Proportional is always better if you can use it. Non-proportional is really just there for a backup in case the proportional doesn't work out. Say you use a ceramic heat emmitter. Most will work great on proportional. Some though will emmit a faint noise as the power is dimmed. So if that is a nuisance you can switch to non-proportional where no dimming occurs. With the non-proportional control of a Herpstat it will help extend life longer than a standard relay based thermostat for heating devices that may be sensitive. The reason is it detects the zero crossing of the AC sine wave each half cycle and would turn the triac on at that point. Without getting too technical just think of this example. Most of us have had an experience where we turn on a light switch in our house at some point and had the light bulb flash for a second and then burn out. The reason is because the filiment was worn but you happened to turn power on when the AC was at its highest voltage (AC swings between 120v and zero constantly) and the abrupt power shot the bulb. With zero crossing detection its turning on voltage while its still low so its less harsh. Any proportional thermostat in proportional mode can still turn the power on when its half way through the wave at 120vac (which is basically 50% power output). However, since it doesn't spend the majority of its time there its still better than a relay based thermostat. Again, the longevity of the heating device is a minor concern. Most will probably never see a difference. The nuisance of certain ceramic heat emmiters (some metal rack systems will also have a noise due to the resonance of the metal holder the heat tape sits on) is really why I wrote it into the code.
Hope that helps!
Dion Brewington
Owner, Spyder Robotics
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