What Is RTD Sensor And How Does It Work?
Introduction
Do you want to know what an RTD temperature sensor is, how it works and how to test it? Then you are in the right spot, we have answered all those questions and many more for you.
Enjoy your reading!
What is an RTD temperature sensor?
An RTD stands for “Resistance Temperature Detector” and it is a sensor whose resistance changes when its temperature changes and it is used to measure temperature. The RTD’s resistance increases linearly when the temperature increases. Many RTDs are called wire wound. They consist of fine wire wrapped around a glass or ceramic core. The wire is made of platinum. Another interesting thing is that the RTD elements are normally housed in a protective probe to protect them from the environment they are immersed in and to make them more robust.
Inexpensive RTDs are called thin film RTDs. They consist of a base ceramic with a fine Platinum track deposited on it. So far we found out what is resistance temperature detector, so let’s now explore how does an RTD work?
How does a resistance thermometer work?
Let’s now explore how an RTD works. As we mentioned an RTD comprises a resistance element and insulated Platinum wires. Sometimes RTDs can have three or even four wires to increase accuracy allowing connection lead resistance errors to be eliminated. The resistance element is made of platinum because it is very long-term stable and it has a linear relationship between temperature and resistance, has a wide temperature range and it has a chemical inertness.
RTD working principle
In terms of how it works, the RTD follows a basic principle. When the temperature of a metal increases, the resistance to the flow of electricity increases as well. An electrical current is passed through the sensor, the resistance element is used to measure the resistance of the current being passed through it. As the temperature of the resistance element increases the electrical resistance also increases.
The electrical resistance is measured in Ohms. The resistance value can then be converted into temperature based on the characteristics of the element. Usually, the response time for an RTD is between 0.5 and 5 seconds. This makes them very suitable for many applications.
RTD sensors types
Resistance Temperature Detectors can be categorised into two types of RTDs. Their type is based on the construction of the temperature-sensing element. The first type contains wire-would elements, while the second type contains thin-film elements.
Thin-film RTDs
The thin-film RTD elements are made by depositing a thin layer of metal which in most cases is platinum on a ceramic substrate material. The metal film is laser cut or etched into an electrical circuit pattern that provides the specified amount of resistance. Lead wires are then attached, and a thin protective glass coating is applied to the entire element.
The advantages of thin-film RTDs are that they are reliable and are produced at a low cost. Moreover, they are more damage resistant from vibrations than the other types of resistance temperature detectors.
Wire-wound RTDs
The other type of RTD is wire-would. Its sensing element comprises a small coil of ultra-thin platinum wire. The wire coil is commonly packaged inside a ceramic or glass tube or the wire can be wound around the outside of a ceramic or glass housing material.
The advantages of wire-wound RTDs are that they are very accurate and those with glass cores can readily be immersed in many liquids, while those with ceramic cores can be used to accurately measure extremely high temperatures.
The disadvantages of wire-would RTDs are that they are more expensive to produce than thin-film and they are more vibration-sensitive.
Resistance temperature detector (RTD) applications
The RTD sensors are primarily used in the following industries:
• Automotive
• Power electronics
• Consumer electronics
• Food handling and processing
• Industrial electronics
• Medical electronics
• Military
• Aerospace
How to test RTD temperature sensor?
To test your RTD sensor set your multimeter to a resistance mode. After that, check the readings across the terminals of the RTD. At room temperature (around 20°C) the reading should be around 110 ohms. Keep in mind that the reading value may be different, which depends on the room temperature.
Finally, place the RTD temperature sensor in ice water. Then, after a couple of minutes check the readings again. Now, you should get a lower number than the room temperature reading. That number should be around 100 ohms.
What is the difference between RTD and thermocouples?
There are a number of differences between thermocouples and RTD sensors. Below we have outlined the main ones.
- Thermocouples are usually smaller than RTDs, making them easier to use.
- Thermocouples (-200 to 2000°C) offer a wider range of temperature operation than RTDs (-200 to 600° C). This means that thermocouples are suitable for more applications.
- Thermocouples offer a response time between 0.1 and 10s which is faster than the response time of RTD sensors.
- RTDs can self-heat while this issue is negligible with the thermocouples.
- Thermocouples are more sensitive than RTD temperature sensors. This is so because these react faster than RTDs with the variation in temperature.
- For thermocouples, the graph between resistance and temperature is not linear, while an RTD is linear.
Resistance Temperature Detector Technical Information
RTDs standard tolerances
Resistance temperature detectors are built to several tolerances and curves, one of the most common is the “DIN” curve. It shows the resistance vs temperature characteristics of a Platinum, 100-ohm sensor, the standardised tolerances, as well as the measurable temperature range.
The DIN standard specifies a base resistance of 100 ohms at 0°C, and a temperature coefficient of .00385 Ohm/Ohm/°C. The nominal output of a DIN RTD sensor is shown below:
There are three standard tolerance classes for DIN RTDs. These tolerances are defined as follows:
- DIN Class A: ±(0.15 + .002 |T|°C)
- DIN Class B: ±(0.3 + .005 |T|°C)
- DIN Class C: ±(1.2 + .005 |T|°C)
RTD Element Types
When you decide the RTD element type, first you should consider what instrument you will be reading the sensor with. You need to choose an element type that is compatible with the instrument’s sensor input. By far the most common RTDs are 100 Ohm Platinum with .00385 temperature coefficient.
RTD Accuracy
Another thing, you need to decide what accuracy is needed in your specific measurement. Accuracy is a combination of both base resistance tolerance (resistance tolerance at the calibration temperature) and temperature coefficient of resistance tolerance (tolerance in the characteristic slope). Any temperature above or below this temperature will have a wider tolerance band or less accuracy. The most common calibration temperature is 0°C.
Why does an RTD have 3 wires?
As we mentioned earlier, most RTDs have two wires, however, others are made with three. This type of construction is used mostly in industrial applications where the third wire provides a method for removing the lead wire resistance from the sensor measurement.
Does an RTD need a power supply?
Resistance Temperature Detectors do require a power source to operate.
Why platinum is used in RTD?
As we mentioned earlier in the article, platinum is used in RTD sensors due to its stability and it provides repeatable and measurable results and has a broad temperature range. Moreover, platinum provides very low fluctuations in temperature readings, resulting in overall precision and stability of temperature measurement.
Conclusion
An RTD stands for “Resistance Temperature Detector” and it is a sensor which is used to measure temperature. It works following a basic principle of when the temperature of a metal increases, the resistance to the flow of electricity increases as well. An electrical current is passed through the sensor, the resistance element is used to measure the resistance of the current being passed through it. As the temperature of the resistance element increases the electrical resistance also increases.
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