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6 TRG-TRC001-EN
notes
period one
The Psychrometric Chart
Finally, humidity ratio describes the actual weight of water in an air – water
vapor mixture. In other words, if one pound of air were wrung completely dry,
comparing the weight of the water to the weight of the dry air would yield its
humidity ratio.
Humidity ratio can be expressed as pounds of moisture per pound of dry air, or
as grains of moisture per pound of dry air. There are 7000 grains of water in a
pound. To appreciate the magnitude of these units of measurement, at sea level
one pound of 70°F air occupies approximately 13.5 cubic feet, and one grain of
water in that air weighs about two-thousandths (0.002) of an ounce.
When any two of these five properties of air are known, the other three can be
quickly determined from the psychrometric chart.
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Figure 11
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Figure 12