What is the difference between a resistance temperature detector (RTD) and a thermocouple? Both RTDs and thermocouples are sensors used to measure heat such as Fahrenheit and Kelvin. These devices are used in a wide range of applications and settings, often presenting people with the dilemma of choosing between RTDs or thermocouples. Each temperature sensor has its own advantages and disadvantages that make it suitable for certain conditions and environments.

RTD vs. Thermocouple

What is RTD?

RTDs are made of metal wires, usually copper or platinum, that offer resistance to the flow of electricity. The RTD’s resistance changes when its temperature changes, allowing it to be used as a gauge for measuring heat. RTDs are considered to be more accurate than thermocouples as they have a linear relationship between resistance and temperature. RTDs are also less affected by electromagnetic fields than thermocouples.

RTD Working Principle

The full English name of RTD is “Resistance Temperature Detector”, so to be precise, it should be translated as “Resistance Temperature Detector”.

RTD is a special kind of resistor whose resistance value increases as the temperature increases and decreases as the temperature decreases. In industry, this feature is used for temperature measurement, so RTD is also commonly known as “thermal resistance”.

RTD Working Principle

Not all metals are suitable for making RTDs. Materials that meet this characteristic need to meet the following requirements:

  • The resistance value of the metal has a linear relationship with the temperature change energy;
  • The metal is more sensitive to temperature changes, that is, the resistance change (temperature coefficient) caused by unit temperature changes is relatively large;
  • The metal can resist fatigue caused by temperature changes and has good durability;

There are not many metals that meet this requirement. Common RTD materials are: platinum (Pt), nickel (Ni), and copper (Cu).

Take platinum thermal resistance as an example. According to the different resistance values, it can be divided into Pt50, Pt100, Pt200, Pt500 and Pt1000.

The numerical value in the name indicates the resistance value of the thermal resistance at 0°C.

For example: Pt100, indicating that the resistance value of the sensor at 0°C is 100Ω.
And Pt1000, it means that the resistance value of the sensor at 0 ℃ is 1000Ω.
The resistance value of RTD thermal resistance at different temperatures can be approximated by the formula: R=R0(1+αT).

in:
1) R0 represents the resistance value of RTD at 0℃;
2) a is called the temperature coefficient, which represents the change value of the resistance at unit temperature;
3) T represents the measurement temperature, the unit is °C;

According to the number of lead wires of RTD thermal resistance, RTD can be divided into two-wire, three-wire and four-wire.

The lead of the two-wire RTD is to directly lead out two wires at both ends of the resistor to the temperature measurement module. The temperature measurement module adopts the principle of bridge balance, and RTD is used as one arm of the bridge to measure.

A three-wire RTD can largely eliminate the influence of the sensor leads themselves on the measurement results. The detection accuracy is greatly improved compared to the two-wire system.

Resistance Temperature Detector advantages

No compensation line is required, and the price is cheap;
It can transmit electrical signals over long distances;
High sensitivity and strong stability;
Good interchangeability and high precision.

Disadvantages of thermal resistance:

Although thermal resistance is widely used in industry. But it requires power excitation.
Temperature changes cannot be measured instantaneously.
The temperature measurement range is limited and the application is limited.

What is Thermocouple?

Thermocouples, on the other hand, are made of two different types of metals that are joined together at the sensor end. The junction between these two metals produces a voltage that is proportional to the temperature difference between the junction and the measuring point. Thermocouples are less expensive than RTDs and can measure a wider range of temperatures. They are also faster at responding to changes in temperature.

Thermocouple Working Principle

Thermocouple Working Principle

A thermocouple is a temperature sensing element. It converts the temperature signal into a thermoelectromotive force signal and converts it into the temperature of the measured medium through an electrical instrument.

The basic principle of thermocouple temperature measurement is that two homogeneous conductors of different compositions form a closed loop. When there is a temperature gradient at both ends, a current will flow through the loop. At this time, there is a seebeck electromotive force – thermal electromotive force between the two ends. This is called the Seebeck effect.

The two homogeneous conductors with different compositions are the hot electrodes, and the end with the higher temperature is the working end. The end with the lower temperature is the free end. The free end is usually at some constant temperature.

According to the functional relationship between thermoelectromotive force and temperature, a thermocouple indexing table is made. The index table is obtained under the condition that the free end temperature is at 0°C. Different thermocouples have different scales.

When a third metal material is inserted into the thermocouple loop. As long as the temperature of both junctions of the material is the same. The thermoelectric potential generated by the thermocouple will remain constant. That is, it is not affected by the access of the third metal into the loop.

Therefore, when measuring the temperature of the thermocouple, the measuring instrument can be connected. After measuring the thermoelectromotive force, the temperature of the measured medium can be known.

Thermocouple Advantages:

  1. High measurement accuracy: The thermocouple is in direct contact with the measured object and is not affected by the intermediate medium.
  2. Fast thermal response time: Thermocouples are sensitive to temperature changes.
  3. Large measurement range: thermocouples can measure temperature continuously from -40 to +1600 °C.
  4. Reliable performance and good mechanical strength.
  5. Long service life and easy installation.

Types and structures of thermocouples

Types of thermocouples Thermocouples include k type (nickel-chromium-nickel-silicon), n-type (nickel-chromium-silicon-nickel-silicon-magnesium), e-type (nickel-chromium-copper-nickel), j-type (iron-copper-nickel) , t-type (copper-copper-nickel), s-type (platinum-rhodium 10-platinum), r-type (platinum-rhodium 13-platinum), b-type (platinum-rhodium 30-platinum-rhodium 6) and so on.

Structural form of thermocouple: The basic structure of a thermocouple is a thermal electrode, an insulating material and a protective tube. Display instrument, recording instrument or computer and other supporting use. In field use, thermocouples suitable for various environments are developed according to various factors such as the environment and the measured medium.

Frequently
Asked
Questions

RTD stands for Resistance Temperature Detector, but is also known as PRT (Platinum Resistance Thermometer).

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

The first step in identifying an RTD is to find out how many lines it has (2, 3 or 4).
Then you can connect the RTD to the multimeter.
If it’s a pt100, it should read between 107-110Ω at room temperature.
But if it’s a pt1000. You should get a reading of 1007 – 1100Ω. This confirms it’s a Pt1000.

PLEASE NOTE: These readings are standard for new RTD sensors. If the sensor is damaged or used continuously. The readings may vary.

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

We get asked this question a lot, but Pt100s and Pt1000s are two types of RTDs (Pt500s are another type of RTD, but now obsolete).

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

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

  • What temperature are you measuring (surface or immersion in solid, liquid or gas)?
  • If a fast response time is a must, see the RTD Technology page for various factors in selecting a response time.
  • Fits the specific dimensions required for your application, such as probe diameter, probe length, compression fittings, required connector types, etc.
  • Do you need special sheath materials?
  • Do you need to calibrate the sensor?
  • Does the sensor need to be resistant to chemicals/abrasion/vibration or any other environmental factors?
  • Is there high electromotive force (electromagnetic interference) in power switching, rectification or radio waves?
  • Any other installation considerations? (eg sensor needs to be bent to form before installation)
  • Distance between sensing area and instrument
  • Sensing ambient temperature over sensor length
  • Connection Preferences
  • Current wiring configurations such as 4-wire sensors will not be compatible with 3-wire configurations.

As a rule of thumb, RTDs should be immersed 4 times the length of the element. (Flat-film elements are typically 2-3mm, while wire-wound elements are about 15mm or more).

We are often asked this question, but Pt100 thermocouples do not exist. A thermocouple is a type of sensor, and a Pt100 is a type of RTD, another type of sensor.

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

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

Conclusions, which one should you use?

It really depends on the specific application and what is more important: accuracy or speed. If you need to measure very high or very low temperatures, a thermocouple is the better choice. If you need more accuracy, then an RTD is the way to go.

Both RTDs and thermocouples have their own advantages and disadvantages, so it really depends on the specific application and what is more important: accuracy or speed. If you need to measure very high or very low temperatures, a thermocouple is the better choice. If you need more accuracy, then an RTD is the way to go.

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Sino-Inst is Manufacturer of RTD & Thermocouples for temperature measurement. We supply more than 20 kinds of RTD & Thermocouples. 40% RTD, 60% Thermocouples.

RTD & Thermocouples for diesel fuel measurement are mainly used for temperature measurement of various meadium.

RTD & Thermocouples enable stable temperature measurement. This greatly meets the measurement needs of many applications.

Sino-Inst’s RTD & Thermocouples for temperature measurement, made in China. Having good Quality, With better price. Our temperature measurement instruments are widely used in China, India, Pakistan, the US, and other countries.

The entire team at Sino-Inst’s has received excellent training, so we can ensure that every client’s needs are met. For assistance with your product requirements, whether it’s a RTD & Thermocouples for temperature measurement, flow sensor, or other device, give us a call.

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