Re: Aquatic Plants Digest V1 #41

>Finally, someone stated that cables that stay on all the time at a low level 
>are better than something that switches them on and off as needed.  The 
>rationale was that the low level ones would be on year around whereas the 
>higher level ones would not come on in the summer (or come on very rarely).  
>This is my first experience with heating cables in my own tanks so my 
>rationale may be messed up (and I'll gladly accept any confirmation to 
>this!), but it seems to me that the opposite would be true.  Let me explain.
>Ideally the system would be designed so that the cables could be on at all 
>times and just provide the required heat with no supplemental heater.  For 
>various reasons this obviously won't work.  The two options then are to 1) 
>Make a system designed to provide less heat than required and have an 
>additional, thermostatically controlled heater to make up the difference 
>(the typical DIY design) and 2) Place a controller on cables that will 
>provide more heat than necessary (the typical commercial design).
>It seems that the second option is better because it is closer to our ideal 
>scanario, i.e. all the heat that is needed is generated by the cables.  If 
>you don't need heat then you don't want the cables powered, no matter how 
>weak they are.  If you do need heat then why generate much of it with an 
>extra heater, thereby wasting a perfectly good opportunity to gain the 
>advantages of the undergravel cables?  Assuming a controller can be made for 
>DIY cables, I see no reason why it would not be superior.
>I agree that too large a heater would result in much more time off than on, 
>and possibly result in noticable temperature transients, but if a person was 
>concerned about their high wattage cables operating for only a small 
>fraction of the time in the summer then perhaps two circuits could be 
>created.  One would be a winter mode with 120 watts and the other would be a 
>summer mode of only, say, 30 or 60 watts.  Set up correctly this would only 
>require flipping a switch or swapping some connectors a couple of times a year.
 Why not run the element in series with a resistor all the time and let the
operate in paralell with the resistor.  That way you would have the low heat
all the time and more when needed all from the cables.  
 Alternately either two controllers and a resistor, or a 2 stage controller
and a resistor could be used to provide either no heat, low heat, or high heat.