How do I check the accuracy of a mechanical tachometer?

Most mechanical tachometers are calibrated to register twice the input RPM (2:1 ratio). When a known speed rotating device (e.g. an electric drill that rotates in the direction to cause the tachometer to indicate) is attached to the drive end of the flexible shaft, an input speed of 500 RPM should register 1000 RPM on the tachometer. This ratio applies to other input speeds, e.g. 1000 input RPM will register 2000 RPM on the tachometer.

NOTE: No ratio adapters are used in this test.

If the instrument does not function correctly, look for possible flexible shaft core breakage. Also, if a ratio adapter is used in the system, it must be checked for function independently.