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 viarelays on a timeproportioning basis to achieve smooth accurate control.

3.2.1HEATING

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.

3.2.2COOLING

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 theincipient pressure rise and switch on the compressor to "pump down".

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Sanyo 550 instruction manual Temperature Control, Heating, Cooling, Refrigeration unit control
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550 specifications

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