On a 4" tube setup I have 8 sets of penetrators installed, plus a few SubConn bulkheads. The penetrators are all made with the 16/2 cable and marine epoxy, following the instructions. The 16/2 cable is then connected to a Tsunami 1200GPH bilge pump.
When I do a vacuum seal test to 15-20 inHg there is a slow leak, probably around 1inHg/minute or so. I’m fairly confident that all of the o-ring seals are solid and holding up the vacuum testing, and believe to have issues with air wicking through the wire cores. The connections have been sealed externally with liquid electrical tape and heat shrink that has thermoplastic built in.
Are there any recommendations for trying to find or fix the leaks on this? How likely is it that this setup would be waterproof, but not airtight (the deepest the ROV will go is 40ft for a maximum of 15 minutes)?
You are more than likely seeing air wicking through the wire as you have speculated. The insulation on the wire is Teflon so unless you acid etch it and use a primer etc. and an encapsulation material that will always be a failure mode. The good thing is if the cable is sealed on both ends or at least as far as the outer jacket is concerned you should be just fine. Even water blocked cable is not designed for vacuum testing. All of the subsea water blocked cable that I use at 1000+ psi is rated for a certain leak rate etc. It is really hard to stop core leakage … hooking a vacuum to it is even worse.
If you are only going to 40ft, I think you are doing way to much testing for way to little pressure that you have to withstand. Just my opinion. You should be just fine.
Just one more comment, the O-ring seal race design is not for a vacuum. That would be a totally different set of tolerances and the O-rings wouldn’t be BUNA-N either. Again, vacuum testing in this case is not really valid.
Thanks for the insight Harold. My ROV team has had a lot of issues in the past with dry housing’s flooding, so it’s become something we’re a little paranoid about I guess.
It’s reassuring to hear that it likely won’t be an issue for us, and we haven’t seen a drop of water leak into the housings yet (though the deepest it’s been is 14ft so far).
Seth, I posted a minor rant in the General Discussion forum about using a PBOF system to ensure that your ROV’s don’t flood. Go read that post and see if that is something you might want to entertain.
Oil filling isn’t practical for us since we have weight restrictions that we’re just going to be meeting (this ROV is for the MATE competition). We have discussed doing some sort of air equalization (or even keeping the inside at a slightly higher pressure than the outside) with an airline and passive pressure relief valve. Given the low depth we’re going to, I doubt it’ll be worth the effort.
I agree with Harold that you are seeing minor leakage through the jackets. I wouldn’t be too worried at 40m. You definitely won’t have any sort of catastrophic failure, so if you are still concerned, pull it up after a minute or two and check for leakage.
@Harold,
Good points all around. The Teflon insulation in our cables is acid etched, so it should adhere to the potting pretty well.
The PBOF is only messy if you spring a leak Actually you setup a system where you have a fill and vent port and you basically vacuum draw the oil through the fill port (which you have lower than your vent) so you are purging air while you are sucking the oil in. Once you get to your fill port area, you will pull on through that point and seal it off. I totally had a senior moment on the issue of specific gravity and the ROV etc. You would have to counter the added weight with foam etc., but everything I have done this for has been attached to a much larger system that had propulsion so it wasn’t an issue.
Seth, you shouldn’t have to worry as long as your cable penetrators are potted correctly, the O-ring in the end caps are in good order and lubed … everything should work just fine.
Actually you setup a system where you have a fill and vent port and you basically vacuum draw the oil through the fill port (which you have lower than your vent) so you are purging air while you are sucking the oil in. Once you get to your fill port area, you will pull on through that point and seal it off. I totally had a senior moment on the issue of specific gravity and the ROV etc. You would have to counter the added weight with foam etc., but everything I have done this for has been attached to a much larger system that had propulsion so it wasn’t an issue.