Casella CEL CEL-368 - Manual (Page 33)

The most obvious difference between the Type 1 CEL-268 and Type 2
CEL-368 is in the microphones and preamplifiers used.
CEL-268 - Type 1
The standard microphone supplied with the CEL-268 (Type 1) is a 1 ⁄
2 " di-
ameter CEL-192 Precision Air Condenser Microphone, available in
either a free field version (suffix F) to meet IEC standards, or in a ran-
dom incidence (R) version for ANSI standards. The microphone capsule
is screwed on to a low noise CEL-225/3 Preamplifier which plugs into
the tapered end of the Sound Level Meter. It can be used remote from
the instrument when the preamplifier is connected to the Sound Level
Meter via an extension cable. C4493/5 (5 m), C4493/10 (10 m), and
C4493/20 (20 m) are standard cables, and a maximum length of 100 m
can be delivered.
The CEL-225/3 Preamplifier is a development of the CEL-225 used with
earlier CEL Type 1 sound level meters. Although the two types of pream-
plifier are physically similar, their functions are not identical. The CEL-
225/3 can be identified by the black key line which aligns with a similar
line on other current CEL sound level meters.
Although no actual damage would occur, the two Type 1 preamplifiers
should not be substituted for each other when measuring sound levels.
However when vibration measurement is to be undertaken, both types
of preamplifier may be used to accommodate a CEL-3025 Vibration At-
tachment screwed on in place of the microphone to accept input signals
from an accelerometer.
When an earlier preamplifier is used with the CEL-268, the noise floor
for linear noise measurement will be raised to about 70 dB. (If an older
series of Type 1 sound level meter is used with a CEL-225/3 Preampli-
fier for sound level measurement, the instrument stabilisation time will
be increased to at least three minutes.) Therefore generally, it is not rec-
ommended to mix old and new preamplifier versions.
The preamplifier applies the 200 V polarization voltage required by the
microphone, provides 30 dB or unity gain, and conditions the output. In-
CEL-268/368 Operator's Handbook - Page 29