Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: Mechanical Temperature Measuring Sensors
Share
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > How It Works > Mechanical Temperature Measuring Sensors

Mechanical Temperature Measuring Sensors

Last updated: August 1, 2018 10:26 am
Editorial Staff
How It Works Temperature Measurement
No Comments
Share
5 Min Read
SHARE

Temperature can be measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic. There are four basic types of temperature measuring devices, each of which uses a different principle:

  • Mechanical (liquid-in-glass thermometers, bimetallic strips, bulb & capillary, pressure type etc.)
  • Thermojunctive (thermocouples)
  • Thermoresistive (RTDs and thermistors)
  • Radiative (infrared and optical pyrometers)

Mechanical Temperature Measuring devices

Principle of Operation

A change in temperature causes some kind of mechanical motion, typically due to the fact that most materials expand with a rise in temperature. Mechanical thermometers can be constructed which use liquids, solids, or even gases as the temperature-sensitive material.

The mechanical motion is read on a physical scale to infer the temperature. The examples include:

  • Liquid-in-glass thermometer

The most common and well-known thermometer is the liquid-in-glass thermometer.

Liquid-in-glass thermometer

As the temperature rises, the liquid expands, moving up the tube. The scale is calibrated to read temperature directly. Usually, mercury or some kind of alcohol is used for the liquid.

  • Bimetallic strip thermometer

Two dissimilar metals are bonded together into what is called a bimetallic strip as figured below.

Bimetallic strip thermometer

Suppose metal A has a smaller coefficient of thermal expansion than does metal B. As temperature increases, metal B expands more than does metal A, causing the bimetallic strip to curl upwards as sketched.

Bimetallic strip thermometer 2

One common application of bimetallic strips is in air-conditioning thermostats, where a bimetallic strip is used as the arm of a switch between electrical contacts. As the room temperature changes, the bimetallic strip bends as discussed above. When the bimetalic strip bends far enough, it makes contact with electrical leads which turn the heat or air conditioning on or off.

Bimetallic strip thermostat

Another common application is for use as oven thermometers or wood burner thermometers. These thermometers consist of a bimetallic strip wound up in a spiral, attached to a dial which is calibrated into a temperature scale.

  • Sealed Bellows

The sealed bellows type is filled with a gas, vapor or liquid, which responds to change in temperature by variation in volume and pressure causing expansion or contraction.

Sealed Bellows Temperature Sensors

  • Bulb and Capillary Sensor

Bulb and capillary elements are used where temperatures are to be measured in ducts, pipes, tanks or similar locations remote from the controller.

The bulb is filled with liquid, gas or refrigerant depending on the temperature range required. Expansion of the fluid in the heated bulb exerts a pressure which is transmitted by the capillary to the diaphragm and there translated into movement.

Bulb & Capillary Temperature Sensor`1

  • Pressure thermometer

A pressure thermometer, while still considered mechanical, operates by the expansion of a gas instead of a liquid or solid. (Note: There are also pressure thermometers which use a liquid instead of a gas.)

Pressure thermometer

Suppose the gas inside the bulb and tube can be considered an ideal gas.

The ideal gas law is PV = m R T

Where

  • P is the pressure,
  • V is the volume of the gas,
  • m is the mass of the gas,
  • R is the gas constant for the specific gas (not the universal gas constant), and
  • T is the absolute temperature of the gas.

The bulb and tube are of constant volume, so V is a constant. Also, the mass, m, of gas in the sealed bulb and tube must be constant. Hence, the above equation reduces to P = kT, where k is constant.

A pressure thermometer therefore measures temperature indirectly by measuring pressure. The gage is a pressure gage, but is typically calibrated in units of temperature instead.

A common application of this type of thermometer is measurement of outside temperature from the inside of a building. The bulb is placed outside, with the tube running through the wall into the inside. The gauge is on the inside. As T increases outside, the bulb temperature causes a corresponding increase in pressure, which is read as a temperature increase on the gauge.

Also Read: Basics of Thermocouples & RTD

Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

Comparison of Temperature Sensors – Industrial Process Control
Float Level Switch Working Principle Animation
Response Time Test of RTD and Thermocouple
What is Thermowell and How-to choose Thermowell Probe Length ?
Types of Thermocouple Measuring Junction
How to Install Thermowell on an Elbow?
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
Leave a Comment

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

Coriolis Meter Explained with Animation
Why 4 wire RTD Measurement Accuracy is better than 2 and 3 wire RTD?
Spiral Bimetallic Thermometer Principle
Ultrasonic Level Switch Working Principle
Bubbler Level Measurement Animation
Thermowell Design Guidelines
What is RTD Sensitivity ?
Pneumatic Temperature Transmitter with Sensor Principle

Keep Learning

Thermocouples Green Rot Effect

Thermocouples Green Rot Effect

Ultrasonic-Level-Transmitter-Animation

Ultrasonic Level Transmitter Animation

Convert 2 wire RTD to 4 wire RTD Sensor

How to Convert 2 wire RTD into 3 or 4 wire RTD?

load cell in deflection

Load Cell Working Principle

RTD Connection Diagram

Difference Between 2 wire RTD, 3 wire RTD, and 4 wire RTD’s

ultrasonic-level-measurement

Ultrasonic Level Transmitter Working Principle

Calibration of Temperature Sensor with Indicator

Calibration of Temperature Sensor with Indicator

Burnout Function in Temperature Transmitter

Burnout Function in Temperature Transmitter

Learn More

Air Pressure Regulator cut-away diagram

Air Pressure Regulator Questions

Air Consumption Calculation for Control Valves

Air Consumption Calculation for ON/OFF Valves

Peak Inverse Voltage of Center Tap Rectifier

Peak Inverse Voltage of Center Tap Rectifier

How to Upload PLC Program in Siemens PLC if Upload Icon is Disabled

Upload Option Disabled in Siemens PLC

OB10 Time of Day Interrupt

Tia Portal – OB10 Time of Day Interrupt Organization Block

Units of Electrical Measurement

Units of Electrical Measurement

Analyzers Questions and Answers

Analyzers Based on Gas Density Questions & Answers

HMI Simulation for PID Controller

PID Controller Output Types – PID_PWM and PID_Output

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?