Shellab SCO6AD - Manual (Page 20)

20 | P a g e
OPERATION (CONTINUED)
CO
2
Atmosphere
The SCO6AD controls the concentration of CO
2
in the incubation chamber atmosphere by
operating an internal electric gas pump connected to the gas input line. The motor pulses CO
2
into
the chamber air stream when deviations from the user-programed CO
2
set point are detected. The
incubator monitors the chamber CO
2
concentration with an infrared sensor in a recirculation box
above the main chamber. The sensor operates on the principle that a specific frequency set of
infrared light is absorbed by CO
2
. The more CO
2
present in the air stream from the chamber, the
more of this band of infrared is absorbed. The sensor is only sensitive to CO
2
, so the measurement
accuracy (
±0.1%) is consistent, regardless of the presence of other gasses. The incubator can
maintain a CO
2
concentration level of 0 – 20% in the chamber, and has a recovery time of less than
5 minutes to restore the concentration level after the inner chamber door has been closed.
The SCO6AD employs proportional-integral-derivative analytic feedback-loop functions when
measuring and controlling the CO
2
concentration. The length of gas injections is proportional to the
difference between the measured concentration and the set point. The frequency of injections is
derived from the rate of change in the difference. Integrator feedback slows the rate of injection as
the concentration approaches the set point, which helps prevent overshoots. During normal
operations CO
2
injections take place in small bursts to correct for deviations of 0.1%. Typically, gas
injections will be long when first setting up the unit for use after installation, and frequent when
recovering from door openings.
The SCO6AD requires periodic calibrations to verify that it is accurately measuring and displaying
the CO
2
level in the chamber. If a measurement error is detected during a CO
2
calibration
procedure (see page 30), the CO
2
display can be calibrated to the actual concentration in the
chamber by placing the display in its calibration mode. Gas calibrations require a gas-analyzer
reference device capable of measuring CO
2
concentrations to at least 0.1% accuracy.
Alarms
The SCO6AD is provided with high and low alarms for temperature and CO
2
deviations. The
alarms have visual indicators and audible components. The temperature alarms thresholds are
deviations of 1
°C or greater from the set point. The CO
2
deviation alarms trip points are
deviations of 1% or greater from the gas concentration set point. There is a 15 minute delay for
the sounding of the low deviation audible alarms to prevent activations when the chamber doors
are opened for brief periods. If the low CO
2
deviation indicator is lit for 20 minutes, a visual No
Gas Supply alarm will activate and show in the CO
2
display as “ngS”.
All audible deviation alarms can be muted, but the visual indicators will remain illuminated for the
duration of a deviation. See the
Muting the Audible Temperature Alarm entry on page 28 and the
Muting the Audible CO
2
Alarm entry on page 32.