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Re: Undergravel Heater -- Getting to the bottom

James Purchase, said a lot of sensible things about heating
cables.  He pointed to two examples of their use:

> I have used two types of under gravel heaters. The first
> was a homemade
> epoxy covered copper manifold which I described in a post
> several years ago.
> My current system uses AZOO cables controlled by an Aqua
> Logic Digital
> Temperature Controller (both obtained from Monolith
> Marine Monsters). Both
> systems produced a measurable temperature differential
> within the substrate
> (higher over the cable or heat source, lower in the space
> between cables or
> tubes). The temperature differential was measured using a
> high resolution
> mercury thermometer originally bought for use in color
> darkroom work - it
> reads to 0.1 C.

At the risk of adding more conjecture, the Azoo cables are
closer to Dupla than Dennerle in watts per inch.  Of
course, how you lay them out counts for a lot too.  Length
alone isn't all that matters.   I don't know the specifics
for James's cases, Dennerle style should produce much less
water current than Dupla style. 
> The question regarding heat differentials within the
> substrate isn't IF they
> are produced, its how BIG a differential is required. In
> both of my systems
> I could see a 3' C gradient - it this enough or too
> little? I certainly
> don't know.

I was too succinct.  To have been clearer, I should have
inserted the term " *significant* temperature
differentials", which would have begged exactly the
question James raises: how much is enough that it matters
at all?  Who knows?  And matters to what purpose?  I'm not
sure about that anymore either, but there's lots I don't

It would be surprising if both Dennerle and Dupla are right
about "optimum" wattage per gallon, but they could be.  And
of course, the anecdotal info most of us have is vague as
are the "control" or comparison cases.  Which is why I ask,
"Cables are good for what, compared to what?"  I don't
dismiss that they are good, I just don't know how good and
for what.  And they are not without their problems, in my
limited experience (and I don't mean just the cost).  So I
personally wouldn't recommended them except as an efficient
method of heating a water-filled tank.  Others have more
experience and recommend other things and other uses.

George seems to have a pretty good idea about what
techniques have which results for him, but then, his
techniques do far more for him than any emulations by me do
for me.  I'll accept that that has more to do with me than
George's techniques.  But in general, there's little real
science regarding cables, at least that is generally

But then, was there ever a lot of science behind laterite? 

Or Fluorite?  I'm not criticising Fluorite; I love the
stuff.  And lots of people use it, I think, because the
word of mouth is so good, the word of mouth supports the
manufacturer's claims, and both are consistent with the
commonly achieved good results.  Less popular are heating
cables because, I conjecture, their benefits are much less
apparent, perhaps only showing up *significantly* after 11
years or so and even then, compared to what alternative
techniques, I'm not sure.

Scott H.

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