1. Introduction

Temperature is a measure of the degree of hotness or coldness of a body. It determines the thermal energy state and direction of heat flow (from higher to lower temperature).

Accurate temperature measurement is essential in:

• Power plants
• Manufacturing processes
• Chemical industries
• HVAC systems
• Research laboratories

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2. Temperature Scales

Common temperature scales:

• Celsius (°C)
• Fahrenheit (°F)
• Kelvin (K)

Conversion Formula

$$K = °C + 273$$K=°C+273 $$°F = \frac{9}{5}°C + 32$$°F=59​°C+32

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3. Classification of Temperature Measuring Instruments

A. Contact Type Instruments

• Measure temperature by direct contact with the object

B. Non-Contact Type Instruments

• Measure temperature without physical contact

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4. Contact Type Temperature Measuring Instruments

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4.1 Liquid-in-Glass Thermometer

Principle

• Thermal expansion of liquid (mercury or alcohol)

4

Features

• Simple and inexpensive
• Limited temperature range

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4.2 Bimetallic Thermometer

Principle

• Different thermal expansion of two metals

4

Working

• Strip bends with temperature change
• Movement converted into dial reading

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4.3 Thermocouple

Principle

Based on Seebeck Effect:

• When two different metals are joined, a voltage is generated due to temperature difference

$$E \propto (T_1 – T_2)$$E∝(T1​−T2​)

$E \propto (T_1 – T_2)$E∝(T1​−T2​)

4

Advantages

• Wide temperature range
• Fast response

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4.4 Resistance Temperature Detector (RTD)

Principle

• Resistance of metal changes with temperature

$$R_t = R_0 (1 + \alpha t)$$Rt​=R0​(1+αt)

$R_t = R_0 (1 + \alpha t)$Rt​=R0​(1+αt)

4

Features

• High accuracy
• Stable output

4.5 Thermistor

Principle

• Resistance varies significantly with temperature

4

Types

• NTC (Negative Temperature Coefficient)
• PTC (Positive Temperature Coefficient)

5. Non-Contact Temperature Measuring Instruments

5.1 Optical Pyrometer

Principle

• Based on brightness comparison

4

Applications

• Furnace temperature measurement

5.2 Infrared (IR) Thermometer

Principle

• Measures infrared radiation emitted by body

4

Advantages

• Fast measurement
• No contact required

6. Comparison of Temperature Sensors

Instrument	Range	Accuracy	Response
Thermocouple	Very wide	Medium	Fast
RTD	Moderate	High	Moderate
Thermistor	Limited	Very High	Fast
Pyrometer	Very high	Medium	Fast

7. Factors Affecting Temperature Measurement

• Calibration errors
• Environmental conditions
• Response time
• Heat transfer losses
• Sensor placement

8. Applications

• Industrial process control
• Power plants
• HVAC systems
• Automotive engines
• Medical instruments

9. Advantages of Modern Temperature Measurement

• High accuracy
• Digital output
• Remote sensing capability
• Automation compatibility

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