12 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 119 



(2) To determine how long it will take a pump with a displace- 

 ment of 27 cubic feet per minute to evacuate a system with a volume 

 of 160 cubic feet to a pressure of 200 microns (at 200 microns the 

 factor is 9.50), calculate as follows: 



9.50X160=152 



^ = 56.29 min. 



Under certain conditions the refrigerated condenser can be 

 eliminated and water vapor can be removed directly through the 

 vacuum pump, provided the pump is capable of removing the vapor 

 at a rate equal to its release from the specimen. To determine 

 whether the condenser can be omitted, the approximate quantity of 

 water vapor being released each minute by the specimens within the 

 chamber must be computed. 



If, for example, a specimen chamber contains three squirrels, two 

 flickers, a toad, and a garter snake (at various drying stages and all 

 of average weight), the average daily release of water vapor wall be 

 approximately 12 grams. 



If 150 microns of pressiu-e were maintained within the chamber, it 

 would be necessary to pump 200 cubic feet of Avater vapor for each 

 gTam of water removed from the system, or 2400 cubic feet per day 

 (1.66 cu. ft./min.). If the vacuum pump cannot handle this volume 

 of vapor, the pressure within the specimen chamber will slowly rise; 

 and, as it rises, the volume occupied by a unit weight of water vapor 

 decreases. At a pressure of 300 microns, two grams of water will 

 occupy the same volume as would one gram at half that pressure. 

 It is obvious, therefore, that overloading a chamber or providing an 

 inadqeuate vacuum pump will cause an increase in operating pressure 

 in the specimen chamber. To avoid this, a refrigerated condenser 

 should be used. 



Since the differential between the water-vapor pressure within the 

 chamber and the vapor pressure of the ice within the specimen provides 

 the driving force for the movement of water vapor through the system, 

 an increase m pressure wdthin the chamber causes a decrease in 

 efficiency. 



An increase in chamber pressure from 150 to 300 microns with a 

 specimen temperature of —10° C. would result in a vapor pressure 

 differential of from 1.80 to 1.65 mm. Hg., or a reduction of a little 

 more than 8 percent. 



While this example demonstrates that increasing water-vapor load 

 is not as deleterious as expected, it nevertheless establishes a relation- 

 ship between water-vapor load and vacumn-pump capacity and 



