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What are the common temperature and humidity sensors?

by:Rika Sensors     2021-09-23
What are the common temperature and humidity sensors?
In the past, temperature and humidity sensors were relatively simple, but with the maturity of technology and technological progress, the development of temperature and humidity sensors is getting better and better now. Since temperature and humidity are closely related to both physical quantities and actual people's lives, a sensor integrating temperature and humidity will be produced accordingly. Temperature and humidity sensor refers to a device or device that can convert temperature and humidity into electrical signals that can be easily measured and processed. Temperature and humidity sensors on the market generally measure temperature and relative humidity.

Combined with the types of sensors currently on the market, even temperature and humidity sensors, this type of sensor will be divided into many types and there are many types. Of course, their application areas are also very different. Let’s take a closer look at what types of humidity sensors are there?

According to the monitoring method, temperature and humidity sensors are divided into contact type and non-contact type.

Contact type: The detection part of the contact temperature sensor and the measured part The object has good contact, also called a thermometer. The thermometer achieves thermal equilibrium through conduction or convection, so that the 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 thermometers include bimetallic thermometers, glass liquid thermometers, pressure thermometers, resistance thermometers, thermistors, and thermocouples.

Non-contact type: Its sensitive components and the measured object do not touch each other, also known as non-contact temperature measuring instrument. This kind of instrument can be used to measure the surface temperature of moving objects, small targets and objects with small heat capacity or rapid temperature changes (transient), and it can also be used to measure the temperature distribution of the temperature field.

The most commonly used non-contact temperature measuring instrument is based on the basic law of black body radiation and is called a radiation temperature measuring instrument. Radiation thermometry includes 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.

The temperature and humidity sensor is also divided into two types: split type and integrated type. The integrated type is introduced above, and the split type is introduced below.

The split type is composed of a temperature sensor and a humidity sensor.

Temperature sensors can be classified into three categories: platinum thermistor temperature sensors, thermocouple temperature sensors, and thermistor temperature sensors.

1: Platinum thermal resistance temperature sensor

Platinum thermal resistance is designed and manufactured by using the basic principle that the resistance value of platinum wire changes with the change of temperature, according to the resistance value R( ℃) is divided into 10 ohms (graduation number Pt10) and 100 ohms (graduation number Pt100), etc., and the temperature measurement range is -200~850℃.

The models that use PT100 platinum thermal resistance as the temperature sensing element include armored type, assembly type, socket type, and end surface thermal resistance. It is mainly used in industries that require small temperature errors or precision instruments.

2: Thermocouple temperature sensor

Thermocouple is the most commonly used temperature sensor in temperature measurement. Its main advantages are wide temperature range and adaptability to various atmospheric environments, and it is strong, low in price, does not require power supply, and is the cheapest. A thermocouple is composed of two different metal wires (metal A and metal B) connected at one end. When one end of the thermocouple is heated, there is a potential difference in the thermocouple circuit. The corresponding temperature change is obtained through the change of electric potential. Thermocouples are the simplest and most versatile temperature sensors, but thermocouples are not suitable for high-precision measurements and applications.

3: Thermistor

is composed of metal oxide ceramics and is a low-cost, most sensitive temperature sensor.

Thermistors are made of semiconductor materials, mostly with a negative temperature coefficient, that is, the resistance decreases with increasing temperature. Temperature changes will cause large resistance changes, so it is the most sensitive temperature sensor. However, the linearity of the thermistor is extremely poor and has a lot to do with the production process. The thermistor measures the absolute temperature on the two lines, with better accuracy, but it is more expensive than a thermocouple, and the measurable temperature range is also smaller than that of a thermocouple. A commonly used thermistor has a resistance of 5kΩ at 25°C, and a temperature change of 1°C causes a resistance change of 200Ω. Note that the lead resistance of 10Ω only causes a negligible error of 0.05°C. It is very suitable for current control applications that require fast and sensitive temperature measurement. Small size is advantageous for applications with space requirements, but care must be taken to prevent self-heating errors.

The humidity sensor of the humidity sensor is divided into two types: resistance type and capacitive type.

The characteristic of the humidity sensitive resistor is to cover a film made of moisture-sensitive material on the substrate. When the water vapor in the air is adsorbed on the moisture-sensitive film, the resistivity and resistance value of the element will change. Use this feature to measure humidity.

Humidity sensitive capacitors are generally made of polymer film capacitors. Commonly used polymer materials include polystyrene, polyimide, butyrate acetate, etc. When the environmental humidity changes, the dielectric constant of the humidity sensitive capacitor changes, and its capacitance also changes. The capacitance change is proportional to the relative humidity.

Common humidity measurement methods are: dynamic method (double pressure method, dual temperature method, split flow method), static method (saturated salt method, sulfuric acid method), dew point method, dry and wet bulb method and various electronic sensors Law.

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