Temperature Sensing Devices

What are Temperature Sensing Devices?

The temperature sensing devices are called thermometers which measures the temperature of a body. The temperature of a body is expressed as a number of a particular thermal scale unit.

A thermometer is composed of two components –

1. A sensor to detect the temperature of the body.
2. A visible scale to get the numerical value for the temperature measured.

Thermometry

The branch of physics which deals with the measurement of temperatures is called thermometry.

• Temperature is a measure of amount of kinetic energy possessed by a body.
• Thermometry is the process of measuring of this energy by using temperature sensing devices called thermometers.

Thermometric property

The property of a material which senses change in temperature and has a linear relation with change of temperature is called thermometric property. A thermometer uses the thermometric property of a material to measure temperature.

Important thermometric properties used for the construction of temperature sensing device are as follows –

1. Linear expansion of liquid column with raise in temperature.
2. Linear increment of electrical resistance of a conductor with raise in temperature.
3. Thermoelectric effect producing electrical signals in thermocouple by maintaining difference in temperature of junctions.
4. Variation in wavelength of radiant energy with change in temperature of a hot body.

1. Expansion of Liquid Column – 

Some thermometers use a bulb filled with a liquid chemical as the temperature sensor. Due to thermal expansion, height of chemical column rises in a glass capillary tube connected with the bulb. The height ( l ) of chemical column changes linearly with the rise in temperature ( T ) .

Governing thermometric relation is expressed as –

T = \left [ \left ( \frac { l_t - l_0 }{ l_{100} - l_0 } \right ) \times 100 \degree C \right ]

1. This property is used in the working principle of a Mercury thermometer.
2. Galinstan is used in the bulb of Hicks thermometer has similar thermometric property.
3. An alcohol thermometer also works in this principle.

A typical mercury bulb thermometer is shown in figure below.

2. Increase of Electrical Resistance – 

The electrical resistance ( R ) of a metal wire changes linearly with rise in temperature ( T ) .

Governing thermometric relation is expressed as –

T = \left [ \left ( \frac { R_t - R_0 }{ R_{100} - R_0 } \right ) \times 100 \degree C \right ]

1. This property is used in the working principle of Platinum resistance thermometers.
2. The same principle is used in working of thermistors of Digital thermometers.

3. Thermoelectric Effect in Thermocouple – 

Electric signals ( ξ ) produced in a thermocouple by Seebeck effect, is proportional to the temperature difference of hot and cold junctions.

Governing thermometric relation is expressed as –

T = \left [ \left ( \frac { ξ_t - ξ_0 }{ ξ_{100} - ξ_0 } \right ) \times 100 \degree C \right ]

1. This property is used in the working principle of Infrared thermometers.

4. Variation of Radiant Energy with Temperature – 

Wavelength of radiated energy ( \lambda_{m} ) emitted from a hot body is proportional to its absolute temperature.

Governing thermometric relation is expressed as –

\lambda_{m} T = b ( Wien’s constant )

1. This property is used in the working principle of pyrometers which are of two types – (1) Total radiation pyrometers and (2) Optical pyrometers.

5. Charle’s Law Effect –

According to Charle’s law, the pressure ( P ) of a gas changes linearly with rise in temperature when the volume is kept constant.

Governing thermometric relation is expressed as –

T = \left [ \left ( \frac { P_t - P_0 }{ P_{100} - P_0 } \right ) \times 100 \degree C \right ] \quad [At constant volume]

1. This property is used in the working principle of a Constant volume gas thermometers.

6. Boyle’s Law Effect –

According to Boyle’s law, the volume of a gas changes linearly with rise in temperature when the pressure is kept constant.

Governing thermometric relation is expressed as –

T = \left [ \left ( \frac { V_t - V_0 }{ V_{100} - V_0 } \right ) \times 100 \degree C \right ] \quad [At constant pressure]

1. This property is used in the working principle of Constant pressure gas thermometers.

Advantages of Gas Thermometers –

1. Gas thermometers such as Helium gas thermometer, Nitrogen gas thermometer etc. are more sensitive than liquid thermometers because gas expansion is large with small change in temperature.
2. These are most accurate and used to calibrate other thermometers.
3. These thermometers can be used for a wide range of measurement such as ( -270 \degree C ) using helium gas and up to ( 1600 \degree C ) using nitrogen gas.

Types of Thermometers

A thermometer is defined as a device used to measure the temperature of a body. Various types of thermometers are in use. Following are of most important –

1. Glass tube Mercury thermometer.
2. Glass tube Alcohol thermometer.
3. Clinical thermometer.
4. Digital thermometer or Thermistor.
5. Infrared thermometer or Thermocouple.
6. Pyrometer etc.

We will discuss in detail about construction and working principle of these devices in separate posts.