RTDs – Definition, Working Principle, Types & Applications

An RTD stands for “Resistance Temperature Detector”. It is a type of temperature sensor used to measure temperature by correlating the resistance of the RTD element with temperature changes. RTDs are commonly used in various industrial, scientific, and commercial applications where accurate and precise temperature measurements are required.

The RTD element is typically made of pure platinum wire wound into a coil or deposited on a ceramic substrate. Platinum is chosen due to its excellent stability, repeatability, and linearity over a wide temperature range. The resistance of the RTD element increases with temperature in a predictable and nearly linear manner, allowing for accurate temperature readings.

RTDs are known for their high accuracy and stability, making them popular choices in various industries such as manufacturing, aerospace, automotive, food and beverage, and environmental monitoring. They are commonly used in temperature measurement devices and systems, and they often interface with data loggers, control systems, and other instruments to monitor and control processes where the temperature is critical.

In RTD devices, different metals are used as the sensing element, and each metal exhibits its unique resistance-temperature characteristics. The relationship between the resistance and temperature is what defines the resistance-temperature characteristic of the RTD.

The three most common types of RTD elements based on different metals are-

• Platinum RTD (PT100 or PT1000)
• Nickel RTD
• Copper RTD

It is worth noting that RTDs are typically more expensive than other temperature sensors like thermocouples and thermistors, but their accuracy and stability make them an ideal choice for critical temperature measurements where precision is paramount.

RTD – Working Principle

The working principle of an RTD is based on the predictable and linear change in electrical resistance of a material with temperature. The mechanism can be summarised in these steps –

Sensing Element – The heart of an RTD is the sensing element, which is typically made of a thin wire or film of pure platinum. The platinum wire is wound into a coil or deposited as a thin film on a ceramic substrate to increase its surface area.

Electrical Resistance – At a reference temperature (usually 0 degrees Celsius or 25 degrees Celsius), the RTD has a known and calibrated resistance value. For a standard platinum RTD, this resistance is typically 100 ohms at 0 degrees Celsius, and it follows a defined resistance-temperature curve.

Temperature Change – When the RTD is exposed to a change in temperature, the temperature of the sensing element also changes.

Resistance Change – As the temperature changes, the electrical resistance of the platinum wire changes predictably and linearly. The resistance increases as the temperature rises and decreases as the temperature falls.

Measurement – The change in resistance is measured using a bridge circuit or a Wheatstone bridge configuration. By measuring the resistance accurately, the temperature of the RTD can be determined with high precision.

Calibration – RTDs are calibrated against known temperature points to establish an accurate resistance-temperature relationship. The calibration data is used to convert the measured resistance into temperature values.

Types of Resistance Temperature Detector

There are several types of RTDs (Resistance Temperature Detectors), each with different configurations and construction methods. The main types of RTDs include-

1. Wire-Wound RTDs – These are the most common type of RTDs and consist of a coil of fine platinum wire wound around a ceramic or glass core. The wire’s resistance changes with temperature, providing a reliable and accurate temperature measurement.
2. Thin-Film RTDs – In this type, a thin layer of platinum is deposited on a ceramic or glass substrate. Thin-film RTDs are known for their fast response times and compact size, making them suitable for various applications.
3. Coiled RTDs – They have a similar construction to wire-wound RTDs but are wound into a smaller, more compact coil, which allows for quicker response times and lower thermal mass.
4. Surface-Mount RTDs – These RTDs are designed for surface mounting directly onto the surface of the object being measured. They are commonly used in electronics and PCB applications.
5. Probe RTDs – Probe RTDs feature a long, slender probe that can be inserted into liquids, gases, or other materials for temperature measurement. They are widely used in industrial processes and environmental monitoring.
6. Mineral-Insulated RTDs – These RTDs consist of a platinum wire encased in a metal sheath, often made of stainless steel, providing excellent mechanical protection and ruggedness. These RTDs are commonly used in harsh environments and high-pressure applications.
7. Glass-Coated RTDs – In this type, the platinum wire is coated with a thin layer of glass for insulation. Glass-coated RTDs are used ins specialized applications requiring resistance to harsh chemicals or extreme temperatures.
8. Thin-Film Glass RTDs – These RTDs are made by depositing a thin layer of platinum on a glass substrate, providing excellent resistance to environmental factors and mechanical stress.

Each type has its advantages and is chosen based on the specific requirements of the application, such as accuracy, response time, environmental conditions, and budget constraints. The most common RTD used is the wire-wound PT100 (platinum 100 ohms at 0 degrees Celsius) due to its widespread availability and high accuracy.

Application Areas of RTDs

RTDs are widely used in various industries and applications due to their accuracy, stability, and reliability. Some of the most common application areas of RTDs include-

• Industrial Processes– RTDs are extensively used in industrial processes for temperature measurement and control. They can monitor temperatures in chemical processing, oil and gas refining, pharmaceutical, food processing, and other industrial operations.
• HVAC Systems – RTDs are employed in heating, ventilation, and air conditioning (HVAC) systems for temperature sensing and control, ensuring comfortable and energy-efficient indoor environments.
• Laboratory and Scientific Research – RTDs play a crucial role in laboratories and scientific research where precise temperature measurements are required for experiments, studies, and calibration of other instruments
• Automotive – In the automotive industry, RTDs are used to monitor engine temperature, exhaust gas temperature, and other critical parameters for engine performance and emission control.
• Aerospace and Aviation – RTDs are utilized in aerospace and aviation applications to monitor the temperature in aircraft engines, airframe components, and other critical systems.
• Power Generation – In power plants, RTDs are employed to monitor temperatures in turbines, boilers, and various power generation equipment.
• Environmental Monitoring – RTDs are used in monitoring systems to measure temperatures in weather stations, climate research, and ecological studies.
• Food Safety and Processing – In the food industry, RTDs are utilized to monitor and control temperatures during food processing and storage to ensure food safety and quality.
• Medical and Healthcare – RTDs are employed in medical devices and equipment, such as patient monitoring systems, incubators, and sterilization processes.
• Cryogenic Applications – RTDs can accurately measure temperatures in extremely low-temperature environments, making them suitable for cryogenic applications like liquefied natural gas storage and research in cryogenics.
• Energy Management – RTDs are utilized in energy management systems to monitor temperatures in buildings, industrial facilities, and power distribution systems.