88 SWEETENED CONDENSED MitK—CoNDENSING 
the vacuum, is expres- 
sed in terms of inches of mercury which the atmospheric pressure 
The pressure or, correctly speaking, 
sustains. ‘Che mereury 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 devrees F., and of 29.922 inches at 32 degrees F. 
The absolute ‘vacuum may be calculated by multiplying the 
atinospheric 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 vacnum pan. The absolute 
pressure may be caleulated as follows: 
Example: The actual vacuum in the pan is 25 ®iches at the 
sea level. What is the absolute pressure? 
14.7 x (30 — 25) 
30 
Relation of Altitude to Atmospheric Pressure.—At altitudes 
higher than the sea level, the atmespheric pressure is reduced 
= 2.45 pounds of absolute pressure per sq. inch. 
and the mercury column is lowered, thoygh 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: 
