Homebuilt ROVs Forum

Right, so I'm planning on making a small (bath-tub / garden pond scale) ROV as a father-son project. We're keeping stuff simple to start off with.

I've played with a MadLab ROV (http://www.madlab.org/kits/rov.html) that had exposed electrical connections and motors without being modified to be waterproof. They were running problem-free at about 3V.

To my question(s)...

What sort of voltage can be run in fresh (ish) water before you get problems with things shorting out?

And, for future reference (assuming #2 son keeps interest in it), what would be the situation in salt water?

Hi there, lots of guys run 12vdc, 24vdc and 36vdc systems ie motors in both fresh and salt water without water proofing and have no issues with shorting.

In any of my tests, I have always used silicone sealant on connections, and sometimes I use a nice machinable wax to seal things in place, which also waterproofs them. But I did so more out of instinct than for scientific reasons. I just popped over to a robotics forum to see what the lore and experiments said.

There's a (mildly prickly) gentleman named Soren who mostly explains things, though technically, even he makes one mistake being repeated again and again. But he backs up his practical assertions with experiments, and reasoning, so it is worth a look.


This first post is interesting for the disagreements, as much as anything.
http://www.societyofrobots.com/robotfor ... ic=13178.0


Here, Soren does some measurements, which are pretty informative:
http://www.societyofrobots.com/robotfor ... 5#msg98035

My gripe is that all of the participants seem to not quite understand resistivity, which is ohm-cm, not ohms/cm. Resistivity is a way to characterize resistance in a way that doesn't depend on the geometry of the conductor. You can wiki it for details, but generally to get resistance, you multiply resistivity by the length of the conductor and divide by the cross sectional area. That way the units cancel properly to get ohms. Being immersed in a liquid, figuring out what length and cross section to consider becomes a numerical computation problem, so the experiments Soren did are golden.

The upshot is that in fresh water, any likely electrical path is going to be pretty resistive compared to the highly conductive wiring in the motor. Sea water is obvious chock full of salt, and will conduct better. Soren's point is that except when you are looking at high impedance circuits, conducting electricity is still going to be much easier in wires than in water, even salt water.

All these things said- one experiment is worth a million calculations, because calculations are only as good as assumptions, and mother nature will not neglect anything, whereas we have to, or spend forever calculating. I just would use a scrap motor- even if I believe I know what is going to happen, I am allergic to wasting my modest resources.

DaveE