Heating Pad v.s. Substrate Heating Cables

     >From: eis at alto1_altonet.com (Paul Nicholson) 
     >Date: Wed, 28 Feb 1996 
     >Subject: Heating Pad v.s. Substrate Heating Cables
     >What are the reaons for using heating wires in the substrate versus a 
     >heating pad under the tank? 
   Though I'm not aware that this has been tested in any systematic 
   way, the hypothesis is that the heating coils inside the tank set 
   up alternating areas of heated substrate and unheated substrate.  
   This is supposed to set up cyclical convection currents which bring 
   fresh water and dissolved materials down to where the roots are. 
   Consensus seems to be that a heating pad underneath would warm the 
   substrate uniformly. Consensus also seems to be that this would be 
   a "good thing" but would not lead to the water/nutrient turnover 
   that is the key beneficial process with coils. A comparison project 
   would be most welcome to many folks on this list, who have argued 
   over this for at least a couple of years.
     Bob_Hoesch at fws_gov
     U.S Fish & Wildlife Forensics Laboratory
     in Ashland, OR, where it's not too cold, but surrounded by beautiful 
     mountains with lots of snow.