88 SWEETENED CONDENSED MILK CONDENSING 



The pressure or, correctly speaking, the vacuum, is expres- 

 sed 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 absolute 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 

 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 calculated as follows : 



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

 sea level. What is the absolute pressure? 



^-r - 2.45 pounds of absolute pressure per sq. inch. 



Relation of Altitude to Atmospheric Pressure. At altitudes 

 higher than the sea level, the atmospheric pressure is reduced 

 and the mercury column is lowered, though the absolute pres- 

 sure 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 table shows the barometric reading in inches 

 of mercury column and the atmospheric pressure in pounds per 

 square inch at different altitudes : % 



