Pressure and vacuum ratings are good for sea level (14.7 psi atm. press.) only. This chart may be used to determine pressure developed at altitudes above sea level. For example, a 5 lb. machine produces 5 psi at sea level. At 5000' elevation, the same machine produces only 4.160 psi.

The following table was derived by using the head formula.

*Variations in pressure with changes in altitude are directly proportional to the relative density of the air.

The following can be used to compute atmospheric pressure at the altitudes indicated according to the relationship: