Basic Concepts 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 among the conventional environmental parameters, humidity is the most difficult parameter to measure accurately. This is because measuring humidity is much more complicated than measuring temperature. Temperature is an independent measured quantity, while humidity is affected by other factors (atmospheric pressure, temperature). In addition, humidity calibration is also a difficult problem. The price of humidity calibration equipment produced abroad is very expensive.

1. Definition of humidity

In the metrology law, humidity is defined as "the quantity of the state of the object". The humidity referred to in daily life is relative humidity, expressed in RH%. In short, it is the percentage of the amount of water vapor (water vapor pressure) contained in the gas (usually in the air) to the saturated water vapor (saturated water vapor pressure) under the same conditions as the air.

Humidity has been closely related to life for a long time, but it is difficult to express it in quantity. There are absolute humidity, relative humidity, dew point, ratio of moisture to dry gas (weight or volume), etc.

2. Humidity measurement methods

Humidity measurement can be divided into 20 to 30 types in principle. However, humidity measurement has always been one of the famous problems in the field of world metrology. A seemingly simple value, in-depth study, 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 reasonable use of sensors . Common humidity measurement methods include: dynamic method (dual pressure method, dual temperature method, shunt method), static method (saturated salt method, sulfuric acid method), dew point method, dry-wet bulb method and electronic sensor method. ① The dual pressure method and dual temperature method are based on the thermodynamic P, V, T equilibrium principle, and the equilibrium time is relatively long. The shunt 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 to operate, they are mainly used as standard measurement, and their measurement accuracy can reach more than ±2%RH. ② The saturated salt method in the static method is the most common method in humidity measurement and is simple and easy to operate. However, the saturated salt method has strict requirements for the balance of liquid and gas phases, and has high requirements for the stability of ambient temperature. It takes a long time to balance, and the low humidity point requires even longer. Especially when the difference between indoor humidity and humidity in the bottle is large, it takes 6 to 8 hours to balance each time it is opened. ③ The dew point method measures the temperature when the wet air reaches saturation. It is a direct result of thermodynamics, with high accuracy and wide measurement range. The precision dew point meter used for measurement can reach an accuracy of ±0.2℃ or even higher. However, the cold mirror dew point meter based on modern photoelectric principle is expensive and is often used in conjunction with a standard humidity generator. ④ The wet and dry bulb method is a humidity measurement method invented in the 18th century. It has a long history and is the most commonly used. The wet and dry bulb method is an indirect method that uses the wet and dry bulb equation to convert the humidity value, and this equation is conditional: the wind speed near the wet bulb must reach 2.5m/s or more. Ordinary dry-bulb and wet-bulb thermometers simplify this condition, so their accuracy is only 5~7%RH. Dry-bulb and wet-bulb thermometers do not belong to the static method. Do not simply think that as long as the measurement accuracy of the two thermometers is improved, the measurement accuracy of the hygrometer is improved. ⑤ Electronic humidity sensor method: Electronic humidity sensor products and humidity measurement belong to the industry that emerged in the 1990s. In recent years, great progress has been made in the research and development of humidity sensors at home and abroad. Humidity sensors are rapidly developing from simple humidity-sensitive elements to integrated, intelligent, and multi-parameter detection, creating favorable conditions for the development of a new generation of humidity measurement and control systems, and also raising humidity measurement technology to a new level. 3. Selection of humidity measurement schemes There are two main modern humidity measurement schemes: dry-bulb and wet-bulb hygrometry and electronic humidity sensor hygrometry. The following compares these two schemes so that customers can choose the humidity measurement method that suits them. Characteristics of wet-and-dry bulb hygrometer: As early as the 18th century, humans invented the wet-and-dry bulb hygrometer. The accuracy of the wet-and-dry bulb hygrometer also depends on the accuracy of the dry-bulb and wet-bulb thermometers themselves; the hygrometer must be in a ventilated state: only when the gauze water jacket, water quality, and wind speed meet certain requirements can the specified accuracy be achieved. The accuracy of the wet-and-dry bulb hygrometer is only 5% to 7% RH. The wet-and-dry bulb hygrometry method uses an indirect measurement method. The humidity value is obtained by calculating the temperature of the dry bulb and the wet bulb. Therefore, there is no strict limit on the use temperature, and the sensor will not be damaged when measuring humidity in a high temperature environment. The maintenance of the wet-and-dry bulb hygrometry method is quite simple. In actual use, it is only necessary to add water to the wet bulb and replace the wet bulb gauze regularly. Compared with electronic humidity sensors, the wet-and-dry bulb hygrometry method will not cause aging, decreased accuracy, and other problems. Therefore, the wet-and-dry bulb hygrometry method is more suitable for use in high temperature and harsh environments. Characteristics of electronic humidity sensors: Electronic humidity sensors have only developed rapidly in recent decades, especially in the past 20 years. Before the products leave the factory, humidity sensor manufacturers must use standard humidity generators to calibrate each one. 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, the long-term use will cause aging and decrease in accuracy. The annual drift of humidity sensors is generally around ±2%, or even higher. In general, the manufacturer will indicate that the effective use time of a calibration is 1 or 2 years, and recalibration is required when it expires. The accuracy level of electronic humidity sensors should be judged in combination with their long-term stability. Generally speaking, the long-term stability and service life of electronic humidity sensors are not as good as those of dry-bulb and wet-bulb humidity sensors. Humidity sensors use semiconductor technology, so there are requirements for the ambient temperature of use. Exceeding the specified operating temperature will cause damage to the sensor. Therefore, the humidity measurement method of electronic humidity sensors is more suitable for use in clean and normal temperature environments. IV. Precautions for selecting humidity sensors ①. Select the measurement range Like measuring weight and temperature, the first thing to do when selecting a humidity sensor is to determine the measurement range. Except for meteorological and scientific research departments, those who engage in temperature and humidity measurement and control generally do not need full-range (0-100%RH) measurement. ②. Select measurement accuracy Measurement accuracy is the most important indicator of humidity sensors. Every percentage point increase is a step up, or even a grade up, for humidity sensors. Because to achieve different accuracies, the manufacturing costs vary greatly, and the selling prices also vary greatly. Therefore, users must tailor their products to their needs and should not blindly pursue "high, precise, and advanced". If humidity sensors are used at different temperatures, their indications must also consider the influence of temperature drift. As we all know, relative humidity is a function of temperature, and temperature seriously affects the relative humidity in a specified space. For every 0.1℃ change in temperature, a humidity change (error) of 0.5%RH will occur. If it is difficult to achieve constant temperature in the application, it is inappropriate to propose too high a humidity measurement accuracy. In most cases, if there is no precise temperature control method or the measured space is not sealed, an 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 ±3%RH or above are selected. The requirement for an accuracy higher than ±2%RH is probably difficult to achieve even for standard humidity generators that calibrate sensors, let alone the sensors themselves. Relative humidity measurement Instrument



















































, even at 20-25℃, it is still difficult to achieve an accuracy of 2%RH. Usually the characteristics given in the product information are measured at room temperature (20℃±10℃) and in clean gas.

③. Consider time drift and temperature drift

In actual use, due to the influence of dust, oil and harmful gases, the electronic humidity sensor will age and its accuracy will decrease after a long time of use. The annual drift of the electronic humidity sensor is generally around ±2%, or even higher. In general, the manufacturer will indicate that the effective use time of a calibration is 1 year or 2 years, and recalibration is required when it expires.

④. Other precautions

The humidity sensor is non-sealed. In order to protect the accuracy and stability of the measurement, it should be avoided as much as possible in acidic, alkaline and organic solvent atmospheres. Also avoid using it in dusty environments. In order to correctly reflect the humidity of the space to be measured, it should also be avoided to place the sensor too close to the wall or in a dead corner where the air is not circulated. If the room to be measured 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 may interfere with each other or even fail to work. When using, a suitable power supply that meets the accuracy requirements should be provided according to the technical requirements.

When the sensor needs to transmit signals over long distances, attention should be paid to the signal attenuation problem. When the transmission distance exceeds 200m, it is recommended to use a humidity sensor with a frequency output signal.