Gas compensation?

#1

Hi Guys, I've been lurking a bit around here lately, but this is my first post. I do not have an ROV yet, but am considering it.I just wanted to contribute with something I was thinking of as a simple solution to be able to go deeper... theoretically, very deep. This is just an idea and may be utter rubbish, but maybe not. You decide.

First, a little background of the idea. I am a rebreather diver and one of my simple rebreather kits in particular uses a principle that could be a simple solution to this problem. The unit is a SCR or semi closed rebreather that operates on the principle of a leaky valve with an over-pressure dump valve. The breathing loop is kept replenished by a small trickle of breathing gas fed at a constant rate. When a slight overpressure occurs in the breathing loop, a shoulder dump valve lets the excess leak out. Simple, and works great.

So... for an ROV, rather than fighting the pressure differential, why not just compensate it on a similar principle. An ROV could carry a very small air tank that would gently leak into the pressure compartments; the paintball crowd have some very small, very high pressure tanks. A release valve that is calibrated to dump at a very slight overpressure would keep the internal pressure just a tiny bit above ambient at all times.

One of the considerations in such a setup is that pressure varies very quickly with very minor ascents and descents. For a diver this means that depending on your orientation in the water, the dump valve can require adjusting during the dive. In my mind, an ROV is meant to keep a fairly stable horizontal position, so this is probably a non-issue. Another consideration is that of buoyancy. As an air tank slowly empties, its buoyancy will change quite significantly. This would make the neutrality of the ROV would change during the dive from negative at the beginning to positive during the end. The last consideration, is the sensitivity of electronics to pressure. I really don't know if this is an issue, but don't expect it would be unless some components have sealed air cavities inside.

Thoughts?

#2

Hi Paul,
I have also been toying with the idea of pressure compensation for dives, more for the prevention/detection of leaks than increasing the dive depth. I am considering a having a bicycle pump valve protruding from the main electronics compartment so that a hand pump could be used to pressurise the ROV from the inside before a dive. The pressure would have to be limited so as not to damage any of the components, especially the ICs or Caps where the cases are sealed. If there are leaks (from poor seals etc) then the internal ROV pressure will drop which could easily be detected by a pressure sensor on the inside of the ROV. Of course, this would only work until the water pressure is greater than the internal air pressure as then the water would leak in with no air coming out...

Essentially, I think compensating for the pressure is a great idea, just be careful not to damage the electronics.

#3

I like the train of thought! Like Kurt suggested, pre-pressurizing sealed containers with a gas may actually induce leaking when pressure equilibrium is reached, because there will be no pressure bias to squeeze the o-ring in either direction against its gland. Additionally, pressurized gases can be dangerous (for obvious reasons).

I like the idea you had, Paul, about having a regulator keep the differential small, but I wonder if the added complexity and mass of the system would be any better then just beefing up the container to handle greater depths. It would be interesting to experiment with this!

I've also been very interested in learning about what pressures various components fail at. In my mind, liquid compensation has always seemed very appealing. Although it's messy to work on the electronics, a plastic bag filled with mineral oil could go to the bottom of the Mariana Trench and do just fine. It seems to me that components you'd have to worry about would be things like electrolytic capacitors, crystal oscillators, and cameras with lens systems that require gas in the optical system.

More experimentation is needed in the realm of pressure testing components, so please let us know if you guys gather any data about this. Hooray for pressure testing!

#4

It's funny I was just thinking this same thing the other day. I was thinking along the lines of C02 canisters as well (paintball/gas type pistols). In fact it would have the added bonus of acting to ensure you never have a fire on board your ROV as well (like if some wiring gets wet and shorts/arcs or a heat sink doesn't quite vent enough).

A C02-filled container won't support a fire. It's the reason the US didn't lose more ships in the Pacific in WW2, the US would flood the lower decks with CO2 and then the fuel and munitions wouldn't burn no matter how many bombs were dropped on them.

An added bonus would be that you'd have a tank on board in case you lose comm and need a backup plan to get your ROV to the surface. Just have it feel a bladder or innertube or something with some of that onboard CO2 and you're on your way up.

#5

Maybe this might work. We use them scuba diving.

http://www.spareair.com/product/models.html

#6

Lots of good ideas, some very cool tests could be done with some interesting or cool catastrophic results. I think the first thing to do is calculate the atmospheric pressure differences from 1 atmosphere down to your planned dive depth and the amount of gas needed, give or a take a bit. Someone please pipe up and correct me, but I am guessing the amount of air in the Etube with the electronics on board would be a volume around 300ml? Not sure what depth you want to go to Paul? But you see where I am going here.
I know spare air get buoyant in a hurry when I have played with them, but they do make you think.
Then again, commercial ROV companies compensate by building things stronger.