Measurement of humidity

a) Station hygrometer:

• It consists of dry and wet bulb thermometer; two thermometers are kept side by side on a single frame called as Stevenson screet at 4 ft height.
• Two thermometers are alike except that in wet bulb thermometer; the bulb is kept moist by means of muslin cloth or wick constantly moistened from a reservoir.
• Dry bulb thermometer is like ordinary air thermometer. Due to evaporation from wick, the temperature of wet bulb drops and reads lower temperature than dry bulb.
• This difference in reading of these two thermometers is called wet bulb depression. Wet bulb depression can be converted into relative humidity with the help of hygrometric table.
• Measurements are taken at a fixed time for the locality in the morning and afternoon. The wet bulb should be kept moist. If there is no depression of the bulb the air is saturated & RH is 100%.

b) Assman psychrometer:

• This is an aspirated type portable hygrometer for the accurate measurement of atmospheric humidity.
• It consists of two thermometers exposed side by side in a steady airflow produced by an attached aspirator.
• One of the thermometers serves as a dry bulb while the other, with its bulb kept moist with wet sleeve acts as a wet bulb. As water evaporates, the temperature of wet bulb is lowered to an equilibrium point.
• This depression of W.B. temperature along with the ambient D.B. temperature is a measure of moistness or humidity of the ambient air which can be readily obtained from a computed table or slide rule specially made to express in units of relative humidity, dew point temperature or vapour pressure.

c) Whirling hygrometer or sling psychrometer:

• It measures relative humidity & temperature directly.
• This is portable instrument consisting of dry and wet thermometers fixed on a wooden frame.
• The wet bulb thermometer has a cotton sleeve wrapped around its base. Distilled water from a small reservoir is used to keep the sleeve wet. The hygrometer is whirled around in the air 20-30 times.
• While this happens, water from the sleeve of the wet bulb thermometer evaporates, and the wet-bulb temperature shown by the thermometer goes down. Wet-bulb depression is matched to the Hygrometric table to read the RH.

d) Hair hygrometer/hygrograph:

• It is a self-recording instrument in which the RH of the air is contiuously traced on a chart fitted on rotating drum.
• Hair hygrographs/hygrometers are based on the hygroscopic nature of human hair.
• The principle of operation is based on the assumption that the hair changes its dimension (length) in response to the changes in humidity of the surrounding air i. e. human hair lengthens as relative humidity increases and contracts as relative humidity decreases.
• The hygrograph is designed such that changes in the length of hair are monitored and calibrated as change in air humidity. It gives satisfactory results in case of extreme temperature and normal humidity.