Physics

Name: Physics
Author: Tower, Willis E. (Willis Eugene)

Tower, Willis E. (Willis Eugene)

Physics

For example, if the dew point is 5°C. and the temperature of the
     air is 22°C., we find the densities of the water vapor at the two
     temperatures, and find their ratio: 6.8/19.3 = 35 per cent. nearly.
     Determinations of humidity may give indication of rain or frost and
     are regularly made at weather bureau stations. They are also made
     in buildings such as greenhouses, hospitals, and schoolhouses to
     see if the air is moist enough. For the most healthful conditions
     the relative humidity should be from 40 per cent. to 50 per cent.

WEIGHT OF WATER (_w_) IN GRAMS CONTAINED IN 1 CUBIC METER OF SATURATED
AIR AT VARIOUS TEMPERATURES (_t_°)C.

  --------+------
   _t_°C. | _w_
  --------+------
    -10   |  2.1
    - 9   |  2.4
    - 8   |  2.7
    - 7   |  3.0
    - 6   |  3.2
    - 5   |  3.5
    - 4   |  3.8
    - 3   |  4.1
    - 2   |  4.4
    - 1   |  4.6
      0   |  4.9
      1   |  5.2
      2   |  5.6
      3   |  6.0
      4   |  6.4
      5   |  6.8
      6   |  7.3
      7   |  7.7
      8   |  8.1
      9   |  8.8
     10   |  9.4
     11   | 10.0
     12   | 10.6
     13   | 11.3
     14   | 12.0
     15   | 12.8
     16   | 13.6
     17   | 14.5
     18   | 15.1
     19   | 16.2
     20   | 17.2
     21   | 18.2
     22   | 19.3
     23   | 20.4
     24   | 21.5
     25   | 22.9
     26   | 24.2
     27   | 25.6
     28   | 27.0
     29   | 28.6
     30   | 30.1
  --------+------

=172. Wet and Dry Bulb Hygrometer.=--A device for indicating the
relative humidity of the air is called an _hygrometer_. There are
various forms. The _wet_ and _dry bulb hygrometer_ is shown in Fig. 149.
This device consists of two thermometers, one with its bulb dry and
exposed to the air, the other bulb being kept continually moist by a
wick dipping into a vessel of water. An application of the principle of
cooling by evaporation is made in this instrument. Unless the air is
saturated so that evaporation is prevented, the wet-bulb thermometer
shows a lower temperature, the difference depending upon the amount of
moisture in the air, or upon the relative humidity. Most determinations
of relative humidity are made with this kind of instrument. It is
necessary in order to make an accurate determination, to fan or set the
air in motion about the thermometers for some time before reading them.
The relative humidity is then found by using tables giving the relative
humidity that corresponds to any reading of the thermometers.

[Illustration: FIG. 149.--Wet and dry bulb hygrometer.]

[Illustration: FIG. 150.--A dial hygrometer.]

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Opening the text

PhysicsTower, Willis E. (Willis Eugene)

Science

Physics

Tower, Willis E. (Willis Eugene)

Physics

For example, if the dew point is 5°C. and the temperature of the
     air is 22°C., we find the densities of the water vapor at the two
     temperatures, and find their ratio: 6.8/19.3 = 35 per cent. nearly.
     Determinations of humidity may give indication of rain or frost and
     are regularly made at weather bureau stations. They are also made
     in buildings such as greenhouses, hospitals, and schoolhouses to
     see if the air is moist enough. For the most healthful conditions
     the relative humidity should be from 40 per cent. to 50 per cent.

WEIGHT OF WATER (_w_) IN GRAMS CONTAINED IN 1 CUBIC METER OF SATURATED
AIR AT VARIOUS TEMPERATURES (_t_°)C.

  --------+------
   _t_°C. | _w_
  --------+------
    -10   |  2.1
    - 9   |  2.4
    - 8   |  2.7
    - 7   |  3.0
    - 6   |  3.2
    - 5   |  3.5
    - 4   |  3.8
    - 3   |  4.1
    - 2   |  4.4
    - 1   |  4.6
      0   |  4.9
      1   |  5.2
      2   |  5.6
      3   |  6.0
      4   |  6.4
      5   |  6.8
      6   |  7.3
      7   |  7.7
      8   |  8.1
      9   |  8.8
     10   |  9.4
     11   | 10.0
     12   | 10.6
     13   | 11.3
     14   | 12.0
     15   | 12.8
     16   | 13.6
     17   | 14.5
     18   | 15.1
     19   | 16.2
     20   | 17.2
     21   | 18.2
     22   | 19.3
     23   | 20.4
     24   | 21.5
     25   | 22.9
     26   | 24.2
     27   | 25.6
     28   | 27.0
     29   | 28.6
     30   | 30.1
  --------+------

=172. Wet and Dry Bulb Hygrometer.=--A device for indicating the
relative humidity of the air is called an _hygrometer_. There are
various forms. The _wet_ and _dry bulb hygrometer_ is shown in Fig. 149.
This device consists of two thermometers, one with its bulb dry and
exposed to the air, the other bulb being kept continually moist by a
wick dipping into a vessel of water. An application of the principle of
cooling by evaporation is made in this instrument. Unless the air is
saturated so that evaporation is prevented, the wet-bulb thermometer
shows a lower temperature, the difference depending upon the amount of
moisture in the air, or upon the relative humidity. Most determinations
of relative humidity are made with this kind of instrument. It is
necessary in order to make an accurate determination, to fan or set the
air in motion about the thermometers for some time before reading them.
The relative humidity is then found by using tables giving the relative
humidity that corresponds to any reading of the thermometers.

[Illustration: FIG. 149.--Wet and dry bulb hygrometer.]

[Illustration: FIG. 150.--A dial hygrometer.]

Public-domain text, read in full here on John Shaqi.

Physics

Physics

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