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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.