Thermocouple
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In electronics, thermocouples are a widely used type of temperature sensor. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy; system errors of less than 1 °C can be difficult to achieve.
A thermopile is a group of thermocouples connected in series.
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Principle of operation
In 1822, an Estonian physicist named Thomas Johann Seebeck discovered that when any conductor (such as a metal) is subjected to a thermal gradient, it will generate a small voltage. Thermocouples make use of this so-called Peltier-Seebeck effect.
Thermocouples produce an output voltage which depends on the temperature difference between the junctions of two dissimilar metal wires. It is important to appreciate that thermocouples measure the temperature difference between two points, not absolute temperature.
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In most applications, one of the junctions — the "cold junction" — is maintained at a known (reference) temperature, whilst the other end is attached to a probe. For example, in the image below, the cold junction will be at copper tracks on the circuit board. Another temperature sensor will measure the temperature at this point, so that the temperature at the probe tip can be calculated.
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The relationship between the temperature difference and the output voltage of a thermocouple is nonlinear and is given by a complex polynomial equation (which is fifth to ninth order depending on thermocouple type). To achieve accurate measurements some type of linearisation must be carried out, either by a microprocessor or by analogue means.
Different types
A variety of thermocouples are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research, etc.).
The "general purpose" thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. They are available in the −200 °C to +1200 °C range. Sensitivity is approximately 41 µV/°C.
- Type E (Chromel / Constantan (Cu-Ni alloy))
Type E has a high output (68 µV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.
- Type J (Iron / Constantan)
Limited range (−40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that cannot accept "modern" thermocouples. J types cannot be used above 760 °C as an abrupt magnetic transformation causes permanent decalibration.
- Type N (Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy))
High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B, R, S) types. Designed to be an "improved" type K, it is becoming more popular.
Thermocouple types B, R, and S are all noble metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approximately 10 µV/°C) they are usually only used for high temperature measurement (>300 °C).
Suited for high temperature measurements up to 1800 °C. Unusually type B thermocouples (due to the shape of their temperature-voltage curve) give the same output at 0 °C and 42 °C. This makes them useless below 50 °C.
- Type R (Platinum / Rhodium)
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use.
- Type S (Platinum / Rhodium)
Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43 °C).
- Type T (Copper / Constantan)
Suited for measurements in the −200 to 0 °C range. The positive conductor is made of copper, and the negative conductor is made of constantan.
Thermocouples are usually selected to ensure that the measuring equipment does not limit the range of temperatures that can be measured. Note that thermocouples with low sensitivity (B, R, and S) have a correspondingly lower resolution.
Applications
Heating appliance safety
Many gas-fed heating appliances like ovens and water heaters make use of a pilot light to ignite the main gas burner as required. If the pilot light becomes extinguished for any reason, there is the potential for uncombusted gas to be released into the surrounding area, thereby creating a risk of explosion.
To prevent such a hazard, some appliances employ a thermocouple to sense when the pilot light is not burning. Specifically, the tip of a themocouple is placed in the pilot flame. If the pilot light goes out, the temperature will fall and the control system for the appliance will trigger a valve to turn off the gas supply.
Radioisotope thermoelectric generators (RTGs)
Thermocouples can also applied to generate electricity in radioisotope thermoelectric generators.
See also
External links
- Thermocouple application notes (http://www.picotech.com/applications/thermocouple.html)
- Temperatures.com page on thermocouples (http://www.temperatures.com/tcs.html)
- Thermocouple design guide (http://www.peaksensors.co.uk/designguide.html) *Thermocouple types (http://www.peaksensors.co.uk/thermocouples.html)
- Omega.com thermocouples (http://www.omega.com/guides/thermocouples.html)de:Thermoelement
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