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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 (300,000
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)
If a Remote Optical Sensor or Magnetic Sensor is used to sense one pulse per
revolution (External mode), the readout will display directly in RPM (FPM)
without any adjustment required.
In instances when you can shut down the device and install a piece of refl ective
tape, then an optical tachometer is easier to use for RPM measurement.
Stroboscopes must be used when you can’t shut down the device. The
human eye is not easily tricked into seeing a stopped image by a stroboscope
when the fl ash rate is slower than 300 FPM. Therefore, stroboscopes are just
about impossible to use below 300 FPM for inspection or to measure RPM.
Stopped Image 1/4 times 1/2 times 1 time 2 times 3 times 4 times
Flash Rate (FPM) 1250 2500 5000 10000 15000 20000
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