Waterproofing a circuit and protection source electronics

[Jonathan Clark]

So lately I've been playing around with creating a housing for the electronics that I'm adding to our ROV and one thing I'm trying to figure out is the best way to protect the ROV in the situation that my water proof, pressure rated container fails. 

 The setup is like this: 

 A 9 pin tether cable comes out of the ROV and into the new custom housing.  In the tether are a pair of 48V DC power lines, a spare twisted pair, a twisted pair acting as a secondary video input, Primary RS-485 communication bus, 12VDC Power Supply, and a common power ground. 

I will primarily be tapping into the 12V power line and the spare pair.  The 12V will power a new micro-controller and some additional sensors, and the micro-controller will send all of it's data back using the spare pare.  I feel pretty confident in being able to SUPER waterproof the main lines coming in and most of the hardware, but I'm a little worried about some of the sensors not being able to handle the pressure at 200ft and literally breaking and causing power lines to short.  I have thought about using some form of humidity/moisture sensor to attempt to detect a water leak before things gets really bad, and to then trip a breaker on the main line in, but I'm still worried that a portion of a sensor could malfunction before a humidity sensor would detect a problem.  

So I'm looking for some information on the best way to protect the ROV's electronics in the event that one of the power lines from a sensor were to become connected to a ground or, heaven forbid, the whole container becomes engulfed in water.  I'm open to any thoughts anyone has.  THANKS! 

[Sean McPherson]

Maybe Don't waterproof it, per se. Prefill the thing with fluorinert? Since the surface tension is lower bubbles exit quicker, too, so filling, shaking, topping off is a fast way to get the gaps. Then less pressure differences, and less chance of a leak :) besides its fun stuff.

Sean

[Jonathan Clark]

That's interesting, I actually looked at some solutions where you fill the electronics with mineral oil.  The only problem is that the ROV actually relies on a lot of the buoyancy created by air in  the sealed container to allow it stay nuetrally buoyant so filling it with a liquid would probably add to much weight.  But I like you're out-of-box thinking though!

[Sean McPherson]

Ooh, maybe you can get a sample of this stuff?

http://www.neverwet.com/videos-news.php

Or these guys?

http://news.discovery.com/tech/coating-protects-cellphone-water-120109.html

Sean

[Sean McPherson]

I figured you might have to add ballast to counteract the buoyancy, normally. Figured the buoyancy was a bad thing most of the time. Hh. Oh well. External displacement bladder an easy fix :) maybe less durable but easy to repair, replace, swap out.

[Jonathan Clark]

Wow, I love the neverwet!  I'm calling them tomorrow morning.

[Aaron VerDow]

What if you made it easy to short the power lines?  The idea is electricity will take the path of least resistance so if you can short the power lines first the electronics should theoretically be protected. You'd have to have a fuse on the ground support and your robot would be dead in the water in the case of a bad leak but at least it wouldn't be dead and damaged.  

[Everseeker]

The oil is less dense then water...

[Brian Wagner]

Katherine, a former LVL1 member, was into underwater ROV's and she would seal her ROV with wax that was melted down from a toilet seal ring.  Don't know if that helps.

[jesco w]

  Techspray  FINE-L-KOTE AR  acrylic conformal coating is what i use for water proofing motherboards for phase change cooling ,        ive used it on ddwrt routers an other pcbs for outdoor use works pretty good ,    

     ive also used plastic dip for tools ,  harbor frieght sells it i think it works good too

On Fri, May 25, 2012 at 8:26 AM, Jonathan Clark <jdc...@gmail.com> wrote:

Well thats interesting.  I wish katherine wasn't a former LVL1 member!

[Pat McCarthy]

Not just waterproofing, at 200 feet, you are looking at ~104 PSIA loading…. That is a significant amount of pressure (and it is VERY cold)

So, if the acrylic/wax/?? Becomes brittle when cold… you may have a problem

 

Patrick McCarthy

 

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[Jonathan Clark]

Actually I need it to be able to handle 500ft which is closer to 273psi so even more reason for the concern

[Tim Miller]

there are pressure rated waterproof connectors

[Pat McCarthy]

Yes, but not many are rated for 237psia!

Although I think that the barrel connectors I saw on the ROV, are.

Patrick McCarthy

 

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[Jonathan Clark]

Everything currently on the ROV is rated for atleast 500PSI as the ROV can handle atleast 1000Ft depth

[Pat McCarthy]

Seeing the fact that you keep bumping at the riff-raff on the bottom… Are you in a wet suit, and prepared to dive down and free the tyke if it gets stuck?

(Yes, I see that it is only at ~30 feet… obviously at 100’+, it is on its own….)

 

Patrick McCarthy

 

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[Jonathan Clark]

I'm actually getting ready to start taking dive lessons just in case.  I'm playing with two new controllers, and some autonomous navigation so you'll definitely see some bumpy navigating now and then.

[Pat McCarthy]

Put a rod in the robotic arm…

Fish for Lunch!

[Pat McCarthy]

Bluegill… Yum!

 

[Raj]

Unless this is strictly for experimentation and won't be used in a practical environment, you may have to worry about contaminating the environment the ROV is operating in.  If you're inspecting a water tank, for example, the risk of contamination may be too great to risk using coatings or inert fillers.  Or if in a petrochemical tank, you may have to worry about the coating dissolving or the inert filling leaking out, changing the composition.

---

From: Jonathan Clark <jdc928@gmail.com>
To: lvl1@googlegroups.com
Sent: Thursday, May 24, 2012 9:39 PM

Subject: Re: {LVL1} Waterproofing a circuit and protection source electronics

[Jonathan Clark]

That's correct raj.  I have to be able to re-coat the surface with a special coating that allows us to enter into chemical levels with higher ph levels.  I also have to be able to submerge the entire ROV in a 400ppm chlorine solution before and after every dive.

[John Coder]

I would think that epoxy would be sufficient.

[Essobi]

They sell spray on rubber sealant too. It's used to waterproof a car under body and trailer/rv roofs. 

Essobi

[Essobi]

Are you running assisted fly by wire then? If so is it an off the shelf solution?

I used to run 1/10th scale 4x4s over a decade ago. 

Was thinking about reviving the beast for something automated. 

Essobi