58 Condensed Milk and Milk Powder 



The pressure, or correctly speaking, the vacuum is expressed in 

 terms of inches of mercury which the atmospheric pressure sustains. 

 The mercury column is not a direct measure of the pressure, but it 

 shows the difference between the atmospheric pressure and the abso- 

 lute pressure in the vacuum chamber. The atmospheric pressure at 

 the sea-level is 14.7 pounds per square inch. It sustains a mercury 

 column in an absolute vacuum of thirty inches at 62 degrees F. and 

 of 29.922 inches at 32 degrees F. The absolute vacuum may be 

 calculated by multiplying the atmospheric pressure by the factor 

 2.04. In case there is only a partial vacuum the mercury column 

 sustained is lowered to the extent of the absolute pressure in the 

 vacuum pan. The absolute pressure may be calculated as follows : 



AV = Absolute vacuum which 'is thirty inches at the 

 sea-level 

 V = Actual vacuum 



P = Atmospheric pressure which is 14.7 pounds at the 

 sea-level 

 AP = Absolute pressure 

 Example : The actual vacuum in the pan is 25 inches at the 

 sea-level. What is the absolute pressure? 



14.7 x (30 — 25) 



— = 2.45 pounds of absolute pressure per sq. inch 



30 



Relation oe Altitude to Atmospheric Pressure. — At alti- 

 tudes higher than the sea-level, the atmospheric pressure is reduced 

 and the mercury column is lowered, though the absolute pressure in 

 the vacuum pan may be the same. Therefore, in factories located 

 at high altitudes the mercury column will show fewer inches of 

 vacuum at a given temperature and with a given absolute pressure. 



The following tables show the barometric reading in inches of 

 mercury and the atmospheric pressure in pounds per square inch at 

 different altitudes, and the altitudes of various cities in the United 

 States. 



