NO. 3549 FREEZE-DRY PRESERVATION — HOWER 15 



the whole section of the tube in unit time (seconds, as applied here), 

 the pressure differential at each end of the tube, the radius, and the 

 length of the tube. 



F= Volume (cm.Vsec.) 



_p= Pressure (dynes/cm.^) ^r* 



r = Radius (in cm.) ~ 8z, 



Z= Length (in cm.) 



The formula states that the volume of vapor transferred through a 

 tube in a specific period of time is proportional to the fourth power of 

 the tube radius. This means that a small increase in tube diameter 

 will produce a considerable increase in vapor conductance. 



To utilize the formula, it is necessary to Imow the coefficient of 

 viscosity, which for air is approximately 1.7X10~^ poises at a pressure 

 of 1 dyne/cm.2= 7.5X10-" mm. Hg. 1. 



For example, to calculate the conductance of vapor through a 

 tube that is 193 cm. long, 7.62 cm. in diameter, and ^^dth a pressure 

 differential between the specimen and the condensing surface of 

 2000 microns, the following values are: 



F=cm.^/sec. 



^=2666.7 dynes/cm.2 (2000)li pressure differential) 

 r=3.81 cm. 

 i=193 cm. 

 z;= 1.7X10"* poises 



y^ (3.14) (2666.7) (3.81)" ^ 1737861.723 



(8) (193) (.017) 26.248 



F= 66209 cm.Vsec. 



Testing for leaks. — The overall system must be tested for 

 vacuum leaks. Leaks in a vacuum system can be detected easUy -with 

 a Peroni pressure gauge used in conjunction with acetone. The 

 vapor pressure of acetone is so high that, when joints or surfaces are 

 brushed with it, any leak will cause a marked increase of pressure 

 within the system. The gauge head should be installed at a point 

 near the vacuum pump and then watched for pressure increases after 

 the system has been evacuated and tested. 



Another test is to inject freon under slight pressure into the system 

 and locate the leaks with a refrigeration leak detector. When the 

 system is initially pumped down, the pressure in the system may be 

 higher than anticipated due to gases being given off by zinc coating 

 or other materials of high vapor pressure that may be inside the sys- 

 tem. The pressure can be effectively reduced by allowing air to 

 reenter the system and by pumping it down repeatedly until these 

 vapors have been evacuated. 



