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A thermocouple is a device consisting of two different conductors (usually metal alloys) thatproduce a voltage proportional to a temperature difference between either end of the pair of 

conductors. Thermocouples are a widely used type of  temperature sensor for measurement and

control[1]

 and can also be used to convert a heat gradient into electricity. They are inexpensive ,[2]

 interchangeable, are supplied with standard connectors, and can measure a wide range of 

temperatures. In contrast to most other methods of temperature measurement, thermocouples are

self powered and require no external form of excitation. The main limitation with thermocouples

is accuracy and system errors of less than one degree Celsius (C) can be difficult to achieve.[3]

 

Any junction of dissimilar metals will produce an electric potential related to temperature.

Thermocouples for practical measurement of temperature are junctions of specific alloys which

have a predictable and repeatable relationship between temperature and voltage. Different alloysare used for different temperature ranges. Properties such as resistance to corrosion may also be

important when choosing a type of thermocouple. Where the measurement point is far from themeasuring instrument, the intermediate connection can be made by extension wires which are

less costly than the materials used to make the sensor. Thermocouples are usually standardized

against a reference temperature of 0 degrees Celsius; practical instruments use electronicmethods of cold-junction compensation to adjust for varying temperature at the instrument

terminals. Electronic instruments can also compensate for the varying characteristics of the

thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include

temperature measurement for kilns, gas turbine exhaust, diesel engines, and

other industrial processes. Principle of operationMain article: Seebeck effect 

In 1821, the German – Estonian physicist Thomas Johann Seebeck  discovered that when anyconductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the

thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves

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connecting another conductor to the "hot" end. This additional conductor will then also

experience the temperature gradient, and develop a voltage of its own which will oppose theoriginal. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar

metal to complete the circuit creates a circuit in which the two legs generate different voltages,

leaving a small difference in voltage available for measurement. That difference increases with

temperature, and is between 1 and 70 microvolts per degree Celsius (µV/°C) for standard metalcombinations.

The voltage is not generated at the junction of the two metals of the thermocouple but rather

along that portion of the length of the two dissimilar metals that is subjected to a temperature

gradient. Because both lengths of dissimilar metals experience the same temperature gradient,the end result is a measurement of the temperature at the thermocouple junction.

[edit] Practical use

[edit] Voltage – temperature relationship

For typical metals used in thermocouples, the output

voltage increases almost linearly with the temperature

difference (ΔT) over a bounded range of temperatures. For precise measurements or measurements outside of the linear

temperature range, non-linearity must be corrected. Thenonlinear relationship between the temperature difference

(ΔT) and the output voltage (mV) of a thermocouple can be

approximated by a polynomial:

The coefficients an are given for n from 0 to between 5 and13 depending upon the metals. In some cases better

accuracy is obtained with additional non-polynomial terms.[4]

 A database of voltage as a function

of temperature, and coefficients for computation of temperature from voltage and vice-versa formany types of thermocouple is available online.[4] 

In modern equipment the equation is usually implemented in a digital controller or stored in a

look-up table;[5] older devices use analog circuits.

Piece-wise linear approximations are an alternative to polynomial corrections.[6]

 

[edit] Cold junction compensation

Thermocouples measure the temperature difference between two points, not absolute

temperature. To measure a single temperature one of the junctions — normally the cold

Polynomial Coefficients 0-500 °C[4] 

 n   a n (for Type K) 

1 25.08355

2 7.860106x10−2

 

3 -2.503131x10−1

 

4 8.315270x10−2

 

5 -1.228034x10−2

 

6 9.804036x10

−4

 7 -4.413030x10

−5 

8 1.057734x10−6

 

9 -1.052755x10−8

 

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 junction — is maintained at a known reference temperature, and the other junction is at the

temperature to be sensed.

Having a junction of known temperature, while useful for laboratory calibration, is not

convenient for most measurement and control applications. Instead, they incorporate an artificial

cold junction using a thermally sensitive device such as a thermistor or diode to measure thetemperature of the input connections at the instrument, with special care being taken to minimize

any temperature gradient between terminals. Hence, the voltage from a known cold junction canbe simulated, and the appropriate correction applied. This is known as cold junction

compensation. Some integrated circuits such as the LT1025 are designed to output a

compensated voltage based on thermocouple type and cold junction temperature.

[edit] Power production

A thermocouple can produce current, which means it can be used to drive some processesdirectly, without the need for extra circuitry and power sources. For example, the power from a

thermocouple can activate a valve when a temperature difference arises. The electrical energy generated by a thermocouple is converted from the heat which must be supplied to the hot side to

maintain the electric potential. A continuous flow of heat is necessary because the currentflowing through the thermocouple tends to cause the hot side to cool down and the cold side to

heat up (the Peltier effect).

Thermocouples can be connected in series to form a thermopile, where all the hot junctions are

exposed to a higher temperature and all the cold junctions to a lower temperature. The output is

the sum of the voltages across the individual junctions, giving larger voltage and power output.In a radioisotope thermoelectric generator, the radioactive decay of  transuranic elements as a

heat source has been used to power spacecraft on missions too far from the Sun to use solar

power.

[edit] Grades

Thermocouple wire is available in several different metallurgical formulations per type,

typically, in decreasing levels of accuracy and cost: special limits of error, standard, and

extension grades.

[edit] Extension wire

Extension grade wires made of the same metals as a higher-grade thermocouple are used toconnect it to a measuring instrument some distance away without introducing additional junctions between dissimilar materials which would generate unwanted voltages; the connections

to the extension wires, being of like metals, do not generate a voltage. In the case of platinum

thermocouples, extension wire is a copper alloy, since it would be prohibitively expensive to useplatinum for extension wires. The extension wire is specified to have a very similar thermal

coefficient of EMF to the thermocouple, but only over a narrow range of temperatures; this

reduces the cost significantly.

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The temperature-measuring instrument must have high input impedance to prevent any

significant current draw from the thermocouple, to prevent a resistive voltage drop across thewire. Changes in metallurgy along the length of the thermocouple (such as termination strips or

changes in thermocouple type wire) will introduce another thermocouple junction which affects

measurement accuracy.

[edit] Types

Certain combinations of alloys have become popular as industry standards. Selection of the

combination is driven by cost, availability, convenience, melting point, chemical properties,

stability, and output. Different types are best suited for different applications. They are usually

selected based on the temperature range and sensitivity needed. Thermocouples with lowsensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria

include the inertness of the thermocouple material, and whether it is magnetic or not. Standard

thermocouple types are listed below with the positive electrode first, followed by the negative

electrode.

[edit] K

Type K (chromel{90 percent nickel and 10 percent chromium} – alumel)(Alumel consisting of 

95% nickel, 2% manganese, 2% aluminium and 1% silicon) is the most common general purpose

thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative toalumel.[7] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C

 / -328 °F to +2462 °F range. Type K was specified at a time when metallurgy was less advanced

than it is today, and consequently characteristics may vary considerably between samples. One

of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made withmagnetic material is that they undergo a deviation in output when the material reaches its Curie

point; this occurs for type K thermocouples at around 150 °C.

[edit] E

Type E (chromel – constantan)[5]

 has a high output (68 µV/°C) which makes it well suited tocryogenic use. Additionally, it is non-magnetic.

[edit] J

Type J (iron – constantan) has a more restricted range than type K (−40 to +750 °C), but higher

sensitivity of about 55 µV/°C.

[2]

 The Curie point of the iron (770 °C)

[8]

 causes an abrupt changein the characteristic, which determines the upper temperature limit.

[edit] N

Type N (Nicrosil – Nisil) (Nickel-Chromium-Silicon/Nickel-Silicon) thermocouples are suitablefor use at high temperatures, exceeding 1200 °C, due to their stability and ability to resist high

temperature oxidation. Sensitivity is about 39 µV/°C at 900 °C, slightly lower than type K.

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Designed to be an improved type K due to increased stability at higher temperatures, it is

becoming more popular, though the differences may or may not be substantial enough to warranta change.

[edit] Platinum types B, R, and S

Types B, R, and S thermocouples use platinum or a platinum – rhodium alloy for each conductor.

These are among the most stable thermocouples, but have lower sensitivity than other types,approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only for high

temperature measurements due to their high cost and low sensitivity.

B 

Type B thermocouples use a platinum – rhodium alloy for each conductor. One conductorcontains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are

suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and

42 °C, limiting their use below about 50 °C.

R 

Type R thermocouples use a platinum – rhodium alloy containing 13% rhodium for one conductorand pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C.

S 

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the

positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R,

type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of  gold (1064.43 °C).

[edit] T

Type T (copper – constantan) thermocouples are suited for measurements in the −200 to 350 °C

range. Often used as a differential measurement since only copper wire touches the probes. Since

both conductors are non-magnetic, there is no Curie point and thus no abrupt change incharacteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.

[edit] C

Type C (tungsten 5% rhenium  – tungsten 26% rhenium) thermocouples are suited for

measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for  vacuum

furnaces at extremely high temperatures. It must never be used in the presence of  oxygen attemperatures above 260 °C.

[edit] M

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Type M thermocouples use a nickel alloy for each wire. The positive wire (20 Alloy) contains

18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. Thesethermocouples are used in vacuum furnaces for the same reasons as with type C. Upper

temperature is limited to 1400 °C. It is less commonly used than other types.

[edit] Chromel-gold/iron

In chromel-gold / iron thermocouples, the positive wire is chromel and the negative wire is goldwith a small fraction (0.03 – 0.15 atom percent) of iron. It can be used for cryogenic applications

(1.2 – 300 K and even up to 600 K). Both the sensitivity and the temperature range depends on the

iron concentration. The sensitivity is typically around 15 µV/K at low temperatures and the

lowest usable temperature varies between 1.2 and 4.2 K.

[edit] Laws for thermocouples

[edit] Law of homogeneous material

A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the

application of heat alone, regardless of how it might vary in cross section. In other words,

temperature changes in the wiring between the input and output do not affect the output voltage,provided all wires are made of the same materials as the thermocouple. No current flows in the

circuit made of a single metal by the application of heat alone.

[edit] Law of intermediate materials

The algebraic sum of the thermoelectric emfs in a circuit composed of any number of dissimilar

materials is zero if all of the junctions are at a uniform temperature. So If a third metal is insertedin either wire and if the two new junctions are at the same temperature, there will be no net

voltage generated by the new metal.

[edit] Law of successive or intermediate temperatures

If two dissimilar homogeneous materials produce thermal emf1 when the junctions are at T1 andT2 and produce thermal emf2 when the junctions are at T2 and T3, the emf generated when the

 junctions are at T1 and T3 will be emf1 + emf2,provided T1<T2<T3.

[edit] Aging of thermocouples

Thermoelements are often used at high temperatures and in reactive furnace atmospheres. In this

case the practical lifetime is determined by aging. The thermoelectric coefficients of the wires inthe area of high temperature change with time and the measurement voltage drops. The simple

relationship between the temperature difference of the joints and the measurement voltage is

only correct if each wire is homogeneous. With an aged thermocouple this is not the case.Relevant for the generation of the measurement voltage are the properties of the metals at atemperature gradient. If an aged thermocouple is pulled partly out of the furnace, the aged parts

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from the region previously at high temperature enter the area of temperature gradient and the

measurement error is significantly increased. However an aged thermocouple that is pusheddeeper into the furnace gives a more accurate reading.[citation needed ] 

[edit] Thermocouple comparison

The table below describes properties of several different thermocouple types. Within the

tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. Forexample, a thermocouple with a tolerance of ±0.0025×T would have a tolerance of ±2.5 °C at

1000 °C.

Type

Temperature

range °C

(continuous)

Temperature

range °C (short

term)

Tolerance class one

(°C)

Tolerance class

two (°C)

K 0 to +1100 −180 to +1300 

±1.5 between −40 °C

and 375 °C±0.004×T between

375 °C and 1000 °C

±2.5 between −40

°C and 333 °C±0.0075×T

between 333 °Cand 1200 °C

J 0 to +750 −180 to +800 

±1.5 between −40 °Cand 375 °C

±0.004×T between

375 °C and 750 °C

±2.5 between −40°C and 333 °C

±0.0075×Tbetween 333 °C

and 750 °C

N 0 to +1100 −270 to +1300 

±1.5 between −40 °Cand 375 °C

±0.004×T between375 °C and 1000 °C

±2.5 between −40

°C and 333 °C

±0.0075×Tbetween 333 °C

and 1200 °C

R 0 to +1600 −50 to +1700 

±1.0 between 0 °Cand 1100 °C

±[1 + 0.003×(T −

1100)] between 1100°C and 1600 °C

±1.5 between 0°C and 600 °C

±0.0025×T

between 600 °Cand 1600 °C

S 0 to 1600 −50 to +1750 

±1.0 between 0 °C

and 1100 °C

±[1 + 0.003×(T −1100)] between 1100

°C and 1600 °C

±1.5 between 0

°C and 600 °C

±0.0025×Tbetween 600 °C

and 1600 °C

B +200 to +1700 0 to +1820 Not Available

±0.0025×T

between 600 °Cand 1700 °C

T −185 to +300 −250 to +400 ±0.5 between −40 °Cand 125 °C

±1.0 between −40°C and 133 °C

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±0.004×T between125 °C and 350 °C

±0.0075×Tbetween 133 °C

and 350 °C

E 0 to +800 −40 to +900 

±1.5 between −40 °C

and 375 °C±0.004×T between

375 °C and 800 °C

±2.5 between −40°C and 333 °C

±0.0075×Tbetween 333 °C

and 900 °C

Chromel/AuFe −272 to +300 n/aReproducibility 0.2% of the voltage;

each sensor needs individual calibration.

[edit] Applications

Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. Theyare 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. For such applicationsthermistors, silicon bandgap temperature sensors and resistance temperature detectors are more

suitable. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

[edit] Steel industry

Type B, S, R and K thermocouples are used extensively in the steel and iron industries to

monitor temperatures and chemistry throughout the steel making process. Disposable,

immersible, type S thermocouples are regularly used in the electric arc furnace process toaccurately measure the temperature of steel before tapping. The cooling curve of a small steel

sample can be analyzed and used to estimate the carbon content of molten steel.

[edit] Heating appliance safety

Many gas-fed heating appliances such as ovens and water heaters make use of a pilot flame toignite the main gas burner when required. If it goes out gas may be released, which is a fire risk 

and a health hazard. To prevent this some appliances use a thermocouple in a fail-safe circuit to

sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame,

generating a voltage which operates the supply valve which feeds gas to the pilot. So long as thepilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. If the

pilot light goes out, the thermocouple temperature falls, causing the voltage across the

thermocouple to drop and the valve to close. Some combined main burner and pilot gas valves(mainly by honeywell) reduce the power demand to within the range of a single universalthermocouple heated by a pilot (25mV open circuit falling by half with the coil connected to

10~12mV @ 0.2~0.25A typically) by sizing the coil to be able to hold the valve open against a

light spring, only after the initial turning on force is provided by a the user pressing and holdinga knob to compress the spring during first lighting. These systems are identifiable by the 'press

and hold for x minutes' in the pilot lighting instructions. (The holding current requirement of 

such a valve is much less than a bigger solenoid designed for pulling the valve in from closed

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would require.) Special test sets are made to confirm the valve let-go and holding currents as an

ordinary milliameter cannot be used as it introduces more resistance than the gas valve coil.Apart from testing the open circuit voltage of the thermocouple, and the near short-circuit DC

continuity through the thermocouple gas valve coil, the easiest non-specialist test is substitution

of a known good gas valve.

Some systems, known as millivolt control systems, extend the thermocouple concept to both

open and close the main gas valve as well. Not only does the voltage created by the pilotthermocouple 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, and a thermopile is used rather than a single thermocouple. Such a systemrequires 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 feature is especially useful for un-powered convection heaters. A similar gas shut-off safetymechanism 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 nothappen.

Out of concern for energy wasted by the standing pilot, designers of many newer appliances have

switched to an electronically controlled pilot-less ignition, also called intermittent ignition. Withno standing pilot flame, there is no risk of gas buildup should the flame go out, so these

appliances do not need thermocouple-based pilot safety switches. As these designs lose the

benefit of operation without a continuous source of electricity, standing pilots are still used in

some appliances. The exception is later model instantaneous water heaters that use the flow of water to generate the current required to ignite the gas burner, in conjunction with a

thermocouple as a safety cut-off device in the event the gas fails to ignite, or the flame is

extinguished.

[edit] Thermopile radiation sensors

See also: bolometer 

Thermopiles are used for measuring the intensity of incident radiation, typically visible orinfrared light, which heats the hot junctions, while the cold junctions are on a heat sink. It is

possible to measure radiative intensities of only a few μW/cm2 with commercially available

thermopile sensors. For example, some laser power meters are based on such sensors.

[edit] Manufacturing

Thermocouples can generally be used in the testing of prototype electrical and mechanicalapparatus. For example, switchgear under test for its current carrying capacity may have

thermocouples installed and monitored during a heat run test, to confirm that the temperature rise

at rated current does not exceed designed limits.

[edit] Radioisotope thermoelectric generators

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

[edit] Process plants

Chemical production and petroleum refineries will usually employ computers for logging and

limit testing the many temperatures associated with a process, typically numbering in thehundreds. For such cases a number of thermocouple leads will be brought to a common reference

block (a large block of copper) containing the second thermocouple of each circuit. Thetemperature of the block is in turn measured by a thermistor. Simple computations are used to

determine the temperature at each measured locati