[APD] Re: Substrate Heating

[Bill Wichers <billw at waveform_net>]

> Wouldnt say a heating pad, or terrarium heating pad
> affixed on the outside of the glass do the exact same
> thing as a substrate heating coil? or is there a
> significance to it's being directly in the substrate? it
> seems to me little would be different in this scenario
> excpt maybe the amount of heat needed, maybe not even
> that? isnt the amount of heat needed to create the
> convection currents relatively low? and i do use sand
> which i believe distributes heat evenly and easily?
> so many ?'s in my posts hehe
> ian

If you distribute the heat evenly then you won't get convection currents. 
Convection currents only occur when an area of higher heat can move against 
an area of lower heat. For example, the water around the heating wire will 
move up and cooler water will replace the heated water as it moves, and 
this happens as a continuous process. If you evenly heat the entire 
substrate, the water won't move since there won't be any localized areas of 
hot/cold.

Even if you use sand, the area around the wire will still be slightly 
warmer than the rest of the sand bed and you will have convection currents 
-- at least until you raise the entire sand bed to the warmer temperature 
which is a condition you normally try to avoid with heating cables.

You need very little thermal gradient to allow for convection currents, but 
there needs to be a *gradient*, which requires localized heating such as 
what you get around a wire. If you're just looking to heat your tank 
evenly, a regular submersible heater will be a lot easier and work just as 
well. If you want convection currents in the gravel use a heating coil. 
IMHO, heating pads underneath the tank aren't worth the effort.

     -Bill


*****************************
Waveform Technology
UNIX Systems Administrator

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