﻿KILN DIcYING HANDBOOK. 



15 



relative humidity, and is what is meant when the term "humidity" 

 is used in this bulletin. The lower relative humidities represent dry 

 air and the higher ones moist air. Air at a temperature of 12'5° F. 

 can hold a maximum of 40 grains of water vapor per cubic foot. If 

 an atmosphere at that temperature had only 10 grains of water per 

 cubic foot it would have only ten- fortieths of the maximum amount 

 of water vapor it could hold, or a relative humidity of 25 per cent. 

 Air with 25 per cent relative humidity is comparatively dry. The 

 relative humidity of air having 30 grains would be thirty-fortieths, 

 or 75 per cent, and would be considered moist. Air at 155° F. can 

 hold 80 grains per cubic foot, or twice as much water vapor as at 

 125°. At 155° air containing 10 grains of water per cubic foot would 





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Fig. 3. — Relation between the moisture in wood and the relative humidity in the sur- 

 rounding atmosphere at three temperatures. Solid lines are based on actual data. 



be very dry, having a relative humidity of only 12f per cent, and air 

 containing 30 grains per cubic foot would still be moderately dry, 

 having a relative humidity of 374 per cent. 



The amount of moisture in the air determines not only the rate 

 but also the extent to which materials will dry. There is a definite 

 balance between the humidity in the air and the moisture content 

 of wood. All kinds of wood, if held long enough in an atmos- 

 phere of constant temperature and humidity, will come to the same 

 moisture content. The time required for this adjustment varies with 

 different species. The relation between humidity in the air and 

 moisture in the wood is an important one, since it forms the basis 

 for some drying schedules and determines the extent to which wood 

 for use under specified conditions of temperature and humidity 

 should be dried. Figure 3 presents curves showing the humidity- 

 moisture relation at three temperatures. The middle curve has been 

 interpolated, actual data having been secured for the outer ones only. 



