Types of Thermometer For Clinical, Laboratory & body Temperature

types of thermometersA thermometer is a device that is used to measure the temperature of a body. There are many types of thermometers, but some common thermometers are laboratory thermometer, clinical thermometer, thermocouple, Bimetallic strip thermometer, Pyrometers, etc.

Some substances have the property that changes with temperature. Substances that show a change in temperature can be used as a thermometric material. For example, some substances expand on heating, some change their colors, some change their electric resistance, etc. Nearly all the substances expand on heating. Liquids also expand on heating and are suitable as thermometric materials. Common thermometers are generally made using some suitable liquid as thermometric material.

Which properties should present in thermometric liquid?

A thermometric  liquid should have the following properties:

types of clinical thermometer

  • It should be visible.
  • It should have uniform thermal expansion.
  • It should have a low freezing point.
  • It should have a high boiling point.
  • It should not wet glass.
  • It should be a good conductor of heat.
  • It should have a small specific heat capacity.

Which liquids are used in a glass thermometer?

A liquid in glass thermometer has a bulb with a long capillary tube of uniform and fine bore. A suitable liquid is filled in the bulb. When the bulb contacts a hot object, the liquid in it expands and rises in the tube. The glass stem of a thermometer is thick and acts as a cylinder lens. This makes it easy to see the liquid level in the glass tube. Mostly Mercury and alcohol are used in glass thermometers.

Why mercury used in thermometers instead of water?

Mercury is used mostly in thermometers because of the following properties:

  • It is visible.
  • It has a low freezing point (-39 °C).
  • It has a very high boiling point (357 °C).
  • It expands linearly.
  • It gives accurate measurements.
  • It is a good conductor of heat.
  • Fast response time.
  • It has a wide range of temperatures.

Properties of Alcohols for thermometers

  • It has a low freezing point (-112 °C).
  • It has a boiling point of (78 °C).
  • It is used to measure high temperatures.
  • It is not a good conductor of heat.
  • It has a bright color.
  • It expands more than mercury.


There are three categories of thermometers which are given these:

  • Types of clinical Clinical  thermometerclinical thermometer

It is used to measure human body temperature in the range of 35 °C  to 42  °C.

  • Types of laboratory thermometer

It is used to measure the room temperature of hot solids and liquids in experiments.It measures temperature in the range of 5 °C to 110 °C and on higher temperatures.

  • Digital thermometers:

These are advanced thermometers used to measure the temperature of a body of a high level of accuracy.

5 Types of thermometer

An ideal thermometer shall have an infinite temperature range. Since no thermometer is ideal, therefore we have a large number of thermometers.

1.Mercury thermometer

mercury thermometer

It has linear expansion property and has a temperature range of -35°C to +500 °C(with compressed nitrogen). It works on the principle:


Where ΔT= Δl/lοα and α is the linear expansion coefficient.

2.     Constant pressure gas thermometer

Constant pressure gas thermometer

In this thermometer, volume increases directly with the increase in temperature. It measures temperature in the range from o k 5oo k.


Where ΔT=ΔV/Vογ=ΔV×273/Vο

And γ=1/273 for ideal gases.

3.    Constant volume thermometer

In this thermometer, pressure increases directly with the increase in temperature. It measures temperature in the range o k to 5oo k.


where ΔT=Δp/P triple point γ

=273 Δp/P triple point

4.Platinum resistance thermometer

In this thermometer resistance of the material in thermometer increases directly with an increase of temperature. It measures temperature in the range of 5oo k to 2300 k.

ΔR α  ΔT

where ΔT=ΔR/Rα ,        α is a thermal coefficient of resistance.

5. Thermocouple thermometer

This thermometer work on the principle:

emf ∈ = αT +β T²

In this scale is nonlinear. Temperature is either matched with a standard curve supplied by the manufacturer or a digital display is provided.α and β depend upon the materials used to form a thermocouple. where:

emf ∈=αT + βT²/2

It measures temperature in the range 500 k to 2300 k


This thermometer work on the principle of radiation. It measures temperature greater than 2000 k. It uses Stefan’s law:

Intensity E= σT4

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