3.4. MEASUREMENT OF
VIBRATION
Vibration measurements may be made as follows:
1. Calibrate
the
meter as described
in
section 3.3.
2. Mount
the
accelerometer by
one
of
the
methods described
in
its
instruction
manual, taking care
to
avoid cable whip.
3.
Hold
the
Sound
Level
Meter as free as possible from
the
vibration
environment and
other
unrequired influences.
4. With
the
transparent
attenuator
knob
in
its fully clockwise position,
turn
the
black
attenuator
knob until a meter deflection
in
the
top
2/3
of
the
scale
is
obtained. If this
is
not possible,
the
transparent
knob
may also be used.
In
all cases
the
transparent
knob
should be kept as
far
clockwise as possible.
The
optimum
signal
to
noise ratio
is
thus
obtained.
5. Taking care
to
avoid overloads as described
in
section 3.2,
point
7,
the
meter reading together with
the
attenuator
setting gives
the
RMS
vibration
in
the
units required.
The
indication on
the
attenuator
cor-
responds
to
full scale deflection
of
the
meter.
Note:
The
frequency range
of
the
2209
will normally extend far above
the
resonance frequency
of
the
accelerometer.
The
user should be careful
that
he
only
uses
the
accelerometer
to
measure signals with frequency com-
ponents
in
the
linear
part
of
its frequency range. Sometimes it may be
necessary
to
use a supplementary low-pass filter.
3.5. ACCURACY OF
VIBRATION
MEASUREMENTS
The
high frequency response of a piezoelectric accelerometer
is
a func-
tion
of
its mechanical characteristics. There are
two
main factors of import-
ance:
the
resonance frequency (f
0
)
of the system which depends mainly
on
the
mode of mounting and
the
frequency range over which
the
response
is
linear.
In
most cases
the
response
is
flat up
to
about
1/5
of
the
resonance
frequency.
The
accelerometer may be used above
this
frequency
as
long as
the
appropriate correction factors are applied
to
compensate for
the
in-
crease
in
sensitivity as
the
resonance frequency
is
approached.
The
lower limiting frequency
of
the
accelerometer and meter
is
set
by
23
To Next Page
Loading...
