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Substrate heating



I would like to hear some opinions or evidence on how substrate heating
coils should function.
 
I have 100W Dupla coils in my 75g tank powered by a DuplaMat
transformer.  They're plugged into an UltraLife temp controller and set
to 83 degrees.  The cables aren't powerful enough to keep the tank temp
that warm though.  Without supplimental heating, the temp continues to
drop below 80.  I haven't allowed the temp to drop any lower, so it's
possible that the temp could drop much further.  I suppliment the coils
with a 100W Ebo Jager set to 82 degrees.  The combination maintains a
very stable temperature.  I could probably use a more powerful
supplimental heater as it takes the Ebo quite some time to raise the
tank temp.
 
With the above setup, the coils never cycle.  They are on 24 hrs a day,
even after the lights have been on all day.  The coils are providing
much warmth as the substrate feels noticably warm to the touch.
 
Am I getting optimum circulation through the substrate with the coils on
all the time or would it be better to allow the substrate to warm up and
cool down.  I would think a stable substrate temp would be best but
Dupla's implementation would suggest that the cycling may be part of the
benefit.  However, I seem to remember reading that a warmer environment
is more conducive to reduction reactions which leads me to think that
the constant heating is best as I believe Dennerle recommends.
 
The UltraLife controller has a 0.1F resolution.  I have tried keeping
both the Ebo and coils on the controller.  The temp drop is as much as
2 degrees at times, probably because the coils take so long to warm up.
Since I currently have Discus in the tank, I really don't know if I want
the temp to drop much below 82.
 
Any thoughts?
 
Mike