## Rule 5

Ascertain from Table 8 the weight of a cubic foot of vapor at that temperature; divide this by the weight of a cubic foot of vapor at the temperature of the atmosphere, and the quotient will be the relative humidity.

## Example

What is the relative humidity when the temperature of the atmosphere is 70° and the dew point is found to be at 45°?

## Solution

From Table 8, the weight of 1 cubic foot of vapor at 45° is .00048 pound. The weight of 1 pound of vapor at 70° is, from the same table, .00115. Hence, relative humidity = .00048 ÷ .00115 = .417, or 41.7 per cent. Ans.

40. It has been found by observation and experiment that a degree of humidity of from 50 to 70 per cent. should be maintained in dwellings, offices, and audience rooms to secure health and comfort. The air in such rooms is usually maintained at a temperature of 65° to 70° F.; therefore, the dew point, as measured by the hygrometer, should be about 55°.

If the dew point is too high, there will be an unpleasant feeling of dampness in the air, the window panes will drip with moisture, and there will be a deposit of dew on all the objects in the room which have a temperature lower than the normal.

41. Rule 6 may be used to find the amount of water which must be evaporated and added to the air supply to maintain any certain degree of humidity.

## Table 8

 Temperature. Degrees F. Pressure per SquareInch.Pounds. Weight per CubicFoot. Pounds. -30 .0049 .000017 -25 .0063 .000023 -20 .0088 .000030 -15 .0106 .000039 -10 .0135 .000050 - 5 .0171 .000063 0 .0216 .000079 5 .0272 .000098 10 .0340 .000121 15 .0423 .000149 20 .0525 .000181 25 .0651 .000222 30 .0806 .000270 35 .0998 .000325 40 .1225 .000400 45 .1470 .000480
 Temperature. Degrees F. Pressure per SquareInch.Pounds. Weight per CubicFoot. Pounds. 50 .176 .00058 55 .212 .00069 60 .253 .00082 65 .302 .00097 70 .358 .00115 75 .425 .00135 80 .502 .00158 85 .589 .00183 90 .692 .00213 95 .809 .00247 100 .943 .00286 105 1.094 .00330 110 1.265 .00380 115 1.462 .00433 120 1.682 .00496 130 2.213 .00640