The air supply is recorded under Cold-air intake. The furnace was provided with a water pan for humidifying the air supply. The amount of water evaporated each day is recorded in the column headed Evap. in 24 hours. The outside temperature ranged from -12°F. to -21°F. The weather was clear and calm except the last day, Jan. 12, which was windy. The higher humidity on that day was no doubt due to the greater amount of heat required from the furnace and the consequent evaporation of the water from the water pan.
The humidity determinations made by a hygrodeik, as before explained, are only approximately correct but sufficiently exact for practical purposes. The temperature is given in degrees Fahrenheit.
In the table it will be noticed that the outside air was used only a part of the time because of the severity of the weather. Attention is called to the quantity of water required to keep the humidity at the amount shown. This averages 27½ quarts per day. At the time these observations were made the physics lecture-room at the North Dakota Agricultural College averaged 18 to 20 per cent. saturation during class hours, with observations made from a similar instrument. This is a steam-heated room with only accidental means of adding water to the air. The result was an atmosphere 3½ per cent. above that of Death Valley.
Hot-air Furnace
Readings taken at 12 o’clock noon each day
| Date | Temp. outside | Wet bulb | Dry bulb | Per cent. saturated | Evap. In 24 hours | Cold-air intake | |
| quarts | pints | ||||||
| Dec. 13 | -13 | 54° | 63° | 53 | Closed 8 a.m. | ||
| Dec. 14 | -18 | 55 | 66 | 47 | Open | ||
| Dec. 15 | -20 | 57 | 68 | 49 | 21 | Closed 7 a.m. | |
| Dec. 16 | -18 | 57 | 67 | 51 | 20 | 1 | Closed 7 a.m. |
| Dec. 17 | -22 | 58 | 69 | 48 | 18 | 1 | Closed 7 a.m. |
| Dec. 18 | -16 | 55 | 65 | 51 | 17 | 1½ | Closed 6:30 a.m. |
| Dec. 19 | -10 | 57 | 68 | 47 | 20 | 1 | Closed 8 a.m. |
| Dec. 20 | 0 | 59 | 70 | 49 | 13 | ¾ | Not open at night |
| Jan. 8 | -12 | 58 | 71 | 43 | 18 | Closed | |
| Jan. 9 | -17 | 57 | 71 | 39 | 25 | Open 24 hours | |
| Jan. 10 | -16 | 58 | 69 | 45 | 27 | 1 | Open 10 hours |
| Jan. 11 | -21 | 60 | 75 | 40 | 30 | Closed | |
| Jan. 12 | -15 | 60 | 73 | 46 | 30 | Closed | |
The amounts of water evaporated may seem large to those who are unaccustomed to quantitatively consider problems in ventilation but the small amount of water in the air at -21° must produce a very dry atmosphere when it is raised to 70° in temperature.
The amount of moisture in air at 20°F. and at 80 per cent. humidity is only 1.58 grains to the cubic foot. If this air is now raised to 70° the moisture will still be 1.58 grains where there should be 4 grains of water to make 50 per cent. humidity. It therefore will require the addition of practically 2.42 grains of water for each cubic foot of entering air in order to bring it up to 50 per cent. humidity.
In a case with the above conditions of atmosphere, suppose it is desired to know the amount of water that would be taken up in humidifying the air for a school-room of size to accommodate 40 pupils. The prescribed quantity of air for this purpose is 30 cubic feet per minute for each pupil. The air is to be maintained at a humidity 50 per cent. saturated. The problem will be one of simple arithmetic. If each pupil is to receive 30 cubic feet of air per minute or 1800 cubic feet per hour, the 40 pupils receiving 1800 cubic feet per hour will require 40 × 1800 = 72,000 cubic feet of air per hour. To each cubic foot of the air is to be added 2.74 grains of water, 72,000 × 2.42 = 164,240 grains of water. Reducing this to pounds, 164,240 ÷ 7000 = 23.46 pounds or 2.77 gallons of water per hour.
In practice the room will show a higher amount than 50 per cent. humidity with this addition of the amount of water, because of the water vapor that is exhaled from the lungs of the pupils. That a considerable amount of water vapor is added to the atmosphere by breath exhalation is made evident from the moisture condensed by breathing on a cold pane of glass. In any unventilated room occupied by a considerable number of people the humidity is thus increased a very noticeable amount.
The change in humidity of the air in a closed room filled with people is very pronounced. The constant exhalation of moisture from the lungs is sufficient to saturate the air in a short time. The heavy atmosphere of overcrowded, unventilated rooms is due to moisture exhalation, body odors and increased carbonic acid gas. As the humidity of the atmosphere is increased a sensation of uncomfortable warmth is the result of the lesser evaporation.