Harmonics; Determing An Object's True Rpm - TESTO 477 Mode D'emploi

Case 4: If you stand behind it and strobe it at a rate of 995 FPM, it
will appear to move in a counterclockwise direction at a rate of 5
RPM.
7.1.3.

Harmonics

If you continuously increase the flash rate while strobing an object,
it may appear to freeze, slow down, speed up, go forward, freeze
again, go backwards, form multiple images, etc. These images
appear at mathematically determined multiples or harmonics of the
object's actual speed.
Example: Assume you wish to slow the motion of the fan used in
the last example, but you want it to be brighter.
Technique: Starting from 1,000 FPM, slowly increase the flash rate.
At 1,500 FPM the image will appear to freeze again. Continue to
increase the rate.
The image will appear to freeze again at 3,000 FPM. At this rate,
the fan appears to be very bright.You can now use the four buttons
to vary the rate above and below 3,000 to make the fan appear to
move both clockwise and counterclockwise.
Helpful Hints:
• Harmonic images appear at both whole number multiples as
well as fractional intervals of the object's actual rate. For
example, a fan rotating at 1,000 RPM will appear to be frozen at
the whole number multiples of 2,000 (2x), 3,000 (3x), 4,000 (4x)
etc., as well as at the fractional rates of 500 (1/2x), 750 (3/4x)
and 1,500 (1 1/2x), etc.
• Some of the harmonic images are "singular" in appearance
while others are "multiple". This becomes important if you want
to determine the objects actual rate as discussed in chapter

Determing an object's true RPM.

7.1.4. Determing an object's true RPM
The testo 477 can be used as a digital tachometer to determine the
true RPM and/ or the reciprocation rate of an object. This is done
by visually "freezing" the object's movement and then reading the
LCD display. As with all stroboscopes, it is important to verify that
this frozen image is not a harmonic of the object's actual rate.
Helpful Hints:
• Knowing the approximate rate of the object in advance gives
you a useful starting point.
• If the object has a uniform shape, like a multi-blade fan or motor
shaft, you must give it an identifying mark (using paint or
7 Application information
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