Working principle of temperature sensor
Temperature sensor is an early development, widely used a kind of sensor, refers to can feel the temperature and converted into available output signal sensor, is the core of the temperature measurement instrument, a wide variety, mainly based on the following working principle:
Sensors designed on the principle of metal expansion: When the temperature of the environment changes, the metal will produce a corresponding extension, so the sensor can signal this reaction in different ways.
Bimetallic sheet sensor: bimetallic sheet is composed of two pieces of metal with different expansion coefficients stuck together. As the temperature changes, material A expands more than another metal, causing the metal sheet to bend. The curvature of the bend can be converted into an output signal.
Bimetallic rod and metal tube sensor: As the temperature increases, the length of the metal tube (material A) increases, while the length of the non-expanding steel rod (metal B) does not increase, so that the linear expansion of the metal tube can be transmitted due to the change of position. In turn, this linear expansion can be converted into an output signal.
Liquid and gas deformation curve design sensor: when the temperature changes, the liquid and gas will also produce corresponding volume changes.
Various types of structures can convert this change in expansion into a change in position, which produces a change in position output (potentiometers, sensing deviations, baffle plates, etc.).
Classification of temperature sensors
The market share of temperature sensor greatly exceeds that of other sensors. Temperature sensors are used in a wide range of applications, including food processing, HVAC environmental control, medical devices, chemical handling, and automotive under hood monitoring (e.g. coolant, intake pipe, cylinder head temperature, etc.). Temperature sensors are used to measure heat to ensure that a process, in response to extreme heat, danger, or inaccessible measurement points, is either stable within a range that makes the application safe or meets the requirements of a mandatory condition. There are different types of temperature sensors according to different classification standards.
Distinguish by method of measurement
According to the measurement method, temperature sensors can be divided into contact and non-contact two categories.
The detection part of the contact temperature sensor has good contact with the measured object, also known as the thermometer.
Thermometers reach thermal equilibrium by conduction or convection, so that the thermometer can directly indicate the temperature of the object being measured.
Generally, the measurement accuracy is higher. Within a certain temperature range, a thermometer can also measure the temperature distribution inside a body. But for the moving body, small target or small heat capacity of the object will produce a large measurement error, commonly used thermometers are bimetallic thermometer, glass liquid thermometer, pressure thermometer, resistance thermometer, thermistor and thermocouple. They are widely used in industry, agriculture, commerce and other sectors. People often use these thermometers in their daily lives.
The non-contact sensitive element and the measured object do not contact each other, also known as the non-contact thermometer table. This instrument can be used to measure the surface temperature of moving objects, small targets, and objects with small thermal capacities or rapidly changing temperatures (transients). It can also be used to measure the temperature distribution of temperature fields.
Commonly used non-contact thermometers are based on the fundamental law of blackbody radiation and are called radiometric thermometers. All kinds of radiometric temperature measurement methods can only measure the corresponding photometric temperature, radiation temperature or colorimetric temperature. Only the temperature measured for a blackbody (an object that absorbs all radiation but does not reflect light) is the true temperature. If the true temperature of the object is to be determined, the surface emissivity of the material must be modified. The surface emissivity of materials not only depends on temperature and wavelength, but also depends on surface state, film and microstructure, so it is difficult to measure accurately.
The advantage of the non-contact temperature sensor is that the upper limit of measurement is not limited by the temperature tolerance degree of the temperature sensing element, so there is no limit to the measurable temperature in principle.
By material and electronic component characteristics
According to the characteristics of sensor materials and electronic components, temperature sensors are divided into thermal resistance and thermocouple.
Thermistors are semiconductor materials, most of which have negative temperature coefficient, that is, the resistance value decreases with the increase of temperature.
Temperature changes can cause large resistance changes, so it is a more sensitive temperature sensor. But the linearity of thermistor is very poor, and has a great relationship with the production process.
Thermistors also have their own measuring techniques. The small size of the thermistor is an advantage because it can be quickly stabilized without causing a thermal load. But it's also very weak, and high currents can cause self-heating. Since a thermistor is a resistive device, any current source on it will cause heat due to power. Power is equal to the product of current squared and resistance. Therefore, use a small current source. If the thermistor is exposed to high heat, damage will result.
Thermocouple is a temperature sensor commonly used in temperature measurement. Its main benefits are wide temperature range and adaptation to various atmospheric environments, but also strong, low price, no power supply, and relatively cheap. Electrocouple is a simple and universal temperature sensor, but thermocouple is not suitable for high precision measurement and application.
According to the output signal mode of the temperature sensor
According to the output signal mode of the temperature sensor, the temperature sensor can be roughly divided into three categories: digital temperature sensor, logical output temperature sensor, and analog temperature sensor.
Digital Temperature sensor Digital temperature sensor produced by silicon process using PTAT structure, this semiconductor structure has accurate and good temperature dependent output characteristics.
Logic output temperature sensor. In many applications, we do not need to strictly measure the temperature value, only care about whether the temperature exceeds a set range, once the temperature exceeds the specified range, an alarm signal will be issued to start or shut down the fan, air conditioning, heater or other control equipment, at this time can choose the logic output temperature sensor.