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Classification of temperature sensors

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Classification of temperature sensors


Temperature sensors are used in automobiles, consumer electronics, household appliances and other products. According to the characteristics of temperature sensing elements, they are mainly divided into thermistors, thermocouples, and resistance temperature detectors as shown in Figure 1. Their measurement temperature range, measurement accuracy, and cost are different.


First, a quick chart comparing thermistors, thermocouples, and resistance temperature detectors


NTC Thermistor




Ceramics (metal oxides)

Two different metals

Pure resistance

Temperature range

-100℃ to 500℃

-270℃ to 2300℃

-250℃ to 900℃

Level of accuracy




Thermal response




Long-term stability












Through the above comparison, everyone can simply understand the differences between several temperature measurement solutions, and these differences also determine different application scenarios. Thermocouples and RTD solutions have a wide temperature measurement range and are complex to use, so they are basically limited to industrial applications. Thermistors NTC are widely used because of their low cost and relatively easy use. For example, the water temperature, oil temperature, engine intake temperature, cylinder temperature, and exhaust temperature in cars are all NTC application environments.



A thermistor is a sensor resistor whose resistance value changes with temperature. According to the temperature coefficient, it is divided into positive temperature coefficient thermistor (PTC thermistor) and negative temperature coefficient thermistor (NTC thermistor). The resistance value of the PTC thermistor increases with the increase of temperature, while the resistance value of the NTC thermistor decreases with the increase of temperature. They are both semiconductor devices. Most thermistors have a negative temperature coefficient, and a few have a positive temperature coefficient. Thermistors are usually made of ceramic materials, such as oxides of nickel, manganese or cobalt plated in glass, which makes them very easy to damage. Compared with the snap-action type, their main advantages are their speed of response to any changes in temp, accuracy and repeatability,as shown in picture 2.


Main features:

①High sensitivity, its resistance temperature coefficient is 10 to 100 times greater than that of metal, and can detect temperature changes of 10-6℃;

②Wide operating temperature range, normal temperature devices are suitable for -55℃ to 315℃, high temperature devices are suitable for temperatures higher than 315℃ (currently up to 2000℃), and low temperature devices are suitable for -273℃ to -55℃;

③ Small size, able to measure the temperature of gaps, cavities and blood vessels in organisms that other thermometers cannot measure;

④ Easy to use, the resistance value can be arbitrarily selected between 0.1 and 100kΩ;

⑤ Easy to process into complex shapes and can be mass-produced;

⑥ Good stability and strong overload capacity.

Figure 3 This PTC thermistor transducer is interchangeable and has fast response characteristics. KTY84 chip has linearity and long-term stability and can be used in diesel injection systems, oil temperature measurement, engine cooling systems, etc.

Figure3 PTC thermocouple sensor.jpg


Thermocouple is a commonly used temperature measuring element in temperature measuring instruments. It directly measures temperature and converts temperature signals into thermoelectric potential signals, which are then converted into the temperature of the measured medium through electrical instruments (secondary instruments). They have a wide temperature operating range, reliability, accuracy, simplicity and sensitivity. This is mainly due to their small size. Thermocouples also have the widest temperature range of all temperature sensors, from below -200 ℃ to well above 2000 ℃.

The appearance of various thermocouples is often very different due to needs, but their basic structure is roughly the same. They are usually composed of main parts such as hot electrodes, insulating sleeve protection tubes and junction boxes. They are usually used in conjunction with display instruments, recording instruments and electronic regulators.

Figure4 thermocouple.jpg

The red and blue segments in Figure 4 are two different materials. The conductor or semiconductor that makes up the thermocouple is called a hot electrode. The end that is welded together will be inserted into the temperature measurement site and become the working end, and the other end is called the cold end, which serves as the reference end. If the temperatures at the two ends are different, this temperature difference will cause the other two ends of the conductor or semiconductor to generate thermoelectric potential, which can be converted into the corresponding temperature using voltage sampling.

Main features:

  1. Simple assembly and easy replacement;
  2. Compression spring type temperature sensing element with good shock resistance;
  3. High measurement accuracy;
  4. Large measurement range (-200℃~1300℃, -270℃~2800℃ under special circumstances);
  5. Fast thermal response time;
  6. High mechanical strength and good pressure resistance;
  7. High temperature resistance up to 2800 degrees;
  8. Long service life.



Thermocouples can work in extremely high and low temperature ranges, ranging from -200℃ to 2300℃. Therefore, thermocouples have found wide applications in the measurement needs of a wide temperature range, such as metallurgy, machinery, chemical industry and other industrial fields, as well as heat treatment, glass manufacturing, etc.

Figure 5 This N-type thermocouple sensor has the advantages of good linearity, large thermoelectric potential, high sensitivity, good stability and uniformity, strong anti-oxidation performance, low price, and is not affected by short-range ordering. It can be used in diesel engine exhaust after-treatment systems.

Figure5 N-type thermocouple sensor.jpg

Resistive Temperature Detector (RTD)

RTD is precise temperature sensors made of a high purity conductive metal such as platinum, copper, or nickel wound into a coil. The resistance change of an RTD is similar to that of a thermistor. Thin film RTD is also available. These devices have a thin layer of platinum paste deposited on a white ceramic substrate. The RTD acts a bit like a thermoelectric converter, converting temperature changes into voltage changes. The resistance-temperature relationship of platinum, copper, or nickel is shown in Figure 6. They have a large temperature coefficient, respond quickly to temperature changes, are resistant to thermal fatigue, and are easily machined into precision coils.

Figure6 Resistance-Temperature Relationship of Platinum Copper or Nickel.jpg

Resistive temperature detectors have a positive temperature coefficient (PTC), but unlike thermistors, their output is very linear, producing very accurate temperature measurements. RTDs are the most accurate and stable temperature transducers. They are better linear than thermocouples and thermistors. However, RTDs are also slower responding and more expensive temp sensors. Therefore, RTDs are best suited for applications where accuracy is critical, but speed and price are not critical.

Figure 7 This RTD Pt200 EGT sensor has the characteristics of standardized linear characteristic curve, high stability and reliability, and short response time under transient conditions. It can be used to control and monitor DPF/GPF systems, monitor heavy-duty diesel engine SCR systems, and protect turbocharger temperature-sensitive components.

Figure7 RTD Pt200 EGT sensor.jpg

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