A type of thermometer that allows the direct electronic monitoring of temperature. A temperature-dependent potential difference exists across the junction between two different metals (the contact potential). If one such junction is placed in the sample, and the other held at constant temperature, the potential difference between the two junctions is a measure of the sample temperature.
In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. They are cheap, interchangeable, have standard connectors, and can measure a wide range of temperatures.
Principle of operation
In 1821, the German-Estonian physicist Thomas Johann Seebeck discovered that when any conductor (such as a metal) is subjected to a thermal gradient, it will generate a voltage. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Using a dissimilar metal to complete the circuit will have a different voltage generated, leaving a small difference voltage available for us to measure, which increases with temperature.
It is important to note that thermocouples measure the temperature difference between two points, not absolute temperature.
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. Another temperature sensor will measure the temperature at this point, so that the temperature at the probe tip can be calculated.
Thermocouples can be connected in series with each other to form a thermopile, where all the hot junctions are exposed to the higher temperature and all the cold junctions to a lower temperature.
Having available a known temperature cold junction, while useful for laboratory calibrations, is simply not convenient for most directly connected indicating and control instruments. They incorporate into their circuits an artificial cold junction using some other thermally sensitive device (such as a thermister or diode) to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals.
Usually the thermocouple is attached to the indicating device by a special wire known as the compensating or extension cable. These cables are less costly than thermocouple wire, although not cheap, and are usually produced in a convenient form for carrying over long distances - typically as flexible insulated wiring or multicore cables. They are usually specified for accuracy over a more restricted temperature range than the thermocouple wires. They use quite different, relatively low cost alloy conductor materials whose net thermoelectric coefficients are similar to those of the thermocouple in question (over a limited range of temperatures), but which do not match them quite as faithfully as extension cables. The combination develops similar outputs to those of the themocouple, but the operating temperature range of the compensating cable is restricted to keep the mis-match errors acceptably small.
The extension cable or compensating cable must be selected to match the thermocouple. It generates a voltage proportional to the difference between the hot junction and cold junction, and is connected in the correct polarity so that the additional voltage is added to the thermocouple voltage, compensating for the temperature difference between the hot and cold junctions.
Ref: "Guide to Thermocouple and Resistance Themometry" pp20 Iss 6.0 TC Ltd.
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.
Different types
A variety of thermocouples are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research, etc.). 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. Type J (Iron / Constantan) Limited range (−40 to +750 °C) makes type J less popular than type K. Type J's have a sensitivity of ~52 µV/°C 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.
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. Type R (Platinum /Platinum with 7% Rhodium) Suited for high temperature measurements up to 1600 °C. Type S (Platinum /Platinum with 10% Rhodium) Suited for high temperature measurements up to 1600 °C. As both conductors are non-magnetic Type T thermocouples are a popular choice for applications such as Electrical Generators which contain strong magnetic fields. Type T thermocouples have a sensitivity of ~43 µV/°CThermocouples are usually selected to ensure that the measuring equipment does not limit the range of temperatures that can be measured.
Applications
Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0--100 °C with 0.1 °C accuracy.
Steel Industry
Type B,S,R and K thermocouples are used extensively in the steel and iron industry to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, Type S thermocouples are regularly used in the electric arc furnace process to accurately measure the steel's temperature before tapping.
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. The thermocouple voltage, typically around 20 mV, 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. 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. Here, a larger voltage is needed than in a pilot flame safety system described above, for which reason a thermopile is used rather than a single thermocouple.
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.
Thermopile radiation sensors
Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink.
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