CPM (RPM) Spectrum

Controls: X-Axis Dialog >> CPM
Macro: CPM

This button toggles the X-axis units in Spectrum modes between Hertz and Cycles Per Minute (CPM). This is intended for use when observing machine vibrations that may correlate with revolutions per minute (RPM). You may toggle this state directly (even if the dialog is not visible) via ALT+SHIFT+C.

By default, CPM mode simply applies a factor of 60 seconds/minute to the former hertz values. That means that 2 kHz becomes 120000 CPM, shown as 120 kCPM or even 0.12 MCPM depending on the total axis range.

However, that only gives true RPM for a source that emits one cycle per revolution. If you are monitoring (say) gear noise that involves one event per tooth, the results would be too high by a factor equal to the number of teeth.

To read true shaft RPM, open the Frequency Counter (from the Options Menu, or directly via CTRL+F). Then set the Cylinders (Cyl) control at the the far right end of the Frequency Counter window to twice the number of gear teeth.

Macro Notes:

CPM=1 sets CPM mode, CPM=0 turns it off, and CPM=x toggles the current state.

See also X-Axis Control Dialog