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
Now we will be exploring the working principle of a RTD. 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?
Now we will exlain everything about RTD testing. 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.
What is the most widely used RTD?
The most widely used RTD is the PT100. PT100 sensors are very popular due to their high accuracy and stability.
Here are some reasons why PT100 RTDs are commonly used:
• Accuracy: PT100 sensors offer high accuracy in temperature measurement, making them suitable for applications where precise temperature control is necessary.
• Stability: PT100 sensors exhibit good long-term stability, meaning they maintain their accuracy over long periods of use.
• Compatibility: PT100 sensors are compatible with a wide range of measurement instruments, making them very versatile for use in different applications.
Why use RTD instead of thermocouple?
There are several reasons why you might choose an RTD (Resistance Temperature Detector) instead of a thermocouple.
• Accuracy and Stability: RTDs offer higher accuracy and stability compared to thermocouples. They have a more predictable and linear relationship between temperature and resistance, leading to more precise temperature measurements.
• Wide Temperature Range: While thermocouples have a broader temperature range compared to RTDs, RTDs often provide better accuracy and stability within their specified temperature range.
• Response Time: In some cases, RTDs may have faster response times compared to certain types of thermocouples, particularly in applications where rapid temperature changes need to be monitored accurately.
Can we use thermocouple instead of RTD?
Yes, it is possible to use a thermocouple instead of an RTD in certain applications, but there are some important considerations that need to be kept in mind.
• Temperature Range: Thermocouples typically have a wider temperature range compared to RTDs. If your application requires temperature measurements outside the range of an RTD, a thermocouple might be the better choice.
• Accuracy and Stability: RTDs offer higher accuracy within their specified temperature range compared to thermocouples.
• Response Time: Thermocouples often have faster response times compared to RTDs, making them suitable for applications where rapid temperature changes need to be monitored.
PT100 Resistance Chart
PT100 Resistance Table
What causes RTD failure?
There are several typical causes of RTD failure. Some of them are:
• Mechanical Damage: Physical damage to the RTD element or its wiring, such as bending, crushing, or abrasion, can lead to failure.
• Excessive Temperature: Exposure to temperatures beyond the RTD’s specified operating range can cause degradation or permanent damage to the RTD element.
• Moisture and Contamination: Moisture ingress or exposure to corrosive chemicals can degrade the insulation or corrode the RTD element, leading to electrical shorts, changes in resistance, or complete failure.
• Vibration and Shock: Excessive vibration or mechanical shock can cause the RTD wiring to break or loosen over time, resulting in intermittent connections or open circuits.
• Ageing and Wear: Like any electronic component, RTDs can degrade over time due to ageing and wear.
• Poor Installation: Improper installation, such as incorrect wiring, inadequate strain relief, or insufficient insulation, can lead to mechanical stress or exposure to environmental hazards, increasing the risk of RTD failure.
RTD Simplex vs Duplex
RTD Simplex and Duplex configurations differ primarily in their design and application suitability. A Simplex RTD has a single sensing element within the probe, making it suitable for applications where only one temperature measurement is needed. This configuration is more cost-effective and is commonly used in standard industrial processes, HVAC systems, and general temperature monitoring where redundancy is not a priority.
In contrast, a Duplex RTD contains two separate sensing elements within the same probe. This design provides redundancy, ensuring that if one element fails, the other can continue to provide accurate temperature measurements. Duplex RTDs are particularly valuable in critical applications where continuous and reliable temperature monitoring is essential, such as in chemical processing, power generation, and other high-stakes industrial environments.
While Duplex RTDs are more expensive due to their additional complexity, they offer enhanced reliability and peace of mind in situations where uninterrupted temperature data is crucial.
Difference Between PT100 and K Type Thermocouple
PT100 sensors and K Type thermocouples are both widely used for temperature measurement but operate on different principles and are suited to different applications. Below we have outlined the key differences between the PT100 and Type K Thermocouple.
Principle of Operation:
• PT100: Uses the change in electrical resistance of platinum with temperature (100 ohms at 0°C).
• K Type Thermocouple: Uses the thermoelectric effect of two different metals (Chromel and Alumel) to produce a voltage corresponding to temperature differences.
Accuracy and Stability:
• PT100: Highly accurate and stable with minimal drift, ideal for precise temperature readings.
• K Type Thermocouple: Less accurate but sufficient for many industrial processes; accuracy can be affected by environmental factors.
Temperature Range:
• PT100: Effective within -200°C to 600°C.
• K Type Thermocouple: Operates over a broader range of -200°C to 1350°C, suitable for high-temperature environments.
Response Time:
• PT100: Slower response due to thermal transfer requirements.
• K Type Thermocouple: Faster response, better for dynamic temperature changes.
Durability and Application Suitability:
• PT100: More delicate, suitable for laboratory and precision applications.
• K Type Thermocouple: More robust, ideal for harsh industrial environments.
Cost:
• PT100: Generally more expensive due to platinum and precision manufacturing.
• K Type Thermocouple: More affordable and cost-effective for a variety of applications.
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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