RTD Sensor Working Principle and Its Applications

Sensors are the devices used by electronic, electrical and mechanical equipment to interact with the external environment. These are used for measuring various types of physical phenomena such as voltages, current, acceleration, etc…Sensors make use of various principles to measure these physical quantities. Such as piezoelectric effect is used for measuring voltage and current, Hall effect is used for measuring magnetic density, etc… RTD – Resistance temperature detector, is a temperature detector sensor that uses the relationship between temperature and resistance of the conductor to measure the temperature. This sensor is rapidly replacing thermocouples.

What is an RTD Sensor?

The term RTD stands for Resistance Temperature Detector. This sensor is also known as Resistance thermometer. This sensor is used to measure the temperature.

Usually, they are available as a length of fine wire made of platinum nickel or copper, wrapped around a ceramic or glass core. This sensor makes use of the temperature/resistance relationship of the wire to measure the temperature.

By using temperature Vs resistance relationship one can find the amount of change happened to the resistance value of the sensor, for a degree change in the temperature. Platinum metal has a stable resistance-temperature relationship across a wide range of temperature.

For Nickel, the amount of change in resistance due to change in temperature becomes non-linear, at a temperature above 300⁰C. Based on their behavior, at different temperature ranges, materials are chosen to make the thin wire, which is used in RTD.

RTD can be constructed in different forms and in some cases, they are better than thermocouples for stability, accuracy, and repeatability. RTD requires a power source to operate. Unlike thermocouple that uses Seebeck effect to generate a voltage, RTD makes use of electrical resistance.

Working Principle

The working of the RTD sensor is based on the resistance- temperature relationship of the material used for its construction. The amount of change seen in the resistance value of the material caused due to per degree rise in temperature is measured and the sensor is calibrated accordingly.

The resistive element is fragile, they always require insulation. Insulator leads are attached to the element. For temperature below 250^oC insulators such as Silicon rubber, PVC is used. A metal alloy which is chemically inert to the temperature is used as a protective sheath, for housing the measuring point and leads.

From the temperature of 0⁰C up to a temperature value where the change is linear, is considered as the temperature range of the sensor. This depends on the material of the wire used in the sensor. For platinum is used the range is up to 660⁰C. Nickel is suitable for temperatures below 300⁰C.

The linear approximation of the resistance-temperature relationship of the metals between 0⁰C and 100⁰C is considered as the significant characteristics of the metal which is used as wire in the sensor.

The temperature coefficient of resistance is given as

α =(R₁₀₀ –R₀)/ (100⁰C.R₀)

Where R₀and R₁₀₀ are the resistance of the sensor at temperature 0⁰C and 100⁰C respectively.

Applications of RTD

• RTD sensor is used in automotive to measure the engine temperature, an oil level sensor, intake air temperature sensors. In communication and instrumentation for sensing the over the temperature of amplifiers, transistor gain stabilizers, etc…
• RTD is used in power electronics, computer, consumer electronics, food handling and processing, industrial electronics, medical electronics, military, and aerospace.

Examples of RTD

Some of the examples of RTD sensor are coolant sensor, transmission oil temp. sensors, intake air temperature sensor, fire detectors, etc..

Due to their accuracy and stability, RTD sensors are rapidly replacing thermocouples in industrial applications. RTD can give higher accuracy values. RTD can be stable for many years compared to the thermocouple, which stays stable only for a few hours of use. There are RTD present in our day to day appliances like coffee machines, cell phones. Which of the application of RTD have you come across?