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The following article will guide you about how to measure and calculate relative humidity. Also learn about the instruments which are used for indirect measurement of relative humidity.
Moisture (water vapour) content in the air can be measured in a number of ways. Most commonly used reference to water vapour is relative humidity (RH), which is the ratio of the amount of water vapour actually in the air as a percentage of that contained in the same volume of saturated air at the same temperature. If a kg of air at constant pressure could hold 12 g of water vapour at a certain temperature, but contains only 9 g at that temperature, it has a relative humidity of 75 percent.
Relative humidity is also defined with reference to the mixing ratio (MR), which is the mass of water vapour in grams per kilogram of dry air. For practical purposes the specific humidity (SH) is identical, being the mass of vapour per kg of air including its moisture.
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The RH can be determined approximately in several ways:
where, the subscript ‘s’ refers to the respective saturation values at the same temperature; and V denotes vapour pressure.
When air temperature and dew point are equal, the RH is 100 per cent and RH can also be determined from:
Direct measurement of actual amount of water vapour in the air is not feasible for ordinary observations.
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Following are the instruments used for indirect measurement:
1. Stationary psychrometer.
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2. Assmann psychrometer.
3. Whirling psychrometer.
4. Hair hygrometer.
5. Hair hygrograph.
Hygrometric tables and saturation vapour pressure tables are necessary for calculating the relative humidity.
1. Stationary Psychrometer:
Stationary or simple psychrometer consists of dry and wet bulb thermometers, housed vertically in the Stevenson screen. In other words, a set of dry and wet bulb thermometers is known as psychrometer. Using the hygrometric table and dew point temperature, humidity can be obtained.
2. Assman Psychrometer:
In Assman psychrometer (Fig. 2.15), the dry and wet bulb thermometers with cylindrical bulbs are fixed vertically. The bulbs are protected from radiation by two coaxial tubes. It is designed to measure temperature and relative humidity, both in the open and in the crop canopy. The aspiration is provided means of a clockwork fan, by which air is drawn at a speed higher than 10 feet per second.
3. Whirling Psychrometer:
Temperature and relative humidity of the air in open and in the crop canopy at different heights can be measured with whirling psychrometer (Fig. 2.15). The dry and wet bulb thermometers are attached horizontally to a rectangular wooden frame and it can be rotated with handle.
Four rotations per second should be given for obtaining desired wind speed around 5 metres per second. With dry and wet bulb temperatures, due point temperature, vapour pressure and relative humidity at different heights can be obtained.
4. Hair Hygrometer:
It is used to obtain humidity at the time of observation by using human hair. Humidity is obtained directly with this instrument (Fig. 2.15). It is ideal for laboratory measurement of humidity at air temperature.
5. Hair Hygrograph:
Continuous record of humidity can be obtained with hair hygrograph (Fig. 2.16). The property of human hair to increase the length with increase in humidity and decrease with decrease in humidity is used in the instrument.
It consists of a bundle of de-oiled human hair tied at both ends and kept tight in the middle by means of a hook attached to one arm of the lever and second arm is associated with pen arrangement which can make marking on graph paper attached on the clock driven revolving drum.
Variations in length of human hair cause displacement of the hook, which is communicated by second arm of the lever to record the changes on graph paper. The circular drum makes one rotation in 24 hours or once in a week. The instrument is placed in a Stevenson’s screen in the observatory.
The sensitive (activating) elements of hygrograph and thermograph are combined in the same case with pens tracing their respective records on the same chart. This combination is called hygrothermograph.