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[APD] Re: Yet another ? about DIY heating cables



I know there have been many many questions about diy heating cables posted but
I still have a question. I have a 12V power supply I was thinking about using
(although I am not opposed to buying/making a new one). I am aiming to get a
total of 50watts for a 65 gallon tank. I would like the cable to be roughly
25' long (obviously this doesn't have to be exact). According to my
calculations in order to achieve this I need cable that is 2.88 ohms or 0.11
ohms/ft. If I lengthen the cable to 30' I only need about 0.09 ohms/ft. At the
moment I don't remember the resistance of copper wire, but I think this puts
me around 30awg. 

31 awg = 0.1309 ohms/foot
30 awg = 0.10371 ohms/foot
29 awg = 0.08121 ohms/foot
28 awg = 0.06531 ohms/foot

Numbers above are from the handy table in the ARRL Handbook. The numbers are for copper wire. For insulated wire it is much easier to find the even gauges (30, 28, 26, etc.). For magnet wire -- which you do NOT want to use in your tank -- you can find either even or odd sizes easily.

Two problems...

Above is the logic most of the diy documentation has used, however, it is
completely ignoring that the documented resistance (or even what you get when
you measure it with your volt-ohm meter) gives you resistance at a specific
temperature. Especially since these are heating cables, the resistance is
going to change as the temperature rises.

Don't worry about it. For normal copper wire you're looking at a small change in terms of percentage, not enough to be of concern in this relatively undemanding application. I have a heated greenhouse door here (to prevent freezeups in the winter from sealing the door shut and preventing access), and the big problem with it is the *elongation* of the wire, not the resistance. It uses about 60 feet of 24 awg magnet wire around the frame running on about 24 volts AC to give around 200-250 watts or so. Works great, makes heat, I don't worry about the resistance change at all.

BTW, very few ohm meters can accurately measure resistances of only a few ohms. You're better off using the published resistance figures and a measuring tape with short lengths of wire.

If you ignore the change in resistance due to temperature, you still have the
problem of finding wire that has the correct resistance. I REALLY don't feel
comfortable putting 30awg wire in a tank full of water... it seems to me it is
very thin and fragile. I realize that a wet human body still has too much
resistance to get enough current from 12v to do any real damage, but I still
like to be careful. I checked into using cable that is designed to be used as
heating cable (where there is a small heating wire wrapped around a bus within
a heavily insulated sheathing), but they are designed to use 120v, therefore
the resistance is too high to work with 12v.

You can safely ignore the resistance change with temperature. Even in more demanding applications it is rarely taken into account. You probably don't want to use "normal" 30 awg insulated wire in your tank since it isn't very durable. I've used 28 awg myself, and it's still a bit smaller than I'd like but it's not a problem if you're careful.

You're unlikely to find 120v cable that you can modify to run at lower voltages. Probably isn't even worth looking.

So two questions...

Does anyone know how much of an impact the change in resistance with change in
temperature makes?

It's negligible -- don't worry about it. You would need some temperature constants for copper (and the right alloy, which for wire is usually "pure" copper, most of the published temperature coefficients you are likely to find are for copper alloys used in things like roofing and they will be different from that of your wire) to really work it out, and it's not worth the time IMHO.

Does anyone (especially those who have made heating cables themselves) know a
good wire to use that will give me about 3 ohms total and is safe to be
submersed in water?

Yes. Use type EE teflon-insulated wire. The EE wire is rated for 1000 volts instead of the usual 300 or 600, and the result is a slightly thicker insulation layer. The teflon won't react with anything in your tank, won't absorb any water over time, and won't be affected (or degraded) by the heat from the wire. It's an *excellent* insulating material for this application but make sure it doesn't get exposed to UV (not usually much of a problem in an aquarium). You'll want to get this wire surplus since it is *very* expensive new. Also, make sure to get stranded wire since it will hold up better.

If you are able to use 28 awg wire email me off list since I have a lot and you can get some from me (it's twisted pair and can be wired to have 2x the resistance for a given length if necessary).

-Bill


Thanks for the help. 


*****************************
Waveform Technology
UNIX Systems Administrator

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