Hello,
We are using a stroboscope to measure the rotation speed of some equipment, but weren't entirely certain that we were reading the meter correctly since it has two scale bars and, because it is old equipment, there is not a manual readily available. It's a Cenco Xenon Stroboscope No. 74678.016, photos below.

 

What is not shown are the units for those numbers. They may be flashes per minute /RPM. If you have a frequency counter it will be easy to count frequency and get revolutions per second. If you have some estimate as to the speed that should assist in determining which scale to use. But an old piece of equipment may not be in calibration. You can use it to read the unloaded speed of an induction motor and compare the indicated speed with the nameplate speed of the motor, it should be within 5% or closer.

 

The stroboscope has two ranges - Low and High. I think the two scales on the readout dial correspond to those two ranges.

 

So if the high is on 54 and the low is on 12, what would that correspond to in revolutions per minute?

 

The stroboscope has two ranges - Low and High. I think the two scales on the readout dial correspond to those two ranges.

So if the high is on 54 and the low is on 12, what would that correspond to in revolutions per minute?

 

Do you have a digital AWG and bright LED ?
Adjust flash frequency until you get a strobe freeze then read the frequency in Hz and multiply by 60.

 

There is only an arbitrary relationship between the two scales. It just depends on which range you have selected. If you select the low range, and the reading is 12 RPM on the right scale, it will also read 12 RPM on the left scale when you select the high range.
The lower range will give you better resolution on lower RPM.

 

There is probably only an arbitrary relationship between the two scales. It just depends on which range you have selected. If you select the low range, and the reading is 12 RPM on the right scale, it will also read 12 RPM on the left scale when you select the high range.
The lower range will give you better resolution on lower RPM.

So, if I had the high range selected when that photo was taken, then it would be recording 5,400 RPM? That is roughly what we expected. Thanks for baring with me.

 

So, if I had the high range selected when that photo was taken, then it would be recording 5,400 RPM? That is roughly what we expected. Thanks for baring with me.

Yes, you are correct. It is an old instrument so the reference frequency may not be very accurate.

 

Use caution with a stroboscope.

You can see a stable mark at half speed or other sub-multiple. If there's one mark on the shaft, when it freezes, you might be reading half-speed instead of the true speed. When the mark freezes, note the speed then double the tach speed. If you were at the actual speed, you should now see two marks on the shaft, 180° apart. If you don't see two marks, you weren't at the true speed.

A turbine generator was destroyed when a strobe was in correctly read when setting the overspeed trip mechanism, resulting in multimillion dollar damage.

The turbine was operating at almost twice the rated speed when the generator rotor expanded enough to contact the stator, resulting in instant destruction.