Principles and Applications of Temperature Sensors

     A temperature sensor is a device used to measure temperature. Its operating principle involves converting temperature into an electrical signal that can be measured and recorded. Based on their measurement principles, temperature sensors available on the market can be categorized into two types according to whether they require physical contact: contact-type and non-contact-type sensors. Alternatively, based on their output signals, temperature sensors can be classified into analog outputs (such as changes in resistance or voltage) and digital outputs (which directly provide digital temperature readings via protocols like I2C, SPI, or 1-Wire). These two classification methods are widely used. Below, we will discuss the working principles and application areas of temperature sensors based on the first classification method.

I. Contact-based

     The following are common types of contact sensors:

 (1) Thermocouple Sensor

     The primary working principle of a temperature sensor is the thermoelectric effect. The thermoelectric effect refers to the phenomenon in which a current flows in a circuit formed by connecting two different materials due to a temperature difference. This current is known as the thermoelectric current. Thermocouples utilize the thermoelectric effect to measure temperature.

     The fundamental principle of a thermocouple is to weld together two metal wires made of different materials, thereby forming a closed circuit. When there is a temperature difference at the junction of the two metal wires, a thermal current will flow in the circuit due to the thermoelectric effect. The magnitude of this thermal current depends on the properties of the two metals and the temperature difference between them. Therefore, by measuring the magnitude of the thermal current, it is possible to determine the temperature difference at the junction of the two metal wires.

(Note: Source: Internet)

      The core advantage of this type of sensor lies in its extremely wide measurement range, as well as its ability to withstand high temperatures, resist vibration, and respond rapidly. Consequently, it is often used in extreme-temperature environments such as industrial kilns, aircraft engines, and nuclear power equipment. However, the measurement accuracy of thermocouples is significantly affected by material consistency and the effectiveness of cold-end temperature compensation; therefore, they typically need to be used in conjunction with dedicated compensation circuits.

 (2) Thermal Resistance Sensor

     A thermistor is a temperature sensor that measures temperature based on the property that its resistance value changes with temperature. It takes advantage of the fact that the resistance of metallic conductors or semiconductors varies with temperature, and calculates the temperature by measuring the change in resistance. Most thermistors are made from pure metal materials; platinum and copper are the most commonly used, and materials such as nickel, manganese, and rhodium have also begun to be employed in the manufacture of thermistors.

     The temperature-measuring principle of resistance thermometers is based on the thermal effect of resistance—that is, the characteristic that the resistance value of the resistive element changes with temperature. For metallic resistance thermometers, the relationship between resistance and temperature can generally be expressed by the following approximate equation: R _t =R _t0 [1 + α(t - t ₀ )]. In the formula, R _t The resistance at temperature t; R _t0 For temperature t ₀ (The resistance value corresponding to t0 = 0℃ usually; α is the temperature coefficient.)

     The main types of resistance thermometers include standard resistance thermometers, armored resistance thermometers, surface-mounted resistance thermometers, and explosion-proof resistance thermometers. Among these, the armored resistance thermometer is a solid device composed of a temperature-sensitive element (resistance body), lead wires, insulating materials, and a stainless steel sheath. It boasts advantages such as small size, absence of air gaps inside, low thermal inertia, minimal measurement lag, and long service life. Resistance thermometers are among the most commonly used temperature sensors in the medium- and low-temperature ranges, characterized by high measurement accuracy and stable performance. Metal resistance thermometers are generally suitable for temperature measurements within the range of -200 to 500°C and are widely used in temperature detection and control applications in household appliances and automobiles. Semiconductor thermistors, on the other hand, typically have a temperature measurement range of around -50 to 300°C. They offer high measurement accuracy, excellent stability, and reliable performance, making them widely applicable in precision measurement scenarios such as process control.

 (4) Transistor Temperature Sensor

       A transistor temperature sensor is a type of sensor that measures temperature by leveraging the property of semiconductor materials, whose electrical characteristics change with temperature. These sensors typically consist of one or more transistors, and the base-emitter voltage (V) of these transistors... _BE ) or collector current (I _C ) will change with temperature variations.

      In a bipolar junction transistor (BJT) temperature sensor, the same transistor operates under two proportional currents, and the corresponding V is measured for each. _BE The voltage difference—this voltage difference is directly proportional to the absolute temperature and can effectively eliminate errors caused by process variations. Ultimately, an internal circuit within the chip converts this voltage signal into a high-precision digital temperature value for output. On the other hand, field-effect transistor (FET) temperature sensors measure temperature by leveraging the characteristic of FET drain current changing with temperature. Featuring high accuracy, fast response, low power consumption, and ease of integration, these sensors are widely used in industries such as industrial automation, healthcare, and environmental monitoring.

 (5) Expansion Thermometer

      This type of thermometer primarily measures temperature by exploiting the thermal expansion and contraction of materials. For example, a glass-tube thermometer uses the property of liquids—such as mercury or alcohol—to expand and contract with temperature changes, and it measures temperature variations based on the resulting difference in volume. A bimetallic thermometer, on the other hand, utilizes the characteristic that two metal strips with different coefficients of thermal expansion bend differently when heated, thereby driving a pointer. These thermometers are mainly used in field applications where high measurement accuracy is not required but where intuitive, stable, and maintenance-free readings are essential.

 (6) Integrated temperature sensor

      An integrated temperature sensor is a temperature-measuring device that integrates a temperature-sensitive element, signal-processing circuitry, and interface circuitry into a single package. Its core principle lies in sensing temperature changes and converting them into quantifiable electrical signals for output—for example, the single-bus temperature sensor DS18B20 can directly convert temperature readings into digital signals and transmit them to a microcontroller for processing, thereby eliminating the need for conventional peripheral circuits such as signal amplification and A/D conversion. These sensors are widely used in consumer electronics, computers, data acquisition systems, environmental monitoring, and other applications.

II. Non-contact

     These sensors measure the surface temperature of an object by detecting the thermal radiation it emits, without requiring any physical contact. During the pandemic, we deployed infrared thermometers. Infrared thermometers and thermal cameras operate based on the law of blackbody radiation: any object above absolute zero emits infrared radiation outward, and the energy of this radiation is proportional to the fourth power of the object’s surface temperature. The sensor uses an optical system to collect the infrared radiation energy, focuses it onto an infrared detection element, converts it into an electrical signal, and then calculates the temperature. They are primarily used for building energy efficiency inspections and medical applications.