13
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
Example: Object Rotating at 5000 RPM
If the speed is outside the full scale range of the stroboscope (20,000
FPM), it can be measured using the method of harmonics and multipoint
calculation. Start at the highest flash rate and adjust the flash rate down.
You will encounter multiple images so be aware of these. Note the flash
rate of the first SINGLE image you encounter, call this speed “A”. Continue
decreasing the flash 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
reflective tape, then an optical tachometer is easier to use for RPM
measurement.
Stroboscopes must be used when you cannot shut down
the device.
The human eye is not easily tricked into seeing a stopped
image by a stroboscope when the flash rate is slower than 300 FPM.
Therefore, stroboscopes are just about impossible to use below 300 FPM
for inspection or to measure RPM.
