Casella CEL CEL-281 - Manual (Page 35)

3 HOW IT WORKS
3.1 Principle of Operation
The CEL-281 consists of a Type 2 sound level meter (IEC 651 and ANSI
S1.4 1983) with very fast time weighting, an analog to digital converter
feeding a microprocessor that samples the signal more than three hun-
dred times per second and makes calculations and predictions based on
the measured levels, and a memory in which all measured and calcu-
lated values are stored.
Either "A" or "C" frequency weightings may be used in the sound level
meter, but once the analog sound input has been converted into a digital
signal and entered the microprocessor, all subsequent manipulation and
calculation is performed by the software, including calculation of Fast
and Slow time weightings. All calculations occur simultaneously and con-
tinuously, and in the manner of a spread sheet, all results are available
for display at any time, and can be downloaded when logging has been
stopped.
While the instrument consists of analog and digital sections, operation
within the digital section is divided into six operational areas that can be
accessed by using appropriate key sequences on the CEL-6012 Keypad,
or by entering commands from a PC connected via the 15-line keypad
connector. The relationship of the operational areas is illustrated in Fig-
ure 5; they are as follows.
3.1.1 Control and Calibration
These are the functions shown above the "Fully Operational" box in the
figure which allow the the operator to control the instrument. They en-
able the instrument to be calibrated, the batteries to be checked, the dis-
play switched on or off, frequency and time weightings to be selected,
and the clock to be set to give predetermined measurement start and
stop times.
When the instrument is switched on, by means of the latching push-
ON/push-OFF switch beside the keypad connector, it starts a 20 second
CEL-281 Operator's Handbook - Page 27
How It Works