As a single-master system, the gage is calibrated to zero or a reference measurement, and users rely on the gage system accuracy to determine tolerance limits. Dual-master systems use two masters to calibrate maximums and minimums. For go/no-go applications, the air gage is usually calibrated with both masters or gages set for both tolerances. The device is available with a range of interchangeable dials for selecting magnifications from 1,250:1 to 10,000:1 in inch or metric scales. Dials are exchanged by a snap-off bezel that allows for easy changes while protecting the gage from contamination. Mahr Federal Inc., 1144 Eddy St., Providence, RI 02905, (800) 333-4243,