3.2. TEMPERATURE CONTROL
Accurate, repeatable temperature control levels are achieved by continuously balancing
heat loss with intermittent modulated heating controlled by electronic controllers.
The conditioned air flows over a dry platinum resistance temperature sensor (BS1904,
lOOohms at 0°C) which feeds signals back to an electronic controller. The actual
temperature is compared with the operator set temperature and errors are corrected by
switching the heaters via relays on a time proportioning basis to achieve smooth accurate
control.
3.2.1 HEATING
One or more "mconel" sheathed electrical heating elements are situated in the treatment
area. These elements normally operate only at black heat, for a long life and avoidance
of excessive heater surface temperatures.
3.2.2 COOLING
All models have CFC-free mechanical refrigeration units with a direct cooling coil to
achieve the minimum temperature specified
All refrigeration compressors are protected for operation at working chamber
temperatures up to +70°C.
Cabinets with a -70°C lower temperature limit (CF7 models) use cascade refrigeration
systems.
The main and auxiliary cooling modes are automatically selected by the control system
to enable:
a) freefall temperature pull-downs from the cabinets maximum temperature, ie the fall
rate is as fast as possible down to +70°C or below.
b) temperature dwells below +40°C (auxiliary cooling is used between +40°C and
+15°C if high humidity control is required).
c) extraction of heat dissipated from the test load between +70°C and the minimum op-
erating temperature.
d) the cooling coil's low surface temperature to dehumidify the cabinet between +5°C
and+70°C.
Refrigeration unit control
The refrigerator is controlled by the switching of solenoid valves which pass refrigerant
to pressure switches, which in turn energise the refrigerator compressor motor. There
is a short delay between switching and motor start up. Similarly, pressure switches act
as safety devices, preventing excessive pressure which might arise when the refrigerator
is not selected, and the refrigerant is heated by the cabinet. In such cases, the pressure
switches detect the incipient pressure rise and switch on the compressor to "pump down".
