The resistance temperature sensor

A resistance temperature sensor, or RTD (Resistant Temperature Detector), is a temperature sensor that uses the resistance of a measuring element for temperature measurement. One of the most commonly used elements is the so-called Pt100.
In a Pt100, the "Pt" stands for the material of the measurement insert (platinum) and "100" stands for the resistance value. A Pt100 has a resistance value of 100 Ω, at zero degrees Celsius.
With increasing temperature, the resistance of the Pt100 will increase, and with decreasing temperature, the resistance will decrease.
The change in resistance occurs at about 0.38 Ω per degree of temperature change.

How do resistance temperature sensors work?

The measuring insert is connected to a measuring system. The method of connection depends on the application, but the measurement principle remains the same.

A change in temperature has a direct effect on the electrical resistance of the measuring insert. With a Pt100, increasing temperature also means increasing resistance, and vice versa. This change in resistance is measurable, and the measured resistance value can be "translated" into a temperature.

The resistance values of Pt100s are internationally defined in a DIN standard (DIN 60751).

Note: There are also resistors that work exactly the other way around, better known as NTCs. With these resistors, as the temperature increases, the resistance actually decreases. However, the method of measurement remains the same, as the temperature is determined from the resistance value measured.

Specifications of a resistance temperature sensor

• Measurement principle: resistance of measuring element.
• Accuracy: very accurate.
• Response time: fast to very fast.
• Durability: less durable at high temperatures.
• Temperature range: suitable from -200°C to +600°C (exceptionally up to +800°C).
• Applications: laboratory, process industry, HVAC systems.
• Price: highly dependent on type and type of connection.

The thermocouple

A thermocouple is a temperature sensor based on the so-called Seebeck effect.The Seebeck effect describes a phenomenon, where a difference in thermoelectric voltage occurs between two different metal conductors, when they are brought to different temperatures.

In other words, if one connects two metal wires of different composition at one end and heats this connection, a measurable voltage difference will occur between the two wires at the other end. The higher the temperature, the higher the measured voltage (millivolts).

Whereas a resistance thermometer translates a resistance (Ω) to a temperature, a thermocouple translates the measured voltage (mV) to a temperature.

There are different types of thermocouples, with different compositions. Which type of thermocouple is best suited depends very much on the application.

Specifications of a thermocouple

• Measurement principle: thermoelectric voltage (Seebeck effect).
• Accuracy: good.
• Response time: fast to very fast.
• Durability: durable, even at extreme temperatures.
• Temperature range: suitable from -200°C to 1800°C (depending on torque type).
• Applications: industrial applications, especially (very) high temperature measurements.
• Price: on average cheaper.

Applications

Resistance temperature sensors

• Laboratory use

Resistance temperature sensors are widely used in laboratories for precise temperature measurements. They are suitable for monitoring experiments, calibrating other sensors and checking ambient temperatures.

• Process industry

Process industries, including the chemical and pharmaceutical sectors, rely on resistance temperature sensors to measure and control temperatures at various stages of production. These sensors provide the accuracy essential for consistent results and safe processes.

• HVAC systems

Heating, ventilation and air conditioning (HVAC) systems use resistance temperature sensors to control the temperature in buildings. This provides comfortable and energy-efficient environments in residential and commercial buildings.

• Food industry

In the food industry, resistance temperature sensors are used to monitor and control the temperature of food and beverage products during production and storage. This contributes to food safety and quality control.

Thermocouples

• Industrial Applications

Thermocouples are ideal for situations where (extremely) high temperatures are measured, such as in melting furnaces and combustion processes. They can accurately measure up to very high temperatures.

• Aerospace Industry

In the aviation industry, thermocouples are used for temperature measurements in aircraft engines, including turbines and exhaust systems. It is essential to monitor the temperature in these critical components to ensure safety and efficiency.

• Oil and gas industry

Thermocouples play a crucial role in the oil and gas industry, where they are used for temperature measurements in wells, production processes and refineries. Monitoring temperatures in these environments is essential for safety and efficiency.

• Laboratory furnaces

In laboratories, thermocouples are often used to measure temperature in furnaces and heating processes.

Which sensor should you choose?

The choice between a resistance temperature sensor or a thermocouple depends on the application.

Consider the following when making your choice:

• What is your temperature range?

• What is your desired accuracy?

• What is your desired response time?

• How aggressive is the environment of measurement (mechanical as well as chemical)?

• What is your budget?

Conclusion

Both types of sensors offer valuable solutions for temperature measurements in a variety of industries. Both have their own strengths and are suitable for different situations.

The choice between resistance temperature sensors and thermocouples depends on the specific requirements of the application and the temperature range in which measurements are to be made.

FAQ

1- What is the difference between a Pt100 and a Pt1000 resistance temperature sensor?
A Pt100 has a resistance of 100 Ω at 0 °C, while a Pt1000 has a resistance of 1000 Ω at 0 °C. The higher standard resistance of a Pt1000 can make it more suitable for application involving high line resistance (resistance in the connecting cable).
Example: for a Pt100, 1 degree ≈ 0.38 Ohm. For a Pt1000, 1 degree ≈ 3.8 Ohm.
A cable resistance of 5 Ohm results in (5/0.38≈) 13 degrees of deviation for a Pt100.
A cable resistance of 5 Ohms results in (5/3.8≈) 1.3 degrees of deviation for a Pt1000.

However, this can be solved in other ways. A Pt1000 is not necessarily more accurate than a Pt100, the way of connection is most important (see below).

2- What are the advantages of three-wire connection in resistance temperature sensors?
When connecting a sensor, there is always extra (unwanted) resistance in the connecting wires. Because it is a resistance measurement, this extra resistance can cause a deviation in the measurement.
The deviation created in the connecting wires can be eliminated by connecting a sensor three-wire. The connected equipment uses the extra wire to measure the resistance in the connecting wires and can then filter out this extra resistance, making the measurement correct again.
The three-wire connection is one of the most commonly used methods of connection.

3- Which type of thermocouple is best suited for high temperature measurements?
For very high temperatures, Pt30Rh-Pt6Rh (Type B) thermocouples are suitable.
These are platinum-rhodium thermocouples, capable of measuring temperatures up to 1700°C.

About the Author

My name is Saskia van der Laan and I have been working at JUMO for quite some time, combining my professional skills with my passion for writing. Always with the aim of using written text to translate technology into practice and to inform and inspire the reader. In my spare time, I like to do sports and enjoy a long walk with the dog.