14 BULLETIN 102, UNITED STATES NATIONAL MUSEUM. 
been reached by man’s physical senses, but both form the bases from 
which all gas volume calculations must be made as shown on 
pages 15 and 16. 
BAltOMETRIC PRESSURE. 
Atmospheric pressure is measured by a barometer — usually in 
inches of mercury, 1 inch of mercury equaling 0.49 pound to the 
square inch pressure — and is synonymous with barometric pressure. 
Sea level is the basis from which atmospheric pressures are 
reckoned. At that point dry air at 32° Fahrenheit exerts a pressure 
of 14.7 pounds to the square inch. 
This pressure varies with altitude and temperature, the pressure 
decreasing with an increase in altitude or temperature. 14.4 pounds 
represents a fair average barometric pressure for most natural gas 
using communities. 
GAGE PRESSURE. 
This is simply the pressure indicated by a pressure gage. Two 
general classes of gages are used for measuring gas pressure : 
1. Spring gages . — Where the effect of the pressure exerted against 
some form of spring is made to move a pointer over a graduated 
dial or scale. 
2. Fluid gages . — Where the effect of the pressure is indicated by 
the height of the column of fluid in a U-shaped tube. One side of the 
U-shaped tube is open to the atmosphere and the other is attached 
to the pipe where the pressure is to be measured. The gas pressure 
in this pipe then lowers the fluid in one side of the tube and raises 
it in the other. The total difference in the heights of the fluid on 
the two sides represents the total fluid pressures. When no pres- 
sure is applied to such a U tube gage other than the prevailing atmos- 
pheric pressure, the liquid will stand at the same level in both tubes. 
The pressures in natural gas distributing plants are almost uni- 
versally measured in ounces to the square inch, while the pressures 
in manufactured gas distributing plants are measured in inches of 
water, 1 ounce equaling 1.73 inches of water. 
Where the word pressure occurs in ordinances or rules it invariably 
means gage pressure. 
ABSOLUTE PRESSURE. 
This is the sum of the gage pressure and the barometric pressure. 
Thus, if the gage pressure is 4 ounces — equaling 0.25 pound — and 
the atmospheric pressure 14.4 pounds to the square inch, the absolute 
pressure will be 14.65 pounds to the square inch, as shown on p. 15. 
This must be used in all gas calculations dealing with change of vol- 
ume due to effect of pressure. 
