Plotting lines of constant relative humidity
Using the skeleton plot thus far developed (Fig. 1), lines of constant relative
humidity can be superimposed on it in the following manner (see Fig. 2).
FIG. 2 Plotting the relative-humidity lines.
For example, let us determine the 60 percent RH line. First, locate the point of 60 percent RH on the 80°F DB line,
as follows, Since (from Table 1) Wsat for 80°F is 156 gr/lb, the moisture content at 60 percent RH
= 0.60 X 156 = 93.5 gr per pound of dry air. This condition is plotted as point A of Fig. 2. To locate another
point note (from Table 1) that at 50°F DB a 60 percent RH results in W = 0.60 X 53.4 = 32.1 gr/lb. Point B on the
60 percent RH line is thus located. After a sufficient number of points have been plotted, a smooth curve is drawn
through them, which is the 60 percent relative-humidity line. The other relative-humidity lines are plotted in the
same manner. Note that, with the saturation line (100 percent RH), these lines constitute a family of curves
sloping generally upward and to the right.