Casella CEL CEL-181 User Manual
Page 10
Casella USA
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Amherst, NH 03031-2839
deviation in results across the sample would
indicate a good correlation to the whole group.
Regulations controlling noise exposure are
normalized to an 8-hour working day and it is
usual practice to monitor for this period.
However, in cases where the noise exposure
pattern is regular it is possible to extrapolate the
full day's exposure from a relatively short
sample. A good example of this is a worker on a
car assembly track who repeats the same
operation on each vehicle as it passes his
workstation. Each operation may be of say 3
minutes duration with a 2-minute preparation
and waiting period before he repeats his
operation on the next vehicle. As each complete
5-minute period is the same, his noise exposure
during each period will be the same. His total
daily noise dose must therefore be the dose
count for one period multiplied by the number of
duty cycles per day. By taking advantage of
such situations it is possible to greatly increase
the number of employees covered by a single
instrument.
A general rule seems to have developed, that of
monitoring for 10 work cycles or in the case of
very fast cyclic machines a minimum period of 5
minutes. As far as the instrument is concerned
the minimum duration of the measurement is
only limited by the resolution of the display. To
obtain a reading accuracy of 10% it is necessary
to ensure that the count reaches double figures
and as the CEL range of dose meters has a
decimal indication this will be achieved very
quickly enabling full advantage to be taken of
situations where short samples are possible.
When shortened sample periods are used the
technique can be validated by looking at the
standard deviation produced from a series of
period dose measurements made on one or two
operators. Interrogating and resetting the
instrument at the end of each of a number of
work cycles can obtain period counts.
In the cases where the work pattern cannot be
divided into even and equal time periods then
the subject must be monitored for the full shift;
examples of such cases are truck drivers,
maintenance personnel, supervisors, etc. There
are no problems associated with work periods
longer than 8 hours as the dose count of 100
will, by definition, represent one daily exposure
whether it is accumulated over a few minutes or
several hours. The maximum period over which
the dose meters may be used depends upon the
noise level. The higher the level the faster the
count and the quicker the dose meter will reach
its maximum count capacity. However, bearing
in mind the capacity of the counter fitted to these
instruments it will be a rare occurrence but
should it occur it will be indicated by the
over-run lamp.
When analyzing the results of surveys carried
out without the direct supervision of the
Industrial Hygienist due regard must be paid to
the possibility of malicious falsification of the
results. Individual employees could well induce
noises into the instrument that are not typical of
their normal work routine. Such occurrences
usually show themselves as an abnormally high
result, which is not typical of the other results,
obtained from similar job functions in the
sample. A repeat measurement would verify
this. As the result is not known to the subject he
does not know by how much he affected the
answers, and experience has shown a very wide
variation in subsequent test because it is very
difficult for the subject to repeat the error signal
with any degree of certainty.
When attempts are made to falsify results
people fail to appreciate the effect of high levels
on the answer that result from the amplitude
weighting function. This invariably results in
answers far higher than would be expected from
even brief knowledge of the noise climate.
Bearing these points in mind it is not difficult to
detect attempts to falsify the results and
eliminate them from the analysis,
As an alternative to use as a personal device
the CEL Noise Dose Meters may be used as
area monitors. This approach is sometimes valid
when employees do not have cause to move
around during their employment. The procedure
is to establish the NDC for the area by placing
the Dose Meter at fixed locations and noting the
levels. Care is needed when this approach is
used for, although it is a quicker method, it is
easy to under-estimate the risk, especially if
people move from the general reverberant noise
field into the near field around the noise
sources.
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