The water vapor present in the atmosphere or the water vapor present in the gas is called humidity. There are many types of humidity such as molal humidity, relative humidity, absolute humidity, saturation humidity, specific humidity, etc.
Molal Humidity DefinitionÂ
The ratio of the number of moles of water vapor present in the gas to the number of moles of dry gas is called molal humidity.
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Hm = No. of moles of vapor in gas/no. of moles of dry gas
Hm = Pv/P-Pv
Molal Humidity Formula
Let x gm moles of gas contain xv moles of vapor.
Molal Humidity = x/xv
On dividing the above equation in the denominator
(xv/x)/xy/x = (Pv/P)/(Pg/P)
Pv = Partial Pressure of Vapour
Pg = Partial pressure of a gas
Total pressure P = Pv + Pg
H = PvMv/(P-Pv)Mg
H = Hm x Mv/Mg
For air and vapor mix.
H = Hm x 18/29
H = Hm x 0.6207
Relative Humidity Definition
The amount of atmospheric moisture is relative to the amount that would be present in the air when it was saturated.
There is the partial pressure of water vapor and vapor pressure ratio of water.
%Hf = (Ps/Pv) x 100
% Hp = (H/Hs) x 100
= pv/P-Ps x Mv/Mg x 100
%Hb = Pv/Ps [P-Ps/P-Pv] x 100
Hb = Hf x 100 x [P-Ps/P-Pv]
Absolute Humidity Definition
In gas and water vapor, the weight of water vapor present in the unit volume of the mixture is called absolute humidity. It is represented by gm/m3 dry gas.
Specific Humidity Definition
It is the actual weight of water vapor, which is added to 1 kg of dry air. It is expressed in grams of water per kg of dry gas.
Dew Point Temperature Definition
It is the temperature of the gas at which condensation of water vapor present in the gas begins. OR If an unsaturated mixture of gas and vapor is cooled, a temperature is attained at which the vapor pressure of the liquid becomes equal to the partial pressure of the vapor. This is called the dew point of the temperature.
Humid Heat and Humid VolumeÂ
Humid Heat
The amount of heat required to raise the temperature of a unit mass of vapor less gas and the amount of vapor present in it is called humidity heat.
Cs = Cpb + Cpa H
Where Cs = Humid heat
Cpb = Specific Heat
Cpa = Specific Heat of vapor
Humid Volume
The total volume of the unit mass of vapor less gas and the amount of vapor present in it at 10 atm pressure and gas temperature is called humid volume.
Vh = 359T/492(1/Mo + H/Ma)
Vh = Humid Volume
T = Absolute Temperature
Adiabatic Saturation Temperature DefinitionÂ
The temperature at which a vapor gas mixture is reached through adiabatic progress to saturate the mixture.
For air, water, steam
Wet bulb temperature = Adiabatic saturation temperature
Also Read:Â
Dry Bulb and Wet Bulb Temperature Definition
Dry Bulb TemperatureÂ
The recorded temperature of a vapor gas mixture by a thermometer (whose bulb is dry) is called dry bulb temperature.
This is the temperature that the ordinary thermometer tells. It has nothing to do with the condition of the air like humidity or water vapor etc.
This principle is not followed at all in a mixture of air and water vapor. With the help of a humidity chart, the humidity of the water, air, and vapor mixture is determined from dry and wet bulb temperatures.
Wet Bulb TemperatureÂ
The wet bulb temperature is the steady state non-equilibrium temperature attained by a small mass placed under inductive conditions in continuous streams of gas. The recorded temperature of the vapor gas mixture by a temperature (which the bulb is wrapped with a wet cloth in the open air) is called wet bulb temperature.
