Shulou(Shulou.com)11/24 Report--

According to physiologists, the temperature and humidity of the environment will directly affect the thermoregulation function and heat conduction effect. So that people's somatosensory fitness is good or bad, which reflects the agility of thinking activities and the excellent state of mind, thus affecting the efficiency of our study and work. After experimental analysis, the most suitable room temperature for human body should be 18 ℃, and the humidity should be 40% to 60%. In people's production and life, there are many different places and environments have specific requirements for temperature and humidity, therefore, reasonable temperature and humidity control has become a necessary means.

Temperature and humidity sensor has a close relationship between temperature and humidity not only from the physical quantity itself, but also in people's life, so it produces an integrated sensor of temperature and humidity.

A temperature and humidity sensor is a device or device that can convert temperature and humidity measurements into electrical signals that can be easily measured and processed. The temperature and humidity sensors on the market generally measure temperature and relative humidity.

Temperature and humidity

Let's first take a look at several physical quantities of temperature and humidity:

Temperature is one of the seven basic physical quantities in the international system of units, which measures the heat and cold of an object. In production and scientific research, many physical phenomena and chemical processes are carried out at a certain temperature, and people's life is also closely related to it.

Humidity has a close relationship with life a long time ago, but it is difficult to express it in terms of quantity.

The most commonly used physical quantity of humidity in daily life is the relative humidity of the air. Expressed in% RH. There is a close relationship between relative humidity and temperature in the derivation of physical quantities. For a certain volume of airtight gas, the higher the temperature, the lower the relative humidity, and the lower the temperature, the higher the relative humidity. It involves complex knowledge of thermal engineering.

Some definitions of humidity:

Relative humidity is defined as "the amount of the object's image state" in the measurement method. The humidity referred to in daily life is relative humidity, expressed by RH%. In short, that is, the percentage of saturated water vapor (saturated water vapor pressure) in a gas (usually air) when it contains the same amount of water vapor (water vapor pressure) as its air.

Absolute humidity refers to the actual amount of water vapor per unit volume of air, usually in grams. Temperature has a direct effect on absolute humidity. In general, the higher the temperature, the more water vapor emitted, the greater the absolute humidity; on the contrary, the absolute humidity is smaller.

Saturated humidity the maximum amount of water vapor that can be contained in the air per unit volume at a certain temperature. If this limit is exceeded, the excess water vapor will condense and become water droplets, and the humidity of the air at this time becomes called saturation humidity. The saturated humidity of the air is not fixed, it changes with the change of temperature. The higher the temperature, the more water vapor per unit volume of air and the greater the saturated humidity.

Dew point refers to air containing a certain amount of water vapor (absolute humidity). When the temperature drops to a certain extent, the water vapor will reach saturation (saturated humidity) and begin to liquefy into water. This phenomenon is called condensation. The temperature at which water vapor begins to liquefy into water is called "dew point temperature" or "dew point". If the temperature continues to drop below the dew point, the supersaturated water vapor in the air will condense into water droplets on the surface. In addition, the wind is closely related to the temperature and humidity in the air, and it is also one of the important factors that affect the change of air temperature and humidity.

Types of temperature sensors temperature sensors have contact and non-contact according to the detection method.

Contact temperature sensor the detection part of the contact temperature sensor has good contact with the tested object, which is also called thermometer.

The thermometer achieves heat balance through conduction or convection, so that the indicated value of the thermometer can directly represent the temperature of the object to be measured.

Generally speaking, the measurement accuracy is high. Within a certain range of temperature measurement, the thermometer can also measure the temperature distribution inside the object. However, large measurement errors will occur for moving bodies, small targets or objects with very small heat capacity. the commonly used thermometers are bimetal thermometer, glass liquid thermometer, pressure thermometer, resistance thermometer, thermistor and thermocouple and so on.

They are widely used in industry, agriculture, commerce and other sectors. These thermometers are also often used in daily life.

Non-contact temperature sensor its sensitive element does not contact with the measured object, which is also called non-contact temperature measuring instrument. This instrument can be used to measure the surface temperature of moving objects, small targets and objects with small or rapidly changing heat capacity, as well as the temperature distribution of the temperature field.

The most commonly used non-contact temperature measuring instrument is called radiation temperature measuring instrument, which is based on the basic law of blackbody radiation.

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 photometric temperature, radiation temperature or colorimetric temperature. Only the temperature measured for the blackbody (an object that absorbs all the radiation and does not reflect light) is the true temperature. If you want to determine the true temperature of an object, it is necessary to correct the surface emissivity of the material. The surface emissivity of materials depends not only on temperature and wavelength, but also on surface state, coating film and microstructure, so it is difficult to measure accurately.

In automatic production, it is often necessary to use radiation temperature measurement to measure or control the surface temperature of some objects, such as steel strip rolling temperature, roll temperature, forging temperature and the temperature of various molten metals in smelting furnace or crucible.

The advantage of non-contact temperature measurement: the upper limit of measurement is not limited by the temperature resistance of the temperature sensor, so there is no limit to the maximum measurable temperature in principle. For the high temperature above 1800 ℃, the non-contact temperature measurement method is mainly used. With the development of infrared technology, radiation temperature measurement has gradually expanded from visible light to infrared. It has been used below 700℃ up to room temperature, and the resolution is very high.

According to the working principle, there are the following forms of temperature sensors:

The sensor metal designed by the metal expansion principle will produce a corresponding extension after the change of ambient temperature, so the sensor can convert the signal of this reaction in different ways.

Bimetal sheet sensor bimetal sheet is composed of two pieces of metal with different expansion coefficient affixed together. With the change of temperature, the expansion degree of material An is higher than that of other metal, causing metal sheet to bend. The curved curvature can be converted into an output signal.

As the temperature increases, the length of the metal tube (material A) increases, while the length of the non-expansive steel rod (metal B) does not increase, so that the linear expansion of the metal tube can be transferred due to the change of position. In turn, this linear expansion can be converted into an output signal.

When the temperature of the sensor designed by the deformation curve of liquid and gas changes, the volume of liquid and gas changes accordingly.

Many types of structures can convert this change of expansion into a change of position, resulting in a change of position output (potentiometer, inductive deviation, baffle, etc.).

Thermistor temperature sensor thermistor is made of semiconductor materials, most of which are negative temperature coefficient, that is, the resistance decreases with the increase of temperature.

The change of temperature will cause a big change in resistance, so it is the most sensitive temperature sensor. However, the linearity of thermistor is very poor, and it has a lot to do with the production process. The manufacturer can not give a standardized thermistor curve.

The thermistor is very small and responds quickly to temperature changes. However, the thermistor needs to use a current source, and its small size makes it extremely sensitive to self-heating errors.

The thermistor measures the absolute temperature on the two lines with better accuracy, but it is more expensive than the thermocouple, and the measurable temperature range is also smaller than that of the thermocouple. It is ideal for current control applications that require fast and sensitive temperature measurement. Small size is beneficial for applications with space requirements, but attention must be paid to prevent self-heating errors.

Thermistors also have their own measurement skills. The thermistor has the advantage of small size, it can be quickly stable and will not cause thermal load. However, it is also very weak, and high current will cause self-heating. Because the thermistor is a resistive device, any current source will cause heat due to power on it. So use a small current source. If the thermistor is exposed to high heat, it will cause permanent damage.

Thermocouple temperature sensor Thermocouple consists of two metal wires of different materials, welded together at the end. When one end of the thermocouple is heated, there is a potential difference in the thermocouple circuit. The measured potential difference can be used to calculate the temperature. Because it must have conductors of two different materials, it is called a thermocouple. Thermocouples made of different materials are used in different temperature ranges, and their sensitivity is also different. The sensitivity of the thermocouple refers to the change of the output potential difference when the hot spot temperature changes by 1 ℃. For most metal-supported thermocouples, this value is about 5 to 40 microvolts / ℃.

Because the sensitivity of the thermocouple temperature sensor is independent of the thickness of the material, it can also be made into a temperature sensor with very fine materials. Also because the metal material for making thermocouple has good ductility, this kind of fine temperature measuring element has extremely high response speed and can measure the process of rapid change.

Thermocouple is the most commonly used temperature sensor in temperature measurement. Its main advantages are a wide temperature range and adaptation to a variety of atmospheric environments, and strong, low price, no power supply, but also the cheapest. Thermocouple is the simplest and most universal temperature sensor, but thermocouple is not suitable for high-precision measurement and application.

Types of humidity sensors humidity sensors are divided into resistive and capacitive humidity sensors.

The characteristic of the humidity sensitive resistor is that the substrate is covered with a film made of humidity sensing material. when the water vapor in the air is adsorbed on the humidity sensing film, the resistivity and resistivity of the element change, which can be used to measure humidity.

Humidity sensitive capacitors are generally made of polymer film capacitors, and the commonly used polymer materials are polystyrene, polyimide, butyrate acetate fiber and so on. When the ambient humidity changes, the dielectric constant of the humidity sensitive capacitor changes, so that the capacitance also changes, and the capacitance change is proportional to the relative humidity.

The common humidity measurement methods are: dynamic method (double pressure method, double temperature method, shunt method), static method (saturated salt method, sulfuric acid method), dew point method, dry and wet ball method and various electronic sensor methods.

The dynamic method is based on the equilibrium principle of thermodynamics P, V and T, and the shunt method is based on the accurate mixing of absolute moisture and absolute dry air. Due to the use of modern measurement and control means, these equipment can be made very precise, but because the equipment is complex, expensive, time-consuming and time-consuming, it is mainly used as standard measurement, and its measurement accuracy can reach ±2% RH-±1.5 RH.

The saturated salt method in the static method is the most common method in humidity measurement, which is simple and feasible. However, the saturated salt method has strict requirements for the balance of liquid and gas phases and for the stability of environmental temperature. When used, it takes a long time to balance, and the low wet point is even longer. Especially when the difference between the indoor humidity and the humidity in the bottle is large, it needs to be balanced for 6 to 8 hours each time.

The dew point method is used to measure the temperature of wet air when it reaches saturation. It is a direct result of thermodynamics with high accuracy and wide measuring range. The precision dew point meter for metering can achieve an accuracy of ±0.2 ℃ or more. However, the cold mirror dew point meter based on modern optoelectronic principle is expensive and is often used with standard humidity generator.

Dry-wet method for measuring dry-wet bulbs, which was invented in the 18th century. It has a long history and is the most commonly used. The dry-wet bulb method is an indirect method, which uses the dry-wet bulb equation to convert the humidity value, and this equation is conditional, that is, the wind speed near the wet bulb must reach more than 2.5m / s. The commonly used dry and wet bulb thermometer simplifies this condition, so its accuracy is only 50.7% RH, which is significantly lower than that of electronic humidity sensors. Obviously, the dry and wet bulb does not belong to the static method, so we should not simply think that improving the measurement accuracy of the two thermometers is equal to improving the measurement accuracy of the hygrometer.

Electronic humidity sensor the accuracy of electronic humidity sensor can reach 2-3% RH, which is higher than that of dry and wet bulb humidity measurement.

The linearity and anti-pollution of the humidity sensor are poor. When testing the environmental humidity, the humidity sensor should be exposed to the environment for a long time, which is easy to be contaminated and affect its measurement accuracy and long-term stability. The method of measuring humidity with dry and wet bulbs is not as good as that with dry and wet bulbs.

Selection of temperature and humidity sensors the following points for attention should be paid to temperature and humidity sensors:

① selects the measurement range

Like measuring weight and temperature, the first thing to choose a humidity sensor is to determine the measurement range. Except for meteorological and scientific research departments, temperature and humidity measurement and control generally do not need full wet range (0-100% RH) measurement.

② chooses measurement accuracy

The measurement accuracy is the most important index of the humidity sensor. Every increase of-percentage point is a step up or even a higher level for the humidity sensor. Because in order to achieve different precision, the manufacturing cost is very different, and the price is also very different. Therefore, users must be tailored to suit their clothes, and should not blindly pursue "high, refined, sharp". If the humidity sensor is used at different temperatures, the indicated value should also take into account the effect of temperature drift.

As we all know, the relative humidity is a function of temperature, and the temperature seriously affects the relative humidity in the specified space. Every change of temperature is 0.1℃. Will produce a humidity change of 0.5 RH (error). If it is difficult to achieve constant temperature in use, it is not appropriate to propose too high humidity measurement accuracy. In most cases, if there is no precise means of temperature control, or if the space under test is unsealed, an accuracy of ±5% RH is sufficient.

For the local space that requires accurate control of constant temperature and humidity, or the need to track and record humidity changes at any time, select a humidity sensor with an accuracy of more than ±3 RH. The accuracy higher than ±2% RH may be difficult even for the standard humidity generator to calibrate the sensor, let alone the sensor itself. Even at 20-25 ℃, it is difficult for relative humidity measuring instruments to achieve an accuracy of 2% RH. The characteristics given in the product data are usually measured at room temperature (20 ℃ ±10 ℃) and in clean gases.

③ considers time drift and temperature drift

In practical use, due to the influence of dust, oil and harmful gases, the electronic humidity transmitter will age and its accuracy will decrease over a long time. The annual drift of the electronic humidity sensor is generally about ±2%, or even higher. In general, the manufacturer will indicate that the effective use time of the first calibration is 1 year or 2 years, and it needs to be re-calibrated when it expires.

Other considerations for ④

The humidity sensor is unsealed. In order to protect the accuracy and stability of the measurement, it should be avoided as far as possible in acidic, alkaline and organic solvent atmosphere. Also avoid using in dusty environment.

In order to correctly reflect the humidity of the space to be measured, you should also avoid placing the sensor in a dead corner that is too close to the wall or where the air is not ventilated. If the room being tested is too large, multiple sensors should be placed.

Some humidity sensors have high requirements for power supply, otherwise it will affect the measurement accuracy. Or the sensors interfere with each other or even fail to work. When in use, suitable and accurate power supply should be provided in accordance with the technical requirements.

When the sensor needs to transmit the signal over a long distance, we should pay attention to the attenuation of the signal. When the transmission distance is more than 200m, it is recommended to choose the humidity sensor with current output signal.

The future temperature and humidity sensor market has a broad prospect, especially in the fields of consumer electronics and the Internet of things. The products of IC semiconductor temperature and humidity sensor with small size, low power consumption, low cost and high integration will be widely used.

This article comes from the official account of Wechat: sensor Technology (ID:WW_CGQJS)

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.