Casella CEL CEL-393 User Manual
Page 34
level. It is continually available throughout the current measurement
period, and may be reset manually or under automatic control.
Valid results may be obtained using either Fast or Slow detector settings
according to the requirements of the measurement protocol being fol-
lowed. In the absence of any specified requirement, the Fast setting
should be used.
Note:
The Slow setting does not fulfill IEC 804 for highly impulsive signals.
The impulse weighted average L
A
I
m
may be obtained by selecting the I
time weighting prior to making a measurement. The P detector setting
should not be used for this type of measurement.
The maximum period for a single current L
eq
answer is 64 hours, due to
the capacity limit of the register. However, individual periods may be
summed following the rules for logarithmic addition, and averaging of
these quantities to form longer periods should this prove necessary.
The algorithm used to compute the L
eq
value uses the minimum scale
deflection value when the sound level is below scale, i.e. if the 50dB
range is selected and the actual sound level drops momentarily to say
45dB, then 50 would be used in the computation in place of 45. The effect
of this will be to produce an answer slightly higher than the true value
when the input signal is close to the bottom of the range. This is
considered preferable to using a result that could be out of tolerance if
the instrument was allowed to run down beyond its linearity range.
Bearing the logarithmic nature of the L
eq
formula in mind, it would be
necessary for the signal to be under range for a considerable proportion
of the measurement period to produce a detectable error. It is good
practice however to ensure that the signal is centred around the middle
of the dynamic range, and to check for under range by reference to the
L
n
values.
For example: if the level was below the range for 10% of the time, and
in the range by 5dB for 90% of the time, the resulting error is only 0.1dB,
and the under range can be checked from the L
90
value.
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393 Handbook