FREEZE-DRY PRESERVATION HOWER 



11 



from volatile materials within the system. Pump displacement or 

 pumping speed must be calculated according to the volume of the 

 system and pressure desired. The time required to pump the system 

 down to the ultimate pressure (about 10 to 100 microns Hg.) should be 

 established. The pump-down chart (fig. 6) may be used to calculate 



Figure 6. — Pump-down factor chart. 



displacement requirements. By way of illustration, the following 

 two examples are cited: 



(1) To determine the required pumping displacement to evacuate 

 a system with a total volume of 15 cubic feet to a pressure of 100 

 microns in 5 minutes, reference to the pump-down chart shows that 

 at 100 microns the factor is 10.9. Multiply 10.9 by the volume 

 (15 cu. ft.) to obtain the total number of cubic feet to be pumped 

 (163.5). To determine the requned pump displacement, divide this 

 total by the number of minutes (5) allowed for the evacuation. 



10.9X15: 

 163.5 



163.5 

 32.7 cu. ft./mm. or 55,563.6 liters/sec. 



