What is Temperature Sensor: Different Type And Application

How does Temperature sensor works?

Does Human body have any Temperature Sensor?

Before you know about the Function & type of Temperature Sensor look temperature sensor of the human body. Do you know human  body  temperature sensors? 

No! do not worry, yes we have temperature sensors in our body this nothing but our upper skin. Let me introduce it for you, any time we want to eat or bathe our brain (controller) needs to know the temperature of food and water. For that we can directly touch the pot or bucket with our skin. What will happen next? Yes our brain receives neural signals from our skin  based on data stored throughout our life. The brain makes decisions whether to eat or not. But be careful touching anything very hot and cold with your skin.

Reflex action and reflex arc when a cook touched a hot pot by accident

What is Temperature ?

Temperature is a quantitative representation of how much a  body or matter is hot and cold.Temperature is the physical property of a body or matter in any state. To measure or represent or convert these properties into a readable form  device used called Temperature Sensor.

Function of Temperature Sensor

Temperature sensors are capable of transforming different process (degree of hot and cold  ) variables into readable form of data. This data may be in digital or analog  depending upon sensor configuration and type of it .Finally data scaled into temperature measurement unit like ℃ or ℉ from voltage,current or strain.

Type of Temperature Sensor

     1.Physical Contact Type
      2.Non Contact Type

 Physical Contact Types:



      3. Thermistor 

      4. RTD

Non Contact Type :

     1. Pyrometer

Working of Different Temperature Sensor

First and most basic design of the temperature measurement device was a mercury or alcohol based thermometer used in daily to daily work to measure body temperature.

A bulb containing Mercury attached to a glass tube of small diameter,volume of mercury in the tube is much less than the volume in the bulb.

Volume of mercury changes with respect to change in temperature;and mercury starts climbing up to tube high based on how much temperature of the contacted body or surface. 

Reading the printed scale we know how hot the body is.

Temperature Measurement Range: 5 °C to 42 °C or 90 °F to 110 °F

Key Takeaway:-Mercury thermometer removed from market because it is dangerous for Human body.

1. Household

2. Medical


Cheap Readily available

 Easy to store 

Mercury is dangerous for Human


Sensitivity Range 

Thermostat or Bi Metallic Temperature Sensor 

Thermostat or Bi Metallic is an electro-mechanical temeprature measurement sensor .This is a device which uses a reference set point to maintain temperature in a closed system.

Construction:- It is made of two dissimilar conducting metal Nickel ,Aluminium Copper and bind together by pressing one another with the help of a pressing machine.

Working Principle:- As small current pass through a closed circuit  of bimetallic stip When bi metallic strip get hot due to there thermal expansion one expand  more than other, result in upward or downward motion of strip. When this happens contact b/w strip open and current flow stops. Based upon the binding process bending motion could be upward or downward.
After temperature came into normal condition, strip contact to each other once again .

Application: Based on their response to change in temperature they are used as ON/OFF  or OFF/ON switches in different industries and in  household devices like  in Automotive radiator , furnace, boilers,AC, Freeze.

Temperature Set Renege: depend upon application requirements 

Key Takeaway:- Resistance of metal strip dependents on Temperature, metal are conductors of heat.


  1. Save Energy by simply cutting the power supply to unit 
  2. Maintain idle comfort temp in room
  3. Easy in Installation
  1. High Hysteresis Loss
  2. Difficult to under stand by new user

How does Thermocouple work?

Thermocouple 'thermo-electric’ temperature measurement device,it is most common from all other temperature sensors available due their  simple design, cost,fast response time and easy to use .

Construction:-Thermocouple are combinations of two dissimilar like iron or copper  and constantan welded at two junctions. One junction is kept at constant temperature (cold or reference point) and the second one in contact with the heat source.

Working Principle:- Thermocouple works on ‘Seebeck effect’ when two dissimilar metals contacted to each other such as  constantan & copper create a ‘thermo-electric’ effect which gives constant potential in millivolts(mA) between them.  Due to temperature difference b/w junction a voltage gradient generated and produced EMF along conducting wires .
After measuring this voltage we can tell the temperature of the source.

Why need a reference?
To make thermocouple resistance of conductive wire need to be known as soon as thermocouple is placed to a heat source difference of voltage recorder and scale down into temperature. But without knowing the reference temperature we will not be able to  compare change in resistance of wire due to change in temperature .
What is an Active sensor?
Sensor which does not require any external supply source or power input to generate an output called active sensor or active devices like Thermocouple .
Thermocouple Ranges :- -200℃ to 2000℃

Key Takeaway:- Need two dissimilar metal along with a known reference point.

Is there any type of Thermocouple?
Thermocouple range depends upon base metal and its reference point base metal change could result in change in range of sensor.

Conducting Metal(+/-)
Temperature Range
British Standard 
Nickel Chromium / Constantan
-200 to 900oC
Iron / Constantan
0 to 750oC
Nickel Chromium / Nickel Aluminium
-200 to 1250oC
Nicrosil / Nisil
0 to 1250oC
Copper / Constantan
-200 to 350oC
Copper / Copper Nickel 
0 to 1450oC

Voltage Vs Temperature Graph

Curie Point or Temperature:- A point or temperature on which magnetic material behaves differently, for thermocouple this may result in deviation in reading from exact  temperature of body or matter. 

Application:- Automobile, Medical industry .

  1. Wide range
  2. Simple in construction
  3. Low cost
  4. Fast response
  5. Small in size and shape
  6. Durable
  1. Reference or cold junction required
  2. Less Accurate
  3. Non-linear
  4. Less stable
  5. Low voltage out-put(0-10mV)
What is Thermistor?
Thermistor is a resistance dependent  temperature measuring device with a wide range and better accuracy , sensitivity compared to other types of sensor.

Construction:- Thermistor are made of  ceramic &  semiconductor made by metal oxide technique such as cobalt,barium titanate.Most of the manufacture used powder pressing technique to form it into disc, ball or rod like shape.   

Working Principle: As soon as the thermistor comes near a heat source  physical resistance of the sensor starts to change with respect to change in temperature .The change in resistance could be positive or negative depending upon material  properties conductor or semiconductor.
Thermistor is a passive device which means it needs a power source to produce output.
Based on output Thermistor are two type :

1.PTC(Positive Temperature Coefficient)
This type of thermistor is made of doped polycrystalline ceramic containing (barium titanate BaTio3) which have the property that their resistance increase with increase in  temperature of body or components. 

Resistance  change in (0 to 120kΩ) Change in Temperature(-40℃ to 180℃ )

2.NTC(Negative Temperature Coefficient )
These thermistors are made from semiconductor material like sintered metal oxide by pressing these into a disc or ball shaped. As temperature of semiconductor increases so thuse the charge carrier in conduction band that mean more current can conduct through the material. This results in a decrease in resistance of the sensor unit .

Resistance change  (0 to 120kΩ) 1/change in Temperature(-40℃ to 180℃ ) 

Thermistor Ranges: -55℃ to 300℃

Key Takeaway:- Use as circuit protector or current limiter 

  1. PTC used as current limiter device in series circuit panels
  2. PTC used in Automobile unit as extra heater during winter
  3. NTC is used as sensor with circuit as current inrush limiting device
  4. NTC use in very much every food and processing industry
  5. Household electrical appliance like toaster, microwave oven freeze use NTC  
Advantages :
  1. Fast response
  2. Small in construction 
  3. Higher sensitivity
  4. Low Cost 
  1. Thermister need external power source to generate output
  2. Self Heating could deflect original output
  3. Narrow temperature range
  4. Non-linear
What is a Resistance Temperature Detector (RTD)?
Resistance temperature detector is a precision temperature sensing device made from highly pure conductive metals, such as platinum or copper. RTD has a positive temperature coefficient but unlike other thermistors their output varies

Construction:- RTD made of conductive metal wire wrapped around a bar or strip insulated from each other through insulating material like mica, to prevent heating. RTD are very fragile so they need a protective unit around them.
RTD is a passive device that means it needs a supply unit to generate output.

Based on wiring configuration RTD are three types
1.Two Wired :- Most basic wiring configured RTD , used when accuracy is not important. Working on bridge balancing by equating R2=R1 and taking R3 middle of range to RTD.

2.Three Wired:- To minimize the effect of lead resistance a three wire configured circuit can be used. Bridge balancing by equating R2=R1 & R3 middle of range to RTD.

 3.Four Wire:- Four wire configuration used to increase accuracy by measuring resistance.By avoiding voltage drop in circuit  effect of lead resistance eliminated.

Working Principle :- Conductor metals have electrons printed a certain way which allow free path to current flow however when temperature of conductor rises  atoms start random motion causes restriction in path of electron flow or current flow, so that the output of the system. By measuring change in output taking current as constant we can tell change in temperature of body or components.
A relation which we follow to measure temperature is called the Van Dusen equation.

𝜶=R100 -R0/100℃ . R0
R0 is temperature of sensor at 0
R100 is temperature of sensor at 100
For example platinum have 𝜶=0.003925Ω/(Ω .℃)  in between 0 to 100range.
RTD has mainly three types based on constriction of  internal circuit thin-film,coiled and wire- wounded. These are mostly used in industry applications.  

Temperature Range:- -200℃ to 500℃ 

Key Takeaway:- Variation in output nearly linear over certain temperatures like for  Copper and Nickel is  150℃ for 300 °C.And change in temperature in respect to temperature is very small. 


  1. RTD is used in the Automobiles industry  to measure engine , air  intake and exhaust temperature. 
  2. Used in  power electronics, computer, food handling and processing, industrial electronics, medical electronics.
  3. Military & Aerospace.

  1. High Accuracy
  2. Large operational range
  3. Low drift
  4. Use in Precision devices
  1. Need external supply to give output
  2. Very difficult to operate over temperature of 600℃
What are Non Contact Type Temperature Sensors?
As the name suggests these sensors or devices measure the temperature of an object without making any physical contact to its surface or body.This sensor is called Pyrometer.

Construction:- These sensors consist of an optik ,sensor unit,electronic processing unit which converts one process variable to readable data and a small display system.

Working Principle:- It is work on principle that ‘intensity of light received by observer or object depends upon distance b/w observer and source along with temperature of source . When an optical system focuses thermal radiation onto an object or detector.Output of optical unit depends upon thermal radiation or irradiation  of target or source through “Stefen Boltzman law”.

E=emitted energy 
𝛔=constant of proportionality 
T= Temperature of object.

Temperature Range:-  250°C to 2760 °C

Key Teakway:-  Measure temperature of movable objects in different environments. 

  1. Pyrometer are mostly applicable when measuring objects in motion or out of reach from operator.
  2. Use in Smelting industries 
  3. Use to measure steam boiler temperature
  4. Use in chemical or nuclear power plants
  5. Applicable in hazardous environment
  1. Non contact required to measure temperature 
  2. Fast response 
  3. More stable reading 
  1. Expensive 
  2. Less accurate 

Different industry or sector even household are required temperature sensor; to preserve , maintain operation readiness of components or units to their optimum level.
Safety is second most important things in any industry it may be Automobile , power plants,food processing. Because things need to operate or stored at certain temperature.
And each temperature measuring device, sensor or transducer need to be operate with its higher  accuracy , sensitivity which result in exact output.
Depending upon application we can chose which one is best after reading specification of sensor.

1.Header image by owlcation.com
2.Mercury Thermometer photo by Jaroslaw Kwoczala

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