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Posted by markg on April 28, 2003 at 10:57:04:
In Reply to: rack system heating question posted by rick s. on April 27, 2003 at 21:21:30:
The Helix is a proportional controller - that is, it pulses the heater as needed to maintain the setpoint temperature. The heat tape temperature will stay within a close range (probably within a degree or less) of where you set it.
The standard waterbed controller is a thermostat - it has an ON-OFF differential. For example, heats to 80, contacts open, cools to 76, contacts close, heats to 80..etc etc. It works for a waterbed because the water temp changes slowly, so the thermostat contacts aren't cycling all the time. Heat tape or heat pads can change temperature very quickly, which means the thermostat contacts will be opening and closing all day long. Not good for metal contacts. (Proportional controllers use solid-state relays which can cycle open and closed millions of times with no wear and tear.) It can work but is just not the best tool for the application when you have a better choice.
Proportional is better for maintaining a more stable temperature on heat tape and ceramic emmiters. The waterbed thermostat will work but not hold the temp as constant and will cycle an aweful lot.