Thermocouple

In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal energy into electrical energy. 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.

Principle of operation

In 1821, 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 note that thermocouples measure the temperature difference between two points, not absolute temperature.

File:Tc-dia.jpg

In most applications, one of the junctions — the "cold junction" — is maintained at a known (reference) temperature, while 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.

File:Tc-db.jpg

Voltage-Temperature Relationship

The relationship between the temperature difference and the output voltage of a thermocouple is nonlinear and is given by a polynomial interpolation.

$T=\sum _{n=0}^{N}a_{n}v^{n}$

The a_n values are given to n between 5 and 9.

To achieve accurate measurements the equation is usually implemented in a digital controller or stored in a lookup table. Some older devices use analog filters.

Different types

A variety of thermocouples are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research, etc.).

Type K (Chromel (Ni-Cr alloy) / Alumel (Ni-Al alloy)): 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).

Type B (Platinum-Rhodium/Pt-Rh): 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 /Platinum with 7% 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 /Platinum with 10% 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 both risk of fire and a health hazard. To prevent such a danger, some appliances use a thermocouple to sense when the pilot light is burning. Specifically, the tip of a themocouple is placed in the pilot flame. This thermocouple electrically operates the gas supply valve responsible for feeding the pilot. So long as the pilot flame remains lit, the thermocouple remains hot and holds the pilot gas valve open. If the pilot light goes out, the temperature will fall along with a corresponding drop in electricity, removing power from the valve. Unpowered, the valve will then automatically shut off the gas, halting this unsafe condition.

Many systems (Millivolt control systems) extend this concept to the main gas valve as well. Not only does the electricity created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Such a system requires no external source of electricity for its operation and so can operate during a power failure, provided all the related system components allow for this. Note that this excludes common forced air furnaces because external power is required to operate the blower motor, but this is feature is especially useful for unpowered convection heaters.

A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.

Out of concern for wasted energy, many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. This eliminates the need for a standing pilot flame but loses the benefit of any operation without a continuous source of electricity.

Radioisotope thermoelectric generators (RTGs)

Thermocouples can also be applied to generate electricity in radioisotope thermoelectric generators.

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