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Thermo-hygrometer is the environmental condition used to determine the temperature and humidity of the environment to determine the production or storage of the product. It is used for temperature and humidity monitoring in vegetable greenhouses, storage rooms, clean rooms, museums, laboratories, etc.
Humidity concept
Scientists often use relative humidity to describe how much water is in the air. Simply put, it is imagined that the air is a towel. If you have a glass of water, you can use a towel to absorb the water. But in fact, the towel can actually absorb more water than a cup. Maybe he can absorb five to ten cups of water. The amount of moisture in the water is only a fraction of the moisture that can be in the air, so the relative humidity is a percentage. When the relative humidity is one hundred percent, the air is saturated. Like a towel that is as wet, the air can no longer take water. When the relative humidity is one hundred percent and the air is saturated, the evaporation and deposition are in an equilibrium state. The amount of evaporation increased as indicated by the equilibrium is reached as a moisture deposit.
Water vapor is called humidity in the air. Because the molecules of water vapor are so small, they cannot be seen, and people who study humidity have developed creative methods to measure the amount of water vapor.
History of hygrometer
Perhaps Leonardo, a man born in Italy in the 15th century, was the first to think of this instrument to measure the water vapor content of the air. He placed a dry cotton on one side of the scorpio. Then he placed an object that was exactly the same weight as cotton on the other side of the scorpio. When the dry cotton absorbs water vapor from the air, it becomes heavier and the side of this day begins to land. The difference between the two weights is a measure of humidity.
Scientists use a device called "psychrometer" to measure relative humidity. "psychrometer" is caused by two thermometers tied together. The bubble of a thermometer is wrapped in a material soaked in water. When starting to measure relative humidity, rotate the psychrometer until the enclosed thermometer maintains a stable temperature that is lower than the dry thermometer. The temperature measure at which the actual air temperature is dried. The difference between the two temperatures is called "wet-bulbdepression" which is the result of evaporation of water from the substance. The scientists recorded the temperature of the low-dry thermometer and "wet-bulbdepression" and then made a chart to calculate the relative humidity. This is also the working principle of the wet and dry bulb hygrometer.
Measuring principle
Humidity definition
In the metrology method, humidity is defined as the "amount of object state". The humidity referred to in daily life is relative humidity, expressed as RH%. In summary, the percentage of the amount of water vapor (water vapor pressure) in a gas (usually in air) and the amount of saturated water vapor (saturated water vapor pressure) in the same condition as air.
Humidity has a close relationship with life long ago, but it is more difficult to express it by quantity. The means of humidity are absolute humidity, relative humidity, dew point, moisture to dry gas ratio (weight or volume), and the like.
Measurement methods
The humidity measurement of the hygrometer has two or thirty kinds in principle. But humidity measurement has always been one of the famous problems in the world of metrology. A seemingly simple measure of value, involving quite complex physics-chemical theoretical analysis and calculations, may involve many factors that must be noted in humidity measurement, thus affecting the rational use of the sensor.
Common methods of measuring humidity include: dynamic method (double pressure method, double temperature method, split method), static method (saturated salt method, sulfuric acid method), dew point method, dry and wet ball method and electronic sensor method.
1 Double pressure method and double temperature method are based on the thermodynamic P, V, T balance principle, and the equilibrium time is long. The split method is based on the precise mixing of absolute moisture and absolute dry air. Due to the adoption of modern measurement and control methods, these devices can be made quite sophisticated, but because of the complexity and cost of the equipment, the operation is time-consuming and labor-intensive, mainly used as a standard measurement, and its measurement accuracy can reach ±2% RH or more.
2 The saturated salt method in the static method is the most common method in humidity measurement and is simple and easy. However, the saturated salt method requires strict balance between liquid and gas phases, and requires high stability to ambient temperature. It is required to wait for a long time to balance, and the low humidity point requires longer. Especially when the indoor humidity and the humidity inside the bottle are large, it needs to be balanced for 6~8 hours each time.
3 The dew point method is to measure the temperature at which the humid air reaches saturation. It is a direct result of thermodynamics, with high accuracy and wide measurement range. The accuracy of the precision dew point meter for measurement can reach ±0.2 °C or even higher. However, the cold mirror dew point meter using the modern light-electric principle is expensive and often used in conjunction with a standard humidity generator.
4 Dry and wet ball method, this is the method of measuring moisture invented in the 18th century. It has a long history and is most commonly used. The dry-wet ball method is an indirect method that converts the humidity value using the dry-wet ball equation, which is conditional: the wind speed near the wet ball must be above 2.5 m/s. The common dry and wet bulb thermometer simplifies this condition, so its accuracy is only 5~7% RH. The dry and wet bulbs are not static. Do not simply think that as long as the measurement accuracy of the two thermometers is increased, the humidity is increased. The measurement accuracy of the meter.
5 electronic humidity sensor method
Electronic humidity sensor products and humidity measurement belong to the industry that emerged in the 1990s, and domestic and foreign companies have made great progress in the field of humidity sensor research and development. Humidity sensors are rapidly evolving from simple humidity sensors to integrated, intelligent and multi-parameter detection, creating favorable conditions for the development of a new generation of humidity measurement and control systems, and raising humidity measurement technology to a new level.
Here are two things to understand: First, because humidity is a function of temperature, temperature changes decisively influence the measurement of humidity. Regardless of the method, accurate measurement and control of temperature is first. It should be noted that even in a well-insulated constant temperature and humidity chamber, there is a certain gradient in the temperature in the working chamber. Therefore, the humidity in this space is also difficult to be completely uniform.
Second, due to the large differences in principles and methods, it is difficult to directly calibrate and identify various measurement methods, and most of them can only be compared by indirect methods. Therefore, it is very difficult to calibrate the measurement results of the full wet range (relative humidity 0~100% RH) between the two moisture measuring methods, or to calibrate the measurement results of each point in all temperature ranges. For example, a ventilated dry and wet bulb hygrometer requires a flow of air at a specified wind speed, while a saturated salt method requires a strict seal, which cannot be compared. The best way is to determine the transmission method and verification procedures stipulated by the state verification system (standard) of the humidity measuring instrument.
Humidity plan
There are two main types of modern humidity measurement programs: wet and dry ball moisture measurement, and electronic humidity sensor moisture measurement. The two options are compared below so that the customer can choose the humidity measurement method that suits them.
The maintenance of the wet and dry ball moisture measurement method is quite simple. In actual use, it is only necessary to periodically add water to the wet ball and replace the wet ball gauze. Compared with the electronic humidity sensor, the wet and dry ball moisture measurement method does not cause aging, and the accuracy is lowered. Therefore, the wet and dry ball moisture measuring method is more suitable for use in high temperature and harsh environments.
Electronic humidity sensor features:
Electronic humidity sensors have developed rapidly in recent decades, especially in the past 20 years. The humidity sensor manufacturer must use a standard humidity generator to calibrate the product before leaving the factory. The accuracy of the electronic humidity sensor can reach 2% to 3% RH.
In actual use, due to the influence of dust, oil and harmful gases, the use time will cause aging and the accuracy will decrease. The annual drift of the humidity sensor is generally about ±2% or higher. Under normal circumstances, the manufacturer will indicate that the effective use time of one calibration is 1 year or 2 years, and it needs to be recalibrated after expiration.
The accuracy level of the electronic humidity sensor should be judged in combination with its long-term stability. In general, the long-term stability and service life of the electronic humidity sensor are not as good as the dry-wet ball humidity sensor.
Humidity sensors use semiconductor technology and therefore have a requirement for the ambient temperature to be used. Damage to the sensor will be exceeded if the specified operating temperature is exceeded.
Therefore, the electronic humidity sensor moisture measuring method is more suitable for use in clean and normal temperature applications.
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