Temperature Measurement

Muhajir Ab. RahimSchool of Mechatronic Engineering

Universiti Malaysia Perlis

Introduction

• Almost all industrial processes need accurately controlled temperature.

• It will determined the desired quality of products (chemical, steel, plastic, nuclear and food industries)- temperature dependent products and processes.

Temperature and Heat

• Temperature is the measure of the thermal energy in a body, which is relative hotness or coldness of a medium and is normally measured in degrees, using Fahrenheit, Celsius and Kelvin scale.

• Heat is a form of energy; as energy is supplied to a system the vibration amplitude of its molecules and its temperature increases. Example unit of heat is BTU and calorie unit.

• The temperature increase is directly proportional to the heat energy in the system.

Temperature Measurement

• Temperature measurement method can be broadly classified as follows;

1) Non-electrical method

2) Electrical method

3) Radiation method

Non-electrical Method

• Example of non-electrical temperature measurement method;

1) Bimetallic thermometer

(based on the principle of solid expansion)

2) Liquid in glass thermometer

3) Pressure thermometers (based on the principle of fluid expansion)

low expansion material

high expansion material

Electrical Method

• Electrical methods are in general preferred for the measurement of temperature as they furnish a signal which can be easily detected, amplified or used for control proposes.

• Example of temperature measurement by electrical methods are;

1) Thermocouple

2) Resistance Temperature Detector (RTD)

3) Thermistor

4) IC Sensor

Temperature (T) Temperature (T)

Temperature (T)Temperature (T)

Vol

tage

(V

)

Res

ista

nce

(R)

Vo

ltag

e o

r C

urr

en

t (

V o

r I)

Res

ista

nce

(R)

Thermocouple Thermistor

IC Sensor

RTD

Thermistor

RTD

IC Sensor

Radiation Method

• The main advantages of radiation method in temperature measurement is because of non contact with the ubject of measurement. So it is applicable to high temperature, moving object..

• Example of radiation method temperature measurement instrument is pyrometer.

How to select?

• You need to consider several factors such as temperture range, required range, accuracy, environment, speed of response, ease of use and cost.

• Thermocouple are used in high temp chemical industries such as glass and plastic process.

• Thermistor and IC sensor usually used in electronic hobby and automotive industry.

• RTD commonly used in lower temp., higher precision chemical industries.