Fan Vibration - API 661

On website http://www.wilcoxon.com/ you will find a down-loadable VIBRATION CALCULATOR. You can only convert mm/s (velocity) to g (acceleration in earth gravities - g = 9.81 m/s/s) at a given frequency, assuming a sinusoidal motion. And make sure you know if the value is rms, "mean", "peak" or "peak to peak" (only about 3 to 1 error between using mean and peak-peak on sine wave, let alone a "peaky" waveform!!). One of the worst things about vibration is how "sloppy" folk are about the units used! You are probably talking converting values at the normal speed (1x, revolutions per second = Hz) and 2x, 3x that speed. Also vibration on large fans at low speed often has such low g values that a transducer which actually measures acceleration (g or mm/s/s) has a very low output - the moving coil or magnet type, which respond to velocity are better = hence values are given in mm/s (velocity).