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Reasons for the preferred open-loop test method are given. A complete test station, built to calibrate precision tachometers in production, is described. A 0.007% precision a-c voltage ratio test console is described in detail giving schematics, critical shielding instructions, and pointers on reducing pickup. An error equation quantitatively describes the main sources of equipment error. Actual performance curves of different types of tachometers are presented with comments on the critical test techniques required.