re: substrate heating

>I've decided on a 24V, 2A transformer, and 30 gauge wire-wrapping
>wire, both from Radio Shack.  The only major problem is that I need
>200 feet of wire to get 30 watts of power.  I'm going to do this using
>4 reels of 50-foot wire, wound in parallel on a frame built of solid
>steel-core, vinyl-insulated clothesline from my local hardware store.
>The ends of the reels will them be soldered together to connect the
>four reels in series.  The frame will be glued to a 46" x 16" piece of
>plastic carpet backing and placed at the bottom of my tank.  Add
>gravel, and I should be all set.

Wire wrap wire is a single strand and is easy to break.  It would be better 
use stranded wire ( several small strands wrapped together in an insulated
jacket).  I used stranded 150 ft of stranded 30 gauge wire, doubled it , 
it and placed it onto an eggcrate frame.

To twist wire, have someone hold the middle, place the two ends in the chuck 
a drill and turn it on.  It will take a while to twist.  Be careful when you 
release it or
it will all wrap up on you.

The two ends should be brought out of the aquarium and soldered directly to 
the transformer ( or first to a fuse).  The 30 gauge wire will get warm but 
not get too hot with 1.2 amps flowing through it.  No matter how well you 
try to seal wire ends which are in water, water will eventually leak and the 
junction will corrode.

>    As an alternative, where might we find some high-resistance, thickly-
>   insulated cable like that used by Dupla and Dennerle, but without the
>    price associated with the (100% pure unobtainium, hand-crafted by
>    Bavarian elves) german products?

I believe the wire Dupla sells is similiar to a wire we use for medical 
applications.  It is silicon coated and very flexible.  It is used for 
electrode wire for ecg, etc.  The price is $1 or $2 per foot, which makes 
the dupla wire fairly similiar in price.  It is more flex resistant than 
ordinary wire, but I don't believe it is any stronger.  If anybody cares, I 
can find the name of the company that makes it.

>Hmm.  I still think the wire-wrap wire is too fragile and brittle for
>such a task, and the amount of work it's going to take you to wrap 200
>feet of wire around the core (I assume this is some sort of rack?)
>is probably worth more than it'll cost to buy the Dupla cable.
>But I want to hear how this comes out because I haven't
>heard any "after" stories with 100% DIY setups yet.  I wonder how
>hard it'd be to find out where dupla gets their flexible resistive wire,
>and just invest in a spool of 100 or 1000 feet...  Or if we could
>somehow insulate nichrome wire, it only costs a few pennies a foot at the
>Chem supply.

I believe nichrome would have far too high of a resistance to be practical. 

If I recall it is about 1 ohm per foot.

>The only problem there might be (not sure) would be corrosion and
>the lead in the solder. The wire is probably copper too so it needs to
>be sealed up. You could use silver solder to replace the lead solder
>but this is overkill. Sealing the connection in epoxy would do it.
>Electrical tape just won't seal. Corrosion would happen where

Even epoxy will leak over time because it makes a mechanical seal only.
A better choice would be to use an acrylic cement because it will weld to 
PVC insulation jacket on the wire.  ( don't use teflon insulated wire, 
nothing will
stick to teflon)  An even better choice if you are not heating the wire too 
much is
to run the ends of the wire out of the tank.  This way you can avoid 
corrosion entirely.