Resistance Thermometers and Thermistors

I. Introduction

Resistance thermometers and thermistors are important components in measurement systems, particularly for temperature measurement. They are widely used in various industries such as industrial processes, HVAC systems, medical applications, and the automotive industry. In this topic, we will explore the fundamentals of resistance thermometers and thermistors, their types, resistance-temperature characteristics, and their applications.

II. Resistance Thermometers

Resistance thermometers are temperature sensors that utilize the change in electrical resistance with temperature. They are commonly used in applications that require high accuracy and stability. There are two main types of resistance thermometers: metal resistance thermometers and semiconductor resistance thermometers.

A. Definition and Working Principle

Resistance thermometers work on the principle that the electrical resistance of a material changes with temperature. This change in resistance can be measured and correlated to the temperature.

B. Types of Resistance Thermometers

1. Metal Resistance Thermometers

Metal resistance thermometers are made of pure metals or metal alloys. They are known for their high accuracy and stability. The most commonly used metal resistance thermometer is the platinum resistance thermometer (PRT).

a. Construction and Materials Used

Metal resistance thermometers consist of a wire made of a pure metal or metal alloy, such as platinum, nickel, or copper. The wire is wound into a coil and enclosed in a protective sheath.

b. Temperature Range and Accuracy

Metal resistance thermometers can measure temperatures ranging from -200°C to 1000°C with high accuracy.

c. Advantages and Disadvantages

Advantages of metal resistance thermometers include high accuracy, stability, and a wide temperature range. However, they are more expensive compared to other types of temperature sensors.

2. Semiconductor Resistance Thermometers

Semiconductor resistance thermometers are made of semiconductor materials, such as silicon or germanium. They are known for their small size, fast response time, and low cost.

a. Construction and Materials Used

Semiconductor resistance thermometers consist of a semiconductor material, such as silicon or germanium, with impurities added to control the electrical properties.

b. Temperature Range and Accuracy

Semiconductor resistance thermometers can measure temperatures ranging from -50°C to 150°C with good accuracy.

c. Advantages and Disadvantages

Advantages of semiconductor resistance thermometers include small size, fast response time, and low cost. However, they have a limited temperature range compared to metal resistance thermometers.

C. Resistance-Temperature Characteristics

The resistance of a resistance thermometer changes with temperature according to a specific relationship. This relationship can be described by the resistance-temperature characteristics.

1. Relationship between Resistance and Temperature

The relationship between resistance and temperature can be linear or non-linear, depending on the type of resistance thermometer.

2. Calibration and Temperature Coefficients

Resistance thermometers need to be calibrated to ensure accurate temperature measurement. Calibration involves determining the relationship between resistance and temperature at various points.

3. Linearization Techniques

Non-linear resistance-temperature characteristics can be linearized using mathematical techniques such as polynomial fitting or look-up tables.

III. Thermistors

Thermistors are temperature sensors that utilize the change in electrical resistance with temperature. They are known for their high sensitivity and fast response time. There are two main types of thermistors: NTC (Negative Temperature Coefficient) thermistors and PTC (Positive Temperature Coefficient) thermistors.

A. Definition and Working Principle

Thermistors work on the principle that the electrical resistance of a material changes with temperature. This change in resistance can be measured and correlated to the temperature.

B. Types of Thermistors

1. NTC (Negative Temperature Coefficient) Thermistors

NTC thermistors are made of semiconductor materials, such as metal oxides. They are known for their high sensitivity and negative temperature coefficient, which means that their resistance decreases as the temperature increases.

a. Construction and Materials Used

NTC thermistors are made of metal oxides, such as manganese, nickel, or cobalt, mixed with a ceramic binder. The mixture is then formed into a desired shape and sintered at high temperatures.

b. Temperature Range and Accuracy

NTC thermistors can measure temperatures ranging from -50°C to 150°C with good accuracy.

c. Advantages and Disadvantages

Advantages of NTC thermistors include high sensitivity, fast response time, and low cost. However, they have a limited temperature range compared to other types of temperature sensors.

2. PTC (Positive Temperature Coefficient) Thermistors

PTC thermistors are made of semiconductor materials, such as doped ceramics. They are known for their positive temperature coefficient, which means that their resistance increases as the temperature increases.

a. Construction and Materials Used

PTC thermistors are made of doped ceramics, such as barium titanate or lead titanate. The ceramics are mixed with a binder, formed into a desired shape, and sintered at high temperatures.

b. Temperature Range and Accuracy

PTC thermistors can measure temperatures ranging from -50°C to 150°C with good accuracy.

c. Advantages and Disadvantages

Advantages of PTC thermistors include high sensitivity, fast response time, and low cost. However, they have a limited temperature range compared to other types of temperature sensors.

C. Resistance-Temperature Characteristics

The resistance of a thermistor changes with temperature according to a specific relationship. This relationship can be described by the resistance-temperature characteristics.

1. Relationship between Resistance and Temperature

The relationship between resistance and temperature for thermistors is highly non-linear.

2. Steinhart-Hart Equation

The Steinhart-Hart equation is a mathematical model that describes the resistance-temperature characteristics of thermistors.

3. Calibration and Temperature Coefficients

Thermistors need to be calibrated to ensure accurate temperature measurement. Calibration involves determining the relationship between resistance and temperature at various points.

4. Linearization Techniques

Non-linear resistance-temperature characteristics can be linearized using mathematical techniques such as the Steinhart-Hart equation or look-up tables.

IV. Applications of Resistance Thermometers and Thermistors

Resistance thermometers and thermistors have various applications in different industries. Some of the common applications include:

A. Temperature Measurement in Industrial Processes

Resistance thermometers and thermistors are widely used for temperature measurement in industrial processes, such as chemical reactions, manufacturing processes, and heat treatment.

B. HVAC Systems and Temperature Control

Resistance thermometers and thermistors are used in heating, ventilation, and air conditioning (HVAC) systems for temperature control and monitoring.

C. Medical Applications

Resistance thermometers and thermistors are used in medical applications, such as patient monitoring, laboratory equipment, and temperature-sensitive medications.

D. Automotive Industry

Resistance thermometers and thermistors are used in the automotive industry for engine temperature monitoring, climate control systems, and battery temperature monitoring.

V. Advantages and Disadvantages of Resistance Thermometers and Thermistors

Resistance thermometers and thermistors have their own advantages and disadvantages.

A. Advantages

1. High Accuracy and Sensitivity: Resistance thermometers and thermistors offer high accuracy and sensitivity, making them suitable for applications that require precise temperature measurement.

2. Wide Temperature Range: Resistance thermometers and thermistors can measure a wide range of temperatures, from -200°C to 1000°C for metal resistance thermometers and from -50°C to 150°C for thermistors.

3. Small Size and Easy Installation: Resistance thermometers and thermistors are compact in size and can be easily installed in various systems.

4. Low Cost: Resistance thermometers and thermistors are relatively low-cost compared to other temperature sensors.

B. Disadvantages

1. Non-linear Resistance-Temperature Characteristics: Resistance thermometers and thermistors have non-linear resistance-temperature characteristics, which require additional calibration and linearization techniques.

2. Limited Self-Heating Capabilities: Resistance thermometers and thermistors have limited self-heating capabilities, which may affect their response time and accuracy.

3. Susceptible to Environmental Factors: Resistance thermometers and thermistors may be affected by environmental factors such as humidity, which can introduce errors in temperature measurement.

VI. Conclusion

Resistance thermometers and thermistors are important components in measurement systems, particularly for temperature measurement. They offer high accuracy, sensitivity, and a wide temperature range. However, they have non-linear resistance-temperature characteristics that require calibration and linearization techniques. Resistance thermometers and thermistors find applications in various industries such as industrial processes, HVAC systems, medical applications, and the automotive industry.

In conclusion, understanding the principles and characteristics of resistance thermometers and thermistors is essential for accurate temperature measurement and control in various applications.

Summary

Resistance thermometers and thermistors are important components in measurement systems, particularly for temperature measurement. They offer high accuracy, sensitivity, and a wide temperature range. However, they have non-linear resistance-temperature characteristics that require calibration and linearization techniques. Resistance thermometers and thermistors find applications in various industries such as industrial processes, HVAC systems, medical applications, and the automotive industry.

Analogy

Resistance thermometers and thermistors can be compared to thermometers used for measuring body temperature. Just like a thermometer measures body temperature by using the expansion of a liquid, resistance thermometers and thermistors measure temperature by utilizing the change in electrical resistance. The resistance of the sensor changes with temperature, allowing us to accurately measure the temperature of the system.