Display Notes for Speed Measurements
Care must be taken to ensure that the reflective mark is providing a 1:1 measurement.
Check that there is only one mark and not two, four, or more stationary marks on the object
under test.
Two or more stopped marks indicate a ‘harmonic’ measurement (2:1, 3:1, 4:1 etc.) and the
instrument will display double, triple, or quadruple the actual speed. A useful method of
avoiding harmonic measurements is to adjust the FINE/COARSE knobs until two images
(reflective marks) appear and then lower the flash rate (via COARSE/FINE knobs) until a
single, stationary image appears. This will be the actual speed.
In the example below, a rotating shaft is affixed with one piece of reflective tape. The
illustrations below represent several possible images that would appear when the strobe
beam is pointed to the shaft.
If more than one reflective mark appears (as in columns 3, 4, and 5 below) the image is
showing a ‘harmonic’. The bold column (column 2) shows the actual speed of the shaft.
Column 1 shows a harmonic whereby the actual speed is divided in half. Because of this, it
is good practice to adjust the ‘FINE’ and ‘COARSE’ knobs until you can see the ‘doubled
harmonic’ (Column 3) and then the ‘halved harmonic’ (Column 1) and then find the setting
in-between (Column 2 in bold).
Shaft speed
Strobe flash rate
Visible reflective
marks
1000rpm
500rpm
1
1000rpm
1000rpm
1
1000rpm
2000rpm
2
1000rpm
3000rpm
3
1000rpm
4000rpm
4
Note that the LED display has an AUTO RANGE INDICATOR (flashing decimal at the far
right) that appears when the readings are over 10,000 RPM. In these cases, multiply the
displayed reading by 10.
Visually inspecting a moving object (stop-motion)
When the speed of the moving object matches the flash rate of the stroboscope, the
moving object appears still. Measure the speed of a moving object as described in the
previous sections to obtain a stop-motion. Once stop-motion is achieved, the FINE adjust
knob is used to vary the viewing perspective of the object under test for visual inspection.
Important note on Test Durations
The Xenon lamp used in this instrument has time restrictions on usage. If the specified test
times and cool-off periods (shown in the table below) are not observed, damage to the
instrument can result and the life span of the lamp can be shortened.
RPM
< 3,000 rpm
> 3,000 rpm
Max. Operating Time
30 minutes
5 minutes
Cool-Off Time
10 minutes
7
Model 461830/461831 Version 2.2 November 2003
EXTECH [ EXTECH INSTRUMENTS CORPORATION. ]
