Basic Terms In Humidity / Dampness estimation

The number of water vapour contained in air or gas is called as humidity. This humidity affects human comforts and many commercial process as in case of chemical industries, garments industries, paper industries, food industries, leather industries, pharmaceutical industries, precision tool manufacturing, etc. Hence, study of humidity is important. Let us define some tasteless terms associated to humidity measurement.

Humidity

Humidity Control

The number of Wv contained in air or gas is called as humidity. It is ordinarily measured as absolute humidity, relative humidity or due point temperature.

Note Wv = water Vapour

Dry air

When there is no Wv contained in the atmosphere, it is called dry air.

Moist air

When there is Wv present in the atmosphere, it is called moist air.

Saturated air

Saturated air is the moist air where the partial pressure of Wv equals the saturation pressure of steam corresponding to the climatic characteristic of air.

Absolute humidity

It is the mass of Wv present per unit volume. In other words, it is the quantity of the Wv present in air and its unit is grams per cubic meter of air.

Relative humidity Rh

Relative humidity = (Wv pressure authentically present)/(Wv pressure required of saturation) at a given.

Here a comparison is made in the middle of the humidity of air and humidity of saturated air at the same climatic characteristic and pressure.
It should be noted that if relative humidity is 100%, it is saturated air. That is, the air contains all the moisture it can hold.
It should also be noted that the degree of saturation (percentage of relative humidity) of air keeps on changing with temperature.

Humidity ratio or exact humidity

For a given volume of air water vapour mixture,

Humidity ratio = (mass of water - vapour)/mass of dry air

Dew point temperature

By continuous cooling at constant pressure if the climatic characteristic of air is reduced, the water-vapour in the air will start to condense at a single temperature. The climatic characteristic at which the water vapour stats condensing is called as dew point temperature.

Dry-bulb temperature

When a thermometer bulb is directly exposed to an air-water vapour mixture, the climatic characteristic indicated by the thermometer is the dry-bulb temperature.
This dry-bulb climatic characteristic is not affected by the moisture present in the air, that is, the climatic characteristic of air is measured in a normal way by the thermometer.
The dry bulb is used to distinguish the normal climatic characteristic measured from the climatic characteristic measured by the wet-bulb.

Wet-bulb temperature

When a thermometer bulb is covered by a constantly wet wick, and if the bulb covered by the wet wick is exposed to air-water vapour mixture, the climatic characteristic indicated by the thermometer is wet-bulb temperature.

When air is passed on the wet wick present on the bulb of the thermometer, the moisture present in the wick strats evaporating and this creates a cooling ensue at the bulb. The bulb now measures the thermo dynamic balance climatic characteristic reached in the middle of the cooling effected by the evaporation of water and heating by convection.

Wet-bulb depression

Wet-bulb depression = (dry bulb temperature) - (wet bulb temperature)

Always dry-bulb climatic characteristic is higher than the wet bulb temperature.

Percentage humidity

Percentage humidity = weight of water vapour in a unit weight of air/ weight of water vapour in same weight of air if the air were thoroughly saturated at the same temperature.

Basic Terms In Humidity / Dampness estimation