What Is Humidity Calibration?

The basic concept of humidity measurement is specified in the metrology method, and humidity is defined as "the quantity of the state of the object". The humidity referred to in daily life is relative humidity, expressed as RH%. In short, it is the percentage of saturated water vapor (saturated water vapor pressure) in the same amount of water vapor (water vapor pressure) in the gas (usually in the air).

Basic concepts of humidity measurement

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The basic concept of humidity measurement is specified in the metrology method, and humidity is defined as "the quantity of the state of the object". The humidity referred to in daily life is relative humidity, expressed as RH%. In short, it is the percentage of saturated water vapor (saturated water vapor pressure) in the same amount of water vapor (water vapor pressure) in the gas (usually in the air).

Industrial and agricultural production, meteorology, environmental protection, national defense, scientific research, aerospace and other departments often need to measure and control environmental humidity. The control of environmental temperature and humidity as well as the monitoring and analysis of the moisture value of industrial materials have become one of the more common technical conditions, but among the conventional environmental parameters, humidity is the most difficult parameter to accurately measure. This is because measuring humidity is more complicated than measuring temperature. Temperature is measured independently, but humidity is affected by other factors (atmospheric pressure, temperature). In addition, the calibration of humidity is also a problem. The humidity calibration equipment produced abroad is very expensive.
First, the definition of humidity is specified in the measurement method, and humidity is defined as "the amount of the state of the object". The humidity referred to in daily life is relative humidity, expressed as RH%. In short, it is the percentage of saturated water vapor (saturated water vapor pressure) in the same amount of water vapor (water vapor pressure) in the gas (usually in the air).
Humidity has a close relationship with life long ago, but it is more difficult to express it in terms of quantity. Humidity is expressed in terms of absolute humidity, relative humidity, dew point, ratio of moisture to dry gas (weight or volume), and so on.

2. Humidity measurement methods Humidity measurement can be divided into two or thirty types in principle. But humidity measurement has always been one of the well-known problems in the world of metrology. A seemingly simple measurement, which involves a lot of complicated physical-chemical theoretical analysis and calculation, may involve many factors that must be paid attention to in humidity measurement, which will affect the reasonable use of the sensor.

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 bulb method and electronic sensor method.

The two-pressure method and the two-temperature method are based on the thermodynamic P, V, and T equilibrium principles. The equilibration time is longer. The split method is based on the precise mixing of absolute moisture and absolute dry air. Due to the use of modern measurement and control methods, these devices can be made quite precise, but because the equipment is complex, expensive, and time-consuming and labor-intensive, it is mainly used for standard measurement, and its measurement accuracy can reach ± 2% RH or more.

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

The dew point method measures the temperature when the humid air reaches saturation, which is a direct result of thermodynamics. It has high accuracy and a wide measurement range. The accuracy of the precision dew point meter for measurement can reach ± 0.2 ° C or even higher. However, the chilled mirror dew point meter using the modern optical-electrical principle is expensive and is often used in conjunction with standard humidity generators.

Wet and dry bulb method, which is a method for measuring humidity invented in the 18th century. Has a long history and is most commonly used. The wet and dry bulb method is an indirect method. It uses the wet and dry bulb equation to convert the humidity value, and this equation is conditional: that the wind speed near the wet bulb must reach more than 2.5m / s. This condition is simplified by the common wet and dry bulb thermometers, so its accuracy is only 5 to 7% RH, and the wet and dry bulbs are not static methods. Do not simply think that as long as the measurement accuracy of two thermometers is increased, it is equivalent to increasing the humidity Meter measurement accuracy.