A reliable method to calibrate gauges, switches or transmitters on Iso-Rings, the calibrating machine is rugged, lightweight and portable, requires no external power and can generate pressure to 400 psi. Instruments are temporarily removed from the Iso-Ring, attached to the calibration machine for verification and then reinstalled on the Iso-Ring. The calibrating machine is available with mechanical or electronic reference gauges, ANSI class-3A accuracy and is traceable to NIST.