Sweetened Condensed Milk — Condensing 79 



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

 in terms of inches of mercury which the atmospheric pressure sus- 

 tains. The mercury column is not a direct measure of the pres- 

 sure, but it shows the difference between the atmospheric pressure 

 and the absolute pressure in the vacuum chamber. The atmospheric 

 pressure at the sea level is 14.7 pounds per square inch. It sus- 

 tains a mercury column in an absolute vacuum of 30 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 calcu- 

 lated 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? 



PX(AV — V) 14.7 X (30 — 25) 



= = 2.45 pounds of absolute 



AV 30 ■ . 



pressure per sq. mch. 



Relation of 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 lo- 

 cated at high altitudes the mercury column will show fewer inches 

 of vacuum at a given temperature and with a given absolute 

 pressure. 



The following table shows the barometric reading in inches of 

 mercury column- and the atmospheric pressure in pounds per square 

 inch at different altitudes : 



