Casella CEL CEL-160 - Manual (Page 15)

The selection of correct starting time constant
(and hence initial paper speed) depends upon
the degree of low frequency content in the
signal and can only be decided from
experience. However, it should be noted that
in the CEL-160 selection of the paper speed
automatically decides the time constant, with
the value selected being related to the start
speed. The final speed will be 10 times faster,
but it should be noted, however, that the
speed never goes above 10 mm/S on serial
analysis.
TimeConstants 0.3 1 3 10 30 100
Paper Speed
mm/S
10 3 1 0.3 0.1 0.03
Figure 6: Paper speed time constant
relationship on serial analysis
6.2 Real Time Analysers
Signals from real time analysers are required to
be in the log DC form and they are connected
and calibrated following the instructions given
in Section 4.2. Having connected the signal
and control lines and selected the filter type on
the keypad the printout is initiated by the
operation of the E key.
On receipt of the analysis command (E)
the CEL-160 sends a reset pulse to the
analyser to set the multiplexer to the first
band, the step control then moves through the
bands allowing the CEL-160 to read each band
and store the level. This takes approximately
1.5 mS for each of the 30 bands and these are
then printed out at 30 mm/S paper speed
giving a complete analysis in around 4 S.
Each band is represented by 3.5 mm of
chart and annotations are as for serial
analysers with the addition of alphanumeric
levels to 0.1 dB printed for each band. This
allows improved resolution; on the 50 dB
range, for example it becomes one part in 500.
It should be noted that for clarity band levels
are not annotated if they represent less than
25% scale deflection.
6.3 Continually Variable Narrow Band
Serial Analysers
Swept band analysis is possible with the Bruel
& Kjar 1621 Analyser. This will provide 3% or
23% analysis and operation is similar to the
use of stepped filters except that the step
pulse moves the filter 1/120 octave at a time.
At the end of a decade the 1621 stops the
CEL-160 from continuing until the 1621 is
manually changed to the next decade and
restarted. This can occur three times to give
three decades of recordings. The setting of
paper speed and modification of time
constants is the same for stepped serial
analysers.
7. Additional Instructions
for Program Level 2
Instruments
7.1 Expanded trace recording
This facility causes the paper speed to
increase to 10 mm/S when the signal level
exceeds 30% scale deflection. The object of
these arrangements is to enable significant
events to be clearly displayed without the
need to consume large amounts of chart paper
in the intervening periods. It also ensures that
the correct real time sequence is maintanied.
This function is only operative in the
RMS mode and is selected by moving the
`EXP. TRACE-OFF-OPTION' switch to the `exp
trace' setting. When the signal level exceeds
the threshold level the time is printed on the
paper and the speed change is made. The
instrument will continue to operate at the
10 mm/S speed until 5 S after the signal level
has fallen below the 30% deflection point.
A full time print is also made at the time
the original speed is resumed, and during the
expansion annotation relative to the 10 mm/S
speed is produced. The expansion facility is
operable with any paper speed in the range
0.01 mm/S to 3 mm/S.
7.2 Transient Capture
The provision of this facility enables the
recorder to operate as a digital transient event
recorder. A 1800 word memory will retain
samples of the waveform which will then be
placed on to the recorder at a slower rate in
such a manner that waveforms having
frequencies up to 10 kHz may be clearly
examined.
As the data store has a finite size there
is a limit to the duration of the signal burst that
can be recorded. This is in turn related to the
sample speed which also determines the
maximum frequency that will be accurately
recorded. At the fastest sample speed the
upper frequency is 10 kHz and the capture
duration is 66 mS.
CEL-160 Graphic Recorder - Page 15