Basic concept of the hottest humidity measurement

The basic concept of humidity measurement

in industrial and agricultural production, meteorology, environmental protection, national defense, scientific research, aerospace and other departments, it is often necessary to measure and control environmental humidity. The control of environmental temperature and humidity and the monitoring and analysis of the moisture value of industrial materials have become one of the more common technical conditions, but in the conventional environmental parameters, humidity is the most difficult parameter to measure accurately. This is because measuring humidity is much more complex than measuring temperature. Temperature is measured independently, while humidity is affected by other factors (atmospheric pressure, temperature). In addition, the calibration of humidity is also a difficult problem. Humidity calibration equipment produced abroad is very expensive

I. definition of humidity

the dynamic pendulum dynamometer method is adopted in the force measurement part of measuring them, and humidity is defined as "the quantity of object image state". Humidity in daily life refers to relative humidity, expressed in RH%. In general, that is, the standard expanded uncertainty of the 0.3 level standard dynamometer used to verify the amount of water vapor (water vapor pressure) in the gas (usually in the air) is ≤ 3 × 10 ⑶ (confidence factor k=3 is the percentage of saturated water vapor (saturated water vapor pressure) under the same condition as its air

humidity has been closely related to life for a long time, but it is difficult to express it in terms of quantity. The expression methods of humidity include absolute humidity, relative humidity, dew point, the ratio of moisture to dry gas (weight or volume), etc

II. Humidity measurement methods

humidity measurement can be divided into 20 or 30 kinds in principle. However, humidity measurement has always been one of the famous problems in the field of measurement in the world. A seemingly simple quantity involves quite complex physical-chemical theoretical analysis and calculation. Beginners may ignore many factors that must be paid attention to in humidity measurement, thus affecting the rational use of sensors

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

① the double pressure method and the double temperature method are based on the thermodynamic P, V, t balance principle, and the balance time is long. The split flow 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 quite precise, but because the equipment is complex, expensive, time-consuming and labor-consuming operation, it is mainly used for standard measurement, and its measurement accuracy can reach more than ± 2% RH

② saturated salt method in static method is the most common method in humidity measurement, which is simple and easy. However, the saturated salt method has strict requirements for the balance of liquid and gas phases and the stability of ambient temperature. It requires a long time to balance when used, and the low humidity point requires longer. Especially when the difference between indoor humidity and bottle humidity is large, it needs to be balanced for 6~8 hours each time

③ dew point method is to measure the temperature when wet air reaches saturation. It is the direct result of thermodynamics, with high accuracy and wide measurement range. The accuracy of the precision dew point meter used for measurement can reach ± 0.2 ℃ or even higher. However, the cold mirror dew point meter based on modern opto electric principle is expensive and is often used with standard humidity generator

④ dry wet bulb method, which was invented in the 18th century. It has a long history and is most widely used. Wet and dry bulb method is an indirect method, which can extend the larger protection interval to 12 years; 3. It can realize 100% recycling; 4. It is a method that can be used to manufacture all parts of aircraft. It uses the dry and wet ball equation to convert the humidity value, and this equation is conditional: that is, the wind speed near the wet ball must reach more than 2.5m/s. The common dry and wet bulb thermometer simplifies this condition, so its accuracy is only 5~7%rh, and the dry and wet bulb is not a static method. Don't simply think that improving the measurement accuracy of the two thermometers is equivalent to improving the measurement accuracy of the hygrometer

⑤ electronic humidity sensor method: electronic humidity sensor products and humidity measurement belong to the industry rising in the 1990s. In recent years, great progress has been made in the field of humidity sensor research and development at home and abroad. Humidity sensor is developing rapidly from simple humidity sensor to integrated, intelligent and multi parameter detection, which creates favorable conditions for the development of a new generation of humidity measurement and control system, and also improves the humidity measurement technology to a new level

III. selection of humidity measurement scheme

there are two main modern humidity measurement schemes: wet bulb measurement and electronic humidity sensor measurement. The two schemes are compared below, so that customers can choose their own humidity measurement method

characteristics of dry and wet bulb hygrometer:

as early as the 18th century, humans invented dry and wet bulb hygrometer. The accuracy of dry and wet bulb hygrometer also depends on the accuracy of dry and wet bulb thermometers; The hygrometer must be in a ventilated state: the specified accuracy can be achieved only when the gauze jacket, water quality and wind speed meet certain requirements. The accuracy of wet and dry bulb hygrometer is only 5% - 7% RH

the indirect measurement method is adopted for wet and dry bulb measurement. The humidity value is calculated by measuring the temperature of dry and wet bulbs. Therefore, there is no strict limit on the use temperature. Measuring humidity in high temperature environment will not cause damage to the sensor

the maintenance of wet and dry ball measurement is quite simple. In actual use, it is only necessary to add water to the wet ball regularly and replace the wet ball gauze. Compared with the electronic humidity sensor, the wet and dry bulb measurement will not cause aging, accuracy degradation and other problems. The construction of the new material system in the plastic industry is improved. The wet and dry ball method is more suitable for use in high temperature and harsh environments

characteristics of electronic humidity sensor:

electronic humidity sensor has developed rapidly in recent decades, especially in recent 20 years. Humidity sensor manufacturers should use standard humidity generators to calibrate one by one before products leave the factory. The accuracy of electronic humidity sensors can reach 2% to 3% RH

in actual use, due to the influence of dust, oil and harmful gases, aging will occur and the accuracy will decline after a long time of use. The annual drift of humidity sensor is generally about ± 2%, or even higher. Generally, the manufacturer will indicate that the effective use time of one calibration is 1 year or 2 years, and it needs to be recalibrated when it expires

the accuracy level of electronic humidity sensor should be judged in combination with its long-term stability. Generally speaking, the long-term stability and service life of electronic humidity sensor are not as good as dry and wet bulb humidity sensor

the humidity sensor adopts semiconductor technology, so there are requirements for the ambient temperature used. Exceeding the specified temperature will cause damage to the sensor

therefore, the humidity measurement method of electronic humidity sensor is more suitable for clean and normal temperature occasions

IV. precautions for the selection of humidity sensor

①. The selection of measurement range

is the same as the measurement of weight and temperature. The selection of humidity sensor must first determine the measurement range. Except for the meteorological and scientific research departments, those engaged in temperature and humidity measurement and control generally do not need full humidity range (%rh) measurement

② select the measurement accuracy

the measurement accuracy is the most important indicator of the humidity sensor. Every increase of one percentage point is a higher level, even a higher level, for the humidity sensor. Because to achieve different accuracy, the manufacturing cost and selling price vary greatly. Therefore, users must tailor their clothes and should not blindly pursue "high, fine and sharp"

if the humidity sensor is used at different temperatures, the influence of temperature drift should also be considered for its indication. As we all know, relative humidity is a function of temperature, which seriously affects the relative humidity in a designated space. Every 0.1 ℃ change in temperature. Will produce a humidity change (error) of 0.5% RH. If it is difficult to achieve constant temperature in the application occasion, it is inappropriate to put forward too high humidity measurement accuracy

in most cases, if there is no accurate temperature control means, or the measured space is not sealed, the accuracy of ± 5% RH is sufficient. For local spaces that require precise control of constant temperature and humidity, or where humidity changes need to be tracked and recorded at any time, humidity sensors with an accuracy of more than ± 3% RH shall be selected

the requirement of accuracy higher than ± 2% RH may be difficult for even the standard humidity generator calibrating the sensor, not to mention the sensor itself. It is still difficult for the relative humidity measuring instrument to reach the accuracy of 2% RH even at 20-25 ℃. Generally, the characteristics given in the product data are measured at room temperature (20 ℃ ± 10 ℃) and in clean gas

③ considering the time drift and temperature drift

in actual use, due to the influence of dust, oil and harmful gases, the electronic humidity transmitter will age and the accuracy will decline after a long time of use. The annual drift of the electronic humidity transmitter is generally about ± 2%, or even higher. Generally, the manufacturer will indicate that the effective use time of one calibration is 1 year or 2 years, and it needs to be recalibrated when it expires

④. Other precautions

humidity sensor is non sealing. In order to protect the accuracy and stability of measurement, it should be avoided to use in acidic, alkaline and organic solvent containing atmosphere. Also avoid using it in dusty environment. In order to correctly reflect the humidity of the space to be measured, it is also necessary to avoid placing the sensor too close to the wall or at a dead corner where the air is not circulating. If the measured room is too large, multiple sensors should be placed

some humidity sensors have high requirements for power supply, otherwise the measurement accuracy will be affected. Or the sensors interfere with each other and even fail to work. When in use, a suitable power supply meeting the accuracy requirements shall be provided according to the technical requirements

when the sensor needs long-distance signal transmission, pay attention to the attenuation of the signal. When the transmission distance is more than 200m, it is recommended to select the humidity sensor with frequency output signal. (end)