factor, i.e. the ratio of peak and RMS. If the crest factor exceeds 9 the additional
evaluation method should be applied.
If the worker being tested is sitting in the right position and vibration exposure has
started measurement can begin.
Select crest factor mode by pressing the ◄ key and start crest factor measurement
by the ► key (Reset). This will
•
reset the RMS values of X/Y/Z, the vector sum a
W
and MTVV to zero
•
restart the measurement timer.
Always press Reset before a measurement
to establish the defined start condi-
tions.
If the displayed crest factors X/Y/Z are below 9, switch to Interval RMS mode by
pressing the ◄ key. At crest factors above 9 you should measure in addition vibra-
tion dose as shown in the next section.
Interval RMS values and crest factors are calculated based on the elapsed measuring
time since the last pressing of the key ► (Reset). This means that the fluctuation of
the displayed values becomes lower the longer the measurement lasts. Single peaks
will have less influence than at the beginning. Relevant for evaluation is only the
value at the end of measurement. The recommended measuring time for whole-body
vibration is at least 2 minutes. To alert you, the timer in the upper right corner re-
mains red until 2 minutes have elapsed.
The displayed interval RMS for X/Y/Z and vibration total values a
w
include the
weighting factors (k) according to table 2. The weighting factors are set to 1 for
maximum RMS (MTVV) and vibration dose value (VDV).
“OVERLOAD” is indicated instead of the measuring values if the current magni-
tude is too high. Even if the overload condition was of short duration the measured
interval RMS value may become invalid because of missing samples. An overload
event during the entire averaging time is indicated by “OVL!” in the upper right
corner after the date. This warning can be deleted by pressing the key ► (Reset).
After measurement you may save the results by pressing the key ▼. Measurement
should be either continued or finished immediately before pressing the key. Other-
wise the measuring values will drop slowly. You will be asked to enter two lines of
ten capital letters or numbers as a comment (Figure 12, page 11). Use the keys
◄▲▼► to select characters and to change the input position. Press F1 to change
the input line. Measurement can be finished before entering a comment because the
results will have already been saved by pressing the key ▼.
Measurements can only be saved if a sensor is detected at X/Y/Z and if there is no
overload condition. In these cases the VM31 will display “Sensor error” or “Over-
load occurred” instead of saving in order to avoid invalid recordings. If an overload
occurred since the last reset (“OVL!” in upper right corner) the instrument will
show a warning “Overload occurred after last reset! Save anyway?”.
If you want to measure several partial exposures you may now do further measure-
ments (see section 3.2).
For the calculation of
vibration exposure A(8
) and to store results on a PC the Ex-
cel macro file vm31.xlsm is provided (see section 13.3).
17
