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Based on my conversation siwth Dion at Spyder, this is an abbreviated version of what is happening:

The Herpstat is programmed to cut power to 50% max when the temperature of your incubator reaches within a few tenths of the set-point temperature. According to him, this helps reduce the temp "spikes" (when the temps shoot up past the set point and then gradually drop back down to it) that you normally see after opening & closing the incubator.

The problem with this is that if your incubator is not very well insulated (like a hovabator, or a HS with a large glass door) only running the heat element at 50% power is not enough make up for the heat loss through the unit, and it will never reach the set-point… it will raise the temp to within a few degrees of set, then cuts power to 50%. At this point, the incubator temps will drop due to the heat loss through the unit, until it drops low enough to kick power back to 100%.... and the cycle keep repeating iteself.

A Helix on the other hand, while still a proportional control, will not cut power to 50%... 100% is still available all the way up to the set point. However, if you have a very well insulated unit, this contributes to the temp spikes when you open & close.

Hope that made sense.