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Do-It-Yourself Substrate Heating Cables

Dan Resler and Uwe Behle

Introduction

There has been much interest and discussion lately concerning the use of heating cables to create gentle currents of nutrient-bearing water through a planted tank's substrate. Many of us who use heating cables have found them to be an excellent aid in providing an optimal environment for plant growth; when combined with the right balance of nutrients and light, the results can be spectacular.

The high cost of commercial systems, however, have discouraged many people from trying heating cables. Others have wanted to build their own systems but have had second thoughts because of concerns about submerging any `homemade' electrical device into their aquariums.

In this article we present the plans for building a safe and inexpensive heating cable system that is well within the capabilities and budget of many plant tank enthusiasts. These systems have been used in the authors' tanks with results equalling commercial systems.

A word of warning before we begin. Electricity and water do not mix well; please be very sure you understand fully what you are doing before you attempt any project of this nature. Your primary concern should always be safety, and a `safe' design is not enough---you must also incorporate safe construction techniques. We strongly urge you to consult with a professional should you have any doubts whatsoever concerning the safety of your design or construction.

There are basically two types of commercial heating cable systems available today. The first method uses a high-wattage (approximately 0.5 watts per liter) heating coil that is intended to be the primary heating source for your tank. This method usually requires a temperature controller in order to limit the heat build-up in the gravel.

The other prominent system proposes low-wattage cables (0.1--0.2 watts per liter). In this case, unless your room temperature is relatively high and stable, additional heating by a submersible heater is required to maintain tropical water temperatures. While the manufacturer of these cables suggests and sells a temperature controller that controls both the main heater(s) and the low wattage cable, it is not strictly necessary. The low wattage makes it possible to leave the cables on permanently for many applications.

We have chosen to develop a low-wattage system for our tanks. This was done primarily to minimize tank heating so that hopefully we would be able to have some flow through the substrate even during the hotter summer months.

The Cables

Step 1 is to determine the wattage and the length of cable that you need. A good rule of thumb is to use 0.1--0.2 watts per liter of water. For example, a tank of 100 cm * 50 cm * 50 cm gives us 250 liters gross volume. If we choose 0.2 watts per liter, the required heating power is 250 * 0.2 or 50 watts.

You also need to determine the length of the cable. Normally the cable follows a zig-zag back and forth pattern across the tank bottom. Note that in layout (a) both connections leave the substrate at the same point; this makes it handy for your external connections, but any configuration that gives good bottom coverage (i.e. your cable traverses the entire length and width of the tank) would be acceptable. The spacing between the parallel lengths of cable should be between 5--10 cm; try to avoid having one part of the heating cable touch another part.

[Figure 1]

Referring to Figure 1, the approximate length of the cable can be calculated as d + (n * w), where n, equal to d/x, is the number of times the cable crosses your tank. (The layout in Figure 1(a) will require a slightly longer cable than the layout shown in 1(b).) Obviously the layouts in Figure 1 require that n be a whole number; this will limit the possible spacings for a given tank size. In addition note that n must be evenly divisible by 2 if you choose to use the first layout (i.e. if you want both cables to exit at the same point). So, for example, if the tank has a bottom surface area of 100 cm * 50 cm and we choose cable spacing x to be 7 cm, it follows that n = 50 / 7, or approximately 7, and your cable length would be 50 + (7 * 100) = 750 cm (7.5 meters).

Using Commercial Cables

At this point you're faced with the decision of whether to use `ordinary' wire for your cables or buy cables specially marketed and tested for use in the substrate of an aquarium. The advantage of using a commercial cable designed for substrate heating is the security that comes from knowing that the cable has been thoroughly tested and, when used as directed, determined to be safe. The disadvantage is that special purpose cables are much more expensive than comparable plain wire cables. One should always err on the side of safety, however; if you have any doubts about the safety of using plain wire as a heating element in water, go with the commercial cables instead.

Table I. Dupla Cables
Cables Wattage
(24 volts)
Length
(cm)
Duplaflex 150 50 n.a.
Duplaflex 300 100 701
Duplaflex 500 150 1001
Duplaflex 750 200 1300
Duplaflex 1000 250 1699

Table I provides some information about the heating cables available from Dupla. (Note that these figures are provided by Dupla and are not values measured by the authors.) The power rating of a cable (measured in watts) is directly related to the voltage applied to the cable and the current (amperage) that flows through it. One watt is said to be equivalent to one amp of current moving through a circuit with the electromotive force of one volt. Therefore if one wanted to use the Duplaflex 300 cables with a 24 volt power supply, the transformer would need to have a rating of at least 4.2 amps.

You are not restricted to applying the manufacturer's suggested voltage to the cables, however. While one should be very leery of exceeding these voltages, there is generally no problem with using them at a lower voltage. Lowering the voltage applied to a cable will lower the power consummed by the cable, as expressed in the equation

	Power(watts) = Volts^2 / Resistance(ohms)	(1)
Therefore applying 18 volts (instead of 24) to the Duplaflex 300 cables would reduce the power rating of the cables from 100 watts to approximately 56 watts.

In our previous example we required 7.5 meters of cable for a 250 liter tank. The closest Duplaflex cable lengths are 7 meters and 10 meters; which would you choose? Total coverage with a 7 meter cable could easily be accomplished with 6 tank traversals of the Duplaflex 300 cable spaced approximately 8 cm apart, or 10 traversals of the Duplaflex 500 spaced at 5 cm. Since both cables would give us acceptable cable spacings, other variables such as cost and desired heat output would have to be considered in making your selection. (In reality, however, it is often your transformer choice that dictates which cable you must use; more on this later.)

Using Ordinary Wire

Equation (1) tells us that for a given voltage, the amount of heat generated by a wire depends indirectly on the resistance of the wire. If the voltage remains the same, increasing the resistance will decrease the power rating of the cable, whereas more heat will be produced if you decrease the resistance of the cable. The resistance of a wire is determined by its thickness and length, with resistance increasing as a wire gets thinner and/or longer.

The trick to `rolling your own' cables is finding the appropriate wire to give you the power rating you need. As an example, consider the 7.5 meter, 36 watt cable we needed for our 250 liter tank example. The commonly available voltages for A.C. step-down transformers are 6, 9, 12, 15, 18, and 24 volts. Rearranging equation (1) solving for resistance gives us

	Resistance(ohms) = Voltage^2 / Power(watts)	(2)
Equation 2 can now be used to derive the following table (with Power = 36 watts, from our previous example):
Volts Volts2 Resistance
(ohms)
Current
(amps)
ohms/meter
(for 7.5 meters)
6 36 1 6 0.133
9 81 2.25 4 0.300
12 144 4 3 0.533
18 324 9 2 1.200
24 576 16 1.5 2.133

In the last column we calculated the resistance per meter value a wire would need if it was 7.5 meters long (as in our example). We did that by dividing column #3 (resistance) by 7.5 (the length of cable). Now we need to find out which AWG wire size has the appropriate resistance per meter; consider the following table:

AWG Diameter
(mm)
Ohms
(per meter)
Ohms
(per 1000')
26 0.405 0.136 41.62
27 0.361 0.172 52.48
28 0.321 0.217 66.17
29 0.286 0.274 83.44
30 0.255 0.345 105.2
31 0.227 0.435 132.7
32 0.202 0.548 167.3
33 0.180 0.692 211
34 0.160 0.873 266

As you can see a 6 volt transformer would require AWG 26 wire (for 7.5 meters) and the 12 volt transformer AWG 32 wire. For 9 volts there is not really a suitable wire size; however, keep in mind that we can tolerate some variation here. A value within 20 percent of that calculated would be fine. (Remember that our initial `rule of thumb' called for 0.1--0.2 watt per liter, which is 0.15 watt plus/minus 30 percent.)

It is always desirable to achieve the heating power with a lowest possible current. In our 6 volt example 6 amps of current were needed for 36 watt cables. This would require the `feeding' cables from the transformer to the heating cable to be relatively thick; for up to 10 amps you can safely use AWG 13 or AWG 12 wire. A better solution for our example would be a 12 volt transformer requiring only 3 amps of current. Be sure to check on the availability of your chosen wire before buying a transformer; some sizes are not universally available. Also be sure to use a sufficient length of `feeder' wire to reach all the way from your aquarium substrate to the transformer.

The cable insulation should be PVC or silicon and be rated to withstand a temperature of at least 80 C. Most cables easily meet these specifications. The cables (heating and feeding) must be spliced and soldered together before being insulated with heat shrink tubing. Silicon aquarium sealer could also be used to insulate the bare metal in your connections.

It is inadvisable to have you cables resting loose on the tank bottom or in your substrate; they could easily be jostled and dislocated through activities such as planting and removing plants and cleaning the substrate. Commercial cable systems advise anchoring the cables approximately 1/4" above the tank bottom. This presumably enhances water flow in the same manner as raising a fire with a grate in a fireplace allows for better airflow to the fire.

We devised special cable anchors using 1/2" diameter PVC pipe and electrical cable ties. Cut the pipe in half lengthwise to provide an appropriate number of 1/4" supports across the width (i.e. front to back) of the tank. (Using a band-saw to cut the pipe; it would be very difficult to get even cuts if you tried to do it by hand). The number of pipe sections needed depends on the size of your tank; you should have no more than 30 cm between supports.

After cutting the cable supports, determine and mark the locations where your cable will cross each pipe section. Two small holes large enough for an electrical cable tie (the kind with grooves that allow you to snug them tight once) should be drilled on each side of these marks. Cable ties are then pushed up through these holes and around the heating cables to secure them to the half-pipe sections (don't forget to trim them!). Since snugging the ties prevented the cables from moving, it is possible to construct everything outside of the tank and then gently lift the whole system into place [see photo?].

One last word of caution---heating cables can get very hot very quickly when powered up. You should only test your cables while they're submerged in water; otherwise they may get too hot and damage the insulation on the wire, not to mention the possiblity of giving you a nasty burn.

The Power Supply

[Figure 3]

Figure 3 shows the schematic for a simple power supply (you can, of course, choose a voltage other than 24). The selection of your transformer is of primary importance when building such a device; there are 3 important things to consider before making your purchase.

First and foremost the transformer must provide insulation between the primary and secondary windings. Transformers can be built in at least two different ways: the primary and secondary windings can be wound together on the same piece of iron core or there can be two separate coils (`bobbins') coupled by a common core.

If the transformer should somehow break down, there is a danger that the insulation between the primary and secondary be violated, thus causing the full mains voltage to be applied to the secondary side of your transformer (and therefore to your cables!). Though the odds of such a failure happening are rather small, one can reduce the risk by using a transformer that uses separate bobbins for each winding. In addition it is a good idea to ground the core---by connecting the core to ground (in a two bobbin transformer), any breakdown in the primary that could possibly get to the secondary has to go through the core and would go harmlessly to ground instead of to the primary.

Second, make sure the transformer will be able to handle the needed power. You should buy a transformer that is rated to handle a 20--30 percent higher wattage than you need. Otherwise it might get too warm and overheat (or even `melt-down' if it's severely overloaded). Finally, be sure that the current rating (amperage) of the secondary winding meets or exceeds the current required.

Don't forget to put a slow-blow fuse into the primary side when you wire everything up. A fuse is your `last line of defense' should your transformer fail. The value of the fuse can be calculated using Amperage = Power/120, volts (e.g. in our running example 36 watts/120 volts = 0.3 amps = 300 milliamps). Use a fuse rated at 1.2 to 1.5 times this calculated value. (Note that it is a good idea to put a fuse on both sides of the primary if you use a grounded core since you never know what side is the `hot' side.) Also be sure to connect the ground wire from the 115 volt source to the metal case of your transformer.

We would like to reiterate that in addition to having a safe design you must also construct your power supply in a safe manner. Building a power supply involves selecting the appropriate parts, choosing how to place your components, drilling mounting holes, ensuring a proper ground, choosing correct connecting wire, soldering, and proper testing. Do not attempt to build a power supply yourself if you are not confident you can do it properly!

The Controller

Most commerical cable systems provide a sophisticated solid-state temperature controller to regulate the on/off cycles of your cable. While these controllers offer superior control of your tank's water and substrate temperature, they are very expensive and generally not required for low-wattage cable systems. A simple mechanical timer (the kind sold to control your house lights while you're away) can be used to switch the cables on and off at preselected intervals. We have found that timers providing 24 on/off cycles in 24 hours work quite nicely with heating cables. Be sure that the timer you select can handle an inductive load; most that are labeled `heavy-duty' fall into this category.

With a little patience it's not difficult to find timer settings that will maintain a correct temperature and provide adequate flow through your substrate. You should choose settings that maximize the number of on/off cycles in a 24 hour period; for example, if you wish your cables to be on 50 percent of the time, 12 cycles of one hour on/one hour off are better than a single 12 hour on/12 hour off cycle. Remember that with a mechanical timer you are the controller; you should monitor your tank's temperature often being particularly alert when the seasons change and your house temperature fluctuates.

Bibliography

George and Karla Booth. ``Live Plants---Lessons Learned.'' Aquarium Fish Magazine, 6(5):52--59, February, 1994.

Kaspar Horst and Horst E. Kipper. The Optimum Aquarium. Aqua Documentia, 1986.

Ludwig Dennerle and Hans Lilge. System for a Problem-Free Aquarium. Dennerle, 1990.

Appendix: Suggested Sources for Components

Heating cables

DALECO
3340F Land Drive
Fort Wayne, IN
phone: 219-747-7376

Transformers

All Electronics
phone: 800-826-5432

Signal Electronics
500 Bayview Avenue
Inwood, NY 11696-1792
phone: 516-239-5777

Wire, lights, etc: your local friendly Radio Shack


Note 1. A leading manufacturer of heating cable systems recommends that the water volume in your substrate be turned over at least once per day.
Note 2. Tank dimensions usually refer to the actual size of the tank and therefore cannot be used to accurately determine the volume of water the tank will contain. If we allow for substrate and equipment displacement, a `250 liter' tank might actually only hold 180 liters of water. This difference might be significant in other applications (for example when dosing a tank with medication) but can be safely ignored with low-wattage cables requiring thermostatically-controlled supplemental heating.

Authors

Dan Resler
2609 Roaring Brook Dr.,
Richmond, VA 23233 U.S.A.
dresler-at-cabell.vcu.edu

Uwe Behle
Schlosstr. 17
71139 Ehningen, Germany
uweb-at-hpbbn.bbn.hp.com

Dr. Resler is a professor of Computer Science at Virginia Commonwealth University; Uwe Behle is an Electrical Engineer working for Hewlett-Packard.

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This page was last updated 29 October 1998