- Specific (or absolute) humidity, humidity ratio or moisture content, w: It is defined as the ratio of the mass of water vapour to the mass of dry air in a given volume of the mixture.Let p, V, T, m = pressure, volume, absolute temperature and mass respectivelyR, M, v, R̅ = gas constant, molecular mass, specific volume, and universal gas constant respectivelySubscripts a, v = air and vapour respectively.
For air,
And 
For water vapour,
Substituting for mv and ma from these equations in Eq. (20.2), we get
Now actual total pressure, pb = pa + pv
For ma = 1 kg, mv = w kg and total mass of moist air
For saturated air, pv = ps and maximum humidity ratio
where ps = partial pressure of air corresponding to saturation temperature i.e. td. - Degree of saturation or percentage humidity, µ: The ratio of actual specific humidity w to the specific humidity ws of saturated air at the same temperature is termed as the degree of saturation.
The degree of saturation is a measure of the capacity of air to absorb moisture. - Relative Humidity (RH or ϕ): It is defined as the ratio of mass of water vapour mv in a certain volume of moist air at a given temperature to the mass of water vapour mvs in the same volume of saturated air at the same temperature.
For a perfect gas pvvv = psvs
Now 
- Pressure of water vapour:
- The partial pressure of water vapour, according to Carrier’s equation, is given by
where pw = saturation pressure corresponding to WBTpb = barometric pressuretd = DBTtw = WBT - Modified Apjohn equation:
- Modified Ferrel equation:
- The partial pressure of water vapour, according to Carrier’s equation, is given by
- Vapour density or Absolute humidity:Let pv = density of water vapour corresponding to its partial pressure and DBT, td.pa = density of dry air.Then mv = Vvρv and ma = Vaρa
Since Va = Vv, therefore,
Or ρv = wρaNow pa Va = maRaTdFor ma = 1 kg, 
PSYCHROMETRIC RELATIONS
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