(a) Describe with the aid of a sketch, how Bi-metallic strips are utilised to measure temperature.(8)
(b) State a typical application for this type of device and its main shortcoming.(2)
Bimetallic Strips for Temperature Measurement
A bimetallic strip is a temperature measurement device composed of two different metals bonded together.
1. The Bimetallic Strip Explained – Fictiv
Principle of Operation:
- Dissimilar Metals: The two metals have different coefficients of thermal expansion. 1. A bimetallic strip is composed of two different materials that have different expansion coefficients. Let’s say these materials are brass and steel, which have coefficients of linear expansion of – Physics physics.bu.edu
- Thermal Expansion: When the strip is heated, the two metals expand at different rates, causing the strip to bend. 1. The Bimetallic Strip Explained – Fictiv www.fictiv.com
- Mechanical Deformation: The bending of the strip can be used to move a pointer on a dial or activate a switch. 1. How does a bimetal thermometer work? – WIKA www.wika.com
Construction:
- Two Metals: Typically, brass and steel or invar and nickel are used. 1. The Bimetallic Strip Explained – Fictiv www.fictiv.com
- Bonding: The two metals are joined together through welding, brazing, or rolling. 1. Bimetallic strip – Wikipedia en.wikipedia.org
- Strip Shape: The strip can be flat, coiled (for increased sensitivity), or spiral-shaped. 1. Bimetallic Strip Thermometers – How Thermometers Work | HowStuffWorks home.howstuffworks.com
Applications:
- Thermometers 1. How does a bimetal thermometer work? – WIKA www.wika.com
- Thermostats 1. Bimetallic Strip Thermometers – How Thermometers Work | HowStuffWorks home.howstuffworks.com
- Overheat protection devices
Advantages:
- Simple construction 1. The Bimetallic Strip Explained – Fictiv www.fictiv.com
- Low cost
- Reliable
- No external power source required
Limitations:
Susceptible to mechanical damage
Lower accuracy compared to other temperature sensors 1. Bimetallic strip thermometer | PPT – SlideShare www.slideshare.net
Limited temperature range
Construction of a Bimetallic Strip
A bimetallic strip is composed of two different metal strips bonded together lengthwise. These metals typically have significantly different coefficients of thermal expansion.
1. The Bimetallic Strip Explained – Fictiv
2. Bimetallic strip – Wikipedia
Materials:
- Common combinations: Steel and brass, or invar and nickel are frequently used due to their differing expansion rates.
- Other combinations: Other metals can be used based on specific application requirements.
Bonding Process:
- Welding: The two metal strips are often joined together by welding. 1. The Bimetallic Strip Explained – Fictiv www.fictiv.com
- Brazing: This method can also be used to bond the strips, but it requires careful control of the heating process.
- Rolling: In some cases, the two metals can be rolled together to form a composite strip.
Strip Configuration:
- Flat: The bimetallic strip can be flat for simple applications. 1. Bimetallic strip – Wikipedia en.wikipedia.org
- Curved: For increased sensitivity, the strip can be coiled into a spiral or helical shape.
Protective Coating:
- To prevent corrosion and damage, the bimetallic strip is often coated with a protective layer.
Key Points:
- The choice of metals and bonding method significantly influences the bimetallic strip’s performance.
- The strip’s shape and dimensions determine its sensitivity and response time.
- Proper construction is essential to ensure the reliability and accuracy of the temperature measurement.
By carefully selecting materials and manufacturing processes, bimetallic strips can be designed for various temperature measurement applications.
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bimetallic strip
Applications of Bimetallic Strips
Bimetallic strips, due to their simple yet effective principle of operation, find applications in various fields.
Common Applications:
- Thermostats: In household appliances, HVAC systems, and industrial processes, bimetallic strips are used to control temperature.
- Thermometers: Older thermometers often employed bimetallic strips to indicate temperature visually.
- Overload Protection: In electrical circuits, bimetallic strips are used in circuit breakers to protect against excessive current.
- Fire Alarms: Some fire alarm systems utilize bimetallic strips to trigger alarms based on temperature rise.
- Pressure Gauges: In certain types of pressure gauges, bimetallic strips can be used to indicate pressure indirectly based on temperature changes caused by adiabatic compression.
- Timers: Simple time-delay mechanisms can be created using bimetallic strips.
Limitations and Considerations:
- Accuracy: Bimetallic strips offer relatively low accuracy compared to modern electronic sensors.
- Response Time: Their response to temperature changes can be slower than other methods.
- Temperature Range: The usable temperature range is limited by the properties of the metals used.
Despite these limitations, bimetallic strips remain a simple and reliable solution for many temperature-related applications.
“Bi-metallic thermometers
The principle of operation of bi-metallic devices is that of differential expansion of two
different materials rigidly joined together, one on the other, as a strip of bi-metallic
material. Figure 1.1 illustrates a typical design usually employed between −40°C and
320°C. Invar (36% Ni, 64% Fe) has a low coefficient of expansion and when welded to a Ni−Mo alloy gives a good bimetallic strip. The helix coils or uncoils with temperature
variation and as one end is fixed the movement rotates shaft and pointer. The range of
the instrument is fixed by the materials used.” Reeds Volume 10 Page 11