What is Bimetallic Strip : Construction and Its Types

Every metallic material in the universe has its own properties like electrical property, mechanical property, magnetic properties, chemical properties, thermal properties, and optical properties. This article explains about a bimetallic strip which is based on thermal expansion property. It is usually observed in applications like an iron box, heaters, kettles, etc. A bimetallic strip converts thermal energy into mechanical displacement.

What is Bimetallic Strip?

Definition: A bimetallic strip works on the principle of thermal expansion, which is defined as the change in volume of metal with the change in temperature. The bimetallic strip works on two basic fundamentals of metals.

• The first fundamental is the thermal expansion, which states that the metals expand or contract based on variation in temperature
• The second fundamental is the temperature coefficient, where each metal (having its own temperature coefficient) expands or contracts differently at a constant temperature.

Properties of Bimetallic Strip

Some of the important properties of the bimetallic strip are

• Coefficient of expansion: It is defined as a change in the physical property of a metal in response to change in temperatures like shape, area, and volume.
• Elasticity modules: It is defined as the ratio of stress to strain in an elastic deformation region.
• Elastic limit on cooling: It is the standard limit at which the metal gets back to its normal state on cooling. This property varies from metal to metal.
• Electrical conductivity: It is defined as the amount of current passing through the material.
• Ductility
• Metallurgical ability.

Construction of Bimetallic Strip

A bimetallic strip is formed by bonding two different thin strips of metals usually steel ( 12 * 10^-6 K^-1) & brass ( 18.7 * 10^-6 K^-1), or copper ( 16.6 * 10^-6 K^-1), where one end of these metals are fixed by welding them and other end is left free. When applying temperature to these materials, then they will start to change their physical state either by expanding or by deforming.

It can be explained in the following two cases,

Case(i): When the temperature increases, it allows the strip to expand towards the metal with a lower value of temperature coefficient, which can be observed in the figure below.

Case(ii): When the temperature decreases, it allows the strip to expand towards the metal with a higher value of temperature coefficient, as shown below.

From this, we can understand that the

Range of deflection = metal used

Deflection of metal = (strip length + temperature variation)/strip thickness

Mathematical Representation

Consider two metals like A and B at two different temperatures ‘T1’ and ‘T2’. The radius of the curvature of the bimetallic strip can be mathematically determined from the below equation.

R = t { 3 (1+m)² + (1+m * n) [ m² +1/ m * n]} / 6(α ‘ _A – α ‘_B) ( T₂ -T₁) ( 1+m)² ……1

Where,

R = curvature radius at temperature ‘T2’

t  = ( t1 + t2) = sum of the thickness of the bimetallic strip

n = E_A / E_B = ratio of elasticity of two metal

m = t1 / t2 = ( Lower thickness – expansion of metal) / (higher thickness – expansion of metal)

α’ _A, α’ _B = Thermal coefficient of expansion metal A and B

T₁= initial temperature

T₂ = Final temperature.

The equation for the metallic strip bending towards the metal with low-temperature coefficient is given as

r = 2 t / [ 6 * (α _A – α _B) (T₂ -T₁)] ……………(2)

In a practical world, the ratio of metals moduli of elasticity and their thickness must be maintained equal so that the metal regains back to its normal position when the applied temperature changes. If the thickness of metal is t/2 then

[r + (t/2)]/r = Expanded Length of Expanded Strip A/Expanded Length of Expanded Strip B

                        = L [1 + α _A (T₂ -T₁)] / L [1 + α _B (T₂ -T₁)]

                        = t/2 [ [1 + α _B (T₂ -T₁)] / [ ( α _A – α _B) (T₂ -T₁)] ]

                        r = t / [2 α _A (T₂ -T₁) ] ………….. (3)

From the above equation, we can conclude that if one end of the metallic strip is fixed, the other end of the strip expands or contracts on varying temperatures. This kind of principle is usually observed in low sensitivity thermometers.

Types of Bimetallic Strips

Bimetallic strips are available in two types, they are

Spiral Strip Type

It consists of a spiral-like structure and a pointer is attached to it, which is used for scaling the temperature. When this spring structure is heated, then the metals exhibit thermal expansion property and it deforms when there is a fall in temperature. At this stage, the pointer records the temperature on the scale. These kinds of thermometers are usually used in recording ambient temperature.

Helical Type

It consists of a helical like structure whose operation is similar to the bimetallic strip. Where the free end of the strip is connected to a pointer. Whenever the strip is heated, it experiences thermal expansion property and contracts on cooling. At this stage, the pointer records the temperature reading. Usually, these kinds of thermometers are used in industrial applications.

Advantages

The following are the advantages of the bimetallic strip

• No external power source is required
• Simple in usage and robust
• Less cost
• Gives accuracy between ± 2 to 5 %

Disadvantages

The following are the disadvantages of the bimetallic strip

• They can measure up to 4000 C
• There will be a change in the quality of metal on regular usage which may lead to an error while measuring.
• At low temperature, the sensitivity and accuracy are not up to the mark.

Applications of Bimetallic Strip

The following are the applications of the bimetallic strip

• Clocks
• Thermistor
• Iron box
• Heat engine
• Heaters

FAQs

1). What devices use a bimetallic strip?

A bimetallic strip is used in devices like a fire alarm, fans, etc.

2). What happens when a bimetallic strip is heated?

• When a bimetallic strip is heated, the metals either expand or deform based on their thermal coefficient properties.
• Case 1: With the increase in temperature  makes the strip expands towards the metal with the lower value of temperature coefficient, which can be observed in the below figure and
• Case 2: When the temperature decreases it makes the strip expands towards the metal with a higher value of temperature coefficient, as shown below.

3). Is bimetallic strip used in fans?

Yes, they are used in fans to convert temperature into mechanical displacement.

4). Why do bimetallic strips bend?

Bimetallic strips bend due to the metal thermal expansion property.

5). Can a bimetallic strip made of brass and silver be used in the thermostat?

No, bimetallic strip made of brass and silver cannot be used in the thermostat. Since they have a negligible difference in their thermal expansion property.

Thus, this is all about an overview of a bimetallic strip which works on two main fundamental thermal expansion and temperature coefficient. It is usually a thermometer device that measures temperature. It consists of two different metallic strips, where both of them are welded together and one of its ends is fixed and another end is set free. Theses metals expand or deform at varying temperatures. They are available in two forms helical and spiral form. Where helical bimetallic strip thermometer is used in industrial areas, and the spiral bimetallic thermometer is used in less sensitive areas. The main advantage is it provides an accuracy between ± 2 to 5 %. Here is a question for you, what is the function of the bimetallic strip?