Addis Ababa University
Addis Ababa Institute of Technology
School of Mechanical and Industrial Engineering
Process Control and Experiment Design for Engineers
Bimetallic Strip
Done by: Shalom Abebaw GSR/6781/14
Submitted to: Dr. –Ing Wondwossen Bogale
Date: 12/24/2021
Table of Contents
List of Figures ................................................................................................................................................ 3
Acknowledgement ........................................................................................................................................ 4
Introduction .................................................................................................................................................. 5
What is a Bimetallic Strip? ........................................................................................................................ 5
Working Mechanism of a Bimetallic Strip..................................................................................................... 6
Types of Bimetallic Strips .............................................................................................................................. 7
Spiral Strip Type ........................................................................................................................................ 7
Helical Strip Type ...................................................................................................................................... 8
Applications of a Bimetallic Strip .................................................................................................................. 9
Thermostat.................................................................................................................................................... 9
Advantages and Disadvantages of a Bimetallic Strip .................................................................................. 11
Advantages.............................................................................................................................................. 11
Disadvantages ......................................................................................................................................... 11
Conclusion ................................................................................................................................................... 12
References .................................................................................................................................................. 13
List of Figures
Figure 1 Bimetallic Strip ................................................................................................................................ 5
Figure 2 Spiral Strip ....................................................................................................................................... 7
Figure 3 Helical Strip ..................................................................................................................................... 8
Figure 4 ON Switch of a Thermostat ........................................................................................................... 10
Figure 5 Off Switch of a Thermostat ........................................................................................................... 10
Acknowledgement
First and foremost, I want to express my gratitude to Almighty God for providing me with this chance.
Next, I'd like to express my gratitude to Dr. –Ing Wondwossen Bogale for entrusting me with this task.
Studying and working with bimetallic strips improved my perspective on the issue and provided me with
a thorough understanding of the applications that lie behind them. Next, I'd like to express my gratitude
to everyone who assisted me in writing this article.
Introduction
What is a Bimetallic Strip?
A Bimetallic Strip is used to transform a temperature change into mechanical displacement. The strip is
made up of two metal strips that expand at different speeds as they are heated. When heated, the flat
strip bends in one direction, but when cooled below its initial temperature, it bends in the opposite
direction. When the strip is heated, the metal with the larger coefficient of thermal expansion is on the
outer side of the curve, and when it is cooled, it is on the inner side (Bimetallic Strip, n.d.).
Figure 1 Bimetallic Strip
Working Mechanism of a Bimetallic Strip
A bimetallic thermometer operates using two fundamental metal properties:
1. The thermal expansion property of the metal
2. The coefficient of thermal expansion of different metals is different for the same temperature.
The bimetallic strip is the most important part of the bimetallic thermometer. The bimetallic strip is
made up of two thin strips of different metals with distinct thermal expansion coefficients.
Fusing or riveting is used to attach the metal strips along their length. On one end, the strips are fixed,
while on the other, they are free to move.
Let's look at brass and steel as examples. These metals' lengths vary at various rates for the same
temperature because of their differing thermal expansion. Because of this feature, as the temperature
changes, one side of the metal strip expands while the other does not, causing bending.
The strip will turn in the direction of the metal with the lower temperature coefficient as the
temperature rises. The strip bends in the direction of the metal with a higher temperature coefficient as
the temperature drops. The temperature variation is indicated by the strip's deflection (How Bimetallic
Thermometers Work, n.d.).
Coefficient of Linear Expansion of Brass and Steel are as follows:
Brass: 19e-6/°C
Steel: 11e-6/°C
The brass expands more than the steel when heated, causing the strip to curve on the outside with the
brass. The strip wraps around the exterior steel as it cools.
Types of Bimetallic Strips
Bimetallic strips are available in two types, they are
Spiral Strip Type
It is made out of a spiral-shaped structure with a pointer connected that is used to scale the temperature.
When the metals in this spring structure are heated, they display thermal expansion and deform when
the temperature drops. At this point, the pointer on the scale records the temperature. These
thermometers are typically used to record the temperature of the environment (What is Bimetallic Strip
: Construction and Its Types, n.d.).
Figure 2 Spiral Strip
Helical Strip Type
It is made up of a helical-like structure that works similarly to a bimetallic strip. Where the strip's free
end is attached to a pointer. When the strip is heated, it has a thermal expansion feature that causes it
to contract when it cools. The temperature reading is recorded by the pointer at this point. These kind
of thermometers are typically utilized in industrial settings (What is Bimetallic Strip : Construction and
Its Types, n.d.).
Figure 3 Helical Strip
Applications of a Bimetallic Strip
1. Air conditioners
2. Thermostats
3. Control devices
4. Heater
5. Ovens
6. Hotwires
7. Refineries
8. Oil burners
For this article we will take a look at the working principle of a thermostat
Thermostat
A bimetallic strip is formed by two pieces of different metals bolted together in a typical thermostat (or
bimetal strip). In an electrical circuit connected to your heating system, the strip acts as a bridge. The
"bridge" is normally down, the strip is carrying power across the circuit, and the heating is turned on.
When one of the metals in the strip heats up faster than the other, the entire strip bends somewhat. It
eventually bends to the point where the circuit is broken open. When the "bridge" is raised, the electricity
is turned off, the heating is turned off, and the chamber begins to cool (Thermostat, n.d.).
But what happens after that? The strip cools and bends back to its original shape when the room cools. It
eventually snaps back into the circuit and restores electricity flow, allowing the heater to turn back on. You
can modify the temperature at which the circuit turns on and off by setting the temperature control.
Because it takes time for the metal strip to expand and contract, the heating doesn't turn on and off every
few seconds, which would be pointless; depending on how well-insulated your home is and how cold it is
outside, it could take an hour or more for the thermostat to turn back on after it's been turned off.
(Thermostat, n.d.).
Figure 4 ON Switch of a Thermostat
Figure 5 Off Switch of a Thermostat
Advantages and Disadvantages of a Bimetallic Strip
Advantages
The following are the advantages of the bimetallic strip
• No external power source is required
• Easy to use and reliable
• Low cost
• Provides accuracy of 2 to 5%
Disadvantages
The following are the disadvantages of the bimetallic strip
• They have a temperature range of up to 4000 degrees Celsius.
• Over time, the quality of the metal will deteriorate, perhaps resulting in a measurement inaccuracy.
• The sensitivity and precision are not up to par at low temperatures.
Conclusion
To summarize, a bimetallic strip is employed in a variety of applications, including thermostats, ovens,
control devices, refineries, and many more. We can use it to match our desired needs and specifications
by adjusting the sorts of metals used. A helical or spiral strip might be used depending on the type of
application.
References
Bimetallic Strip. (n.d.). Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Bimetallic_strip
How Bimetallic Thermometers Work. (n.d.). Retrieved from TAMESON: https://tameson.com/bimetallic-
thermometer.html
Thermostat. (n.d.). Retrieved from Explain That Stuff:
https://www.explainthatstuff.com/thermostats.html
Thermostat. (n.d.). Retrieved from Explain That Stuff:
https://www.explainthatstuff.com/thermostats.html
What is Bimetallic Strip : Construction and Its Types. (n.d.). Retrieved from ELPROCUS:
https://www.elprocus.com/what-is-bimetallic-strip-construction-and-its-types/
