14 



PROCEEDINGS OF THE NATIONAL MUSEUM vol. ng 



Table 4. — Metric conversion table 



1 in. 

 1 ft. 

 1 in.2 

 1 ft.2 

 1 in.3 

 lft.3 



10 mm. 

 010 cm. 

 100 mm.2 

 100 cm.2 

 010 ml. 

 010 cl. 

 010 dl. 

 010 1. 



Table 5.-~Conversion multipliers (e.g., liters per second times the factor equals 

 cubic feet per minute) 



From 



liters per second 



liters per minute 



liters per second 



cu^ic centimeter per second 



cubic feet per minute 



late the vacuum-line dimensions required to permit adequate 

 conductance. 



^ The flow of fluids (or vapors) wdthin a vacuum system is usually 

 either molecular flow or viscous flow. Molecular flow occurs at 

 pressures at which the molecule's mean free path is greater than the 

 average diameter of the tubing through which it flows. 



Since the mean free path of a molecule does not approach the 

 diameter of any large vacuum line at above 1 -micron pressure, we 

 are concerned only with viscous flow. 



It should be noted that restriction of tubing to the viscous flow of 

 vapors is significant only when the volumes to be moved are very 

 large or the pressure differential is small. The law that expresses the 

 viscous flow of fluids through a tube was first deduced by Jean L. M. 

 Poiseuille, the French physicist (Daniels et al, 1949, p. 71). This law 

 establishes relationships between the coefficient of viscosity (vis- 

 cosity = poises/dyne sec./cm.2), the volume of the fluid flowdng through 



