Casella CEL CEL-181 - Manual (Page 11)

Casella USA
17 Old Nashua Road #15
Amherst, NH 03031-2839
www.casellausa.com Page 11 of 14 tel: ( 800) 366-2966
[email protected] 27 Jan 2006 fax: (603) 672-8053
5 Analyses of Results
The results of dose meter surveys are
completely self-explanatory and give a direct
measure of the degree of auditory hazard
associated with any job function. If the dose
meter has been employed in an area noise
monitor mode it is then necessary to establish
the individual exposure duration in each zone
and then compare this with the permitted
exposure in that zone. The most convenient
format is to express each exposure as a fraction
of the permitted time at that level and then sum
the fractional exposures relating to that day. If
the answer is less than unity there is no auditory
hazard. Where q = 3 devices are used it is
possible to express NDC in terms of an Leq
level in decibels. Noise Dose Count (NDC) is
related to Equivalent Continuous Level (Leq) by
the formula:
Leq = k + [10 log
10
(NDC)]
T
It is, therefore, a simple matter to express dose
results as an Leq as long as the time over which
they were taken is known. A slide rule is
provided with CEL instruments to make this
calculation and all that is necessary is to set the
NDC figure on the black scale against the time
scale. The Leq can then be read from the red
scale through the window. If the high range is
used the result will always be correct; however,
if the low range is selected 30 dB must be
subtracted from the result. Instruments
operating on a q = 3 basis are often referred to
as equal energy devices and proposed
standards are suggesting that the calibration
criteria should therefore be expressed in terms
of Pascal
2 hours. In this context, therefore, it
should be noted that 90dB for 8 hours is 3.2
pa
2 h whilst 87dB is 1.6 pa 2 h and 85dB is 1.0
pa 2 h.
In cases where levels below the recommended
criteria are obtained the results need only be
held on file for future reference. It is important,
however, that where overexposures are
defected action is taken to correct matters.
Usual practice is to refer the subject in question
for audiometric examination and to make further
more detailed measurements of their work
environment to isolate the significant noise
sources. Such investigations usually include
level against time traces to link noise levels to
individual sources. Having identified the noise
sources these are then analyzed for the
frequency component. This analysis is essential
for the correct specification of noise screens and
the prescription of adequate ear protectors. Full
information is given in the (UK) Department of
Employment's 'Code of Practice'.
Suitable instrumentation for these further studies
comprises:
CEL-193
Precision Integrating Impulse
Sound Level Meter
CEL-178 Octave Band Analyzer
CEL-196 Automatic 1/3 Octave Band
Analyzer
Full details of these instruments and the
applications are available upon request.
6 Care and Maintenance Procedures
The Instruments are designed in line with
general criteria for socially acceptable
monitoring instruments (SAMI's) and within this
context will withstand the general industrial
environment. In situations where protective
clothing is worn the instrument should be
located within this clothing and care taken that
the microphone is not exposed to excessive
temperatures or high dust concentrations.
Details of dust protection for the microphone are
available on request.
When not in use the battery should be removed
and the instrument kept in its presentation case.
Other than the calibration procedure described
earlier in this manual no routine maintenance is
necessary.