Single Master Air Gaging Systems

Single Master Air Gaging Systems
George Schuetz, Mahr Federal Inc.

 As we have noted on numerous occasions in these pages, air gaging is the granddaddy of precision measurement, with systems employed on the shop floor since the 1940s. Air gaging was the earliest form of submicron measurement available, and many of the air plug configurations developed during this period have not changed dramatically since then.

 What has changed are the readouts and their capabilities. Today's digital readouts offer more range and higher resolutions. Also, modern amplifiers can provide additional functions such as displaying actual part size, performing dynamic checks so the operator does not have to calculate results, and changing the display from ID to OD with just a single switch. This has made the job faster and easier for the user and provides better results at the end of the day.

 But one air gaging issue that has remained cloudy over the years is whether to choose a single or dual master system. Both have their advantages (and disadvantages) and both are available and in use worldwide. Understanding these differences in performance, price, and philosophy will help users make the best choice for their application. We'll start here with single master systems.

 The principle of air gaging is simple: if you blow a jet of air onto an object, the pressure of that air drops as the distance of the object increases. In a tightly configured system, this generates a pressure distance curve as shown in the Figure. The pressure distance curve has a linear area, "b", which can be used for distance measurements, and if the components of the system are controlled precisely enough in terms of pressure, air jet, location, machined characteristics, etc. the pressure distance curve can be repeated and reproduced very exactly in a manufacturing environment.

 This means that the midpoint of the curve, the "O", can be determined very precisely. And by using a fixed magnification display (which is what converts pressure to a measurement reading), you get a system in which only a single master is required to set zero within the pressure distance curve and the resulting displacement readout unit.

 This philosophy is not unique to air gaging. The same concept is used in certain amplifier and LVDT systems. Both are manufactured to known electrical characteristics resulting in a predictable voltage displacement curve. Manufacturing to a standard allows for interchangeability while still maintaining overall system performance. And knowing this, we need only use a single master to set zero for comparative measurements. Only on a regular calibration cycle does the span need to be verified.

 With air gaging, single master systems offer a number of advantages, including:
• One master means very easy set-up
• Tooling and displays of the same fixed magnification are interchangeable without the need to
    adjust magnifications
• Accuracy is built in and thus has known linearity over entire range
• Measurements are not susceptible to small pressure changes. They have excellent stability, and
    readings do not drift after being set
• There are easy ways to monitor tooling for performance without the need for additional masters
• Good speed of response
• Uses medium/high air pressure (30 psi) which helps clean parts
• Recessed jets offer greater jet clearance for longer tooling life, i.e., tooling body wear does not 
    affect magnification
• Recessed jets reduce potential for damage or clogging
• There are certifiable tools and restrictor kits available to verify the performance of the display units


 But the big "IF" with single master systems is that these benefits accrue only if the tooling and readout devices are manufactured to the highest standards, and include such things as atmospherically balanced air systems that are not susceptible to small pressure changes (this is done by incorporating precision reference orifices into the tooling). This, of course, means these systems cost more. Having a fixed magnification system also means you need a specific amplifier for each tool, and you need extra tools to verify system calibration.