FREEZE-DRY PRESERVATION — HOWER 



9 



At the Smithsonian, a length of copper tubing, 3^ inches in diameter 

 (later replaced with steel), was brazed into an opening, 3% inches in 

 diameter, near the top of the 30-gallon tank. The opposite end of the 

 tubing was connected to the specimen-chamber vapor line. At the 

 base of the tank was an opening into which a l}^-inch steel pipe was 

 welded, connecting it to the vacuum pump. 



Inside the condensing chamber, a large complex of coUs within coils 

 was fashioned from 200 feet of copper refrigeration line % inches in 

 diameter. The expansion valve was mounted inside the chamber and 

 the refrigeration line was brought into and out of the chamber through 

 openings in the side of the chamber, which were sealed with soft sol- 

 der as in the specimen chamber (see fig. 4). 



Refrigeration of the condenser. — The force underlying the 

 motion of water vapor from the ice, through the specimen chamber to 

 the refrigerated condenser, is the vapor-pressure difference. This 

 force is produced by the difference in temperature between the ice 

 crystals within the specimen and the refrigerated condenser. Table 2 



Table 2. — Relationship between temperature (° C) and vapor pressure (mm. Hg.) 



demonstrates the fixed relationship between temperature and the 

 vapor pressure of water. The temperature difference between the 

 specimen and the refrigerated condenser need not be extreme in order 

 to produce a substantial difference in water-vapor pressure; however, 

 it should be noted that lowering the condenser temperature produced 

 less gain logarithmically in vapor pressm^e difference. 



219-947 66—2 



