Casella CEL CEL-181 User Manual
Page 9
Casella USA
17 Old Nashua Road #15
Amherst, NH 03031-2839
the calibration cycle as this will restart the
internal timer and result in an incorrect noise
dose being delivered by the calibrator.
The result obtained from the calibration
sequence should be compared against the
count given on the calibration certificate at the
end of this handbook. In the event of the
calibration drifting due to climate changes, etc,
the instrument may be adjusted by means of the
CAL control. This is accessible through the top
printed circuit panel when the rear cover has
been removed. Using the screwdriver provided
the control should be rotated clockwise to
reduce the count and vice-versa. Before
adjusting the unit it should be switched OFF and
after adjustment the calibration sequence
repeated. When calibrating Dose Meters it must
be borne in mind that NDC is a linear function
whilst it is normal to express acoustic accuracy
in terms of dB, a logarithmic unit. Hence, a
calibration accuracy of 0.5dB is the same as a
tolerance for NDC of approximately 10%.
Having completed the calibration the instrument
should be switched OFF in order to remove the
calibration information from the counters, it is
then ready to make noise dose measurements.
4 Measurement
Methods
Having completed the calibration and battery
checks described in section 3 of this manual the
instrument is ready for the measurement of
noise exposure. Check that the correct
measurement range has been selected and then
mount the microphone on the subject. In
selecting the mounting position for the
microphone due consideration should be given
to the parameter we are trying to measure.
Noise deafness is directly proportional to the
amount of noise entering the ear and the ear's
total noise immission depends upon the ear's
position relative to the noise sources. It is
necessary, therefore, to have due regard to the
subject's work pattern and choose a 'worst case'
condition. For example, in some circumstances
there is a difference in level at each ear, and in
some cases there can be a difference between
right and left handed workers especially when
employed out of doors in conditions approaching
an acoustic free field environment.
The design of the instrument is such that friction
of clothing against the cable and microphone
body will not significantly affect the result;
however, it is good practice to ensure that the
microphone is not banged during the
measurement sequence as this could induce an
erroneous signal into the system. There can
also be a significant difference between the
noise level at the breast pocket and the ear due
to body shielding effects, etc, and this is,
therefore, not a good position to mount the
microphone. By definition the microphone
should be as close to the ear as possible and
experience has shown that clipped to the lapel
at the apex of the shoulder or attached to the rim
of a safety helmet are the best positions. The
signal received by the microphone in these
positions is typical of the actual level at the ear
and by carefully tailoring the cable run under the
collar and mounting the Dose Meter in an inside
pocket or clipping it to the belt the complete
system is the unobtrusive and causes minimum
interference with the normal work pattern.
Ensuring that the instrument is 'accepted' by the
subject will ensure that he soon forgets its
presence and works normally thereby allowing
the Dose Meter to integrate typical noise signals
- a good maxim when mounting the unit on a
subject is 'out of sight is out of mind'
.
Having mounted the microphone the Dose
Meter should be switched on and then the
instrument returned to its location on the
subject. The instrument may then be left to
accumulate the noise dose information required.
With the large number of people who need to be
surveyed and the relative expense of the
instruments it is not practical to permanently
monitor for noise exposure, so a sampling
system must be used. The number of people in
any one area to be sampled will, obviously,
depend upon the similarity of their work pattern.
For example, 100 operators employed in a
weaving shed all working on similar machines
and keeping the same hours must have
comparable noise exposure. A 10% sample
would probably give a good indication of the
exposure of all of the subjects within the group.
On the other hand in the construction industry,
where each employee is an individual
tradesman, such widespread samples would not
be valid and it would be necessary to look at
each job function in more detail. By applying
normal mathematical tests to the sample results
obtained it is possible to place a confidence
factor on the results obtained and thereby
validate the sample size chosen, e.g. a small
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