Single Master Air Gaging Systems

Choosing a single master or dual master air gaging system can be difficult. Understanding the similarities and differences--in performance, price and philosophy--can help a user make the best choice for their application.

Columns From: 3/15/2010 Modern Machine Shop,

Editor's Commentary

To determine which type of air gaging system is right for you, tune in next month when we look at dual master systems and add up all the pluses and minuses in terms of performance. (For a link to the next column, click on the article in the "Editors Picks" section.

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George Schuetz

FIG. 1: 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.

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 submicronic measurement available, and many of the air plug configurations developed during this period have not changed dramatically since then.
 
However, the readouts and their capabilities have changed. 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 a single switch. This makes the job faster and easier for the user and provides better results at the end of the day.
 
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 for worldwide use. 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 similar to the one shown in Figure 1. The pressure distance curve has a linear area, “b,” which can be used for distance measurements. If the components of the system are controlled precisely enough in terms of pressure, air jet, location, machined characteristics and other considerations, the pressure distance curve can be repeated and reproduced in a manufacturing environment.
 
This means that the midpoint of the curve, the “¤,” can be determined precisely. Also, by using a fixed magnification display, which is what converts pressure to a measurement reading, the system requires only a single master 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. Knowing this, users only need 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 various advantages:
 
• Setup is eased through the use of one master.
 
• Tooling and displays of the same fixed magnification are interchangeable, without the need to adjust magnifications.
 
• Accuracy is built in, so linearity is known throughout the entire range.
 
• Measurements are not susceptible to small pressure changes. They have excellent stability, and readings do not drift after being set.
 
• Tooling performance can be monitored without the need for additional masters.
 
• Response speed is good.
 
• Medium/high air pressure (30 psi) is used to help clean parts.
 
• Recessed jets offer greater jet clearance for longer tooling life. Tooling body wear does not affect magnification.
 
• Recessed jets reduce potential for damage or clogging.
 
• Certifiable tools and restrictor kits are available to verify the performance of the display units.
 
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 things such as atmospherically balanced air systems that are not susceptible to small pressure changes. (This is done by incorporating precision reference orifices into the tooling.) Of course, these systems cost more. With a fixed magnification system, a specific amplifier is needed for each tool, and extra tools are needed to verify system calibration.
 
To determine which type of air gaging system is right for you, tune in next month when we look at dual master systems and add up all the pluses and minuses in terms of performance.

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