Casella CEL CEL-193 User Manual
Page 8
Casella USA
17 Old Nashua Rd #15
Amherst, NH 03031 USA
the attenuator setting and, when activated, the 30dB range expansion. The detailed measurement
procedure concerning the location of the measurement position, frequency weighting selection and the time
constant to be used are usually laid down in the various applications standards along with the necessary
information relative to the interpretation of the results obtained (ISO R 1996, 1999, etc.). This manual is,
therefore, more concerned with the actual operation of the instrument rather than the detailed meas-
urement procedure, and the points given below should be taken as general considerations when using the
instrument.
3.1 Attenuator Setting
The ATTENUATOR control (g) should be set to a range where the sound levels are clearly indicated on the
meter scale, i.e. without failing off the bottom of the scale or driving the instrument into overload. With the
60dB scale, this should be possible in almost all conditions and unlike older designs of meters which had a
less accurate secondary scale, the CEL-193 has precision grade accuracy across its complete 60dB scale. At
no time should the overload light come on, even momentarily, as this indicates that the reading is low due
to peak clipping and the meter should be switched to a higher range so that this does not occur. The lowest
measurement range is 20-80dB RMS and the highest is 100-160dB RMS with intermediate ranges selectable
in 10 dB steps. Instantaneous levels of up to a minimum of 60dB above scale reference level can be
correctly handled by the detector without saturating the voltage squarers. The instrument does not,
therefore, suffer from the crest factor and impulse capability limitations to the same degree as traditional
designs. Full details of the instrument's response to impulses is covered in the relevant standards.
Range changing may cause temporary fluctuations in the indicated value particularly when switching
between the 80dB and 90dB ranges and vice versa. The instrument should, therefore, be allowed to settle
for a few seconds after range changing before measurements are taken.
3.2 Time Constant Selection
For the vast majority of applications, switch (c) would normally be set to FAST or SLOW. These are time
constants specified in the various National and International Standards as 125msecs and 1 second
respectively. The FAST time constant enables sound level fluctuations to be more easily seen on the meter
movement, whilst SLOW averages these fluctuations out to give a steadier and more easily read answer.
The IMPULSE time constant is as defined in IEC 179A and provides a 35msec constant for rising edges and
a 1.5 second response for falling edges. This arrangement has the effect of causing the meter to 'dwell'
around the maximum value enabling it to be read from the display. An immediate increase in sound level of
20dB will cause the meter to read within 1 dB of the final steady state after 200msecs from the increase
when on FAST. On SLOW this time period is 1.6 seconds. When the input signal is dropped by 20dB or more
the RMS level indicated will fall at a rate of 3dB every 87.5msec on FAST and every 0.7 second on SLOW.
Similar factors can be applied to the impulse response bearing in mind the different rise and fall times.
Selection of PEAK gives a time constant of approximately 50 µsec and this is used to determine the
amplitude of actual peak levels in conjunction with the maximum hold memory (see Section 4 of this
manual). It is not advisable to select the peak time constant when in the SPL mode as it is difficult to
accurately read the rapid fluctuations of the meter and in these conditions it is also possible that a situation
could arise when misleading answers would be obtained due to the fact that the dominant time constant, in
the system will be the actual meter movement and not the detector thereby causing incorrect averaging of
the signal.
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2 Mar 2009
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