Temperature sensor definition
A temperature sensor is a sensor that senses temperature and converts it into a usable output signal. Temperature sensors are the core part of the temperature measurement appearance, and there are many varieties. The temperature sensor is very accurate in measuring the ambient temperature, and is widely used in agriculture, industry, workshops, warehouses and other fields.
The prehistory of temperature sensors
In 1600 A.D., Galileo Galilei developed a gas thermometer. One hundred years later, the final thermometer and mercury thermometer were developed. With the needs of the development of modern industrial technology, metal wire resistance, temperature difference electromotive force elements, and bimetallic temperature sensors have been developed one after another. Since 1950, semiconductor thermistors have been developed and manufactured. Recently, with the rapid development of raw materials and processing technology, various types of temperature sensors have been developed one after another.
Classification of temperature sensors
According to the measurement method, it can be divided into two categories: touch type and non-touch type.
1. Touch type
The detection part of the touch temperature sensor has an outstanding touch with the measured guide, also known as the thermometer.
The thermometer reaches thermal equilibrium by conduction or convection, so that the thermometer indication can directly represent the temperature of the measured policy.
Generally, the measurement accuracy is high. Within a certain scale of temperature measurement, the thermometer can also measure the temperature dispersion inside the object. However, the commonly used thermometers include bimetal thermometers, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors and thermocouples. They are widely used in industrial, agricultural, commercial, and other sectors. These thermometers are also often used in everyday life.
2. Non-touch
Its sensitive components do not touch each other with the measured guide, also known as non-touch temperature measurement appearance. This appearance can be used to measure the surface temperature of moving objects, small routes, and small heat capacity or rapid temperature changes (transient) directions, as well as to measure the temperature dispersion of the temperature field.
*The commonly used non-touch temperature measurement surface is called the radiation temperature measurement surface according to the basic law of blackbody radiation. All kinds of radiation temperature measurement methods can only measure the corresponding photometric temperature, radiation temperature or colorimetric temperature. Only the temperature measured on a black body (an object that absorbs all radiation but does not reflect light) is the actual temperature. If the actual temperature of an object is to be determined, the apparent emissivity of the data must be corrected. The surface emissivity of the data not only depends on the temperature and wavelength, but also depends on the surface condition, coating film and microscopic arrangement, so it is difficult to measure.
The advantage of non-touch temperature sensors is that the upper limit of measurement is not constrained by the temperature resistance of the temperature sensing element, so there is no restriction on the highest measurable temperature in principle.
According to the sensor data and the characteristics of electronic components, it is divided into two categories: thermal resistance and thermocouple.
1. Thermal resistance
Thermistors are made of semiconductors, and most of them have a negative temperature coefficient, that is, the resistance value decreases with the increase of temperature.
Temperature changes can result in large resistance changes, making it the most sensitive temperature sensor. However, the linearity of thermistors is extremely poor and has a lot to do with the production process.
Thermistors also have their own measurement techniques. The advantage of the thermistor is its small size, it stabilizes quickly and does not form a thermal load. However, it is also very unrobust, and high currents will form self-heating. Because a thermistor is a resistive device, any current source will generate heat on it due to power. The power is equal to the product of the square of the current and the resistance. Therefore, a small current source should be used. If the thermistor is exposed to high heat, it will cause permanent damage.
2. Thermocouples
Thermocouples are the most commonly used temperature sensors in temperature measurement. Its main advantages are a wide temperature scale and adaptability to various atmospheric environments, and it is robust, inexpensive, does not require power supply, and is also the most inexpensive. Thermocouples are the most simple and universal temperature sensors, but thermocouples are not suitable for high-precision measurements and applications.
According to the form of the output signal of the temperature sensor, it can be roughly divided into three categories: digital temperature sensor, logic output temperature sensor, and analog temperature sensor.
1. Digital temperature sensor
It is a digital temperature sensor produced in a silicon process with a PTAT structure, which is a semiconductor structure with the best temperature-dependent output characteristics.
2. Logic output temperature sensor
In many applications, we do not need to measure the temperature value strictly, only care if the temperature exceeds a set scale, once the temperature exceeds the specified size, an alarm signal is issued, and the fan, air conditioner, heater or other control equipment is activated or closed, in this case the logic output temperature sensor can be selected
3. Imitation temperature sensor
Mimicking temperature sensors such as thermocouples, thermistors, and RTDS to monitor temperature, the linearity is not good in some temperature scales, and cold junction compensation or lead compensation is required; The thermal inertia is large, and the response time is slow. Compared with the integrated analog temperature sensor, it has the advantages of high sensitivity, good linearity, and fast response speed, and it also integrates the drive circuit, signal processing circuit and necessary logic control circuit on a single chip, which has the advantages of small practical scale and easy application. Common analog temperature sensors are LM3911, LM335, LM45, AD22103 voltage output type, and AD590 current output type.
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