Shel lab SMI11, Installation And Operation Manual, Page: 19 / 36

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OPERATION
T
HEORY OF
O
PERATION
The SMI general purpose incubators are engineered to provide constant temperature incubation
environments. Each unit can obtain a stable, uniform temperature in its chamber, ranging from the
room temperature (ambient) +8°C up to 70°C for incubation applications. Each incubator features an
inner glass viewing door that allows visual inspection of samples without compromising the chamber
temperature environment. Each incubator unit in a SMI12 is independently powered and heated.
Heating
Each incubator is controlled by a microprocessor board with heating elements attached to the incubation
chamber walls and a solid-state temperature sensor probe inside the chamber. The processor employs
proportional-integral-derivative analytical feedback-loop functions when measuring and controlling the
chamber air temperature. PID-controlled heating pulse intensities and lengths are proportional to the
difference between the measured chamber temperature and the set point. The set point is the desired
operating temperature entered by the user. The frequency of pulses is derived from the rate of change in
the difference. The integral function slows the rate of pulses when the temperature nears the set point in
order to prevent overshooting.
The PID functions are also used to optimize incubator warming rates for hotter or cooler environments.
Heat loss from leaving the incubator doors open for long periods of time (an hour or more) can trick the
controller into operating as though in a cool environment. This can result in a period of temperature
overshoots.
Each incubator relies on natural heat radiation for cooling. An incubator can achieve a low-end
temperature of the ambient room tempe8°C. The fan inside in the incubator aids in maintaining air
circulation and a uniform air temperature in the incubation space.
The chamber door is self-heating to bolster the thermal uniformity and stability of the chamber and to
minimize condensation on the glass viewing door. The glass door will cool while the chamber door is
opening, eventually leading to condensate on the door and impacting the chamber temperature
stability and uniformity. Minimize sample viewing times when possible.
The Over Temperature Limit System
The OTL is a mechanical heating cutoff included with each incubator and operates independently of the
incubator microprocessor controller. The OTL helps safeguard samples by preventing runaway heating in
the event of a hardware failure in the microprocessor controller or a sudden external heat spike. The OTL
is connected to a hydrostatic sensor probe located inside the incubator and is intended to be set by the
user to approximately 1 ° C above the current operating temperature set point.
If the incubator temperature exceeds the OTL cutoff setting, the OTL will route power away from the
incubator heating elements. It will continue to do so as long as the incubator air temperature is higher
than the present OTL cutoff setting. A red indicator illuminates when the OTL is rerouting power.