Why have a special up-to-air valve?
An up-to-air valve provides a way of equalizing air pressure in a vacuum system so that the vacuum system may be opened. External air pressure presses on any surface with a 14.7 pounds per square inch force. That force is considerable on even a small 2″ valve, for example, which would make that size valve impractical as a way to allow air to enter a vacuum system: Force = Area x psi = pi x r 2 x 14.7 = 3.14 x 1 x 14.7 = 46 lbs. An up-to-air valve allows air to enter a system gradually without the turbulence of a large influx of air that would accompany a bigger valve.
