7.8.4 Resistance Thermometers or Thermistors

In our modern world, many temperature measurements are made electronically. Digital thermometers, which are everywhere from hospitals to kitchens, rely on the fact that the electrical resistance of certain materials changes predictably with temperature. These devices, called resistance thermometers or thermistors, are among the most accurate and reliable temperature sensors available.

Definition

Resistance Thermometers (or Thermistors) are temperature-measuring devices that work on the principle that the electrical resistance of a material changes with temperature. By measuring the resistance of a material, the temperature can be determined.

Main Idea

The main idea is that the electrical resistance of a material varies with temperature. In a resistance thermometer, a material (usually a metal like platinum or a semiconductor) is used as the sensing element. As the temperature changes, the resistance of the material changes in a predictable way. By measuring this resistance (using a circuit), the temperature can be accurately determined.

Core Concept

The Principle The text states: "It can measure temperature due to change in its resistance (change in opposition to flow of current through it) due to variation of its temperature."

• Resistance: The opposition to the flow of electric current.
• Thermometric Property: The change in electrical resistance with temperature.

• Resistance Temperature Detectors (RTDs): Made of metals, usually platinum, which have a positive temperature coefficient (resistance increases with temperature). They are very accurate and stable.
• Thermistors: Made of semiconductor materials (metal oxides) that have a large (and usually negative) temperature coefficient (resistance decreases with temperature). They are very sensitive but less linear than RTDs.

• A small current is passed through the sensing element (the resistor or thermistor).
• The voltage across the element is measured.
• By Ohm's law (V = IR), the resistance is calculated.
• The resistance is then converted to a temperature using a calibrated relationship.

Applications The text states: "It is used to measure temperature."

• Digital thermometers: Used in medical thermometers, weather stations, and industrial thermometers.
• Thermostats: Used to control heating and cooling systems.
• Automotive: Used to measure engine coolant temperature, intake air temperature, etc.
• Industrial processes: Used in manufacturing, chemical processing, and food production.
• Electronics: Used to monitor the temperature of electronic components (e.g., CPUs).

• High accuracy: RTDs are among the most accurate temperature sensors available.
• Wide range: Can measure from very low temperatures (cryogenic) to high temperatures.
• Stability: RTDs are very stable over time.
• Fast response: Thermistors can respond quickly to temperature changes.
• Electronic output: Easy to integrate with digital circuits and displays.

• Non-linear: Thermistors are highly non-linear, requiring calibration or linearization circuitry.
• Self-heating: The current passing through the sensor can cause it to heat up, affecting the reading. This is minimized by using a small current.
• Limited range: Thermistors have a limited temperature range (typically -50°C to 150°C).
• Cost: RTDs (especially platinum) can be expensive.

Detailed Explanation

Resistance thermometers are widely used in science and industry because they are accurate, reliable, and easy to interface with electronic measuring systems.

Resistance Temperature Detectors (RTDs) RTDs are made of pure metals, most commonly platinum. Platinum has a very stable and nearly linear resistance-temperature relationship over a wide range. The most common type is the PT100, which has a resistance of 100 ohms at 0°C.

The resistance of a metal increases with temperature because the atoms vibrate more vigorously, scattering the electrons and impeding their flow.

Thermistors Thermistors are made of semiconductor materials (metal oxides like manganese, nickel, and cobalt). They have a much larger temperature coefficient than metals, making them very sensitive. However, their resistance-temperature relationship is highly non-linear.

• NTC (Negative Temperature Coefficient): Resistance decreases as temperature increases. These are the most common type of thermistor.
• PTC (Positive Temperature Coefficient): Resistance increases as temperature increases.

The Measurement Circuit A resistance thermometer is typically connected in a circuit (like a Wheatstone bridge) to measure its resistance accurately. The resistance is then converted to a temperature using a calibration curve or equation.

• High accuracy and precision: Especially RTDs.
• Good stability: Platinum RTDs are very stable over time.
• Wide temperature range: From cryogenic temperatures to high temperatures.
• Electronic output: Easy to connect to digital displays and computers.

• Self-heating: The current through the sensor can cause it to heat up. This is a source of error.
• Non-linearity: Thermistors require calibration or linearization.
• Cost: Platinum RTDs are relatively expensive.

Intermediate Level

In physics, resistance thermometers and thermistors work on the principle that the electrical resistance of a material changes with temperature. For metals (RTDs), resistance increases with temperature due to increased atomic vibrations scattering electrons. For semiconductors (thermistors), resistance usually decreases with temperature because more charge carriers are released.

RTDs (like PT100) are very accurate and stable, making them ideal for precision measurements and calibration. Thermistors are more sensitive but less linear and have a narrower temperature range. They are used in applications requiring high sensitivity and fast response, such as digital thermometers and electronic thermostats.

The resistance is measured using a circuit (e.g., a Wheatstone bridge or a voltage divider), and the temperature is calculated using a known relationship (e.g., the Callendar-Van Dusen equation for RTDs).

Important Points

• - Resistance Thermometers: Use change in electrical resistance with temperature.
• - RTDs: Metal (platinum) sensors, positive temperature coefficient, very accurate.
• - Thermistors: Semiconductor sensors, usually negative temperature coefficient, very sensitive but non-linear.
• - Applications: Digital thermometers, thermostats, automotive, industrial.
• - Advantages: High accuracy, wide range, electronic output.
• - Limitations: Self-heating, non-linearity (thermistors), cost (RTDs).

Remember

• - Resistance Thermometers: Resistance changes with temperature.
• - RTD: Platinum, positive coefficient, accurate.
• - Thermistor: Semiconductor, negative coefficient, sensitive.
• - Applications: Digital thermometers, thermostats.
• - Advantages: Accurate, electronic output.
• - Limitations: Self-heating, non-linearity.

Common Mistakes

• - Thinking all resistance thermometers are the same: RTDs and thermistors have different characteristics (linearity, sensitivity, range).
• - Forgetting self-heating: The current used to measure resistance can heat the sensor, causing an error. This is minimized by using a small current and pulsed measurement.
• - Assuming thermistors are linear: Thermistors have a highly non-linear resistance-temperature relationship, requiring calibration.

Quick Revision

Resistance thermometers and thermistors are temperature sensors that work on the principle that the electrical resistance of a material changes with temperature. RTDs use metals (like platinum) and have a positive temperature coefficient. Thermistors use semiconductors and usually have a negative temperature coefficient. They are used in digital thermometers, thermostats, and many other electronic devices because of their accuracy and ease of integration with electronic circuits.

60-Second Revision

• RTD: Platinum, positive coefficient, accurate.
• Thermistor: Semiconductor, negative coefficient, sensitive.
• Applications: Digital thermometers, thermostats.
• Advantages: Accurate, electronic output.
• Limitations: Self-heating, non-linearity.

Frequently Asked Questions (FAQs)

Q1. What is a resistance thermometer? Answer: A resistance thermometer is a device that measures temperature by measuring the change in electrical resistance of a material. The resistance changes predictably with temperature.

Q2. What is the difference between an RTD and a thermistor? Answer: An RTD (Resistance Temperature Detector) is made of a metal (usually platinum) and has a positive temperature coefficient (resistance increases with temperature). It is very accurate and stable. A thermistor is made of a semiconductor and usually has a negative temperature coefficient (resistance decreases with temperature). It is more sensitive but less linear.

Q3. How does a resistance thermometer work? Answer: A small current is passed through the sensing element, and the voltage across it is measured. The resistance is calculated using Ohm's law (R = VI), and the temperature is determined from a known resistance-temperature relationship.

• Q4. What are the applications of resistance thermometers?
• Answer: They are used in digital thermometers, thermostats, automotive sensors, industrial processes, and electronic devices.

• Q5. What are the advantages of resistance thermometers?
• Answer: Advantages include high accuracy, wide temperature range, good stability (RTDs), fast response (thermistors), and easy electronic output.

• Q6. What are the limitations of resistance thermometers?
• Answer: Limitations include self-heating (caused by the measurement current), non-linearity (thermistors), and cost (RTDs).

• Q7. What is a PT100?
• Answer: A PT100 is a type of RTD made of platinum that has a resistance of 100 ohms at 0°C. It is a very common and accurate temperature sensor.

Multiple Choice Questions (MCQs)

• Q1. Resistance thermometers measure temperature by measuring changes in:
• A. Volume
• B. Pressure
• C. Electrical resistance
• D. Color

• Correct Answer: C
• Explanation: The text states that resistance thermometers measure temperature due to change in resistance.

• Q2. An RTD is typically made of:
• A. Silicon
• B. Platinum
• C. Carbon
• D. Glass

• Correct Answer: B
• Explanation: RTDs are typically made of platinum, which has a stable and nearly linear resistance-temperature relationship.

• Q3. A thermistor is typically made of:
• A. Metal
• B. Semiconductor material
• C. Glass
• D. Plastic

• Correct Answer: B
• Explanation: Thermistors are made of semiconductor materials (metal oxides).

• Q4. The resistance of an NTC thermistor:
• A. Increases with temperature
• B. Decreases with temperature
• C. Remains constant with temperature
• D. Is zero at all temperatures

• Correct Answer: B
• Explanation: NTC (Negative Temperature Coefficient) thermistors have resistance that decreases as temperature increases.

• Q5. The resistance of a metal RTD:
• A. Increases with temperature
• B. Decreases with temperature
• C. Remains constant with temperature
• D. Is infinite at all temperatures

• Correct Answer: A
• Explanation: Metals have a positive temperature coefficient, so their resistance increases with temperature.

• Q6. Which of the following is an application of resistance thermometers?
• A. Fever strips
• B. Digital thermometers
• C. Liquid-in-glass thermometers
• D. Bimetallic strips

• Correct Answer: B
• Explanation: Resistance thermometers (thermistors) are used in digital thermometers.

• Q7. What is a limitation of thermistors?
• A. They are very linear
• B. They have a highly non-linear response
• C. They have a very wide range
• D. They are very expensive

• Correct Answer: B
• Explanation: Thermistors have a highly non-linear resistance-temperature relationship.

• Q8. The self-heating effect in resistance thermometers is caused by:
• A. The ambient temperature
• B. The current passing through the sensor
• C. The material of the sensor
• D. The color of the sensor

• Correct Answer: B
• Explanation: The current passing through the sensor can cause it to heat up, affecting the reading.

• Q9. A PT100 sensor has a resistance of 100 ohms at:
• A. 0°C
• B. 100°C
• C. 273°C
• D. -100°C

• Correct Answer: A
• Explanation: A PT100 sensor has a resistance of 100 ohms at 0°C.

• Q10. Resistance thermometers are popular because they:
• A. Are cheap
• B. Provide an electronic output easy to interface
• C. Have no limitations
• D. Are large and bulky

• Correct Answer: B
• Explanation: Resistance thermometers provide an electronic output (resistance/voltage) that is easy to interface with digital circuits and displays.

Frequently Asked Questions (FAQs)

Q1. What is the difference between a thermistor and an RTD? Answer: An RTD (Resistance Temperature Detector) uses a metal (like platinum) and has a positive, nearly linear temperature coefficient. A thermistor uses a semiconductor and has a large, usually negative, non-linear temperature coefficient. RTDs are more accurate and stable; thermistors are more sensitive and cheaper.

Q2. Why is platinum used in RTDs? Answer: Platinum is used because it is chemically stable, has a high melting point, and its resistance changes in a very predictable and nearly linear way with temperature.

Q3. What is the self-heating effect? Answer: The self-heating effect is the error caused by the current passing through the resistance thermometer. The current can heat the sensor above the ambient temperature, leading to a reading that is too high. This is minimized by using a small current.

Q4. How is the resistance of a thermistor converted to temperature? Answer: The resistance is measured, and the temperature is calculated using a calibration equation (e.g., the Steinhart-Hart equation) or a look-up table that relates resistance to temperature.

• Q5. What is the typical temperature range of a thermistor?
• Answer: Thermistors typically have a temperature range of about -50°C to 150°C, although some can go higher or lower.

Q6. Why are resistance thermometers preferred over liquid-in-glass thermometers in industry? Answer: Resistance thermometers provide an electrical output that can be easily digitized, displayed, recorded, and used for control. They are also more accurate, more durable, and can be used for remote monitoring.

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