June 08, 2021
Temperature sensors are the core part of temperature measuring instruments. There are many varieties. According to the measurement method, they can be divided into two categories: contact type and non-contact type.
Contact temperature sensor
The detection part of the contact temperature sensor has good contact with the measured object, and achieves thermal equilibrium through conduction or convection, so that the indication value of the thermometer can directly indicate the temperature of the measured object.
Generally, the measurement accuracy is high. Within a certain temperature measurement range, the thermometer can also measure the temperature distribution inside the object. But for moving objects, small targets or objects with small heat capacity, larger measurement errors will occur.
Commonly used contact temperature sensors include bimetal thermometers, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors and thermocouples.
Non-contact temperature sensor
The non-contact temperature sensor is not in contact with the measured object. It can be used to measure the surface temperature of moving objects, small targets and objects with small heat capacity or rapid temperature changes (transient). It can also be used to measure the temperature distribution of the temperature field.
The working principle of the most commonly used non-contact temperature sensor is the radiation temperature measurement method based on the basic law of black body radiation.
Radiation thermometry includes the luminance method (see optical pyrometer), radiation method (see radiation pyrometer) and colorimetric method (see colorimetric thermometer). All kinds of radiation temperature measurement methods can only measure the corresponding luminosity temperature, radiation temperature or colorimetric temperature. Only the temperature measured for a black body (an object that absorbs all radiation and does not reflect light) is the true temperature. The surface emissivity of the material not only depends on the temperature and wavelength, but also on the surface state, coating film and microstructure, so it is difficult to accurately measure.
Among them, infrared temperature sensor is a particularly common one. After the infrared sensor receives the energy emitted by the object, through its optical system, it can convert the infrared energy into an electrical signal, and then convert it into a temperature value.
Four temperature sensors commonly used in life
Temperature sensor is one of the most commonly used sensors. All types of equipment use temperature sensors, including computers, cars, kitchen appliances, air conditioners, and home thermostats.
The most common temperature sensors are mainly divided into four types, including thermocouples, thermistors, resistance temperature detectors (RTD) and IC temperature sensors. Among them, IC temperature sensors include analog output and digital output.
In industrial facilities, more than 90% or even more of the temperature monitoring is done by thermal resistance (RTD) and thermocouple (T/C).
Thermocouple: wide temperature measurement range, cheap
The thermocouple is composed of two metal wires of different materials, which are welded together at the ends to form a loop. This end connection point is called Hot Junction, and the other end of the metal wire connection is called the cold junction.
When we heat the hot end, because the two metal wires have different materials (different thermal conductivity), the temperature of the cold end is different from the temperature of the hot end. When there is a temperature difference between the hot end and the cold end, an electromotive force will be generated in the circuit. The direction and magnitude of the electromotive force are related to the material of the conductor and the temperature of the two ends (thermoelectric effect). Connect the cold junction with the display instrument to display the thermoelectromotive force generated by the thermocouple. By querying the thermocouple index table, the temperature of the measured medium can be obtained.
Thermocouples are the simplest and most versatile temperature sensors. The main benefits are wide temperature range and adaptability to various atmospheric environments, and they are robust, low in price, and do not require power supply, especially the cheapest.
Commonly used thermocouples can measure continuously from -50 to +1600°C, and some special thermocouples can measure as low as -269°C (such as gold, iron, nickel and chromium), and up to +2800°C (such as tungsten-rhenium).
But the disadvantage is that the accuracy is not high enough, so it is not suitable for high-precision applications.
Thermistor: small size, fast response
Thermistor is a kind of temperature sensor device, made of semiconductor, its resistance changes with temperature, and its resistance value also changes. For different metals, every degree of temperature changes, the change in resistance value is different, and the resistance value It can also be directly used as an output signal.
Because the thermistor is a semiconductor, it is mostly a negative temperature system, that is, the resistance decreases with increasing temperature, and a small change in temperature will cause a large resistance change, so it is the most sensitive temperature sensor.
However, the thermistor has extremely poor linearity and exhibits a highly non-linear resistance-temperature curve. This has a lot to do with the production process, so the manufacturer cannot give a standardized thermistor curve.
The thermistor is very small in size, responds quickly to temperature changes, is extremely sensitive to self-heating errors, and has high accuracy, but it needs to use a current source, the temperature measurement range is small, and the price is more expensive.
RTD (Resistance Temperature Detector): the most accurate and stable
RTDs are similar to thermistors in that the resistance changes with temperature. As long as the resistance of the temperature-sensitive thermal resistance changes, the temperature can be measured. The difference is that the thermistor is made of semiconductor material, while the RTD is made of metal.
RTD is usually made of platinum, so RTD is also called platinum resistance, and some parts are made of nickel or copper. RTD can take many different shapes, such as winding, film and so on.
RTD is the most accurate and stable temperature sensor, its linearity is better than thermocouple and thermistor. But RTD is the most expensive temperature sensor. Therefore, RTD is most suitable for applications that have strict requirements on accuracy, but speed and price are not critical.
IC type temperature sensor: direct reading
IC temperature sensor (temperature integrated circuit) is a digital temperature sensor, which has a very linear voltage/current-temperature relationship. Some IC sensors even have a digital output form that represents temperature and can be directly read by a microprocessor.
Current sensor (left) and voltage sensor (right)
Temperature ICs provide easy-to-read readings that are proportional to temperature. They are also very cheap, but the disadvantages are that they are limited by configuration and speed, temperature measurement range is very limited, self-heating, unstable, and the need for external power supply problems.
Because the temperature IC needs an external power supply, it is usually embedded in the circuit and not used for detection.