What is a Thermocouple? A Thermocouple is a sensor used to measure measure temperature. Thermocouples consist consist of two wire legs made from different metals. The wires legs are welded together at one end, creating a junction. This junction is where the temperature is measured. When the junction experiences a change in temperature, a voltage is created. The voltage can then be interpreted using thermocouple reference tables to tables to calculate the temperature. There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility. Type J, , T, ! " are #$ase %etal& thermocouples, the most common types of thermocouples.Type thermocouples.Type ', (, and $ thermocouples are #)oble %etal& thermocouples, which are used in high temperature applications *see thermocouple temperature ranges for details+.
Thermocouples are used in many industrial, scientific, and "% applications. They can be found in nearly all industrial mar-ets /ower 0eneration, il10as, /harmaceutical, $ioTech, $ioTech, 2ement, /aper ! /ulp, etc. Thermocouples are also used in everyday appliances li-e stoves, furnaces, and toasters. Thermocouples are typically selected because of their low cost, high temperature limits, wide temperature ranges, and durable nature.
Types of Thermocouples: $efore discussing the various types of thermocouples, it should be noted that a thermocouple is often enclosed in a protective sheath to isolate it from the local atmosphere. This protective sheath drastically reduces the effects of corrosion.
1 Type K Thermocouple (Nickel-Chromium Thermocouple (Nickel-Chromium / Nickel-Alumel): The type is the most common type of thermocouple. 3t4s inexpensive, accurate, accurate, reliable, and has a wide temperature range. Temperature Rane:
Thermocouple grade wire, 5676 to
•
8,9::; *58<: to =8>:2+ •
"xtension wire, 98 to 9?8; *: to 8::2+
Accuracy (!hiche"er is reater): •
(tandard @1 8.82 or @1 .<7B
•
(pecial Cimits of "rror @1 =.=2 or :.6B
#
Type $ Thermocouple (%ron/Constantan): The type J is also very common. 3t has a smaller temperature range and a shorter lifespan at higher temperatures than the Type . 3t is equivalent to the Type in terms of expense and reliability. Temperature Rane:
Thermocouple grade wire, 96> to
•
=,6::; *8=: to <>:2+ •
"xtension wire, 98 to 9?8; *: to 8::2+
Accuracy (!hiche"er is reater): •
(tandard @1 8.82 or @1 .<7B
•
(pecial Cimits of "rror @1 =.=2 or :.6B
& Type T Thermocouple (Copper/Constantan): The Type T is a very stable thermocouple and is often used in extremely low temperature applications such as cryogenics or ultra low freeDers. Temperature Rane:
Thermocouple grade wire, 676 to <::;
•
*8<: to 9<:2+ •
"xtension wire, 98 to 9?8; *: to 8::2+
Accuracy (!hiche"er is reater): •
(tandard @1 =.:2 or @1 .<7B
•
(pecial Cimits of "rror @1 :.72 or :.6B
' Type Thermocouple (Nickel-Chromium/Constantan): The Type " has a stronger signal ! higher accuracy than the Type or Type J at moderate temperature ranges of =,:::; and lower. (ee temperature chart *lin-ed+ for details. Temperature Rane:
Thermocouple grade wire, 676 to =>::;
•
*8<: to E<:2+ •
"xtension wire, 98 to 9?8; *: to 8::2+
Accuracy (!hiche"er is reater): •
(tandard @1 =.<2 or @1 :.7B
•
(pecial Cimits of "rror @1 =.:2 or :.6B
Type N Thermocouple (Nicrosil / Nisil): The Type ) shares the same accuracy and temperature limits as the Type . The type ) is slightly more expensive. Temperature Rane:
Thermocouple grade wire, 676 to
•
89::; *8<: to 9?82+ •
"xtension wire, 98 to 9?8; *: to 8::2+
Accuracy (!hiche"er is reater): •
(tandard @1 8.82 or @1 .<7B
•
(pecial Cimits of "rror @1 =.=2 or :.6B
N*+, TA, T.R*C*0, (Type 2R2 3 +): )oble %etal Thermocouples are selected for their ability to withstand extremely high temperatures while maintaining their accuracy and lifespan. They are considerably more expensive than $ase %etal Thermocouples.
4 Type Thermocouple (0latinum Rho5ium - 167 / 0latinum): The Type ( is used in very high temperature applications. 3t is commonly found in the $ioTech and /harmaceutical industries. 3t is sometimes used in lower temperature applications because of its high accuracy and stability. Temperature Rane:
Thermocouple grade wire, 7E to 8<::;
•
*7: to =6E:2+ •
"xtension wire, 98 to 9?8; *: to 8::2+
Accuracy (!hiche"er is reater): •
(tandard @1 =.72 or @1 .87B
•
(pecial Cimits of "rror @1 :.>2 or :.=B
8 Type R Thermocouple (0latinum Rho5ium -1&7 / 0latinum): The Type ' is used in very high temperature applications. 3t has a higher percentage of 'hodium than the Type (, which ma-es it more expensive. The Type ' is very similar to the Type ( in terms of performance. 3t is sometimes used in lower temperature applications because of its high accuracy and stability. Temperature Rane:
Thermocouple grade wire, 7E to 8<::;
•
*7: to =6E:2+ •
"xtension wire, 98 to 9?8; *: to 8::2+
Accuracy (!hiche"er is reater): •
(tandard @1 =.72 or @1 .87B
•
(pecial Cimits of "rror @1 :.>2 or :.=B
9 Type + Thermocouple (0latinum Rho5ium &67 / 0latinum Rho5ium 47): The Type $ thermocouple is used in extremely high temperature applications. 3t has the highest temperature limit of all of the thermocouples listed above. 3t maintains a high level of accuracy and stability at very high temperatures. Temperature Rane:
Thermocouple grade wire, 98 to 9=::;
•
*: to =<::2+
•
"xtension wire, 98 to 8=8; *: to =::2+
Accuracy (!hiche"er is reater): •
(tandard @1 :.7B
•
(pecial Cimits of "rror @1 :.87B
Thermocouple Junctions: ;roun5e5 Thermocouples: This is the most common junction style. A thermocouple is grounded when both thermocouple wires and the sheath are all welded together to form one junction at the probe tip. 0rounded thermocouples have a very good response time because the thermocouple is ma-ing direct contact with the sheath, allowing heat to transfer easily. A drawbac- of the grounded thermocouple is that the thermocouple is more susceptible to electrical interference. This is because the sheath often comes into contact with the surrounding area, providing a path for interference. nroun5e5 Thermocouples (*r nroun5e5 Common Thermocouples): A thermocouple is ungrounded when the thermocouple wires are welded together but they are insulated from the sheath. The wires are often separated by mineral insulation.
are !ire thermocouples): A thermocouple is exposed when the thermocouple wires are welded together and directly inserted into the process. The response time is very quic-, but exposed thermocouple wires are more prone to corrosion and degradation. Fnless your application requires exposed junctions, this style is not recommended. nroun5e5 ncommon: An ungrounded uncommon thermocouple consists of a dual thermocouple that is insulated from the sheath and each of the elements are insulated from one other.
Thermocouple Sheath Comparison: &14 (stainless steel): This is the most common sheath material. 3t is relatively corrosion resistant and is cost effective. &6': This sheath is not as corrosion resistant as 9=>((. The cost difference between 9=>(( and 9:6(( is nominal. %nconel (reistere5 tra5emark) 466: This material is recommended for highly corrosive environments.
What are Special Limits of Errors (SLE)? pecial ,imits o@ rror: These thermocouples are made with a higher grade of thermocouple wire, which increases their accuracy. They are more expensive than standard thermocouples. tan5ar5 ,imits o@ rror: These thermocouples use standard #thermocouple grade& wire. They are less expensive and more common.
What is M.I. Cale? %.3. *%ineral 3nsulated+ cable is used to insulate thermocouple wires from one another and from the metal sheath that surrounds them. %3 2able has two *or four when duplex+ thermocouple wires running down the middle of the tube. The tube is then filled with magnesium oxide powder and compacted to ensure the wires are properly insulated and separated. %3 cable helps to protect the thermocouple wire from corrosion and electrical interference.
What is System Error? (ystem error is calculated by adding the accuracy of the temperature sensor *thermocouple+ and the accuracy of the meter used to read the voltage signal together. ;or example, a Type thermocouple has an accuracy of @1 8.82 above :2. Cet4s say the meter has an accuracy of @1 =2. That means the total system error is @1 9.92 above :2.
Thermocouple !s. "T#
Temperature rane: ;irst, consider the difference in temperature ranges. )oble %etal Thermocouples can reach 9,=:: ;, while standard 'TGs have a limit of >:: ; and extended range 'TGs have a limit of =,=:: ;. Cost: A plain stem thermocouple is 8 to 9 times less expensive than a plain stem 'TG. A thermocouple head assembly is roughly 7:B less expensive than an equivalent 'TG head assembly. Accuracy2 ,inearity2 3 ta>ility: As a general rule, 'TGs are more accurate than thermocouples. This is especially true at lower temperature ranges. 'TGs are also more stable and have better linearity than thermocouples. 3f accuracy, linearity, and stability are your primary concerns and your application is within an 'TG4s temperature limits, go with the 'TG. ura>ility: 3n the sensors industry, 'TGs are widely regarded as a less durable sensor when compared to thermocouples. However, '"T"%/ has developed manufacturing techniques that have greatly improved the durability of our 'TG sensors. These techniques ma-e '"T"%/4s 'TGs nearly equivalent to thermocouples in terms of durability. Response Time: 'TGs cannot be grounded. ;or this reason, they have a slower response time than grounded thermocouples. Also, thermocouples can be placed inside a smaller diameter sheath than 'TGs. A smaller sheath diameter will increase response time. ;or example, a grounded thermocouple inside a =1=>& dia. sheath will have a faster response time than a 'TG inside a I& dia. sheath.
What is mBiB ca>le? %ineral insulated cable is manufactured from completely inorganic material. The copper sheath and conductors, insulated with magnesium oxide ensure that the cable is able to withstand the effects of fire and is fully usable afterwards. The m.i. cable system provides a simple solution to many difficult wiring problems and ma-es for a dependable and permanent installation for virtually all types of electrical circuits. The copper cable sheath meets )"2 grounding requirements. (uitable for 9:: and >:: volt applications. /ressure tested to 8::: psi. %ineral insulated cable complies with Articles 99:, 7:: and 7:= and all other applicable provisions of the )ational "lectric 2ode. To produce assemblies, the cable is cut to the desired length, pigtails braDed on and terminated with union type glands with male )/T threads.
.o! is mineral insulate5 ca>le ma5e (olid copper conductor rods are first inserted into a vertical 8.7 diameter copper tube 9: foot long, -ept at the correct spacing with mineral insulated bloc- inserts. The tube is pac-ed with magnesium oxide, which acts as a seal and fireproof insulator. The end of the copper tube is sealed off and a steel draw wire attached. Fsing special machinery, the filled tube is put through a swaging process which reduces the sheath and conductor diameter proportionately while lengthening the cable. ;or example, a two conductor cable K=> AW0. has a nominal length of =88= feet. (waging or drawing down loc-s the conductors into place so the position of the conductors inside the sheath never varies, even when formed or bent. Guring the swaging process, the cable is put through an annealing or heating process to align the copper molecules and -eep the cable flexible.
aterial speci@ications %ineral insulated cable is manufactured from high conductivity copper A(T% $6 or $7L insulated with compressed magnesium oxide powderL a seamless phosphorous deoxidiDed copper sheathL maximum continuous operating temperature of 87:M2L a wor-ing voltage of :9:: volts A21G2 and :>:: volts A21G2L factory test voltage =7:: volts '%( *9:: volt cable+ and 87:: volts '%( *>:: volt cable+L a power factor of
:.=BLdielectric constant *(32+ at 9.< and dielectric strength <: volts1ml at 6<7M2. NOTE:
;or electrical data of Alloy E87 outer sheath with solid nic-el conductors, please contact our sales department.
ineral insulate5 ca>le "sB con5uit $y using mineral insulated cable, you not only eliminate the uncertainty of conduit and wire, but also the cost of additional materials, including pull boxes, unions, grounding wires, conduit seals and other fittings. /ressure tested to 8::: psi, mineral insulated cable delivers superior performance by bloc-ing the passage of vapors, gases, liquids and flamesL eliminates pressure piling and confines any exploding gases to their immediate area. With conduit, seals must be placed within =E of the arcing device or instrument to be connected to the conduit system. 3f a conduit seal fails, gas or liquids can migrate freely and it is for this reason that some companies require single or double conduit seals in haDardous areas to be annually xrayed and inspected. %322 cable assemblies are exempt from these requirements and are FC approved for use in 2lass 3, groups $, 2 and G, Givision 3 haDardous areas and meet the requirements of )"2 Article 7:=7 *f+*9+ for canned pumps etc.
ses o@ mBiB ca>le %322 cable assemblies are ideal for power, control, instrumentation and thermocouple applications in classified areas. The copper outer sheath is approved as a ground conductor and the high current rating can satisfy virtually all power applications. 3n addition m.i. cable features a low physical profile that allows unlimited bends without
pull boxes. %322 offers four types of cable assemblies o
o
o
o
copper sheathed cable, copper conductorsL FC fire rated for two hours at =E7:M;. twisted pair and shielded twisted pair with copper conductors, inner shield and outer sheath, FC fire rated for two hours at =E7:M;. alloy E87 sheath, solid nic-el conductors, fire rated for 9: minutes at 88::M; or nic-el clad copper conductors, fire rated for 9: minutes at 8:::M;. stainless steel 9=> thermocouple extension cable, all types
All cable assemblies are appropriate for the following conditions
ire an5 heat resistant The inorganic construction of mineral insulated cable ma-es it extremely fire and heat resistant. With an operating limit equal to the melting point of copper, =?E8M;, =:E9M2 it can easily withstand high temperatures and heavy current overloads. 3t emits no smo-e or toxic substances and allows no flame propagation. 3n contrast, a third party test proved standard conduit and wire failed at 687M;. These fireproof properties ma-e it the best choice for connecting motor operated valves, control stations, plant shutdown systems, instruments and power devices that must remain operational under actual fire conditions. %ineral insulated cable and cable assemblies are extremely reliable as power and control cables in hospitals, hotels, high rise buildings for emergency generators, fire pumps, smo-e and vent exhaust fans, communication circuits, multiplex alarm systems and instrumentation. ther applications include wiring connections to electric heaters on boilers, tan-s and furnace wiring, metal smelters and lumber -ilns, where regular pvc insulation hardens and crac-s and has to be replaced regularly.
Waterproo@ The cable is solid and constructed with an extruded copper sheath that ma-es it impervious to liquids. )either water, process liquids or corrosive solutions can migrate into the system. 3n copper and brass corrosive environments, or for direct burial an extruded outer HG/" jac-et may be specified to provide additional protection. /N2 boots cover the brass connectors. "poxy end termination4s provide lifetime sealing to
moisture ingress. 3ndustrial applications include light fitting connections in oil tan-s that are periodically washed out with -erosene. %.3 cable assemblies are F( 2oast 0uard approved and are currently used to connect warning beacons, fog horns and lanterns on offshore platforms. They also solve many electrical wiring problems on oil tan-ers and marine vessels that operate under the worst of climatic conditions.
Temperature sta>le %ineral insulated cable does not deteriorate at low temperatures and assemblies are unaffected by continuous, even extreme changes in temperature. Gaily temperature changes cause condensation and moisture to accumulate in exposed conduit above the seal pac-ing, causing rust and corrosion to build up and bloc- the conduit drain. 3n cold temperatures this moisture can freeDe, splitting the conduit or seal and possibly damaging the instrument or electrical device, especially if the wiring is top entry. $ecause m.i. cable assemblies are solid and completely moisture resistant, this ris- is eliminated. 3ndustrial applications include power wiring to motors in C)0 tan-s, freeDer rooms and instrument connections in harsh climates such as Alas-a and 2anada.
Ra5iation resistant %agnesium oxide, the insulation in mineral insulated cable, provides superior radiation resistance over any other material and for this reason, %322 cable assemblies are an excellent choice for nuclear power plant applications.
What is a Thermocouple ! How Goes it Wor-O written by Haresh hemani P edited by Camar (tonecypher P updated =:18918:=9 A thermocouple is a device used extensively for measuring temperature. Learn how the device works here.
A thermocouple is comprised of at least two metals joined together to form two
•
junctions. One is connected to the body whose temperature is to be measured; this is the hot or measuring junction. The other junction is connected to a body of known temperature; this is the cold or reference junction. Therefore the thermocouple measures unknown temperature of the body with reference to the known temperature of the other body.
orking !rinciple
•
The working principle of thermocouple is based on three e"ects# discovered by $eebeck# !eltier and Thomson. They are as follows% $) Seeec% e&ect: The $eebeck e"ect states that when two di"erent or unlike metals are joined together at two junctions# an electromotive force &emf' is generated at the two junctions. The amount of emf generated is di"erent for di"erent combinations of the metals. ') eltier e&ect: As per the !eltier e"ect# when two dissimilar metals are joined together to form two junctions# emf is generated within the circuit due to the di"erent temperatures of the two junctions of the circuit. ) Thomson e&ect: As per the Thomson e"ect# when two unlike metals are joined together forming two junctions# the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit. (n most of the cases the emf suggested by the Thomson e"ect is very small and it can be neglected by making proper selection of the metals. The !eltier e"ect plays a prominent role in the working principle of the thermocouple.
•
)iagrams
•
*ow it orks The general circuit for the working of thermocouple is shown in the +gure , above. (t comprises of two dissimilar metals# A and -. These are joined together to form two junctions# p and # which are maintained at the temperatures T, and T/ respectively. 0emember that the thermocouple cannot be formed if there are not two junctions. $ince the two junctions are maintained at di"erent temperatures
the !eltier emf is generated within the circuit and it is the function of the temperatures of two junctions. (f the temperature of both the junctions is same# eual and opposite emf will be generated at both junctions and the net current 1owing through the junction is 2ero. (f the junctions are maintained at di"erent temperatures# the emf3s will not become 2ero and there will be a net current 1owing through the circuit. The total emf 1owing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions. The total emf or the current 1owing through the circuit can be measured easily by the suitable device. The device for measuring the current or emf is connected within the circuit of the thermocouple. (t measures the amount of emf 1owing through the circuit due to the two junctions of the two dissimilar metals maintained at di"erent temperatures. (n +gure / the two junctions of the thermocouple and the device used for measurement of emf &potentiometer' are shown. 4ow# the temperature of the reference junctions is already known# while the temperature of measuring junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the value of unknown temperature directly.
)evices 5sed for 6easuring 768
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The amount of emf developed within the thermocouple circuit is very small# usually in millivolts# therefore highly sensitive instruments should be used for measuring the emf generated in the thermocouple circuit. Two devices used commonly are the ordinary galvanometer and voltage balancing potentiometer. Of those two# a manually or automatically balancing potentiometer is used most often. 8igure / shows the potentiometer connected in the thermocouple circuit. The junction p is connected to the body whose temperature is to be measured. The junction is the reference junction# whose temperature can be measured by the thermometer. (n some cases the reference junctions can also be maintained at the ice temperature by connecting it to the ice bath &see +gure 9'. This device can be calibrated in terms of the input temperature so that its scale can give the value directly in terms of temperature.
