What Are Thermocouples?
A thermocouple is a component used in electronic and certain mechanical circuits. It is composed of two dissimilar metals joined together at one end. A voltage develops across the metals. This voltage changes with temperature. There may be thermocouples at several locations in your home, such as in a stereo receiver (used to monitor overheating of its circuitry) or in your gas log fireplace.
Discovery
In 1821, physicist Thomas Seebeck discovered that when two wires made of different metals were connected at both ends, and heat was applied to one of the joined ends (at the "junction"), a small electric current continuously flowed through the circuit. This phenomenon bears his name, as the energy generated is called "the Seebeck voltage".
All Metal Alloys Produce The Thermocouple Effect
The effect is observed when all dissimilar metals are joined, though certain combinations exhibit more desirable characteristics for practical use. For example, in harsh environments, alloys that are more resistant to corrosion are used. Some metal combinations work in different temperature ranges. Alloy combinations that produce consistent and predictable temperature-to-voltage relationships are most desirable for measuring temperatures.
Applications
The name "thermocouple" is aptly applied to this device, as it couples two dissimilar metals together to produce a voltage that varies with temperature. Obviously then, they are used as thermal sensors in science and industrial applications. Examples include monitoring the exhaust temperature of diesel engines and gas turbines, measuring the temperature in kilns, and in measuring temperature during the making of steel. Thermocouples can be made to measure a wide range of temperatures, yet they are inexpensive.
Safety Applications
Thermocouples are used to detect the presence of a pilot flame in gas and propane log fireplaces, and in home heating gas furnaces. The flame from the unit's pilot light engulfs a thermocouple. Should the flame go out, the thermocouple will become cold, and signal the shut-off of gas, thus preventing gas from leaking into the building. In a sense, the thermocouple is converting heat energy into electrical energy, which is then used to control a solenoid that opens or closes a value to the main gas line.
Design Considerations
Thermocouples are very simple devices, yet engineers must take many factors into consideration when designing circuits that use them. If a voltmeter or other wire is connected to a thermocouple, the metal of the voltmeter's probe or other circuit wires will no doubt be a dissimilar metal, and that junction will exhibit the thermocouple effect. In order to obtain accurate temperature measurements using thermocouples, electrical engineers must take the effect of connecting wires into consideration, and design circuits that compensate for these effects.
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