Air gaging is a technology that employs the use of air flow volumes and air pressure to determine the size of measured part dimensions. To achieve this measurement, it relies on the laws of physics, which state that flow and pressure are directly proportionate to clearance and react inversely to each other. As clearance increases, air flow also increases and air pressure decreases proportionately. As clearance decreases, air flow also decreases and air pressure increases accordingly.
In all air gaging applications, regulated air flows through a restriction of some kind — a needle valve, jeweled orifice, etc. — prior to being expelled through the nozzles of a particular air tool. When the nozzle of an air tool is unobstructed and blowing to the atmosphere, maximum air flow occurs and the air pressure — called back pressure — between the restriction and the nozzle is at a minimum. As an obstruction is brought closer to the front of the nozzle, air flow from the nozzle diminishes and back pressure builds. When the nozzle is completely obstructed, air flow is zero, and back pressure reaches the pressure supplied by the regulated air supply. In this scenario, air flow moves from maximum flow to minimum flow, while back pressure moves proportionately in the opposite direction: minimum to maximum.
These values of flow and pressure can each be plotted and graphed against the nozzle's clearance from the obstruction. The extremes of both back pressure and flow notwithstanding, these graphed values will always represent straight-line curves and establish the straight line or "linear" proportions that form the basis of all air gaging.
A Brief History of Air Gaging. The first air gages were developed in France before World War II by a carburetor company that sought a reliable method of measuring its carburetor jets. Through the war and the patents that followed, improvements in back pressure bleed, differential, and flow technologies progressed steadily. Modern air-to-electronic amplification has raised the resolution of air gaging to the level of millionths of an inch while allowing modern data collection and processing technologies to generate and download information for statistical process control.
Master Gages: One vs. Two Master Systems. Both single-master and two-master setups are used in air gaging. The two-master system establishes linearity at both tolerance extremes during calibration; the single-master system is faster to set up but assumes a known linear response. Most modern Edmunds air gaging systems use two masters for the highest accuracy in production.
Types of Air Gaging Systems. Back Pressure Bleed System — uses a bleed orifice to dump excess flow and measure resulting pressure. Back Pressure System — measures the back pressure directly without bleed. Differential System — compares back pressure on the measurement side against a reference circuit for improved sensitivity. Flow System — measures volumetric flow directly through a calibrated float or transducer.
Why Use Air Gaging Today? Air gaging is an efficient and reliable method of measurement that's ideal for measuring dimensions with tolerances smaller than 0.005 in. When gaging tight tolerances, a resolution as small as 0.000002 in. can be achieved. It is non-contact, repeatable, and well-suited to high-volume production.
About the Author: Rob Edmunds III is involved with many facets of the gaging industry, and participates in national and regional industry associations. Currently Vice President of Edmunds Gages, he can be reached during normal business hours at Edmunds Gages 860-677-2813 or via email at REdmundsIII@edmundsgages.com.