What electrical connectors to use underwater?

I have come up with an idea for an inexpensive underwater connector system, using PEX swivel adapters, that might be of use to you:

http://edwardmallon.wordpress.com/2015/01/29/a-simple-diy-underwater-connector-system/

Those adapters are rated to 100 psi, so should be good to a reasonable depth. The key is scoring the inside surfaces of the barbs with a wire brush so that the epoxy that seals the barb onto your cable gets some good mechanical adhesion. Be careful not to hurt the oring seats though. I also make sure that the wire insulation does not pass continuously through the epoxy, so that water can not wick down the conductors into the join if you accidentally cut jacket/insulation on the cable.

Links to the parts I am using are in the post, but there are plenty of companies making the parts. I will be testing these guys at depths up to 30m in a couple of months, as part of some new sensor assemblies that I am building. They will be left underwater for several months, but that means I won't have the final word for you on how well they handle long term depth exposure till mid year.

Addendum 2015-0-901

I just thought I should add a note that the connectors made it through their first real world tests: spending more than four months at between 7-10 m depth: http://edwardmallon.wordpress.com/2015/08/26/field-report-2015-08-12-success-with-ds18b20-temperature-strings/

We put both of those loggers back in the water, with one unit stretching from 10 to 20m. We will leave them in for 4-6 months on this run. Will post that update next year.

SeaCon is the company we use at work, but at hundreds of dollars per connector in most cases, are probably way out of your league.

That SubConn connector is a lot more than you need as well. It's designed for underwater mating, which I doubt you'll do. You're probably planning on connecting everything up in air and then dunking it in water. Underwater mating connectors also have ridiculously high mating/unmating forces and are rather large.

Given that your max depth is 20 feet, and pressure is about 1/2 PSI per foot, you're looking at 10 PSI. So any connector that can maintain a hard vacuum (14.5 PSI or so) would be feasible.

There's not a huge market for these things (most UUV projects are research or oil exploration related, can afford Seacons or equivalent, run to thousands of PSI of pressure, use pressure-compensated or water-blocked cables, and buy a lot more than you), so most of the products you can afford and that might work will not say anything about pressure resistance.

I'd recommend starting with "harsh environment" connectors. They are usually variants of standard connectors like USB or Ethernet, and have an extra outer shell, screw lock, and an O-ring seal. Be careful with O-rings. It's real easy to pinch one and ruin it if you treat it like a normal connector, and sometimes they need a lubricant/sealant to work well.

You should probably rig up a leak detecting circuit, get a harsh environment connector, put it in 20 ft. of water, and see if it's up to snuff.

Rather than using an expensive connector specifically designed for this situation, some penny-pinching fishermen report success with an approach something like this:

• drill a hole through the hull (while in dry dock, so water doesn't immediately begin rushing through the hole)
• snap a rubber grommet in the hole (so the hard edges of the hole don't directly rub against the outer insulation of the cable)
• run the cable through the hole in the grommet
• (optional) route a slot in what will be the "hull side" of a small polyethylene board, from one edge to a little past the center. Fill the slot with sealant and cover the rest of the "hull side" of the board with a thin layer of adhesive/sealant. Arrange things so the cable runs out the hole in the hull, then the cable runs inside the slot to the edge of the board, and then out into the water. Press the board against the hull, with the hole in the hull more-or-less centered and covered up by the board.
• Seal the hole with silicone caulk / marine epoxy / marine GOOP / 3M-5200 marine adhesive. In theory, you only need 2 rings: one to seal between the cable and the grommet, and one to seal between the grommet and the boat hull. In practice, most people end up making a huge blob on the inside and the outside, completely covering up the rubber grommet.
• put whatever connector you prefer (it doesn't need to be waterproof) on the "dry" end of the cable.
• wait at least a day for the sealant to dry

(Many boat manufacturers and fishfinder manufacturers discourage people from drilling a hole below the waterline. Instead, they recommend running the cable up and over the lip of the hull, or at least up and through a hole above the waterline -- but neither one is an option with a completely submerged UUV / AUV).

How to Install a Fish Finder; How to Install a Through-Hull; How to Install Depth Finders; Drilling Holes in Fiberglass; Use epoxy to fill in holes below the water line.