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 Square

Inch.

Pounds.

Weight per Cubic

Foot. 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 Square

Inch.

Pounds.

Weight per Cubic

Foot. 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