Shel lab SRI3, Operation Manual

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OPERATION
T
HEORY OF
O
PERATION
Refrigerated incubators are engineered to provide constant temperature incubation environments
suitable for BOD applications. The unit can obtain a stable, uniform temperature in its chamber,
ranging from 0°C up to 40°C.
Achieving and Maintaining the Temperature Set Point
When the unit is powered, its refrigeration compressor runs continuously. The chamber temperature
controller is wired to a solid-state temperature probe located in the chamber airstream on the chamber
back wall. When the controller detects that the chamber temperature has dropped below the set point,
it pulses power to a heating element. The element is located adjacent to the compressor chiller coil in
an air recirculation duct at the bottom of the unit.
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
current set point. The frequency of pulses is derived from the rate of change in that difference. The
integral function slows the rate of pulses when the temperature nears the set point to avoid
overshooting.
A circulation fan provides even air distribution throughout the chamber and plays an important role in
maintaining temperature uniformity around the shelf space.
Temperature Low Limit Cutoff
The refrigeration system comes with a cut off limit set at 1°C. This helps prevent the chamber
temperature from dropping below freezing and endangering samples and fluid-filled sample containers
in the chamber.
The Over Temperature Limit System (OTL )
The OTL is a backup mechanical heating control system that operates independently of the
microprocessor controller. When set, the OTL prevents runaway heating in the event of a failure of
the main temperature control system or an external heat spike by rerouting power away from the
heating element.
The system consists of a mechanical thermostat control wired by a fluid capillary to an independent
hydrostatic temperature probe located on the back wall of the incubation chamber. The OTL is set
by the end-user at approximately 1°C above the current chamber temperature when operating
stabilized at your application temperature.
Because of its nature as a cutoff and its lack of PID analytics, the OTL cannot deliver the same
degree of temperature stability and measurement precision as the digital display and controls. The
OTL System should only be used as a means of heating regulation for the incubation chamber until
samples can be safely transferred to another incubator.