9
3.3.1 True RPM Measurement
General
:
Stroboscopes are highly stable industrial quality instruments for non-contact RPM mea-
surement as well as for inspection and observation of moving parts.
They give the operator the illusion of “stopped motion” where in actuality the equipment
under observation is in a moving state. By adjusting the flash rate, equipment in motion
appears to be standing still. With a slight adjustment, movement can be viewed in appar-
ent slow motion, which enables the operator or observer to study the process in action.
All stroboscopes can measure rotational (rpm) or reciprocating (strokes per minute)
speeds with the same high precision as with an electronic digital tachometer.
To measure true revolutions per minute (rpm):
1. “Mark” the object to be measured by either visually noting an inherent distinguishing
characteristic (such as a label, scratch, etc.) or physically marking the object with a
small piece of tape, pencil mark, etc.
2. Firmly plug in power cord.
3. Turn power switch on.
4. Turn setter from highest rpm downward.
5. The true rpm can be noted once the action appears frozen
and
the first
single
image
of the “mark” appears (see chart below and accompanying diagram for further
explanation).
6. To verify this, double the flash rate by pressing
; you then should see 2 images.
If the flash rate, however, is below the actual RPM value by 1/even num-bers, a
stopped motion with only one image will appear.
7. To verify rpm reading, press
; a single image should appear again.
Measuring higher RPM than the stroboscopes’ range using harmonics
:
All instruments can be used also to measure higher RPM by using the images effect.
For example: - Range up to 20,000 rpm
- Rotating object 60,000 rpm
- Images 3 times seen
Shaft
Rotation
(rpm)
Flashes
(rpm)
Flashes/
rpm
Shaft
Stopped
Images
At
1500
rpm
6000
4 times
4
4500
3 times
3
300
2 times
2
1500
1 times
1
750
1/2 times 1
500
1/3 times 1
