digital readout.

Because of its composition, a thermocouple’s performance is affected by different environmental determinants. The temperature range being measured is perhaps the most important factor, and users should consult a manufacturer for help in proper selection. Reference tables are also available on the web and in pertinent literature. The chemical, abrasion and vibration resistance of the thermocouple are also factors. High lead resistance can cause sensitivity to noise. Also, the conductive metals can be altered by temperature extremes and the influence of external chemicals. If the makeup of the measuring wire is changed, the device will become decalibrated.

Common materials used in thermocouples include tungsten, Chromel, Alumel, platinum, rhodium, iron and copper. For many applications, thermocouples are the best choice for instrumentation.