Casella CEL CEL-160 - Manual (Page 11)

should be changed by using the D key, see
Section 3.1.
Calibration is performed using the
CEL-177 Precision Acoustic Calibrator with the
sensitivity of the amplifier being trimmed to
suit the microphone in use by means of the
SLM gain control. This control should be
adjusted with the screwdriver provided and
rotated clockwise to increase the gain. The
system is then ready for operation as a
completely self contained recording sound
level meter.
The full range of sound level measure-
ments may be undertaken in accordance with
established procedures for precision sound
level meters. By the addition of the CEL-3025
Vibration Attachment the CEL-160 is converted
into a comprehensive recording vibration
meter. Further information is given in Section 5
of this manual whilst full details of the
Vibration Attachment are published separately.
4.2 DC Inputs
DC inputs should be connected via the DC
socket and should be in the log DC form that is
provided by most modern designs of sound
level meter. 0 V is taken by the instrument to
represent minimum scale deflection on the
50 dB range. Signals may be in a +ve or -ve
sense, and to accomodate this choice the +/ -
switch should be set accordingly. The
sensitivity of the DC amplifier is then adjusted
by means of the GAIN-CAL control to match
the scale deflection to the input signal
sensitivity. The instrument will accept DC input
signals having a sensitivity in the range 40 -
140 mV/dB range setting, with the nominal
sensitivity of 140 mV/dB occurring in the
GAIN-CAL position.
On the 50 dB dynamic range setting 0 V
will always represent minimum scale
deflection with the full scale value being set by
the GAIN-CAL control. On the 20 dB and 10 dB
dynamic range settings the scale is expanded
from the full scale down giving coverage of
FSD -20 dB and FSD -10 dB respectively, ie on
20 dB and 10 dB dynamic ranges 0 V is always
off scale.
The writing system in the DC mode is
similar to that in the RMS level configuration
(see 5.3) and hence a continuous trace will be
synthesised from the discreet samples.
Signals having a rise time of >5 mS or sources
with a time constant of >10 mS will be
satisfactorily recorded.
Similarly there will be an upper limiting
frequency of 50 Hz, however, the best that can
be satisfactorily resolved at the highest paper
speed is around 20 Hz.
One of the main applications of the DC
input configuration is when it is required to
make level recordings using the Type 1 P
response RMS detector or impulse time
constants found in the CEL-193 Precision
Integrating Sound Level Meter. In such cases
the sound level meter is connected to the
recorder via the cable type CEL-3696 and the
input polarity set to -ve and the input selector
to DC. A CEL-177 Precision Acoustic Calibrator
should be applied to the sound level meter and
this calibrated in the normal manner. The
recorder GAIN-CAL control should then be
adjusted so that the full scale deflection points
of the sound level meter and the recorder
coincide when the later is set to have a 50 dB
dynamic range. It will then be necessary to set
the full scale annotation using the D key to
match the sound level meter range with a
similar adjustment being made each time the
sound level meter range is changed. The main
RANGE (dB) control on the recorder should not
be adjusted as this will also affect the scale
annotations without changing the systems
gain. Should it become necessary to increase
the resolution of the display the dynamic range
switch may be used to give either 20 dB or
10 dB across the complete trace. This
arrangement enables a resolution of up to
0.1 dB to be obtained.
The CEL-175/-187/-245 and -345
instruments have their DC outputs with a +ve
sense and also use the CEL-3696 cable to
connect to the recorder, whilst the CEL-314
Precision Impulse Sound Level Meter has a -ve
signal and requires the CEL-3726/2 cable.
Inputs will also be accepted from certain
Bruel & Kjaer and other makes of sound level
meters but it is necessary to ensure that they
have their DC output in terms of mV/dB as
incorrect scaling will occur for older designs of
meter that have outputs linear in mV.
The CEL-160 will also accept DC inputs
from a wide range of other transducers such
as anemometers, thermometers, gas
detectors etc that have conditioning amplifiers
to provide DC signals at the required levels.
CEL-160 Graphic Recorder - Page 11