Shellab SCO31 - Manual (Page 20)

20 | P a g e
OPERATION
T
HEORY OF
O
PERATION
Heating and CO
2
Control
The incubator uses a microprocessor controller board wired to a solid state temperature probe, a
heating element located in a recirculation duct, as well as a blower fan in the duct, to monitor and
regulate the temperature within the incubation chamber. SCO Incubators rely on natural heat
radiation for cooling. When powered and with the door closed, the incubation chamber temperature
cannot go below the ambient environmental temperature, plus the waste heat generated by
internal electrical and mechanical operations (room temperature +8

C).
The same microprocessor board controls the concentration of CO
2
in the incubation chamber by
operating an internal gas-injection solenoid valve connected to the gas input line. The processor
monitors CO
2
concentration level in the incubator using an infrared sensor in the recirculation duct.
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 that band of infrared is
absorbed. The sensor is only sensitive to CO
2
, so measurement accuracy is consistent, regardless
of the presence of other gasses in the incubator.
The microprocessor controller employs proportional-integral-derivative analytical feedback-loop
functions when measuring and controlling both temperature and CO
2
levels. The rate of PID-
controlled heating is proportional to the difference between the measured chamber air temperature
and the user-programed temperature set point. The rate of heating slows as the air temperature
nears the set point to prevent overshooting. 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 heating pulses and CO
2
injections take place in small bursts to correct for
deviations of 0.1 °C and 0.1% in gas concentration. Heating pulses and gas injections will be
frequent when first setting up the unit for use after installation, and when recovering from door
openings.
Automatic Door Cutoff
Whenever the incubation chamber door is opened the SCO Incubator stops the flow of CO
2
into the
chamber, depowers the heater element, and ceases operation of the internal blower fan. This limits
the amount of CO
2
released into the workspace around the incubator. It also prevents the heater
from attempting to counteract the continual inflow of cooler air, which would cause a significant heat
spike once the door is closed. Normal CO
2
injections, heating, and fan operation all resume
automatically when the door is closed.