General Definition

The operation of air gages (the tools of pneumatic gaging) is based on phenomena that occur when pressurized air, while escaping through an orifice of controlled size, is impeded in its free flow by impinging against a solid surface. The distance of that surface from the air escape opening will affect the air flow velocity and create back pressure upstream of the orifice. By maintaining, at a constant level, the pressure and the volume of the air that is introduced into the system, variations in the upstream air will be a function of the obstruction to the air escape. To create an effective obstruction, the solid surface facing the orifice must be located close enough to actually reduce the area of air escape.

Within a specific dimensional range, which depends on the design characteristics of the gage, variations in the air flow and pressure will be almost proportional to the changes in the distance between the office and the obstruction.

In actual gage applications the orifices are the bores of nozzles mounted into appropriate gage bodies, which keep the air escape openings at a positive distance from the object wall, when the dimension being measured is at nominal size. Variations in the actual size of the part will modify the distance between the orifice and the obstruction, changing the velocity and the pressure of the air in the system. The changes caused by differences in the escape rate of the air can...