Despatch 926E Equipment Manual (C-155)Rev 2/11 - Manual (Page 31)

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THEORY OF OPERATION
This section contains a functional as well as a detailed circuit description of the
temperature control circuits in all models of these chambers.
Functional Description
The Despatch 900 Series Environmental Bench Chamber uses a closed airflow system
to transfer heat to or from objects under test. A low inertia electric coil heater is used
for elevated temperatures in the chamber. Cooling of the chamber is accomplished by
injecting liquid carbon dioxide or liquid nitrogen into the test chamber, where the coolant
rapidly evaporates. The operating temperature of the test chamber is determined by
the setting of the Watlow EZ-Zone PM control.
Temperature sensing elements in the chamber allow the control to automatically
maintain chamber temperature at the value indicated on the set point switches. A dual
PID control loop in the Watlow EZ-Zone PM control activates heating or cooling as
necessary.
The dual output control ensures that only one function, heating or cooling, is in
operation at any one time. This eliminates the possibility of the heat control circuit
opposing the cool control circuit.