Wednesday, December 29, 2010
【 Weak current College 】 humidity technology, the use of humidity approaches 】
The basic concept of humidity measurement in industrial and agricultural production, meteorology, environmental protection, defense, research, and aerospace sectors, often need to ambient humidity measurement and control. On the environmental temperature and humidity control, as well as industrial material moisture value of monitoring and analysis have become more common technical specifications, but in general the environmental parameters, humidity is the most difficult to accurately measure of a parameter. This is because the measurement of humidity than the much more complex measuring temperature, temperature is a measure of independence, and the humidity is affected by other factors (atmospheric pressure, temperature). In addition, humidity calibration is also a challenge. Overseas production of humidity calibration equipment prices are expensive. First, the definition in the measurement of humidity in the law, the humidity is defined as "an image of the State". Daily life within the meaning of the humidity as relative humidity, RH% said. In short, the gases (typically air) containing a quantity of steam (water vapour pressure) and air the same circumstances the amount of saturated steam (saturated vapor pressure).
Humidity long ago and life there is a close relationship, but the number used to represent the more difficult. Representation on humidity absolute humidity, relative humidity, dew point, moisture and dry gas ratio (weight or volume), and so on. Second, the humidity measurement humidity measurement Division from principle has 20 or 30. But the humidity measurement is always the world metrology area one of the well-known problem. A seemingly simple measure, deep dive, involve very complex physical-chemical analysis and calculations, the new entrants may be ignored in the humidity measurement is essential to note that many factors affect the sensor of fair use. Common humidity measurement methods are: dynamic method (dual-pressure method, dual temperature method, method), static (saturated salt method, sulfuric acid method), dew point, wet and dry bulb and electronic sensors. ① double pressure method, dual temperature method is based on thermodynamic P, V, T balance theory, balance time, streaming method is based on the absolute humidity and absolutely dry air of the precise mix. Thanks to modern measurement and control instruments, these devices can be pretty accurate, but because of equipment complex, expensive, time-consuming and requires a major operation, as a standard measurement, the measurement accuracy of up to ± 2% RH. ②-static method in the saturated salt method, is the humidity in the measurement of the most common way is easy. But saturated salt method for liquid, gas two-phase balance requires very strict environmental temperature stability requirements. Use request wait a long time to balance, low moisture demands an even longer. Particularly in indoor humidity and bottles humidity difference is large, each time you open the need to balance 6 ~ 8 hours. ③ dew point method is the measurement of moist air becomes saturated when thermodynamic temperature, is the direct result of the measurement accuracy is high, wide range. Measurement of dew-point hygrometer with precision accuracy up to ±0 0.2 ° c or even higher. But with the modern theory of optical-electronic hygrometer is expensive, often and standard humidity generator use. ④ wet and dry bulb method, which is 18 century invention of moisture. A long history, the most commonly used. Wet and dry bulb method is an indirect method, which uses psychrometric humidity value conversion out of the equation, the equation is conditional: that is, in the wet-bulb nearby wind speed required to achieve 2.5m/s or more. General use of wet and dry bulb thermometers simplifies this condition, so its accuracy only 5 ~ 7% RH, wet and dry bulb does not belong to a static method, not to simply think that raising two thermometers measuring accuracy is to improvethe accuracy of the measuring Chamber. Dry and wet bulb measurement using the wet method, the indirect measuring methods by measuring the dry bulb and wet bulb temperature after the calculated humidity value. Therefore, there is no strict restrictions on the use of temperature, high temperature measuring humidity will not cause damage to the sensor. Wet and dry bulb hygrometer's features:
As early as the 18th century man invented the wet and dry bulb hygrometer, wet and dry bulb hygrometer accuracy depends on the dry bulb and wet bulb thermometer itself of two precision; hygrometer must be ventilated State: only the gauze jacket, water, wind speed will meet certain requirements in order to achieve the required accuracy. Wet and dry bulb hygrometer accuracy only 5 per cent of a 7% RH. ⑤ electronic humidity sensor method electronic humidity sensor products and humidity measurement is the rise of industry in the 90s, in recent years, both at home and abroad in the field of humidity sensor research and development has made tremendous progress. Humidity sensors are simple humidity sensor to integrated, intelligent, multi-parameter detection of direction of rapid development, to develop a new generation of humidity measurement and control system for creating favorable conditions to humidity measurement technology to the next level. Third, the humidity measurement programmes to choose the most modern humidity measurement programmes mainly in two ways: wet and dry bulb and wet measurement, electronic humidity sensor wet. Following these two programmes, so that the customer chooses the suitable humidity measurement method. Wet and dry bulb measuring wet maintenance is simple, in actual use, you only need to regularly add water to wet-bulb and wet bulb gauze can be replaced. Compared with electronic humidity sensor, wet and dry bulb moisture method does not generate a ageing, precision decline. So wet and dry bulb moisture method is more suitable for high temperature and harsh environments. Electronic humidity sensor characteristics: and electronic humidity sensor is in recent decades, especially in the last 20 years to develops rapidly. Humidity sensor production plant in products manufactured before the adoption of standard humidity generator to calibration-by-team, electronic humidity sensor accuracy can reach a 3% 2% RH. In actual use, due to dust, oil and harmful gases, use over time, will produce a decline in ageing, precision, humidity sensor, the excursion is generally in ± 2% or even higher. Under normal circumstances, the manufacturer may indicate a 1 time calibration of efficient use of time for 1 year or 2 years due to calibration again. Electronic humidity sensor accuracy level in conjunction with its long-term stability to judge, generally speaking, the electronic humidity sensor for long term stability and useful life than psychrometric humidity sensor. Humidity sensor is used in semiconductor technology, the use of ambient temperature is required, more than its working temperature will cause damage to the sensor. So electronic humidity sensor wet method is more suitable in the cleansing and use at ambient temperature. 4. humidity sensor selection considerations ①. choose measuring range and measuring weight, temperature, select humidity sensor first determine the measurement range. In addition to meteorology, scientific research departments, temperature,Humidity measurement and control in General does not require all wet range (0-100% RH) measurement. ②, select measurement accuracy
The humidity sensor for measuring accuracy is the most important indicators, improving-percentage of humidity sensor is the next level, or even previous grades. Because you want to achieve different accuracy, their manufacturing costs vary greatly, prices are also very different. Therefore, users must be tailored to be blind pursuit of "high-precision, advanced".
As at different temperatures using humidity sensor, the indication should also consider the effect of temperature drift. We all know, the relative humidity is a function of temperature, temperature severely affects the relative humidity within a specified space. Temperature change 0.1 ℃. Will produce 0.5% RH of humidity change (error). Use the occasion if difficult constant temperature is raised too high moisture accuracy is not appropriate. In most cases, if there is no precise means of temperature control, or the measured space is a non-sealed, ± 5% RH precision is sufficient. For precise control of temperature and humidity in the local space, or you need to keep track of humidity changes, and then choose ±1% RH above precision humidity sensor. The precision is greater than ± 2% RH requirements even calibrate sensor standard humidity generator also difficult, not to mention the sensor itself. Relative humidity measuring instruments, even in 20-25 ° c, to reach 2% RH accuracy remains very difficult. Usually given in the product information of the characteristics is at room temperature (20 ° c ± 10 ° c) and clean gas measurement. ③, consider drift and drift
In actual use, due to dust, oil and harmful gases, use over time, electronic humidity sensor that produces an aging, precision, electronic humidity sensing device, the excursion is generally in ± 2% or even higher. Under normal circumstances, the manufacturer may indicate a 1 time calibration of efficient use of time for 1 year or 2 years due to calibration again. ④, other considerations for humidity sensors are sealed, for the protection of the measurement accuracy and stability, you should avoid in acidic, alkaline and containing organic solvents used in the atmosphere. Also avoid the dust in the larger environment. To correctly reflect the desire of humidity measurement space, you should also avoid the sensor placed too close to the walls or poorly ventilated corners. If the test room is too large, you should place more than one sensor. Some humidity sensor power supply power supply requirements is relatively high, otherwise will have an impact on measurement accuracy. Or sensor mutual interference, even not work. Should be used in accordance with the technical requirements for the provision of appropriate, consistent with the accuracy requirements of power supply.
The sensor requires long-distance signal transmission, it is necessary to pay attention to the problem of signal attenuation. When the transmission distance of more than 200m above, it is proposed to adopt frequency output signal of the humidity sensor.