7.8.5 Bimetallic Strip Thermometers

Have you ever wondered how a thermostat knows when to turn the heating on or off? Or how an old-fashioned oven thermometer works without any liquid or electronics? The answer, in many cases, is a bimetallic strip. This simple but ingenious device uses the different expansion rates of two metals to measure temperature. It is one of the most common and reliable thermometric devices, found in everything from household thermostats to industrial control systems.

Definition

A Bimetallic Strip Thermometer is a temperature-measuring device that uses the differential thermal expansion of two different metals bonded together to indicate temperature. When heated, the two metals expand by different amounts, causing the strip to bend. The amount of bending is proportional to the temperature change.

Main Idea

The main idea is that different metals expand at different rates when heated. When two metals with different expansion rates are bonded together into a strip, heating the strip causes one metal to expand more than the other. This differential expansion causes the strip to bend. The amount of bending is predictable and can be calibrated to indicate temperature. Bimetallic strips are used in thermometers, thermostats, and many other temperature-sensitive devices.

Core Concept

The Principle The text states: "It can measure temperature by variation in volume expansion of [two different metals]."

• Thermometric Property: Differential thermal expansion of two different metals.
• Mechanism: Two metals with different coefficients of thermal expansion are bonded together. When heated, the metal with the higher expansion coefficient expands more, causing the strip to bend toward the metal with the lower expansion coefficient.

Construction A bimetallic strip consists of two thin strips of different metals (e.g., brass and steel, or copper and iron) bonded together along their entire length. The strip is usually coiled into a spiral or formed into a straight strip, depending on the application.

• When the temperature changes, both metals expand or contract.
• Because the metals have different expansion rates, one expands more than the other.
• This causes the strip to bend (or the spiral to coil/uncoil).
• The movement of the strip is used to move a pointer on a scale, or to open/close an electrical contact.

Applications The text mentions bimetallic strips as a thermometric property. Common applications include:

• A pointer is attached to the end of the strip. As the strip bends, the pointer moves across a calibrated scale, indicating the temperature.
• Used in ovens, room thermometers, and industrial dial thermometers.

• The bimetallic strip is part of an electrical circuit. When the temperature reaches a set point, the strip bends and opens or closes the circuit, turning a heating or cooling device on or off.
• Used in home heating systems, air conditioners, refrigerators, and ovens.

3. Bimetallic Strip Thermometers in Cars: - Used to measure engine coolant temperature and other temperatures.

4. Electrical Switches and Circuit Breakers: - Used to protect circuits from overheating. When the temperature rises too high, the strip bends and breaks the circuit.

• Simple and robust: No liquids, no electronics, no power required.
• Inexpensive: Cheap to manufacture.
• Wide temperature range: Can be designed for various temperature ranges.
• Direct mechanical output: The bending can be used directly to move a pointer or operate a switch.
• Reliable: Few failure modes.

• Limited accuracy: Not as accurate as liquid-in-glass or resistance thermometers.
• Calibration drift: The strip can deform permanently over time or with repeated temperature cycling.
• Limited range: The range is limited by the properties of the metals used.
• Slow response: Takes time to reach thermal equilibrium.
• Mechanical wear: Moving parts can wear out.

Detailed Explanation

The bimetallic strip thermometer is a classic example of using a physical property (thermal expansion) to measure temperature.

• Brass: α ≈ 19 × 10^-6 K^-1
• Steel: α ≈ 12 × 10^-6 K^-1

If brass and steel are bonded together and heated, the brass expands more than the steel. This causes the strip to bend toward the steel side (the side with lower expansion).

• The difference in expansion coefficients of the two metals.
• The length of the strip.
• The temperature change.
• The thickness of the strips.

By choosing appropriate metals and dimensions, the strip can be designed to bend by a specific amount for a given temperature change.

Bimetallic Strip in a Thermostat In a thermostat, the bimetallic strip is part of an electrical circuit. When the temperature falls below a set point, the strip bends and closes the circuit, turning on the heater. When the temperature rises above the set point, the strip bends the other way and opens the circuit, turning off the heater. This is a simple and effective way to control temperature.

Bimetallic Strip in a Dial Thermometer In a dial thermometer, the bimetallic strip is coiled into a spiral. When heated, the spiral coils or uncoils, rotating a pointer on a dial. The dial is calibrated to indicate temperature.

Advantages Over Other Thermometers Bimetallic strip thermometers are simple, rugged, and require no power. They are ideal for applications where a direct mechanical indication or control is needed (e.g., thermostats, oven thermometers).

Limitations The main limitation is accuracy. Bimetallic strip thermometers are typically accurate to about ±1°C to ±2°C, which is sufficient for many applications but not for precise scientific measurements.

Intermediate Level

In physics, a bimetallic strip thermometer uses the differential thermal expansion of two different metals. The two metals (e.g., brass and steel) are bonded together. When heated, the metal with the higher expansion coefficient expands more, causing the strip to bend. The amount of bending is proportional to the temperature change.

Bimetallic strips are used in thermometers (with a pointer and scale) and in thermostats (as an electrical switch). They are simple, robust, and inexpensive, but they are not as accurate as other types of thermometers. Their accuracy is typically ±1°C to ±2°C.

The bending of the strip can be calculated using the formula for a bimetallic strip, which depends on the difference in expansion coefficients, the temperature change, and the dimensions of the strip.

Important Points

• - Bimetallic Strip: Two different metals bonded together.
• - Thermometric Property: Differential thermal expansion.
• - Mechanism: Heating causes the strip to bend due to different expansion rates.
• - Applications: Thermometers, thermostats, electrical switches.
• - Advantages: Simple, robust, inexpensive, no power required.
• - Limitations: Limited accuracy, calibration drift, slow response.

Remember

• - Bimetallic Strip: Two metals bonded together.
• - Principle: Differential expansion.
• - Bending: Proportional to temperature change.
• - Applications: Thermometers, thermostats, switches.
• - Advantages: Simple, robust, cheap.
• - Limitations: Limited accuracy.

Common Mistakes

• - Thinking both metals expand by the same amount: They expand by different amounts, which is what causes the bending.
• - Thinking bimetallic strips are very accurate: They are not as accurate as liquid-in-glass or resistance thermometers. They are best for control applications (thermostats) and indicative measurements.
• - Forgetting that the strip can be coiled: Coiling the strip amplifies the movement, making it easier to read on a dial.

Quick Revision

A bimetallic strip thermometer uses two different metals bonded together. When heated, the metals expand by different amounts, causing the strip to bend. The amount of bending is proportional to the temperature change and can be used to move a pointer or operate a switch. Bimetallic strips are used in thermometers, thermostats, and electrical switches. They are simple, robust, and inexpensive, but their accuracy is limited compared to other types of thermometers.

60-Second Revision

• Principle: Differential expansion.
• Bending: Proportional to temperature.
• Applications: Thermometers, thermostats, switches.
• Advantages: Simple, robust, cheap.
• Limitations: Limited accuracy.

Frequently Asked Questions (FAQs)

Q1. What is a bimetallic strip? Answer: A bimetallic strip is a device made of two different metals bonded together. Because the metals have different expansion rates, the strip bends when heated. This bending is used to measure temperature or operate a switch.

Q2. How does a bimetallic strip thermometer work? Answer: When the strip is heated, one metal expands more than the other, causing the strip to bend. The amount of bending is proportional to the temperature change. A pointer attached to the strip moves across a calibrated scale to indicate the temperature.

Q3. What are the applications of bimetallic strips? Answer: Bimetallic strips are used in thermometers, thermostats (to control heating and cooling), electrical switches and circuit breakers (to protect against overheating), and oven thermometers.

• Q4. What are the advantages of bimetallic strip thermometers?
• Answer: Advantages include simplicity, robustness, low cost, no power requirement, and direct mechanical output.

• Q5. What are the limitations of bimetallic strip thermometers?
• Answer: Limitations include limited accuracy, calibration drift over time, slow response, and a limited temperature range.

Q6. Why does the bimetallic strip bend? Answer: The strip bends because the two metals have different coefficients of thermal expansion. When heated, the metal with the higher expansion coefficient expands more, causing the strip to curve toward the metal with the lower expansion coefficient.

Q7. What metals are commonly used in bimetallic strips? Answer: Common combinations include brass and steel, copper and steel, and nickel-iron alloys. The choice depends on the required temperature range and expansion characteristics.

Multiple Choice Questions (MCQs)

• Q1. A bimetallic strip consists of:
• A. One metal
• B. Two different metals bonded together
• C. Two identical metals
• D. A metal and a plastic

• Correct Answer: B
• Explanation: The text states that a bimetallic strip measures temperature by variation in volume expansion of two different metals.

• Q2. The bending of a bimetallic strip is due to:
• A. The color of the metals
• B. The different expansion rates of the two metals
• C. The weight of the metals
• D. The electrical conductivity of the metals

• Correct Answer: B
• Explanation: The two metals have different coefficients of thermal expansion, causing the strip to bend when heated.

• Q3. A bimetallic strip is used in:
• A. Liquid-in-glass thermometers
• B. Thermistors
• C. Thermostats
• D. Gas thermometers

• Correct Answer: C
• Explanation: Bimetallic strips are commonly used in thermostats to control temperature.

• Q4. When a bimetallic strip is heated, it bends toward:
• A. The metal with the higher expansion coefficient
• B. The metal with the lower expansion coefficient
• C. The metal with the higher melting point
• D. The metal with the lower density

• Correct Answer: B
• Explanation: The metal with the higher expansion coefficient expands more, causing the strip to curve toward the metal with the lower expansion coefficient.

• Q5. What is the main advantage of a bimetallic strip thermometer?
• A. High accuracy
• B. Simple and robust
• C. Wide temperature range
• D. Fast response

• Correct Answer: B
• Explanation: Bimetallic strips are simple, robust, and require no power.

• Q6. What is a limitation of bimetallic strip thermometers?
• A. They require electricity
• B. They are very expensive
• C. They have limited accuracy
• D. They are fragile

• Correct Answer: C
• Explanation: Bimetallic strip thermometers have limited accuracy compared to other types of thermometers.

• Q7. In a thermostat, the bimetallic strip is used to:
• A. Display the temperature
• B. Open or close an electrical circuit
• C. Measure pressure
• D. Measure humidity

• Correct Answer: B
• Explanation: In a thermostat, the bimetallic strip bends to open or close an electrical circuit, controlling the heating or cooling system.

• Q8. A bimetallic strip thermometer is often used in:
• A. Medical thermometers
• B. Ovens and room thermometers
• C. Laboratory experiments
• D. Deep-freeze monitoring

• Correct Answer: B
• Explanation: Bimetallic strip thermometers are commonly used in ovens and room thermometers.

• Q9. The thermometric property of a bimetallic strip is:
• A. Electrical resistance
• B. Volume expansion of a liquid
• C. Differential thermal expansion of two metals
• D. Color change

• Correct Answer: C
• Explanation: The thermometric property is the differential thermal expansion of two different metals.

• Q10. Which of the following is NOT an application of bimetallic strips?
• A. Thermostats
• B. Oven thermometers
• C. Circuit breakers
• D. Digital thermometers

Correct Answer: D Explanation: Digital thermometers typically use thermistors or RTDs, not bimetallic strips. Bimetallic strips are used in thermostats, oven thermometers, and circuit breakers.

Frequently Asked Questions (FAQs)

Q1. How does a bimetallic strip thermostat work? Answer: A bimetallic strip is part of an electrical circuit. When the temperature falls below a set point, the strip bends and closes the circuit, turning on the heater. When the temperature rises above the set point, the strip bends the other way and opens the circuit, turning off the heater.

Q2. What is the difference between a bimetallic strip thermometer and a liquid-in-glass thermometer? Answer: A bimetallic strip thermometer uses the differential expansion of two metals to bend a strip, which moves a pointer. A liquid-in-glass thermometer uses the expansion of a liquid in a glass tube. Bimetallic strips are more robust and require no liquid, but they are less accurate.

• Q3. Why is a bimetallic strip coiled in some thermometers?
• Answer: Coiling the strip amplifies the movement. A small change in temperature causes a larger rotation of the coil, making it easier to read on a dial.

• Q4. What is the typical accuracy of a bimetallic strip thermometer?
• Answer: Bimetallic strip thermometers typically have an accuracy of about ±1°C to ±2°C.

Q5. Can a bimetallic strip be used to measure very high temperatures? Answer: Yes, bimetallic strips can be designed to measure high temperatures, depending on the metals used. However, they are more commonly used for moderate temperature ranges (e.g., -50°C to 500°C).

Q6. What is calibration drift in a bimetallic strip? Answer: Calibration drift is the gradual change in the strip's calibration over time, caused by repeated temperature cycling or mechanical stress. This can lead to inaccurate readings.

Q7. What metals are commonly used in bimetallic strips? Answer: Common combinations include brass and steel, copper and steel, and nickel-iron alloys. The choice depends on the required temperature range and expansion characteristics.

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