Thermoelectric Transducers: Principles, Types, and Applications

Introduction

A temperature transducer is a device that converts a thermal quantity (temperature) into another physical quantity, such as mechanical energy, pressure, or an electrical signal, to allow for measurement or control. This conversion enables temperature to be easily monitored and utilised in various applications.

A thermocouple is a common type of temperature transducer. It works based on the Seebeck effect, where an electric potential difference (voltage) is generated between two dissimilar metals joined at two points, with a temperature difference across these points. This voltage is directly related to the temperature difference, enabling temperature measurement.

Types of Temperature Transducers

Temperature Sensors can be broadly categorised into Contact and Non-Contact types, based on how they interact with the thermal source.

Contact Temperature Sensors

In contact temperature sensors, the sensing element is in direct contact with the thermal source, meaning that heat is transferred through conduction. These sensors are ideal for measuring the temperature of solids, liquids, or gases that are in direct physical contact with the sensor.

Examples of Contact Temperature Sensors:

• Thermocouples: Generate a voltage based on the temperature difference across two metal junctions.
• Resistance Temperature Detectors (RTDs): Change their electrical resistance with temperature.
• Thermistors: Similar to RTDs but use semiconductor materials for more sensitivity in a specific temperature range.
• Bimetallic Strips: Use the expansion of two different metals to measure temperature mechanically.

Non-Contact Temperature Sensors

In non-contact temperature sensors, the sensing element does not touch the thermal source. Instead, these sensors use convection or radiation to sense temperature changes from a distance. Non-contact sensors are well-suited for measuring the temperature of moving objects or sources that are difficult to access.

Examples of Non-Contact Temperature Sensors:

• Infrared (IR) Sensors: Detect the thermal radiation emitted by an object, which correlates with temperature.
• Thermal Cameras: Capture infrared radiation across a surface to create a visual map of temperature variations.

Thermistor

A Thermistor (short for Thermal Resistor) is a temperature-sensitive device whose resistance varies with temperature. Due to their high sensitivity, thermistors are considered ideal for precise temperature measurement and are widely used as temperature transducers.

Key Properties of Thermistors

Most thermistors have an Negative Temperature Coefficient (NTC), meaning their resistance decreases as temperature increases. This makes them especially responsive to small temperature changes. Thermistors are made from metallic oxide mixtures combined with other materials, giving them semiconductor properties that make them highly sensitive to temperature.

Thermistors are more sensitive to temperature changes than other temperature sensors, like Resistance Temperature Detectors (RTDs) and Thermocouples, making them ideal for applications requiring precise temperature monitoring.They have a wide resistance range, typically from 0.5 Ω to 0.75 MΩ, allowing flexibility in various applications and are typically used in applications with a temperature range from -60°C to 150°C, making them suitable for low to moderate temperature applications.

Applications of Thermistors

Thermistors are commonly used in applications such as:

• Temperature Measurement and Control: Widely used in thermostats, medical devices, and climate control systems.
• Circuit Protection: Used in electronic circuits to protect against overheating.
• Battery Management: Employed in batteries to monitor and prevent overheating, improving battery safety and performance.

Resistance Thermometers

The Resistance Temperature Detector (RTD) is a highly precise temperature transducer that utilises a pure conducting metal, typically platinum, copper, or nickel, wound into a coil. Similar to thermistors, RTDs operate on the principle that the electrical resistance of the metal changes with temperature.

The resistance of an RTD at a given temperature T is defined by the formula:

R = R₀ (1 + αΔT)

where: R  = Resistance of the element at the measured temperature. ( Ohms Ω), R₀  = Resistance of the element at a reference temperature (usually 0ºC).( Ohms Ω), α = Temperature coefficient of resistance for the metal, which quantifies how much the resistance changes with temperature.(K^-1), ΔT = Change in temperature from the reference temperature.(K)

Main Features of RTDs: RTDs provide accurate, stable temperature readings and are very responsive to small temperature changes. Compared to thermistors and thermocouples, RTDs are relatively inexpensive, especially when high precision is required. They are capable of measuring temperatures from -182.96°C to 630.74°C, making them suitable for a broad range of applications.

Common Applications of RTDs: RTDs are commonly used in Industrial Temperature Control for precise temperature monitoring in manufacturing, processing, and HVAC systems. Laboratory and Research where  high accuracy in a wide temperature range is essential. Electronics and Instrumentation for temperature compensation and calibration due to their stability and reliability.

Thermocouples

Thermocouples are temperature transducers that rely on the Seebeck effect to measure temperature differences between two junctions. They consist of two dissimilar metals, for example, copper and constantan joined together at two junctions. These metals produce a potential difference, or emf (electromotive force), when their junctions are kept at different temperatures.

A thermocouple has two junctions, the reference (cold) junction, kept at a constant temperature, and the measuring (hot) junction, which is exposed to the temperature being measured.

When the junctions of two dissimilar metals are at different temperatures, a temperature gradient along the wires generates an emf. This phenomenon, known as the Seebeck effect, causes a voltage to be developed between the two junctions, which varies with temperature differences.

The voltage output from the thermocouple correlates directly with the temperature difference, allowing the temperature at the hot junction to be measured.

Main Features of Thermocouples

Thermocouples can measure extreme temperatures, from -200°C to over +2000°C, providing a range much broader than that of Resistance Temperature Detectors (RTDs) and Thermistors.

As self-generating transducers, thermocouples do not require an external power source for operation, unlike RTDs and thermistors.

Thermocouples are relatively inexpensive, making them a popular choice for industrial and general-purpose temperature measurement.

Compared to RTDs and thermistors, thermocouples generally have lower accuracy and are less suitable for applications requiring high precision.

Applications of Thermocouples

Due to their wide temperature range and durability, thermocouples are commonly used in:

• Industrial Furnaces and Kilns: For measuring high temperatures in harsh environments.
• Automotive and Aerospace: Monitoring exhaust, engine, and turbine temperatures.
• Household Appliances: Used in ovens, water heaters, and other appliances requiring temperature monitoring.

Integrated Circuit Temperature Transducers

Integrated Temperature Transducers are temperature sensors that combine a temperature-sensing element with monolithic electronic circuits in a single package for accurate and efficient temperature measurement. These transducers are commonly used in applications that require compact, low-cost solutions with moderate temperature ranges.

Integrated temperature transducers provide a linear relationship between temperature and output signal, making them easy to interpret and ideal for straightforward measurement and control applications. These transducers are very cheap, offering an affordable solution for basic temperature measurement needs. Their temperature range is typically 0°C to 200°C, which limits their use in high or extremely low-temperature applications and is one of their primary disadvantages.

Applications

Integrated temperature transducers are commonly found in:

• Consumer Electronics: Used for temperature monitoring in devices like computers and mobile devices.
• HVAC Systems: Ideal for ambient temperature control within buildings.
• Battery Management: Useful for monitoring moderate temperatures in battery packs and other power systems.

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