Monarch Instrument Nova-Strobe PBL - Manual (Page 20)

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button. You should again see a single image (which may be phase shifted
with respect to the fi rst image seen).
For example, when viewing a shaft with a single key way you will see one
stationary image of the key way at the actual speed and at 1/2,1/3,1/4, etc, of
the actual speed. You will see 2 images of the key way at 2 times the actual
speed, 3 key way at 3 times, etc. The FPM equals the shafts Revolutions Per
Minute (RPM) at the highest fl ash rate that gives only one stationary image
of the key way.
Example: Object rotating at 3000 RPM
If the speed is outside the full scale range of the stroboscope (500,000
FPM), it can be measured using the method of harmonics and multipoint
calculation. Start at the highest fl ash rate and adjust the fl ash rate down.
You will encounter multiple images so be aware of these. Note the fl ash rate
of the fi rst SINGLE image you encounter, call this speed “A”. Continue
decreasing the fl ash rate until you encounter a second SINGLE image. Note
this speed as “B”. Continue decreasing the speed until you reach a third
SINGLE image at speed “C”.
For a two point calculation the actual speed is given by:
RPM = AB/(A-B)
For a three point calculation:
RPM = 2XY(X+Y)/(X-Y)2 where
X = (A-B) and Y = (B-C)
Stopped Image 1/4 times 1/2 times 1 time 2 times 3 times 4 times
Flash Rate (FPM) 1000 1500 3000 6000 9000 12000
Actual RPM