FREEZE-DRY PRESERVATION — HOWER 



much vapor-pressiire difference, the only limitation to the effective- 

 ness of a condenser is the surface area (fig. 2). 



Pressure reduction. — When vapor transfer occurs at atmospheric 

 pressure, water molecules are impeded by collisions with air molecules. 

 At atmospheric pressure, the mean free path (the average distance a 

 vapor molecule can travel before colliding with another molecule) is 

 approximately .005 microns (5X10~^ mm.). The mean free path of 

 a water-vapor molecule with relation to pressure is as follows : 



pressure m ean free path 



0.0034 mm. 

 0.034 mm. 

 0.34 mm. 

 3.4 mm. 

 34.0 mm. 



The transfer of water molecules from an ice crystal to a condenser 

 can be accelerated by increasing the length of their mean free paths; 

 this can be done by reducing the pressure in the chamber with a 



Figure 2. — Water-vapor differential (^= specimen chamber; 5= condenser chamber; C= 

 cut-off valve; Z)= vacuum pump. 



vacuimi pump. Not all vapors, however, are condensable; therefore, 

 a pmnp must be used to remove these vapors so that chamber pres- 

 sure will remain low enough to permit efficient removal of the water 

 to its condenser. 



