Temperature Measurement

What is a temperature sensor and what are their types?

The measurement of thermal energy in a body is the temperature, temperature is a major concern in most of the process industries. The major property of the temperature is that the increase or decrease of the temperature would affect the quantity of the energy of motion of the particles in a component. A bowl of hot water would have huge energy when compared to a bowl of cold water. We can define temperature as the measurement of the molecular kinetic energy in a system. The measurement of temperature is really important in an industrial process, due to the variations in temperature level there could be changes in physical or chemical state. So temperature measurement is really important to protect the equipment. If the temperature is not controlled then it will cause deterioration in pipelines and also in tanks.

Types of temperature sensors

What is the importance of temperature measurement?

  • The temperature would affect the reaction rate
  • It would affect the viscosity
  • The increase or decrease in temperature could change the state of matter
  • Material strength would be affected
  • Process safety is dependent upon this
  • Efficiency and product quality can be improved by controlling the temperature

What are the types of temperature sensors?

We can also classify the temperature sensors as electrical and nonelectrical methods.

Nonelectrical methods

  • In this method, the sensor would measure the temperature by the change in volume of liquid due to the temperature.
  • Temperature measurement can also be done according to the change in the pressure of the gas due to temperature variations.
  • Measurement is also be done by the changes in vapor pressure during the temperature variation

Electrical methods

  • Thermocouples
  • Measurement is also done by the variation in the resistance of the material due to the change in temperature
  • Measurement is also done by determining the energy which is received by radiation

Mechanical method (Non-electrical)

1. Bimetallic Thermometer

Bimetallic Thermometer diagram

Thermometers are the oldest temperature sensors and they are widely used to do temperature measurements. In this type of thermometer, bimetallic strips are used and it is constructed by welding two strips of metals that have different thermal expansion coefficients. So when this strip is subjected to heat then the metal would change its length according to its thermal expansion properties. So when the heat is applied to it then the metal with a higher coefficient would expand quicker than the metal which has a low thermal coefficient. So because of this, the whole metallic strip would bend in the direction of the metal that has a lower thermal coefficient.

Advantages of Bimetallic Thermometer

  • Maintenance is easy
  • Low cost
  • No need for a power source
  • Good temperature range

Disadvantages of bimetallic thermometer

  • Local mounting
  • Calibration could change of rough handling
  • Mostly indicating type is available
  • Low accuracy
  • Limited applications

What are the applications of Bimetallic thermometer

  • It is used for the automatic control of the home appliances
  • Engine temperature measurement
  • Reaction chambers
  • Furnace
  • To control air temperature
  • Process parameters can also be measured
  • Thermostat switches
  • Wall thermometers

2. Gas thermometer

Gas thermometer representation

In this type of thermometer, the temperature measurement is done by determining the expansion of gases. The liquids and solids would expand in an irregular manner while in the case of gases it is different, gases would expand in a regular manner according to the rise in temperature. So due to this feature of gases, it is used to do the measurement of the temperature. In this sensor, the gas would be enclosed in a thermostatic substance. When this sensor is subjected to heat then there would be a change in pressure. The working principle of gas thermometer is according to the Charles law which states that due to the increase in temperature of gas the volume of the gas would increase too. When the sensor is heated then the gas molecule would achieve more energy and it would move faster this would cause the volume to increase.

So when the sensor is not subjected to temperature then, the gas molecules only have low energy and these molecules would be packed together and thus less volume. Mostly used gases in this thermometer will be nitrogen and helium. Nitrogen is less expensive and it won’t react with steel bulb material at high temperatures. Nitrogen won’t react properly at low temperatures and for this purpose, we can use helium.

Advantages of Gas thermometer

  • Highly accurate
  • Wide range
  • Highly sensitive
  • It is not dependent on the type of gas that is used
  • Temperature response would be quick while comparing it with the liquid-filled type

Disadvantages of Gas thermometer

  • It’s not easy to handle
  • Highly sensitive to temperature change and also mechanical vibrations
  • In order to do the measurement, it would take some time
  • Expensive
  • It can’t be used to measure the rapid temperature changes
  • Not portable

Applications of gas thermometer

  • Due to its high accuracy, it can be used to do the calibration of other thermometers
  • It is used in piping and tanks to do the temperature measurement

Electrical method

1. RTD

RTD representation

RTD is a temperature sensor and its operating principle is dependent on the fact that the resistance of the metal would increase due to the increase in temperature. So that’s why the name resistance temperature detector, mostly used metals for the temperature measurement is nickel and platinum. These temperature sensors are available in many forms. The RTD would be basically a thin wire of platinum coil, mostly RTD’s are manufactured in two ways using wire or a film. The output of the RTD will be electrical and due to this, it can be used wherever feedback is needed and corrective actions can be done in an automated system.

Types of RTD

Wire wound RTD

Wire wound RTD

We can classify wire wound RTD into two types one of them is composed of coils of wire which will be encapsulated by the glass and we can see this type of RTD for most industrial applications. The other type of wire wound RTD will be wound around a ceramic or glass core and then it will be covered with additional glass. So in this, the sensing wire will be connected to the larger lead by soldering.

Thin-film RTD

Thin film RTD

This type of RTD is created by depositing a thin layer of platinum onto a ceramic substrate, and then it will be stabilized. The deposition of platinum would be in the resistance pattern, mostly with provisions so that the resistance can be adjusted by cutting the circuit in a trim area. After that lead wires would be attached. These, RTD’s are coated with glassy coating for mechanical and moisture protection. 

Coil element RTD

Coil element RTD

This type of RTD is created by using a helical coil of platinum sensing wire, this wire will be inserted into the bores of an insulating mandrel and thus provides a strain-free sensing element.

Hollow annulus RTD

This type of RTD uses an open-ended metal winding mandrel and this would increase the fluid contact and decrease the thermal mass and thus provide a faster response. This type of RTD is completely sealed but it is costlier when compared to the other types of RTD.

Hollow Annulus RTD

What are the advantages of RTD?

  • Highly accurate
  • Stable
  • Linear resistance change
  • Good operating temperature range
  • It would work properly at a high temperature
  • The response is faster than thermocouple
  • Good interchangeability
  • Absolute temperature can be measured
  • It can measure very low to high temperature
  • The output of the RTD is electrical so that it can be used with PLC

What are the disadvantages of RTD

  • Power source is needed
  • Expensive
  • Shock and vibration could affect the measurement
  • It would heat itself

Applications of RTD

  • Textile production
  • Processing of plastics
  • Microelectronics
  • Petrochemical
  • It can be used to do the temperature measurement of air, gas, and liquid
  • Stoves and grills
  • It can be used for the temperature measurement of exhaust gases
  • Medical and chemical laboratories

2. Thermistor

Various types of Thermistors

A thermistor can be described as a resistive material, which is composed of metal oxides and it will be coated with glass. Thermistors are made of semiconductor materials from certain mixtures of oxides such as nickel, manganese, copper, cobalt, and other materials and these materials will be sintered at very high temperatures. In this type of temperature sensor, its resistance would decrease by an increase in the temperature. Thermistors are made from semiconductor material, the semiconductor material which is used would vary the characteristics of the thermistor like sensitivity, temperature range, resistance range, etc.  Thermistors can be classified into two types. One has the negative temperature coefficient and the other which has the positive temperature coefficient. The PTC thermistors only have a limited range and because of this NTC thermistors are mostly used.

Thermistors have good sensitivity it would show a large change in resistance for small temperature change. So the thermistors are highly sensitive. The major difference between a thermistor and an RTD is the material that is used for their construction. The materials which are used in the thermistor could be ceramic or polymer while in the case of the RTD it would use pure metals. The sensitivity of the thermistor is really high when compared to an RTD.

What are the advantages of thermistors

  • Low cost
  • Small in size
  • High sensitivity
  • Point sensing, this sensor can be converted into a small-sized pin for point sensing
  • They are compatible with many devices

What are the disadvantages of thermistor

  • Nonlinear output
  • The temperature range is limited
  • Fragile
  • Self-heating

What are the applications of thermistors

  • It can be used for temperature trip alarms because they are highly sensitive
  • It can be used to monitor the temperature of the battery pack
  • Incubator temperature monitoring
  • PTC thermistors can be used as a current limiting device for circuit protection

3. Thermocouple

Thermocouples are constructed by joining two pieces of dissimilar materials at their end. The metals would be joined by soldering or welding, the working principle of this device is by Seebeck effect. It states that when two dissimilar metals are joined together and if the two junctions are at different temperatures, then there will be a current flow in closed circuits of these, two dissimilar materials. So when heat is applied to this device then a voltage will be generated, the voltage which is produced will be dependent upon the metal which is used and also on the temperature between the junction.

Thermocouple representation

A thermocouple is capable to do the conversion of thermal energy to electrical energy. If the temperature that exists at the two junctions are the same then the voltage that is produced at each junction would cancel each other and there won’t be any current flow. So if there is a different temperature in each junction different voltages are produced and also there will be current flow in the circuit. So in order to do the temperature measurement by using a thermocouple then one end of the thermocouple should be kept in contact with the process and the other end must be kept at a constant temperature. Reference junction is the part of the sensor where the end part would have a constant temperature and the part of the sensor which is in contact with the process is the measurement junction.

Temperature measurement by thermocouple

What are the advantages of thermocouples?

  • It is widely used in many industries
  • Construction is simple
  • Rugged
  • High-temperature operation
  • Low cost
  • Quick response to temperature changes
  • Remote temperature measurement
  • It can be used for a wide range of temperature measurement
  • Calibration checking can be easily done
  • Long transmission distance can be achieved

What are the disadvantages of thermocouple

  • Nonlinear response
  • Not stable
  • Low repeatability
  • Sensitivity is lower for small temperature changes
  • Low accuracy
  • The extension wire which is used should be the same as the thermocouple type
  • The wire must be shielded otherwise it could pick up the radiated electrical noise
  • It requires amplifiers for many applications
  • For most of the control applications, it would need expensive accessories

What are the applications of thermocouple

  • Medical equipment
  • Industrial heat treating
  • Packaging equipment
  • Semiconductor processing
  • Food processing equipment


1. Radiation Pyrometer

Pyrometer diagram

We can measure the temperature of an object by using a pyrometer and it will do the non-contact measurement. Most objects would have above zero absolute temperature and these objects would emit heat the emitted heat energy which is radiated by the body is dependent on the temperature of the body and this is Stefan-Boltzman’s law. So a pyrometer does the measurement according to this law. This device would do the measurement by measuring the radiation which would be radiated from an object.

A radiation pyrometer would receive the radiation from the object and it will be focused by using mirrors to the detector. The detector could be mostly a thermocouple and the thermocouple would create an EMF which is proportional to the temperature and this will be transmitted to a millivoltmeter.

2. Optical pyrometer

Optical pyrometer representation

There will be a filament lamp in this pyrometer, the radiation from the object would be focused by the lens to the lamp. In the image, we can see a red filter between the eyepiece and the lamp, and this is placed to allow only narrow-band wavelength to pass through it. The eyepiece would be adjusted till the lamp is in sharp focus. By varying the value of the rheostat the lamp is heated due to the current that passes through it. If the current increases, then the temperature and also the resistance of the lamp increases. If the radiation intensity of the lamp is the same as that of the target or temperature source then the filament will be indistinguishable. So the multimeter which is connected to the lamp would measure the current that passes through it and by this we could determine the temperature of the filament and thus the target temperature can be determined too.

What are the advantages of pyrometers?

  • There is no contact with the product
  • Response time is really fast
  • There is no corrosion or oxidation to affect the measurement accuracy
  • Stability
  • High repeatability
  • Measurement of moving objects
  • We can measure certain objects which is difficult to access

What are the disadvantages of the pyrometer

  • High initial cost
  • It is really complex due to the required electronics support for the measurement
  • The accuracy of the measurement would be affected by dust or due to smoke background
  • Radiation
  • The application of the sensor would be limited to the field of view and also the spot size
  • It can’t be used to get the continuous value for the temperature

What are the applications of the pyrometer

  • It is used in automation and feedback control
  • These sensors are used for manufacturing processes like glass, metals, cement, semiconductors, etc
  • This sensor would be useful in fire fighting

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