What types of temperature sensors are there?

Thermocouples are among the most widely used temperature-sensing elements in industry. Their advantages include: ① High measurement accuracy, as thermocouples make direct contact with the measured object and are unaffected by intermediate media. ② A broad measurement range; standard thermocouples can continuously measure temperatures from –50°C to +1600°C, while certain specialized types can extend down to –269°C (e.g., gold–iron–nickel–chromium) or up to +2800°C (e.g., tungsten–rhenium). ③ Simple construction and ease of use; thermocouples typically consist of two dissimilar metal wires, with no restrictions on size or configuration, and are encased in a protective sheath, making them highly convenient for practical applications.

Principle of Operation of Temperature Sensors and Thermocouples

Thermocouple temperature sensors are among the most widely used temperature-sensing elements in industry. Their advantages include:
① High measurement accuracy. Since the thermocouple temperature sensor is in direct contact with the measured object, it is not affected by any intermediate medium.
② Wide measurement range. Commonly used thermocouple temperature sensors can continuously measure over a range of −50 to +1600°C; certain specialized thermocouples can even measure as low as −269°C (e.g., gold–iron–nickel–chromium) and as high as +2800°C (e.g., tungsten–rhenium).
③ Simple construction and easy to use. Thermocouple temperature sensors typically consist of two different metal wires and are not limited by size or configuration; they are encased in a protective sheath, making them very convenient to operate.
1. Basic Principle of Thermocouple Temperature Measurement Using a Temperature Sensor
When two conductors or semiconductors of different materials, A and B, are welded together to form a closed circuit, an electromotive force is generated between the two junctions, 1 and 2, if there is a temperature difference between them. This results in a current flowing through the circuit. This phenomenon is known as the thermoelectric effect. Temperature sensors based on thermocouples operate by exploiting this effect.
2. Types and Structural Configurations of Temperature Sensor Thermocouples
(1) Types of Temperature Sensors and Thermocouples
Common temperature‑sensing thermocouples can be classified into two major categories: standard thermocouples and non‑standard thermocouples. Standard thermocouples are those for which the national standard specifies the relationship between thermoelectric potential and temperature, the permissible error, and a unified calibration table; they are also accompanied by compatible indicating instruments. Non‑standard thermocouples, by contrast, fall short of standard thermocouples in terms of application range and magnitude of measurement; they typically lack a uniform calibration table and are used primarily for measurements in certain specialized applications. With respect to standardized thermocouples, since January 1, 1988, China has produced all thermocouples and resistance temperature detectors in accordance with IEC international standards, designating seven types—S, B, E, K, R, J, and T—as the country’s unified standard thermocouple series.
(2) Structural configurations of thermocouple temperature sensors
To ensure the reliable and stable operation of temperature‑sensing thermocouples, the following structural requirements apply:
① The welds between the two thermoelectric electrodes of the thermocouple used in the temperature sensor must be secure.
② The two thermoelectric electrodes must be well insulated from each other to prevent short circuits.
③ The connection between the compensation lead and the free end of the thermocouple temperature sensor shall be convenient and reliable.
④ The protective sheath shall ensure adequate isolation of the thermoelectric electrodes from harmful media.
3. Temperature Compensation for the Cold Junction of Thermocouple Temperature Sensors
Because the materials used for thermocouple temperature sensors are generally quite expensive—especially when precious metals are employed—and because the distance between the measurement point and the instrument can be considerable, compensation leads are typically used to extend the cold junction (free end) of the thermocouple to a control room where the temperature is relatively stable, connecting it to the instrument’s terminals. It must be emphasized that the role of thermocouple compensation leads is merely to extend the thermoelectric electrodes, relocating the cold junction to the instrument’s terminals in the control room; they do not eliminate the influence of cold-junction temperature variations on temperature measurement and thus provide no true compensation. Consequently, additional correction methods are still required to account for the effect of a non‑zero cold-junction temperature, t0 ≠ 0°C, on the measurement.
When using thermocouple compensation leads with temperature sensors, it is essential to ensure that the lead type matches the sensor and that the polarity is correctly connected. The temperature at the connection point between the compensation lead and the thermocouple must not exceed 100°C.
Principle of Operation of Resistance Temperature Detectors
Resistance temperature detectors are the most commonly used temperature sensors in the medium- and low-temperature ranges. Their key features include high measurement accuracy and stable performance. Among them, platinum resistance thermometers offer the highest accuracy; they are not only widely employed in industrial temperature measurement but are also used to fabricate standard reference instruments.
1. Principle and Materials of Temperature Measurement Using Resistance Temperature Detectors
Temperature measurement using resistance temperature detectors (RTDs) is based on the principle that the electrical resistance of metallic conductors increases with rising temperature. RTDs are typically made from pure metals; platinum and copper are the most widely used, while materials such as nickel, manganese, and rhodium are also increasingly employed. For example, Omega’s PT100 temperature sensor incorporates a 100‑ohm platinum resistance temperature probe.
2. Structure of the Temperature Sensor Thermistor
(1) Precision Temperature Sensors: Resistance Thermometers The temperature-sensing element (resistor body) of commonly used industrial resistance thermometers operates on the principle that changes in the measured temperature are directly reflected in variations of the resistor’s resistance. Consequently, any resistance introduced by lead wires or other connecting conductors can affect the accuracy of temperature measurements. To eliminate the influence of lead‑wire resistance, a three‑wire or four‑wire configuration is typically employed.
(2) Armored Temperature Sensor Thermoresistors An armored temperature sensor thermoresistor is a solid‑state device composed of a temperature‑sensing element (resistive body), lead wires, insulating materials, and a stainless steel sheath. Its outer diameter typically ranges from φ2 to φ8 mm, with the smallest possible size down to φ mm.
Compared with conventional resistance temperature detectors, it offers the following advantages: ① Small size, with no internal air gaps, resulting in low thermal inertia and minimal measurement lag; ② Excellent mechanical properties, vibration resistance, and impact resistance; ③ Flexible, making installation convenient; ④ Long service life.
(3) End-face Temperature Sensor Thermoresistor The temperature-sensing element of the end-face temperature sensor thermoresistor is wound from specially treated resistance wire and closely attached to the thermometer’s end face. Compared with conventional axial temperature sensor thermistors, it can more accurately and rapidly reflect the actual temperature of the measured end face, making it suitable for measuring the end-face temperatures of bearing shells and other machine components.
(4) Flameproof Temperature Sensor RTD The flameproof temperature sensor RTD, via a specially designed junction box, confines any explosion that might occur inside the enclosure—caused by sparks or arcs—to the junction box itself, thereby preventing ignition of the surrounding explosive atmosphere. This type of temperature sensor RTD is suitable for temperature measurement in hazardous areas classified as Zone Bla–B3c.
3. Composition of the Temperature Sensor and Resistance Thermometer Measurement System
A resistance‑temperature detector (RTD) temperature measurement system typically consists of an RTD sensor, connecting wires, and a display instrument. The following two points must be observed:
② The thermoresistance of the temperature sensor and the graduation code of the indicating instrument must be identical.
② To eliminate the influence of variations in the resistance of connecting leads, a three-wire connection method must be employed.

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