What does RTD stand for?

October 10, 2021

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This article is just an introduction, and does not represent the AMPFORT company's products and any meaning.


Q-What does RTD stand for?

A-RTD stands for Resistance Temperature Detector, but it is also called PRT (Platinum Resistance Thermometer)


Q-What is RTD?

A-Temperature sensor, including a resistor (usually 100 or 1000 ohms). Resistance changes with temperature; it can then be correlated to provide a temperature reading. RTD is considered to be one of the most accurate temperature sensors. In addition to providing very good accuracy, they also have excellent stability, repeatability, and have a high immunity to electrical noise. This means that RTD sensors are very suitable for applications in process and industrial automation environments, especially motors, generators and other high-voltage equipment.


Q-What is PRT?

A-A platinum resistance thermometer (PRT) is an RTD that uses platinum as a sensing element. The most common PRTs are Pt100, Pt500 and Pt1000. (PRT is just a more specific name for RTD)


Q: I need a new RTD sensor, but I don’t know what type I have been using. Can you help?

A-The first step to identify RTD is to find out how many wires it has (2, 3 or 4), then you can connect RTD to a multimeter, if it is pt100, it should read between 107-110Ω at room temperature . However, if it is pt1000, you should get a reading of 1007-1100Ω, which confirms that it is Pt1000.

Please note: These readings are the standard for new RTD sensors. If the sensor is damaged or used continuously, the readings may be different.


Q-What is A-level or B-level accuracy?

A-The international standard IEC 60751:2008 defines the resistance and temperature characteristics of RTDs. Within this standard, in order to provide good interchangeability, there is an accuracy standard. Class A and Class B are two accuracy standards. We provide a tolerance reference table.


Q-What is the difference between RTD and Pt100?

Answer-We are often asked this question, but Pt100s and Pt1000s are two types of RTDs (Pt500 is another type of RTD, but it is now obsolete)


Q-Does my RTD sensor need an extension cord?

A-RTDs use cables because they detect temperature by calculating resistance changes in the material, so you can simply order RTDs with long leads or purchase additional cables to expand on your own.


Q-How do I choose the right RTD sensor for my application?

A-When choosing an RTD, the following factors must be considered:

• What is the temperature you are measuring (surface or immersed in solid, liquid or gas)?

• If you must have a fast response time, to understand the various factors in choosing a response time, please refer to the RTD technology page.

•Suitable for the specific dimensions required by your application, such as probe diameter, probe length, compression fittings, required connector types, etc.

•Do you need a special sheath material?

• Do you need to calibrate the sensor?

• Does the sensor need to be resistant to chemicals/wear/vibration or any other environmental factors?

• Does the power switch, rectification or radio waves have high electromotive force (electromagnetic interference)?

• Any other installation considerations? (For example, the sensor needs to be bent and formed before installation)

•The distance between the sensing area and the instrument

• Sensing ambient temperature within the length of the sensor

• Connection preferences

• The current wiring configuration, such as 4-wire sensors will not be compatible with 3-wire configurations


Q-What is the difference between Pt100 and Pt1000?

A-If your Pt100 and Pt1000 are immersed in 0°C water and ice, the reading of Pt100 is 100Ω (ohm), and the reading of Pt1000 is 1000Ω (ohm). The most common type is Pt100 (3-wire), although Pt100 4-wire is still commonly used in laboratories and applications that require accurate readings.

A simple way to think about it is that in Pt100, 1°C is equal to 0.39Ω, and in Pt1000, 1°C is equal to 3.9Ω (100 and 1000 are just the reference resistances of the RTD at 0°C). Our mineral insulated RTD has a temperature rating of up to 200°C, so we can use this number. At 0°C, the pt100 resistance will measure 100Ω (ohms), and every 1°C increase will increase 0.39Ω.

At 0°C, the pt1000 resistance will measure 1000Ω (ohm), and an increase of 1°C will increase by 3.9Ω. Therefore, if we compare them at 600°C, you will see that the output of Pt100 is 313.59Ω, while the output of Pt1000 is 3135.9Ω.

This shows us that the output of Pt1000s is very high, so Pt100 is obviously a better choice for accurate readings, because it will provide more accurate and sensitive readings, which is very suitable for applications where heating at low speeds but depth measurement is required, while Pt1000s is more suitable for For applications that apply a lot of heat at once and require less depth measurement (eg 67°C instead of rough numbers such as 66.72°C), it is also worth noting that these factors can depend on the wire configuration used.


Question-What is the difference between 2, 3 and 4 wire rtds?

The A-2 wire RTD is the least accurate of the three types, because the lead resistance cannot be eliminated or calculated. This creates uncertainty in the readings, so 2-wire RTDs are usually used with short leads, where accuracy is not the main priority, they are mainly used for Pt1000 to reduce sensitivity and uncertainty (try to get a stationary reading because it actually Too accurate readings will not stay the same) but this means that compared to 3 or 4 wires, it will not provide very accurate readings.

3-wire RTD is the most commonly used RTD sensor. Assuming that all three leads are the same, the third lead calculates the average lead resistance of the entire circuit and removes it from the sensor measurement. This makes the 3-wire RTD more accurate than the 2-wire alternative, but not as accurate as the 4-wire, but in circuits with long leads, the distance between the detector and the reading is very long, by using a 3-wire structure.

4-wire RTDs are used in applications that require high accuracy. In a 4-wire RTD sensor, the actual resistance in each lead can be measured and eliminated, leaving the precise resistance of the detector. The 4-wire circuit uses the first two leads to power the circuit, while the 3rd and 4th wires read the resistance in each lead to compensate for any difference in lead resistance.


Q-How far should my RTD be immersed in the measured medium?

Answer-Based on experience, the RTD should be immersed 4 times the length of the element. (The flat membrane element is usually 2-3mm, and the winding element is about 15mm or more).


Q-What is a mineral insulated RTD cable? (Why should I use it?)

A-The mineral insulated RTD cable consists of an outer metal sheath that houses the RTD conductor; then the sheath is filled with highly compacted magnesium oxide powder to prevent the conductor from contacting anywhere other than the fixed joint. This structure allows the sheath to be easily bent or formed into the shape required by the user, but remains rigid during use. The entire assembly is airtightly sealed, so that the conductor is completely protected from the operating environment.

The mineral insulated RTD probe is durable and can be shaped and shaped to suit any application without affecting the sensor. These sensors have a long length and a wide diameter. They can be used at a higher temperature than the manufactured RTD


Q-What is the best, RTD or thermocouple? (RTD and thermocouple)

A-RTDs and thermocouples have their advantages and disadvantages. The main advantages of RTD sensors are that they are very accurate over a wide range and have excellent stability. Compared with thermocouples, thermocouples have lower accuracy and poor stability, and are known to change over time. drift. RTD sensors are more suitable for lower temperature ranges and require higher accuracy and repeatability. They also need to be immersed in the medium due to conduction errors in the catheters.


Q-What is the difference between RTD and thermistor?

A-Both RTDs and thermistors are electrical devices designed to measure temperature. The main difference between the two is their materials. Thermistors are usually made of ceramic or polymer materials, while RTDs are made of pure metal. RTDs can read temperatures up to 600 °C, but they are usually used lower to take advantage of high accuracy. The thermistor can read temperatures up to 200 °C and has a faster response time than RTD.


Q-What is a Pt100 thermocouple?

Answer-We are often asked this question, but the Pt100 thermocouple does not exist. A thermocouple is a sensor, and Pt100 is an RTD, which is another sensor type.


Q-What is a Pt200 sensor?

A-Pt200 sensor is an RTD. The resistance of Pt200s at 0ºC is 200 ohms (Ω). The Pt200 sensor is now obsolete and has been replaced by the Pt100 and Pt1000 sensors. The Pt500 sensor is also an obsolete RTD.


Q-What is a Pt500 sensor?

A-Pt500 sensor is a kind of RTD, the resistance of Pt500s at 0ºC is 500 ohms (Ω). The Pt500 sensor is now obsolete and has been replaced by the Pt100 and Pt1000 sensors. The Pt200 sensor is also an obsolete RTD.