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Re: heating cable sources

I have been following this thread with interest. I have never used a cable
heating system, and I wonder: can substrate heating users here on the list
say that they have noticed a definite improvement in plant growth/health
over standard submersible heaters?

Another thing - there has been some discussion regarding the use of homemade
cables.  Bill Wichers said:
> Yes, I do think Dupla has their cable specially made.
> It is a heavy-walled silicone-insulated wire similar to UL Style #3239
> (high voltage silicone wire), but with a resistive element inside that
> looks like nichrome and some nylon/spectra (not sure which) fibers
> presumably to prevent stretching from breaking the conductor.

I imagine that the resistive element included in this wire is the critical
factor to making it work.  Simply hooking up some 26 - 30 ga. wire to a 12
volt supply is not, in my view, enough to generate sufficient heat for the
purpose without a very high current. For example, 26 gauge wire (AWG) has a
resistance of .0480 ohms per foot (source ARRL Handbook).

If we were to use this example in a 18" x 48" tank, snaking the cable from
front to back every 1" would give us approximately (18 x 48)=216 inches of
cable, with a resistance of (216 x .0480)=10.37 ohms. At 12 volts, the
current through the wire is E/R or (12/10.37)=1.15 amps. So, the total power
through this system is E x I (12 x 1.15) or around 13.8 watts, which doesn't
sound like much of a heater to me!

This is done totally off the top of my head, and I may have missed
something, so please correct me if I'm mistaken.  The point I am trying to
make is that plain wire is unlikely to work in this application without
something to increase the resistivity of the conductor. Therefore, I'd keep
looking for the special wire that Bill Wichers mentioned.

Best regards,
Michael Clark
mdc at ivc_com