# Re: Dupla Cables

```As some one who is presently making his own cable system this very
moment and as an Electrical Engineer I thought I would add my own 2
cents in - [sorry but in this digest mode I will have to put in breaks
for the author since the carrots are gone]
Chris Wells

From: Mortimer Snerd <n9720235 at cc_wwu.edu> aka Justin Collins
My understanding was that both voltage and amps could kill you, if they
were
high enough, and that the higher voltage of the Dupla system was a
reasonable
compromise, as it allows a lower amperage.

>Comment from Chris Wells
Well it is the current, but the voltage is what pushes the current so
they go hand in hand.
Think about Static Discharge - did you know that the voltage can reach
25,000 volts?? (typical is closer to 10,000) but the current is very
small and the event is very very short (.0000000005 Seconds)
My understanding is that the Dupla system has a cable source voltage of
24V and the cable is a high resistance type.  Due to the higher
resistance than a standard control wire, the current will be less.  You
can use the following simple formulas to determine what will flow
Volts = Resistance (ohms) X Current
With a Radio Shack \$10 volt/resistance meter you can measure the
resistance of the cable or if Dupla publishes it you can solve for
current and get
Current = 24 Volts /  Resistance specified or measured
Wattage can be calculated as:
Watts = Voltage X Current or
Watts = Current X Current X resistance
In the Dupla system the current will be lower since the resistance is >>
than standard cable.
At the same time this means the cable will heat more per unit length and
may be tough to handle.  That is one reason why I am choosing to use
standard teflon coated 26 gauge stranded wire.  The bigger reason though
is that it is cheap.  Note that teflon is very heat resistant (most
cables coated this way are rated to ~200F) and very strong relative to
abbrassion.  I could go to a smaller gauge but it becomes rather
delicate.

>From: Mortimer Snerd <n9720235 at cc_wwu.edu>aka Justin Collins
As for overheating the aquarium, I'm not terribly worried about that as
I plan
to use a temperature controller to turn off the cables when the water
gets too
hot.  My hood is also well ventilated, so I don't run into problems with
heat
due to that.  Also, is there really a good way to know how much water is
cycling
through the substrate?

>Comment from Chris Wells
Very good point since this is the whole point of the excersize.  We want
a slow but steady exchange of water through the substrate.  In an
experimental stage one could use dye to see the flow but who wants to
muck up a tank that way?  I will be thinking of a way - perhaps the
temperature differential from the water to the gravel could be an
indirect way to indicate the flow since it is the mechanism that is
driving the process.  I suggest you (we) put a thermometer in the gravel
and then one 6 inches above.  If you want a way to throtle your input
you could use a "Variac" autotransformer in front of the step down
transformer and size your cable for the higher wattage range.  This
special transformer can deliver 0 - 140% of the voltage from the wall by
the twist of a knob.  They can be found in most electrical component
suppliers like Newark ( http://www.newark.com/ ) and cost will most
likely be about \$45.  This way you can adjust the differential and see
how the plants respond.  This is what I plan to do.

>From: Mortimer Snerd <n9720235 at cc_wwu.edu>aka Justin Collins
If there is, I would really, really like to know so I
can set up a couple of 10 gallon aquariums with just cables and and
substrate
and spend a couple of days messing around with this stuff.  Basically
what I'm
trying to find out is if there is a good reason to use a lower wattage
cable
that is on more vs. a high wattage cable that may only be on half as
much.

>Comment from Chris Wells
As I said before the issue is most likely cable temperature.  It is
easier to handle a cooler wire.  Of course this gets back to the whole
issue of water movement.  I do think that you want the concentration and
spacing that Dupla has recommended since that will help move/circulate
the water through the substrate.  So I have doubled up my regular wire
to give me this result.  After calculating the proper length I find the
mid point and  parallel the wire.  As a benefit this causes a
cancellation in the electrical/magnetic field since the currents are
running in opposite directions.  With the regular cable it becomes
impracticle to use 24 volts since the cable must be too long.  That is
why most DIY cable heaters use 12V.  If you come up with resistive
heating cable like Dupla then you can keep the wire relatively short by
using a higher voltage.  Its sort of a balancing act of compromises.

>From: Mortimer Snerd <n9720235 at cc_wwu.edu> aka Justin Collins
Also, the opinion you have expressed about \$150 being too much for wire
and
suction cups is just that.  I can rationalize it, seeing as I have a
really
small desire to stick my hand in the tank one day and get a nasty shock
because
I made a mistake assembling the cables.  \$150 is minimal when compared
to
personal safety.

>Comment from Chris Wells
Another good point - when you are not sure about something rely on
others
Still \$150 is a lot of money.
If you look for Teflon or Silicon coated wire you should be more than
ok.
Still the best recommendation relative to personal safety is
PUT AG GROUND WIRE IN THE TANK!!!!!
PUT THIS WHOLE CIRCUIT ONA GROUND FAULT INTERRUPTER OUTLET!!!!!
If the wire insulation ever fails the GFI will trip and shut down the
system - Neat yes?
These can be purchased at any Hardware store for about \$10 and will
replace the standard wall outlet.
I would never run any aquarium circuit without this - period.  The only
down side is if you have some stray leak from lighting or the like you
can get in a salt water system from water spray and the system shuts
down on you without a warning.  One way to deal with this is to
construct an alarm that will go off  (email me if you want some ideas on
this)
Also I would fuse the primary of the step down transformer with a 3A
slow blow fuse.
Remember that the Voltage is high and the current low on the primary
(wall) side of the circuit and the Voltage is low and the current high
on the secondary.  I believe the current will be well below 1 amp on the
primary circuit.  This primary circuit fuse (put it in the hot lead)
will protect from any catastrophic failure of the transformer or if you
totally short out the secondary.

> Justin,
>> One thing is that power is drawn, not pushed.  If you think about it,
if you
> put a 40 watt light bulb in a socket it draws 40 watts even though the

> circuit will push 10 times that.  So the transformer has only to
exceed the
> amount of amps being called.   My setup probably draws something like
4 amps
> and my transformer will put out like 10. This is good because the
> transformer never runs near capacity and consequently runs cool.

>Comment from Chris Wells
Good point - allways oversize transformers!
I think your analogy or push and drawn sort of works but becarefull how
far you take it.
The load is a path that allows a particular amount of electrical current
to flow, a 40 W bulb is more restrictive than a 100 W bulb and so people
say it "draws" less current.  However the current flows because of the
Voltage against the load.  The Voltage is the Electromotive Force and so
is very similar to pressure against the input to a pipe that makes water
flow and the ID/length of the pipe is analogous to the resistance of the
wire.  In the water analogy I believe you would agree that a pressure
differential is needed across the pipe to make the water flow.  And
likewise I would argue that a voltage differential  "pushes" the current
or Amps through the wire.

> As to the safety issue a car battery puts out 220 amps at 12 volts. A
> transformer to run your system using non Dupla cables will put out 10
amps
> at 12 volts. I have never heard of anyone being electrocuted by a car
> battery.  I asked the electrician on staff at the radio station I work
at
> and he said it was absolutely not a dangerous current.  The Dupla
system
> that uses a higher voltage is actually more current, and hence in
theory
> more dangerous.
>
>Comment from Chris Wells
Note it doesn't take much current to stop ones heart and 12 V can source
>>>>>more than enought to do so.  Typicall though the resistance of
ones skin ~ 80,000 ohms  or so (clean and dry)depending on the condition
and what is on the skin, is enough to keep 12 volts from allowing
(pushing) more than 150 mico Amps from flowing.  Note that the limit is
not too far away though.  In many of my designs I am required to limit
the voltage to less than 500 micro Amps for safety.  Now if you have a
cut in the skin or if the skin is coated with water or worse yet salt
water in an aquarium then the resistance can go way down and yikes you
will get a bad poke or worse!!
I've been poked many times by a car battery and it hurt!

> For safety it is more important that the primary (house side) and
secondary
> (tank side) windings are separated.  You could tape both live leads to
your
> forehead with no I'll effect.  If you zip tied them to your tongue
though it
> would most certainly be unpleasant.  Most all new transformers will
have
> separate primary/secondary windings, my ACME transformer is made for
outside
> use and does.
>
>Comment from Chris Wells
Yes good separation is important to keep the 120 volt primary side from
breaking over to the safer secondary side.  One way to get this is to
take a look at the construction of the transformer.  You can get
transformers with split bobbins where the two windings are clearly
separated.  However you can get good results on the same bobbin if you
build it correctly.

> I think that 150 watts in a 100-gallon are to many, especially if you
are
> using powerful lights under a hood.  The combined effect will over
heat the
> aquarium.  I really believe you only need to cycle the water through
the
> substrate 1 or 2 times a day.

>Comment from Chris Wells
I don't know the right amount of cycling - you are probably right but
one does not have to leave the cable on all the time.  There are several
ways to throttle the system.  I mentioned the Variac or
Autotransformer.  I hear of  many who just put their unit on a timer on
for so many hours, off for so many varying the time on to get proper
flow.  I also was thinking of using a heater I got from "thatpetplace"
as the controller.  It cost about \$40 and is called "TitronX" and has a
300 W separate heating element and a separate sensor and controller.  I
opened up the controller and it is a simple circuit like a thermostat on
your house that compares the thermocouple sensor to a reference that is
adjustable.  The comparator drives a relay that cycles the power to the
heater.  This heater easily draws 2.5 Amps and so it can handle the
primary of my cable (about 90W) heater since it should be about 3/4 of
an Amp.  That way it will adjust for any seasonal variations.  I may
even combine it with the Variac so that when it is on the heat can be

> I'm not sure if the analogy is totally accurate but... If electricity
were
> water, amps would be pressure (PSI), and volts would be volume (GPH).
> Combine these to get Watts.

>Comment from Chris Wells
I think you have the right idea - just reverse volts and amps.
Amps is the electrical current or flow
Volts is the Electromotive Force or Pressure.

<snip>......
>> The Dupla cable is just a bigger pipe so it needs a greater volume of

> electricity to expand/heat it.  Hence more power = more dangerous.
>
>Comment from Chris Wells
No - you are close but the Dupla cable will draw less current,......but
because of the greater resistance will disipate more heat.  The greater
voltage is more dangerous because if it finds a better path than the
cable, like your hand, it will push a larger current.  Note that both 24
volts and 12 volts can both kill.  The best solution is to put in the
protection I talked about - both the ground wire and the GFI

> I used to do irrigation for a big, big farm.  I used to set up green
house
>> \$150 is too much for suction cups and wire, even if you do have the
money.
> Regards,

>Comment from Chris Wells
I agree - the water analogy works and \$150 is tooooo much to pay for
cable and suction cups.
I hope you both are successfull in your cable systems - I think either
way (24 or 12 V) can work.
I would love to talk more off line - I'm sure we can share our
findings.  Sorry this post is so long
Regards
Chris Wells
cdwells at concentric_net

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