VEGETABLE AND FRUIT DEHYDRATION 49 



_ 0.625P, (percent RH) ^ 0.625P S (percent AH) 

 a ~~lOOB-P s (percent RH) 100(P-P S ) 



< i. ati\ mn fl 100p(B-P s ) 100a(B-P s ) _ 



(percent AH) = 100-= Ps (£_^ = .625P S " 



100 (percent RH)(B-P S ) _ (percent RH) (a +0-625) -100 a 

 100P-P S (percent RH) ' 0.625 



(percent 2?#) = 100-p- = (percent AH)^^ = 



IQOg / g \ lQOa+0.625 (percent Aff) 



P s V + 0.625/ a + 0.625 



In these formulas, the symbols have the following meanings : 



a= absolute humidity, pounds water vapor per 



pound dry air. 

 a s = absolute humidity of saturated air. 

 B = barometric pressure, inches of mercury; for the 



standard chart B =29.92 inches. 

 p= partial pressure of water vapor in the air, 

 inches of mercury ; note that p is equal to the 

 vapor pressure of water at the dewpoint of 

 air of the given composition. 

 P s = vapor pressure of water at the given tempera- 

 ture, inches of mercury, 

 (percent AH) = percent absolute humidity, 

 (percent RH) = percent relative humidity. 



The unfortunate multiplicity of terms for expressing the concep- 

 tion of moistness of air is reflected in some confusion in the existing 

 literature on dehydration. The older discussions referred almost 

 entirely to percent relative humidity; the term is still used to some 

 extent, particularly in the analysis of the moisture-vaporproofness 

 of packaging materials. In work on the drying step alone, however, 

 there is little occasion to use the concept of "percent humidity" 

 (either relative or absolute) at all. Dehydrator operation can be as 

 effectively controlled through the use of two temperatures (dry-bulb 

 and wet-bulb) as through the derived term "percent humidity." 

 Dehydrator design does not necessitate the use of "percent humidity" 

 in any form. 



Volume of Mixtures of Air and Water Vapor 



The volume, in cubic feet, occupied by a pound of any gas, such as 

 air, is known as its specific volume. Air and mixtures of air and 

 water vapor, like other gases, occupy more volume per pound weight 

 at higher temperatures than at lower ones, and at lower barometric 

 pressures than at higher ones. The specific volume of dry air at 

 normal sea-level barometric pressure (29.92 inches of mercury) is 

 shown in table 7. Correction to a different barometric pressure, for 

 example the lower pressures which prevail at higher altitudes, may 



569074—44 4 



