Thermocouple connected to a multimeter displaying room temperature in °C.
Thermocouple
From Wikipedia, the free encyclopedia
A thermocouple consists of two dissimilar conductors in contact, which produce a voltage when heated.
The size of the voltage is dependent on the difference of temperature of the junction to other parts of the circuit. Thermocouples are a widely ud type of
temperature nsor for measurement and control [1] and can also be ud to convert a temperature gradient into electricity. Commercial thermocouples 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 lf powered and require no external form of excitation. The main limitation with thermocouples is accuracy; 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 alloys are ud 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 the measuring instrument, the intermediate connection can be made by extension wires which are less costly than t
he materials ud to make the nsor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments u electronic methods 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 ud in science and industry; applications include temperature measurement for kilns, gas turbine exhaust,diel engines, and other industrial process.
Contents
1 Principle of operation
1.1 Derivation from Seebeck effect 1.2 Properties of thermocouple circuits 2 Practical u
2.1 Voltage–temperature relationship 2.2 Cold junction compensation 3 Grades 4 Types
4.1 K 4.2 E 4.3 J 4.4 N
4.5 Platinum types B, R, and S 4.6 T 4.7 C 4.8 M
4.9 Chromel-gold/iron 5 Aging of thermocouples 6 Thermocouple comparison 7 Applications
7.1 Steel industry
7.2 Heating appliance safety 7.3 Thermopile radiation nsors 7.4 Manufacturing 7.5 Power production 7.6 Thermoelectric cooling 7.7 Process plants
leavefor7.8 Thermocouple as vacuum gauge 8 See also 9 References 10 External links
Principle of operation
Main article: Seebeck effect
In 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor 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 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 oppo the original. Fortunately, the magnitude of the effect depends on the metal in u. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a smal
l difference in voltage available for measurement. That difference increas with temperature, and is between 1 and 70 microvolts per degree Celsius (µV/°C) for standard metal combinations.
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. Becau both lengths of dissimilar metals experience the same temperature gradient, the end result is a measurement of the difference in temperature between the thermocouple junction and the reference junction.
Derivation from Seebeck effect
Upon heating, the Seebeck effect will initially drive a current. However, provided the junctions all reach a uniform internal temperature, and provided an ideal voltmeter is ud,then the thermocouple will soon reach an equilibrium where no current will flow anywhere (). As a result, the voltage gradient at any point in the circuit will be given simply by , where is the Seebeck coefficient at that point, and is the temperature gradient at that point.
A thermocouple measuring circuit with a heat source, cold junction and a measuring instrument.
Polynomial Coefficients 0-500°C [5] (for Type K)125.08355小学生大队委竞选稿
27.860106x10−23−2.503131x10−1
48.315270x10−25−1.228034x10−269.804036x10−47−4.413030x10−58 1.057734x10−69−1.052755x10−8
Cold Junction Compensation inside a Fluke CNX t3000 temperature meter.Note the thermistor to measure the junction temperature. And the large pads and large thermal mass contacts.
bureaucratThis leads to a characteristic voltage difference independent of many details (the conductors' size, length do not matter):
where and are the Seebeck coefficients of materials A and B as a function of temperature, and and are the temperatures of the two junctions. If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated as:
It is important to note that the emf is not generated at the junctions themlves, but rather in the wires leading between the hot and cold junctions (where ). As a result,the nature and composition of the junctions (where is internally constant) itlf does not influence the measured voltage. Converly, if there are variations in the composition of the wires in the thermal gradient region (due
to contamination, oxidation, etc.), outside the junction, this can lead to changes in the measured voltage.
Properties of thermocouple circuits
The behavior of thermoelectric junctions with varying temperatures and compositions can be summarized in three properties:[4]
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 ction. 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.Intermediate materials —the algebraic sum of the thermoelectric EMFs in a circuit compod of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature. So if a third metal is inrted in either wire and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.
Successive or intermediate temperatures —if two dissimilar homogeneous materials produce thermal EMF1 when the junctions are at T1 and T2 and produce thermal EMF2 when the junctions a
re at T2 and T3, the EMF generated when the junctions are at T1 and T3 will be EMF1 + EMF2,provided T1<T2<T3.existence
Practical u
Voltage–temperature relationship
For typical metals ud in thermocouples, the output voltage increas almost linearly
with the temperature difference (ΔT) over a bounded range of temperatures. For
preci measurements or measurements outside of the linear temperature range, non-linearity must be corrected. The nonlinear relationship between the temperature difference (ΔT) and the output voltage ( a few mV) of a thermocouple can be approximated by a polynomial:
The coefficients a n are given for n from 0 to between 5 and 13 depending upon the metals. In some cas better accuracy is obtained with additional non-polynomial terms.[5] A databa of voltage as a function of temperature, and coefficients for computation of temperature from voltage and vice-versa for many types of thermocouple is available online.[5]
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In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table;[6] older devices u analog circuits.
Piece-wi linear approximations are an alternative to polynomial corrections.[7]
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 junction—is maintained at a known reference temperature, and the other junction is at the temperature to be nd.[8]qsi
Having a junction of known temperature, while uful for laboratory calibration, is not convenient for most measurement and control applications. Instead, they incorporate an artificial cold junction using a thermally nsitive device such as a resistance thermometer,thermistor 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. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation. Some integrated circuits are designed for cold junction temperature compensation for specific thermocouple types.
Grades
Thermocouple wire is available in veral different metallurgical formulations per type, typically, in decreasing levels of accuracy and cost:special limits of error, standard, and extension grades.
Extension grade wires made of the same metals as a higher-grade thermocouple are ud to connect 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 ca of platinum thermocouples, extension wire is a copper alloy, since it would be prohibitively expensive to u platinum 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.
The temperature-measuring instrument must have high input impedance to prevent any significant current draw from the thermocouple, which would in turn produce an undesired resistive voltage drop across the wire and/or junction. Changes in metallurgy along the length of the thermocouple (such as termination strips or changes in thermocouple type wire) will introduce another thermocoupl
e junction which affects measurement accuracy.
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 lected on the basis of the temperature range and nsitivity needed. Thermocouples with low nsitivities (B, R, and S types) have correspondingly lower resolutions. Other lection 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 (assuming ) first, followed by the negative electrode.
K
Type K (chromel {90% nickel and 10% chromium}—alumel {95% nickel, 2% mangane, 2% aluminium and 1% silicon}) is the most common general purpo thermocouple with a nsitivity of approximately 41 µV/°C (chromel positive relative to alumel when the junction temperature is higher than the reference temperature).[9] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C / -330 °F to +2460 °F range. Type K was specified at a time when metallurgy w
as less advanced than it is today, and conquently characteristics may vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 350 °C . Wire color standard is yellow (+) and red (-).
E
Type E (chromel–constantan)[6] has a high output (68 µV/°C) which makes it well suited to cryogenic u. Additionally, it is non-magnetic. Wide range is −50 to 740 °C and Narrow range is −110 to 140 °C. Wire color standard is purple (+) and red (-).
J
Type J (iron–constantan) has a more restricted range than type K (−40 to +750 °C), but higher nsitivity of about 55 µV/°C.[2] The Curie point of the iron (770 °C)[10] caus an abrupt change in the characteristic, which determines the upper temperature limit. Wire color standard is white (+) and red (-).
N
Type N (Nicrosil–Nisil) (nickel-chromium-silicon/nickel-silicon) thermocouples are suitable for u between −270 °C and 1300 °C owing to its stability and oxidation resistance. Sensitivity is about 39 µV/°C at 900 °C, slightly lower compared to type K.
Designed at the Defence Science and Technology Organisation (DSTO), Australia, by Noel A Burley, type N thermocouples overcome the three principal characteristic types and caus of thermoelectric instability in the standard ba-metal thermoelement materials:[11]
1. A gradual and generally cumulative drift in thermal EMF on long exposure at elevated temperatures. This is obrved in all ba-metal thermoelement materials and is mainly due to compositional changes caud by oxidation, carburization or neutron irradiation that can produce transmutation in nuclear reactor environments. In the ca of type K, mangane and aluminium elements from the KN (negative) wire migrate to the KP (positive) wire resulting in a down-scale drift due to chemical contamination. This effect is cumulative and irreversible.
2. A short-term cyclic change in thermal EMF on heating in the temperature range ca. 250–650 °C, which occurs in types K, J, T and E thermocouples. This kind of EMF instability is associated with structural changes like magnetic short range order.
3. A time-independent perturbation in thermal EMF in specific temperature ranges. This is due to composition-dependent magnetic transformations that perturb the thermal EMFs in type K thermocouples in the range ca. 25-225 °C, and in type J above 730 °C.
Nicrosil and Nisil thermocouple alloys show greatly enhanced thermoelectric stability relative to the other standard ba-metal thermocouple alloys becau their compositions substantially reduces the thermoelectric instability described above. This is achieved primarily by increasing component solute concentrations (chromium and silicon) in a ba of nickel above tho required to cau a transition from internal to external modes of oxidation, and by lecting solutes (silicon and magnesium) that preferentially oxidize to form a diffusion-barrier, and hence oxidation inhibiting films.[12]
Platinum types B, R, and S
Types B, R, and S thermocouples u platinum or a platinum–rhodium alloy for each conductor. The are among the most stable thermocouples, but have lower nsitivity than other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually ud only for high temperature measurements due to their high cost and low nsitivity.
B
Type B thermocouples u a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. The thermocouples are suited for u at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their u below about 50 °C.
R
Type R thermocouples u a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are ud 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 cond wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are ud up to 1600 °C. In particular, type S is ud as the standard of calibration for the melting point of gold (1064.43 °C).
T
Type T (copper – constantan) thermocouples are suited for measurements in the −200 to 350 °C ran
ge. Often ud 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 in characteristics. Type T thermocouples have a nsitivity of about 43 µV/°C.
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 ud in the prence of oxygen at temperatures above 260 °C.
M
Type M thermocouples u a nickel alloy for each wire. The positive wire (20 Alloy) contains 18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. The thermocouples are ud in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. It is less commonly ud than other types.
Chromel-gold/iron
In chromel-gold/iron thermocouples, the positive wire is chromel and the negative wire is gold with a
small fraction (0.03–0.15 atom percent) of iron. It can be ud for cryogenic applications (1.2–300 K and even up to 600 K). Both the nsitivity and the temperature range depends on the iron concentration. The nsitivity is typically around 15 µV/K at low temperatures and the lowest usable temperature varies between 1.2 and 4.2 K.
Aging of thermocouples
Thermoelements are often ud at high temperatures and in reactive furnace atmospheres. In this ca the practical lifetime is limited by aging. The thermoelectric coefficients of the wires in a thermocouple that is ud to measure very high temperatures change with time, and the measurement voltage accordingly drops. The simple relationship between the temperature difference of the joints and the measurement voltage is only correct if each wire is homogeneous. As thermocouples age in a process their conductors can lo
homogeneity due to chemical and metallurgical changes caud by extreme or prolonged exposure to high temperatures. If the inhomogeneous ction of the thermocouple circuit is expod to a temperature gradient the measured voltage will differ resulting in error. For this reason, aged thermocouples cannot be taken out of their installed location and
recalibrated in a bath or test furnace to determine error. This also explains why error can sometimes be obrved when an aged thermocouple is pulled partly out of a furnace—as the nsor is pulled back, inhomogenous ctions may e exposure to incread temperature gradients from hot to cold as the inhomogeneous ction now pass through the cooler refractory area, contributing significant error to the measurement. Likewi, an aged thermocouple that is pushed deeper into the furnace might sometimes provide a more accurate reading if being pushed further into the furnace caus the area of inhomogeneity to be located in an area of the furnace where it is no longer expod to a temperature gradient.[13]
Thermocouple comparison
陈慧娴飘雪日文版The table below describes properties of veral different thermocouple types. Within the tolerance columns, T reprents the temperature of the hot junction, in degrees Celsius.For example, 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)IEC Color code
BS Color code
ANSI Color code
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 °C and 1200 °C J 0 to +750−180 to +800±1.5 between −40 °C and 375 °C
±0.004×T between 375 °C and 750 °C ±2.5 between −40 °C and 333 °C
±0.0075×T between 333 °C and 750 °C N 0 to +1100−270 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 °C and 1200 °C
R 0 to +1600−50 to +1700±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×T between 600 °C and 1600 °C Not defined.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×T between 600 °C and 1600 °C
Not defined.
B
+200 to +1700
0 to +1820Not Available
±0.0025×T between 600 °C and 1700 °C No standard
u
copper wire No standard
u
copper wire
Not
defined.
T −185 to +300−250 to +400±0.5 between −40 °C and 125 °C
±0.004×T between 125 °C and 350 °C ±1.0 between −40 °C and 133 °C
±0.0075×T between 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×T between 333 °C and 900 °C
Chromel/AuFe −272 to +300n/a Reproducibility 0.2% of the voltage; each nsor needs individual calibration.
Applications
Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. Applications include temperature measurement for kilns, gas turbine exhaust, diel engines, and other industrial process. 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. For such applications thermistors, silicon bandgap temperature nsors and resistance temperature detectors are more suitable.suck my dick什么意思
Steel industry
Type B, S, R and K thermocouples are ud extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process. Disposable,immersible, type S thermocouples are regularly ud in the electric arc furnace process to accurately measure the temperature of steel before tapping. The cooling curve of a small steel sample can be analyzed and ud to estimate the carbon content of molten steel.
Heating appliance safety
Many gas-fed heating appliances such as ovens and water heaters make u of a pilot flame to ignite the main gas burner when required. If it goes out, gas may be relead, which is a fire risk and a health hazard. To prevent this, some appliances u a thermocouple in a fail-safe circuit to n 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 the pilot 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 clo. Some combined main burner and pilot gas valves (mainly by Honeywell) reduce the power demand to within the range of a single universal thermocouple heated by a pilot (25 mV open circuit falling by half with the coil connected to a 10–12 mV, 0.2–0.25 A source, 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 the ur pressing and holding a knob to compress the spring during first lighting. The 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 clod would require.) Special test ts are made to confirm the valve let-go and holding currents as an ordinary milliameter cannot be ud 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 clo the main gas valve as well. Not only does the voltage 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, and a thermopile is ud rather than a single thermocouple. 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 becau external power is required to operate the blower motor, but this feature is especially uful for un-powered 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 energy wasted by the standing pilot, designers of many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. With no standing pilot flame, there is no risk of gas buildup should the flame go out, so the appliances do n
ot need thermocouple-bad pilot safety switches. As the designs lo the benefit of operation without a continuous source of electricity, standing pilots are still ud in some appliances. The exception is later model instantaneous (aka "tankless") water heaters that u 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.
Thermopile radiation nsors
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Thermopiles are ud 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. It is possible to measure radiative intensities of only a few μW/cm2 with commercially available thermopile nsors. For example, some lar power meters are bad on such nsors. The principle of operation of a thermopile nsor is distinct from that of a bolometer, as the latter relies on a change in resistance.
Manufacturing
Thermocouples can generally be ud in the testing of prototype electrical and mechanical apparatus. 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 ri at
rated current does not exceed designed limits.
Power production
Main article: Thermoelectric generator
A thermocouple can produce current to drive some process directly, without the need for extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when a temperature difference aris. 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 transfer of heat is necessary becau the current flowing through the thermocouple tends to cau the hot side to cool down and the cold side to heat up (the Peltier effect).
Thermocouples can be connected in ries to form a thermopile, where all the hot junctions are expod 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 ud to power spacecraft on missions too far from the Sun to u solar power.
Thermopiles heated by kerone lamps were ud to run batteryless radio receivers in isolated areas.[14] There are commercially produced lanterns that u the heat from a candle to run veral light-emitting diodes, and thermoelectrically-powered fans to improve air circulation and heat distribution in wood stoves.
Thermoelectric cooling
Main article: Thermoelectric cooling
不喜欢的英文The Peltier effect can be ud for cooling, in the rever process to a thermoelectric generator. Instead of generating electric power, the thermocouple consumes it, working as a heat pump.
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 the hundreds. For such cas a number of thermocouple leads will be brought to a common reference block (a large block of copper) containing the cond thermocouple of each circuit. The temperature of the block is in turn measured by a thermistor. Simple computations are ud to determine the temperature at each measured location. Thermocouple as vacuum gauge
A thermocouple can be ud as a vacuum gauge over the range of approximately 0.001 to 1 torr. The temperature detected at the thermocouple junction depends on the thermal conductivity of the surrounding gas, which depends on the pressure of the gas. Thus, the potential difference measured by a thermocouple is proportional to the logarithm of pressure in low-medium vacuum. At higher and lower pressures, the thermal conductivity of air and other gas is esntially independent of pressure. The thermocouple was first ud as a vacuum gauge by Voege in 1906.[15]
See also
Heat flux nsor
Bolometer
Giuppe Domenico Botto
Resistance thermometer
Thermistor
List of nsors
