What is temperature

Temperature is property of any matter which tells the sensitivity of hotness that we will feel when we touch it.How ever it does not signify how much total energy it contains.For example of same mass of water and coal at same temperature coal has absorbed more heat because its heat capacity is higher.But since temperature is aproperty which is associated with average of its molecules of the substance is having you feel the same sensitivity when either coal or water at same temperature is touched with bare hands.







From maxwell distribution we see that as temperature increases the the curve tends to be more flat that means average energy of molecules increases with rise in temperature.

It is different from entropy(a measure of randomness of molecules).For example if water and coal are at same temperature and have same mass and same amount of heat is supplied to both,coal will undergo more entropy change and less temperature change ie energy distribution among their molecule will be more random and average energy per molecule will be less compared to water. That means energy distribution among its molecules will be less uniform.So if heat is given to a substance and its temperature does not rises and substance does not do any work then it entropy increases which means the whole energy given is not distributed uniformly among its molecules which indicates the molecules are in random movement so transfer of energy among its molecules do not take place very often and therefore most of the energy is concentrated among few number of molecules.

Thermocouple Wire and Extension

Thermocouple are simply dissimilar wires joined at hot end.Thermocouple wire is composed of wire from sensing tip to the cold junction compensation end.But for practical purpose  grade of these wires are different.It means that there will difference between thermocouple wire and extension wire.Features which is given for the wire of which sensing part is made will be different from extension wire.For ex  Maximum ambient temperature of extension wire will be far less then thermocouple probe wire.

Some basic procedures to install a thermocouple wires  are.

1.)Wires are fabricated  into an accurate and dependable thermocouple by joining the Thermo elements at the sensing ends.

2.)To extend thermocouples  extension wire of the same type must be used to indicating or Control and instrumentation.

3.)The color code for negative is RED through out.

4.) Connectors are used to extend the wire if required, they are made of the

same alloys and have the same color codes as extension wire.

5.) Exception to these codes is  Hi-Temp connectors which are colored red regardless of

T/C type.)


For extension Wire to be used are of same metals as that of thermocouples.These wires are used to connect the thermocouple to the measuring instruments.It  is taken into consideration to not introduce any additional joints as it would create unwanted voltages.Since the connecting wires are of same metals they do not generate voltages.It is not necessary to use same metal but the thermal coefficient of  emf the metals connecting should be same for a  narrow range of temperature.For example platinum thermocouples, extension used is copper alloy because using platinum extension would be quite expensive.

Total allowable length

Theory of circuit says the input impedance of the measuring instruments(amplifying instruments) should be high so that the it draws minimum current from thermocouple.This will prevent voltage drop in wires due to wires own resistance.But still while selecting the allowable length of thermocouple wire two factors have to considered.

Total loop resistance-Generally kept less then 100 ohms.Measured by multiplying resistance per unit length and wire length.

Electrical noise immunity-Wires should be kept away from electromagnetic field source.Since the voltage generated by thermocouple is few millivolts.

Also any change in system  like termination type ,size   will cause unwanted  junction and will effect its accuracy.

Hoe thermocouple calibrationis done

Thermocouples calibration is not the same as instruments because they cannot be calibrated as they are non adjustable.However its functioning can be validated by using simple process.

At home or lab we can do it by following procedure

We use Standard temperature source large enough so that the thermocouple can be inserted into it like ice bath.We can use  the boiling and/or freezing point of a series of pure solvents as those temperatures are well known at STP.We measure the voltage output of thermocouple.Plot that on the Voltage temperature graph.And see if it fits the graph obtained from the thermocouple chart.Since thermocouple chart  use reference junction to be Zero degree.We need to find the room temperature(Reference junction) using thermometer etc and covert it to voltage using charts  and add that to voltage being measured by voltmeter.It comes in temperature range of about 100 to 1200 degC as thermocouple are more accurate in higher range temperatures.

Another such heat source is thermocouple calibration furnace.They have fast heating time.Accurate temperature display about+-0.5decC.Also they can be used to calibrate multiple thermocouples at the same time.Thermocouple calibration furnace


Calibration at Industries.

Temperature of heating equipment,furnace ,solvents etc shown by Thermocouple can be read directly by SCADA.SCADA stands for (supervisory control and data acquisition.)These are systems which are used to collect data from various sensors locates at different areas of the factory.It then sends them to main computer which moniters and control them.There is a seperate hardware interface (Controllers like PLC Input output cards etc),which is used is used for controlling of various parameters .It can also be controlled manually through HMI(Human machine Interface) or the main computer with the necessary software installed.

And to validate the temperature we measure the millivolts at thermocouple end using voltmeter .Voltage is measured at two ends of thermocouple near the main probe.
Now  using ASTM E230-03 (Standard Specification and Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples”) find the temperature in degree centigrade  for the measured voltage or use a thermocouple calibrator (a digital thermometer with type setting)which will tell the temperature of according to the voltage measured for the type of thermocouple set in the meter.For ex,if it is set to Type K it will read temperature according to type K thermocouple.The same temperature should be shown by SCADA instruments.While using reference tables Diiference in hot and cold junctio is considered. To deter mine the temperature at thermocouple probe ,junction temperature where voltmeter is used  can be considered to be room temperature.


Some of the top SCADA instruments manufacturers are

Intellution,Rockwell Software,Wonderware,Citect,GE/Fanuc,Siemens,Iconics
USDATA,National Instruments,Indusoft.


How to read the chart

It consist of rows of values showing millivolt.

The reference junction(cold junction) is Zero degree C for all data.

Each row consist of  eleven values

Voltage value is for temperature corresponding to the row containing that voltage value plus the temperature corresponding to column containing that voltage value.

Typical Thermocouple table

Thermocouple manufacturers

Omega All range and types of Tc and other sensors.Complete range of instruments for industrial instrumentation.

KIMO instruments

Conac tecnologies Temperature sensors and compression seal fittings solution.

ABB TC and TC transmitters.

Danfoss Sensors valvesHeating and refrigeration  control of appliances at home.sensors switches and instrument for Industrial automation.

Endress hauser

Fluke  Handheld instruments loogers,Calibratior,thermometer,Surface Tc probe and local reading instruments.

Fluke calibration

Watlow Electric heaters and Temperature sensors and temperature controllers

Agilent technologies TC pressure or vacuum gauge.Industrial automation and lab service.

Parker Instrumentation
Calex electronics limited

United electrics controls

Gefran Industrial automation,controllers,inverter for drives and sensors(temperature,pressure,position,force transducers(load cell) etc)

Thermal detection Temperature sensors for process and manufacturing industries.(Pharmaceuticals temperature probe,industrial straight TC,fittingMadison level pressure and temperature sensor(Tc,RTD,switch,thermisters) manufacturingVulconic

Marsh bellpfram

Lab facility Complete ange of instruments for TC and TC accesories.Connectors,cables,sensors,transmitters,fittings etc
bocon Temperature sensors.Limited variity of  TC designed
Ust sensor industrial sensors(tempearture,pressure etc)
West control solution
Tecpel  Handheld TC for multimeters
H.heinz meb wider stande gmbh
Phoenux Tm gmbx
digitron  Haldheld reading and data logging
Hayashi denko
Author grillo
Peak sensors
CMR Sensors for desiel and gas engines.Bearing temperature,exhaust gas tempearature etc.
B+B Temp,pessure,humidity sensors
Rossel mestechnik Temp sensors for various industries.
Temsens instruments   TC and other tempearture sensors
Desin instruments
Pixsys  Bayonate type TCs
Murphy Stainless steel straight TCs
Athena All range of TC design(straight,wires ,extruders etc)
EDand D
Ihne and tesch
Fischer messtecnik Straight TC for process control and Tc transmitters.
Turk hillinger
Lm therm
Pico technology TC with handle or plug type or expose wre with connectors for use with data loggers at loacal reading
Hanna instruments
Engineer Cy
Rototherm Mineral insulated straight Tc with assemblies.
Nor cal products
Hoover dam technologies
LAPP group
teledyne technologies
acim jouanin
Nuova fima
fast heat
Suchy messtechnik
Alpha wire

Thermocouple types

Thermocouples are categorized based on their use,Sensitivity and temperature range.They can be easily identified by using the color chart which is given here.

Type K (Chromel / Alumel)

Sensitivity -41 µV/°C.

Temperature range –270 to 1372°C.

Use: General purpose use like process plants etc.It is oldest and therefore most commonly used and popular type.Its probes are of all varieties and have its temperature range is quite wide making it applicable relatively vast process.Its can readily be used in Oxidizing or inert environment. However in reducing or vacuum it can deteriorate. Nickel part of the probe  is magnetic so its show abrupt variation in emf above temperature around 220.

Type E (Chromel / Constantan)

Sensitivity (68 µV/°C) Highest sensitivity in terms of  EMF /°C or °F of any of the thermocouples

Overall range –270 to 1000°C useful range 95-900 degrees C

It is mainly used for cryogenics or very low temperature measurement.Another advantage is Its none of the wires are magnetic.Suitable in oxidizing environment which are clean and not partially oxidizing or frequent change in environment from oxidizing to reducing.

Type J (Iron / Constantan)
Usefulrange (-45 to +760 °C)

Use: Equipment that can not accept ‘modern’ thermocouples.it is magnetic so above curie temperature around  750 it can loose calibration..Reducing, Vacuum, Inert environment .It can deteriote at high temperature at high temperature in oxidizing or sulphurous environment.below zero temperature applicationis also not suitable because of iron conductor may rust or form embritllement.So it is used upto 450 °C.

Type N (Nicrosil / Nisil)

650 -1260 degrees C –270 to 1300°C

Use: It is stable as it does not show abrupt emf change like type k.Also at high temperature it does not oxidizes. was made as a better replacement of type K.Type B s or R are TC  are also stable type of thermocouple but their sensitivity is not so good at low temperature so the are mostly used for high temperature.Also N  type is much cheaper.

Type D (

Type R/S/B (Platinum type thermocouple)

These are noble metal thermocouple and one of the most stable types.But they are  best applicable at high temperature because of the flat or constant emf  response  at temperature below 40-50°C Doe snot oxidizes in oxidizing condition like hot air. It is not enclosed in metal tubes because of high susceptibility to contamination ie diffusion of unwanted material etc.

They are mainly used in glass,semiconductor or steel industries to monitor the temperature of arch of this melting chamber of furnace where glass or steel is melted.They are generally enclosed in ceramic tube.

R (Platinum / Rhodium)
Overall range –50 to 1768°C.Useful range 870-1450 degrees C

S (Platinum / Rhodium)

Overall range –50 to 1768°C Useful range 980-1450 degrees C

sensitivity (10 µV/°C)

They are particularly used as standard for calibrating in melting pont of gold.

 B (Platinum / Rhodium)

Overall range 0 to 1820°C.Useful range1370-1700 degrees C (approx 10 µV/°C)


Type T (copper-constanan)

Overall range –270 to 400°C.Useful Range 200-350 degrees C.

Sensitivity 43 µV/°C.

They are good at low temperature so cryogenics very low temperature) application is its  standard characteristics. They are Particularly useful where long presence of moisture in unavoidable such as food industries.Can be used in all types of environment inert,oxidizing or reducing provided they are mild and not extreme.

Type C tungsten 5% rhenium – tungsten 26% rhenium)

Overall range 0 to 2320°C.Useful range 1650-2315 degrees C

Well-suited for vacuum furnaces at extremely high temperatures.Not useful in presence of oxygen above 250 °C.they are used vehicle for space,neclear reactor core,heating in industrial process etc. Tungeston is combustible at very high temperature so cannot be used in oxidizing environment.They are used in inert or hydrogen environment,Also in rigorous process of heating and cooling may cause embrittlement which then may cause breakage of wire due to vibration.This  limits it to be used in all type of industrial heating.

Type M nickel alloy for each wire

0-1287 degrees C

Use: These thermocouples are used in vacuum furnaces for the same reasons as with type C.

This chart shows the various color codes for identifying the type of wire and its polarity based on the type of wire.These color codes are different in different countries and Also different for different types of thermocouples. You can identify the type of thermocouple by noticing the insulation color and the color used for different polarities wire.


Thermocouple drift and decalibration

When thermocouple is placed in the high temperature region for considerable long time we cannot be sure of the reading of the temperature shown by it.This is because the emf produced by the TC can change over time even though it is at same temperature.This change in emf is called thermocouple drift.

Causes This  is change is due to change in chemical and metallurgical properties of metal. of thermoelements at its  surface or at volume or both.It result in change in seebeck coefficient in the some or entire regions of the thermoelements.

At surface following may occur

  • Oxidation of surface material mainly if the thermocouple wires are not protected by sheath or where open wire configuration is used.
  • Corrosion or depletion because of the gas or chemical around the thermoelements.
  • Chemical reaction with the insulator or sheath.

Inside of the thermocouple ie in its bulk changes may occur becuase

  • The metal of the thermoelement may change is metallic phase like.it may undergo allotropic change,diffusionof foriegn mateial.
  • Metal can undergo grain growth
  • Continous stress may cause the internal lattice to undergo deformation and develope voids etc
  • Ordering transformation which is changee in order or symmetry to disorder of the lattice when heated.


Things to be noted about thermal drift is

It is not 100 % avoidable.

Drift is not caused if  seebeck constant changes near the hot end of the thermoelement.

Drift s mainly caused if the thermoelements in between the thermocouple wire undergo change in seebeck constant.

This is because the temperature profile of is some thing like this








As it is clear that at hot and cold end region of Tc the temperature in uniform.So any

No effect of change in seebeck coefficient.But at the middle region the temperature is not uniform so if seebeck cooefficient changes drift may occur.