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Measuring Vibration
It is advisable to validate your instrument prior to, and after taking
measurements using a known vibration source such as the Castle GA606
Vibration Calibrator.
To ensure measurements are as accurate and as repeatable as possible
always ensure that your cable is tightened securely to your accelerometer
and that the accelerometer is mounted as securely and as flush as possible
to the vibration source. The trailing cable of the accelerometer should also
be attached to the vibration source without creating a potential hazard for
the operator or other people. Where practicable it may be beneficial to
permanently attach the accelerometer cable.
Where possible always mount the accelerometer as near to the centre of
where the operator holds and grips the vibration source. In reality this is not
always possible and the best compromise must be achieved.
Measurement durations are not governed but as a rule of thumb use an
absolute minimum time period of 15 seconds for Hand Arm Vibration. It is
recommended depending on the process that a minimum period of 3 to 15
minutes is used for Hand Arm vibration. These increased durations will
undoubtedly increase the accuracy and repeatability of your measured
results.
The vibration is measured in one axis only and once all axes have been
individually recorded you will have to assess how long the operator is exposed
to the machine.
Taking the Vector sum acceleration is the preferred method of calculating
vibration dose although it is not the only option. It is possible to look at the
Dominant Axis. This means that once you have taken the X, Y and Z readings
you may take the largest one providing that it is larger than the other axes by
a factor of two or more.
The parameter A(8) is actually the partial vibration dose based on 8 hours. It
is calculated as follows: -
This parameter is given by the GA2003 providing the measurement period is
equal to the duration of the work.
Alternatively the AV (Linear Average Value) may be used in the above
equation (a
h,w
) and the time (t) is the daily exposure time for this operation
where i is the reference for the individual measurement and 8 is for the 8
hour day.
8
)
8
(
,
t
A
a
w
h
i
