Relative Humidity Lines:

The relative humidity φ is described like the ratio of the mole fraction of water vapour in moist air to mole fraction of water vapour within saturated air at the similar pressure and temperature. From perfect-gas relationships an additional expression for φ is

The associated humidity lines are curved lines & follow the saturation curve.

In general, these lines are drawn having values 10%, 20%, 30% etc. and up to

100%.The saturation curve shows 100% relative humidity. Generally the values of associated humidity lines are given along the lines themselves as illustrated in given figure

                   Relative Humidity Lines

The lines on psychrometric chart for any other wanted value of R_H might he constructed as. Taking 50 % RH as an instance, the point on the 20°C line correspondent to this RH ought to be at the intersection C along the line representing a vapour pressure of

Drawing 50 percent RH Line

p_v = 0.5 × 2342 = 1171 N/m2

At this

p_a= 101325 - 1171 = 100154 N/m2

ω = 0.622(1171)/ 100154

 = 0.00727 kg w.v. /kg d.a.

Similarly, points for other temperatures might be plotted to construct the full 50 % RH line. It may be noted down that ω = 0 line also corresponds to zero % RH.