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Dan Resler's Heating Coils


  1. DIY cables
    by (Dan Resler) (Mon, 21 Feb 94)

DIY cables

by (Dan Resler)
Date: Mon, 21 Feb 94

I recently set up a hi-tech 90gal plant tank following the Dupla & Booth
plans. Since there's been several queries concerning heating cable systems I
decided to report on the setting up and installing of my cable system.

The Cables

I tend to err on the safe side when it comes to anything I put into the
water, so I decided to pay the extra money and get the Dupla cables (as
compared to using standard wire as recently proposed by Uwe Behle). I'm
using the 24 volt Dupla 300 cables which Dupla says are 7m long and are rated
at 100 watts. A resistance measurement gives them an actual rating of about
94 watts, while the actual length of the `hot' section is 6.96m (22'10").

My objective was to get the minimum wattage that would provide water flow
through the substrate; I did not want the cables to contribute much to
heating the tank as I anticipate trouble with keeping the tank cool in the
summer. Unfortunately, as has been mentioned in this group previously, the
lower the power rating of the cables the shorter Dupla makes them (this
applies to both the `hot' and `cold' parts of the cables; my leads to the
transformer box were about 3' too short). IMHO, a cable length of ~7m is the
absolute minimum for a tank with a 4' X 18" footprint.

Cables 7 meters long allow for 6 traversals of the tank bottom with ~6cm
between the cables. The Dupla cable anchors are ridiculously expensive, so I
devised my own using PVC pipe and electrical cable ties. I cut 1/2" PVC
pipe (use a bandsaw!) in half lengthwise to provide 6 ~17" supports across
the width (i.e. front to back) of the tank. Then two small holes were drilled
on each side of where a cable would cross the supports, and cable ties were
pushed through these holes and around the heating cables to secure them to the
half-pipe sections. Since snugging the ties prevented the cables from moving, I
was able to construct everything outside of the tank (a big advantage with a
24" high tank) and then gently lift the whole system in.

The Power Supply

The Dupla cable controller was way outside my budget; I decided early on
that I would have to build my own. This posed a problem, however, as I know
very little about electronics! After many discussions with net people and
attendance at the Booth School for the Electrically Impaired (with guest
lectures by Uwe Behle and Anthony Tse), I decided to build a very simple
transformer box with no temperature controller; here's a rough ASCII schematic
of what I built:

                               *      *
           O----+--------------+      +----------fuse------O
                |              *      *
  to cables     |        24v   *      *  120v              to mains
              lamp   secondary *      * primary
                |              *      *
                |              *      +--------------------O
           O----+--------------+      *
                               *      *         earth------O

My first design involved adding a light dimmer on the primary side to
control the power delivered to the cables. I experimented with several
light and ceiling fan dimmers but they all proved unsatisfactory; the EE
types on the net I consulted with reckoned they were throwing out funny
wave forms that messed with the transformer (I was unable to confirm this
since I don't have access to a 'scope).

If you build anything like this, be sure to add a slo-blo fuse to the hot
(black) lead to the primary. I'm told transformers do fail. (BTW, please do
not attempt to build something like this unless you're sure of your design
and you know what you're doing; I didn't, so I spent lots of bandwidth asking
questions and getting people to check my very simple design.)

The Controller

Since I didn't know how to build a temperature controller and I couldn't
afford to buy one, I decided to use a simple mechanical timer to control
the cables. After much searching I found one that allowed up to 24 on-off
cycles in 24 hours (the timer must be able to handle an inductive load; all
of the ones I've seen labeled `heavy-duty' can handle such loads).

The Results

I installed the cables under about 3.5" of gravel. With the cables off, the
water measured 72 F. I first set the timer for 12 on-off cycles in 24 hours
(i.e. 1 hour on followed by 1 hour off). Within 10 hours the tank stabilized
at 76 F (with lights off the entire time). With the cables on (45 minutes into
their 1 hour cycle) the temperature at the gravel surface was measured to be
the same as the water temperature near the top. I took 5 measurements around
the tank about 1" deep into the gravel and got a reading of 76 F (a couple
might have been closer to 76.5 F). To confirm that the cables were indeed
working I turned them off at 10pm one evening; by 7am the water temperature
had fallen to 72 F.

I wished to maintain the water temperature at about 79 F, so I added 2 75
watt Ebo-Jager submersible heaters and began experimenting with the heater
thermostats and the cable on/off cycles. In order to provide a temp drop of
1 - 2 degrees (I might go to 2-3 degrees) at night I turned the cables off
from 2 - 5 am and set heater A to go on at ~76 F (in case of a cold spell)
and heater B at ~78 F. So far only heater B has been on in the morning and the
tank has stabilized at 77.5 to 78 F at 8am and 79 F at 9pm. The cables are
on 75% of the time: from 5am - 8pm, 9 - 10pm, 11 - 12pm, 1 - 2am. The ambient
room temperature varies from 70 - 72 F, I am using a trickle filter and an
Ehiem canister, 160w fluorescent lighting, and the tank is partially open at
the top with no insulation at the bottom.

The Cost

Dupla 300 cables              US$95.00
PVC pipe & cable ties             4.00
Transformer box                 ~50.00
24 hour timer                     9.00

The most difficult parts to find were the 24v, 4amp transformer (I had to
order it) and the timer (most I saw had only 2 - 4 on/off cycles). I
eventually stumbled upon an `Ingraham Model 12-201E Multiple Program Timer'
from Toastmaster, Inc, at Wal-Mart!

If one was willing to forgo the Dupla cables, it then becomes possible to
have a working heating cable system for under $75.


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