Uncover the basics of “What are bimetallic strips?” in this informative blog post. Let us explore their composition, uses, and intriguing characteristics to gain a deeper understanding of how bimetallic strips work and their significance across different sectors.

Introduction

Have you ever wondered how certain everyday gadgets and devices seem to magically adapt to changes in temperature? From thermostats to electric irons, these objects owe their functionality to a remarkable phenomenon called thermal expansion, with one of its key players being the bimetallic strip.

The effects of thermal expansion are pervasive in our daily lives, influencing the design and functionality of countless objects and structures.

Yet, it is in the realm of bimetallic strips that this phenomenon finds one of its most ingenious applications, offering a glimpse into the intricate interplay between thermal physics and practical engineering.

Thermal expansion is the fundamental property of matter and is governed by the behaviour of atoms and molecules within a material. On the other hand, bimetallic strips represent a remarkable engineering invention that benefits from the principles of thermal expansion to achieve specific mechanical responses.

What is Thermal Expansion?

Thermal expansion can be defined as the tendency of a matter to change its shape, volume, or density in response to a change in temperature.

When a substance is heated, its particles gain kinetic energy, causing them to vibrate more vigorously. As a result of this increased energy, the particles exert greater pressure on their surroundings. In solids, this increased molecular motion leads to the expansion of the material in all directions.

Conversely, cooling a material results in contraction or a decrease in volume.

Understanding Thermal Expansion in Solids

Thermal expansion manifests differently across various states of matter. When heat is applied to the solid, the particles absorb energy and undergo vibrational motion, causing the material to expand.

This expansion occurs in all directions, increasing the dimensions of the solid. However, the extent and nature of expansion vary depending on factors such as the

• crystal structure
• temperature range
• composition of material

Role of Thermal Expansion in Everyday Objects

Thermal expansion silently influences the design and operation of countless everyday items, seamlessly integrating into our lives with its subtle yet indispensable role. The manifestations of thermal expansion are manifold and often profound. For instance,

• The snug fit of metal lids on glass jars to ensure a secure seal.
• Expansion of railroad tracks to the cracking of pavement on hot summer days.
• Expansion joints in bridges and highways caused by changing weather conditions.
• Thermometer utilising a liquid or gas within a narrow tube to measure temperature accurately.

Types of Thermal Expansion

Thermal expansion in solids can be classified into several distinct types, each characterised by specific mechanisms and properties. Among these, the most common forms of thermal expansion include:

• Linear Expansion
• Volumetric Expansion

Linear Expansion

In linear expansion, the material expands uniformly along one or more axes, increasing its length or linear dimension. This phenomenon is prevalent in one-dimensional structures such as rods, wires, and beams.

For a substance whose original length is L, the change in length (∆L) of the material due to temperature change (∆T) can be expressed mathematically using the coefficient of linear expansion (α):

∆L = α L ∆T

Volumetric Expansion

Volumetric expansion involves changes in the volume of a solid material due to temperature fluctuations. This three-dimensional expansion is observed in bulk materials and is characterised by alterations in both length and width.

For a substance whose original volume is V, the change in length (∆V) of the material due to temperature change (∆T) can be expressed mathematically using the coefficient of volumetric expansion (β):

∆V = β V ∆T

Coefficient of Expansion

The concept of the coefficient of expansion is a main point in the study of thermal expansion. It quantifies the extent to which a material expands or contracts per unit change in temperature. Each material displays a unique coefficient of expansion, reflecting its intrinsic thermal properties.

Coefficient of Linear Expansion (α) vs Coefficient of Volumetric Expansion (β)

The coefficient of linear expansion (α) is defined as the fractional change in length per degree Celsius (℃) change in temperature and is typically expressed in units of reciprocal temperature (℃^–1).

Similarly, the coefficient of volumetric expansion (β) describes the rate of change in volume per unit change in temperature and is also expressed in units of reciprocal temperature (℃^–1).

This knowledge forms the foundation for exploring more specialised applications, such as the utilisation of bimetallic strips in temperature-sensitive devices and systems.

Bimetallic Strips

Initially, these strips were conceived as a means to address the challenges of temperature measurement and regulation. However, they soon found their way into a myriad of mechanical devices and systems, revolutionising industries ranging from manufacturing to aerospace.

Historical Background

The history of these strips is entangled with a broader description of scientific inquiry and technological advancement.

One of the earliest documented references to bimetallic structures dates back to the work of French physicist Jean Charles Athanase Peltier in the 1830s, who observed the curious behaviour of two dissimilar metals when subjected to thermal gradients.

Over the ensuing decades, researchers and inventors around the world experimented with different metal combinations and configurations, seeking to harness the unique properties of bimetallic materials for practical purposes.

The advent of precision manufacturing techniques and metallurgical advancements further propelled the development and refinement of bimetallic strip technology, paving the way for its widespread adoption in miscellaneous industrial and commercial applications.

What are Bimetallic Strips? (Definition)

Metallic strips consisting of two dissimilar metal layers that are fused together and possess the unique ability to bend or deform in response to changes in temperature.

Principle of Bimetallic Strips

The idea of differential thermal expansion, which states that two dissimilar metals bonded together exhibit differing rates of expansion or contraction in response to temperature changes, forms the basis for the functioning of these strips.

The principle of these strips demonstrates the synergy between materials science and mechanical engineering, offering a versatile and reliable solution for thermal sensing, actuation, and control in diverse applications.

Composition of Bimetallic Strips

Typically, these strips are composed of two dissimilar metal layers that are bonded together to form a composite structure. The choice of metals for a strip depends on factors such as:

• thermal conductivity
• mechanical properties
• coefficient of thermal expansion

Some common combinations of these strips include;

• brass-steel
• steel-copper
• aluminium-steel

Each metal layer contributes unique characteristics to the strip, influencing its overall performance and behaviour under varying temperature conditions.

For example, the outer layer (brass) may possess a higher coefficient of thermal expansion (19×10⁻⁶ /℃), enabling greater deformation in response to temperature changes, while the inner layer (steel, 11×10⁻⁶ /℃) provides structural support and stability.

Structure and Design

The design of these strips is governed by principles of mechanical engineering and materials science, with careful consideration given to factors such as;

• thickness
• bonding technique
• geometry

By optimizing these parameters, engineers can mould the properties of a strip to meet specific application requirements, whether it be precise temperature sensing, actuation, or control.

How Do Bimetallic Strips Work?

These strips operate on the principle of differential thermal expansion. The distinct thermal response of two bonded metals experiences mechanical deformation in the composite structure due to temperature changes. In a wide array of devices and systems, this mechanism enables these strips to function as:

• actuators
• control mechanisms
• sensitive temperature sensors

Functional Anatomy of Bimetallic Strips

The inner workings of a strip are governed by the interplay between its constituent metals and their respective coefficients of thermal expansion.

A typically strip consists of two layers of different metals that are bonded together physically, and each layer exhibits unique thermal properties. When subjected to temperature variations, these metals either expand or contract at varying rates, resulting in the bending or curvature of the strip.

This bending of the strip controls the conduction process and thus finds its application in many household appliances and safety mechanisms.

Temperature Changes and Curvature of Bimetallic Strips

The curvature of a strip is directly influenced by temperature changes. When a bimetallic strip is heated, the strip bends towards the metal layer, exhibiting a higher coefficient of thermal expansion.

Conversely, cooling the strip causes the strip to bend in the opposite direction, as the metal layer with the lower coefficient of expansion holds greater influence.

This reversible bending behaviour forms the basis for the mechanical motion observed in bimetallic strips.

For example, the coefficients of linear expansion of steel and brass are 11×10⁻⁶ /C and 19×10⁻⁶ /C, respectively. Hence, upon heating, the strip will bend toward brass, as shown, and toward steel while cooling (not shown here).

Applications of Bimetallic Strips

These strips play a multidimensional role in our daily lives. They facilitate precise temperature control, safety monitoring, and protection against thermal hazards. They illustrate the versatility and reliability of thermal expansion principles in increasing functionality and safety.

These strips serve as integral components in various household appliances and also serve as critical components in various safety devices and mechanisms. Note that all these applications follow the “break the connection, build the connection” strategy.

Thermometers

A bimetallic thermometer employs a bimetallic strip as its core sensing element. This strip consists of two different metal layers bonded together. Typically, these layers are made of materials with different coefficients of thermal expansion (e.g., brass and steel).

Components of Bimetallic Thermometer

The components of a bimetallic thermometer include the following:

Helical Shape Strip

• The strip in a bimetallic thermometer is often shaped into a helix (coil) to enhance its sensitivity to temperature changes.
• When exposed to varying temperatures, the metal strip either expands or contracts at different rates, causing the strip to bend.

Pointer and Scale

• The bimetallic strip is connected mechanically to a pointer (needle) through a metallic rod.
• The pointer moves along a calibrated temperature scale.
• As the strip bends due to temperature fluctuations, the pointer’s position changes accordingly.

Calibrated Scale

• The temperature scale is carefully calibrated during the manufacturing process.
• Each division on the scale corresponds to a specific temperature value.
• For example, one division might represent 1 °C or 2 °F. The calibration ensures that the pointer accurately reflects the temperature.

Reading Temperature

• When the ambient temperature rises or falls, the bimetallic strip responds by bending or curving.
• As a result, the pointer moves to a new position on the scale, indicating the current temperature.
• Users can read the temperature directly on the scale.

Precision and Reliability

Bimetallic thermometers are known for their precision and reliability. They are commonly used in various applications, including household thermometers, industrial processes, and HVAC systems.

Thermostats

A thermostat is a crucial component in heating, ventilation, and air conditioning (HVAC) systems. Its primary function is to maintain a desired temperature within a space. Whether it is keeping our home cosy during the winter or ensuring a cool environment in the summer, thermostats play a vital role.

Bimetallic Strips in Thermostats

The heart of many thermostats is the bimetallic strip. This strip consists of two different metal layers bonded together. These metals have distinct coefficients of thermal expansion. When exposed to temperature changes, they expand or contract at different rates.

Temperature Regulation Process

The temperature-regulating process of a thermostat is discussed here:

Heating Mode

• Imagine we set the thermostat to a comfortable temperature (say 70 °F) during a cold winter.
• As the room temperature drops below 70 °F, the strip senses the change.

Bending of Bimetallic Strip

• The strip bends due to the differential expansion of its metal layers.
• This bending motion activates a switch connected to the thermostat.

Heating Activation

• The switch, in turn, signals the heating system (furnace, heat pump, etc.) to turn on.
• Warm air begins to circulate, gradually raising the room temperature.
• Once the room reaches the desired 70 °F, the strip straightens out, deactivating the heating system.

Cooling Mode

Now assume we want the room to stay cool in a hot summer at a comfortable temperature (say 75°F).

Bending of bimetallic strip

• As the room temperature rises above 75°F, the strip responds to this change in temperature.
• It bends in the opposite direction this time.

Cooling Activation

• The switch triggers the cooling system (an air conditioner or heat pump) to kick in.
• Cold air flows, lowering the room temperature.
• Once the room cools down to 75°F, the strip straightens and turns off the cooling system.

Accuracy and Calibration

Thermostats are carefully calibrated during manufacturing. Each division on the temperature scale corresponds to a specific temperature value. The behaviour of the bimetallic strip ensures precise temperature control.

Fire Alarms

Bimetallic strip is an integral components of fire alarms and play a vital role as a temperature sensor in early fire detection. When there is a fire, the high temperatures deform the strip, triggering an alarm, thus alerting the occupant to leave the premises of a building.

Working of Bimetallic Strips in Fire Alarms

The working of a heat-activated fire alarm with a bimetallic strip is discussed here:

Normal State

• Under normal conditions, the strip remains straight or slightly curved.
• It is positioned within the fire alarm unit.

Temperature Rise

• When a fire occurs nearby, the ambient temperature rises significantly.
• The heat reaches to the strip.
• Due to the differential expansion of its metal layers, the strip begins to bend.
• This bending motion is crucial for triggering the alarm.

Alarm Activation

• As the strip continues to bend, it eventually reaches a critical point.
• At this threshold temperature, the strip activates a switch or relay.
• The switch completes an electrical circuit, setting off the alarm.
• The alarm may include loud sirens, flashing lights, or both.

Alerting Occupants

• The activated fire alarm alerts occupants to the danger.
• People can evacuate the building promptly, minimising the risk of injury or loss of life.

Maintenance and Testing

Regular maintenance and testing are essential to ensure fire alarms function correctly. Check batteries, replace expired units, and verify alarm functionality periodically.

Electric Irons

Electric irons are household appliances used for removing wrinkles and creases from fabric through the application of heat and pressure. The main components of electric irons consist of:

• flat metal plate (typically coated with a non-stick material)
• handle (for easy manoeuvrability)
• heating element
• bimetallic strip

Bimetallic Strips in Electric Irons

Bimetallic strip is an integral components of electric irons, playing a crucial role in temperature regulation and safety. Within the iron, a bimetallic strip is strategically positioned to monitor and control the plate’s temperature.

Temperature Regulation Process

Here is how the strip operates within an electric iron:

Initial Heating

• When you plug in the iron and turn it on, electricity flows through the heating element, generating heat.
• As the temperature rises, the bimetallic strip, which is close to the heating element, begins to react.

Temperature Sensing

• The strip consists of two metal layers with different coefficients of thermal expansion.
• As the iron heats up, these layers expand at different rates, causing the strip to bend or curve.

Regulation

• The bending motion of the strip is crucial for controlling the temperature of the iron.
• If the temperature exceeds the desired level, the strip bends in a way that triggers a mechanism to reduce the heat output.

Cooling

• Once the iron reaches the set temperature or is turned off, the heating element cools down.
• The strip returns to its original shape, ensuring the iron remains at a safe operating temperature.

Safety Features

Electric irons are equipped with safety features to prevent overheating and potential hazards. The bimetallic strip serves as a reliable sensor for maintaining the temperature of iron within a safe range.

By incorporating these strips into their design, electric irons achieve precise temperature control, ensuring efficient wrinkle removal while safeguarding fabrics from damage.

Thermal Circuit Breakers

Thermal circuit breakers are essential safety devices found in electrical systems. Their primary purpose is to protect circuits and equipment from damage due to excessive current. When a circuit experiences an overload or short circuit, the thermal circuit breaker intervenes to prevent further harm.

Bimetallic Strips in Thermal Circuit Breakers

At the core of a thermal circuit breaker lies a bimetallic strip. This strip consists of two different metal layers bonded together. These metals have distinct coefficients of thermal expansion. When subjected to heat (caused by excessive current flow), they expand at different rates.

Overload Protection Process

Here we shall see how a thermal circuit breaker with a bimetallic strip works:

Normal Operation

• Under normal conditions, the electrical current flows through the circuit without any issues.
• The bimetallic strip remains in its original position.

Excessive Current Flow

• Suppose an overload situation due to a short circuit or excessive current.
• The increased current heats the bimetallic strip.
• As the strip’s temperature rises, the metals expand.
• The differential expansion causes the strip to bend.
• This bending motion triggers the circuit breaker mechanism.

Circuit Interruption

• The bent strip activates a switch or relay within the circuit breaker.
• The switch rapidly opens the circuit, breaking the electrical connection.
• By doing so, the thermal circuit breaker prevents further current flow.
• This interruption safeguards the wiring, appliances, and other components downstream.

Cooling and Resetting

• Once the excessive current situation is resolved (e.g., the short circuit is fixed), the strip cools down.
• As it cools, it returns to its original shape, allowing the circuit breaker to reset.
• We can manually reset the breaker to restore power.

Safety and Reliability

The behaviour of the bimetallic strip ensures that the thermal circuit breaker responds predictably to overloads. These breakers are widely used in residential, commercial, and industrial settings. Thermal circuit breakers play a critical role.

Conclusion

In conclusion, bimetallic strips represent a remarkable fusion of scientific principles and practical engineering, offering versatile solutions for temperature sensing, control, and safety applications.

Fundamentally, a bimetallic strip technology works on the principle of differential thermal expansion. According to this, two dissimilar metals bonded together exhibit distinct responses to temperature changes. As a result, there is a predictable mechanical deformation.

As we reflect on the significance of these strips, it becomes evident that their impact extends far beyond their humble appearance. These seemingly simple components embody the ingenuity of human creativity and scientific discovery, offering elegant solutions to complex challenges in an ever-evolving world.

Through the use of thermal expansion principles and the distinctive characteristics of bimetallic materials, we unlock new possibilities for enhancing efficiency, safety, and sustainability in diverse applications.

Frequently Asked Questions (FAQs)

What are bimetallic strips?

Bimetallic strips are comprised of two dissimilar metals that are bonded together. The two metals possess different coefficients of thermal expansion. When strips are subjected to temperature changes, they bend differently compared to the metals they are made up of.

How do bimetallic strips work? Or what is the mechanism behind bimetallic strips?

Bimetallic strips operate based on the principle of differential thermal expansion. When two bonded metals with different coefficients of expansion are heated, they respond differently to temperature changes, causing the strip to bend or deform.

Where are bimetallic strips commonly used?

Bimetallic strips find widespread application in household appliances, safety devices, industrial equipment, and educational demonstrations, among other fields.

Are bimetallic strips environmentally friendly?

While bimetallic strips offer valuable functionality, considerations regarding material composition, manufacturing processes, and end-of-life disposal should be taken into account to minimise environmental impact.

How are bimetallic strips used in thermostats?

Bimetallic strips are used in thermostats to regulate temperature.  When the temperature rises above a certain point, the strip bends, breaking or creating a contact to turn a heating or cooling system on or off.

Are bimetallic strips used in thermometers?

Yes, these strips are normally used in mechanical thermometers. As the temperature changes, the strip bends, causing the pointer on the thermometer scale to move and indicate the temperature.

How do bimetallic strips contribute to the safety of electrical appliances?

They play a crucial role in improving safety by preventing overheating in electrical appliances. They help regulate temperature and can trigger shutdown mechanisms when necessary.

Can bimetallic strips be adjusted for different temperature thresholds?

Yes, the properties of the metals and the dimensions of the strip can be adjusted to tailor the bending behaviour to specific temperature thresholds, making them versatile for various applications.

Are there any limitations or drawbacks to using bimetallic strips?

While bimetallic strips offer simplicity and reliability, they may not be as precise as electronic sensors in some applications. Additionally, factors like mechanical fatigue over time can affect their performance.

If two metals have different coefficients of expansion, in which direction will the bimetallic strip bend?

A bimetallic strip will bend in the direction of the metal with a smaller coefficient of expansion when heated.

The strip bends toward the metal with the lower coefficient because of the difference in expansion rates between the two metals, which happens because the lower metal expands less than the other metal. in preventing electrical fires and equipment damage. They’re an essential part of electrical safety protocols.