Idea = Pressure compensation with scuba dive tank

[bigbadbob]

Interesting idea!!
A couple of things to watch out for-
The metal cans of electrolytic capacitors can squash under pressure so you might need to replace them with tantilum ones.
I'm not sure how high above the internal waterline you intend to have your electronics but they could be prone to splashing from below. potting them in might be an idea but that would make it top heavy and prone to roll.
The volume of air will decrease under pressure so there's a limit to how deep you can go before the internal water level reaches your sensitive bits. balancing the pressure doesn't balance the volume.
If you want to run in salt water, the water content of the air above will be high.

This sounds negative, i don't mean to be... just a few things you'd need to work around.

[dtrip]

Interesting idea!!
A couple of things to watch out for-
The metal cans of electrolytic capacitors can squash under pressure so you might need to replace them with tantilum ones.
I'm not sure how high above the internal waterline you intend to have your electronics but they could be prone to splashing from below. potting them in might be an idea but that would make it top heavy and prone to roll.
The volume of air will decrease under pressure so there's a limit to how deep you can go before the internal water level reaches your sensitive bits. balancing the pressure doesn't balance the volume.
If you want to run in salt water, the water content of the air above will be high.

This sounds negative, i don't mean to be... just a few things you'd need to work around.

Thank you!
Im aware of the capacitor issue, but for now I can not do anything about it, since I have already ordered an Arduino controller.
So for now I will just ignore it and hope it does not become fatal for the project in the future !

The electronics will be well above the waterline, inside their own PVC tube, closed except a tiny hole to equalize pressure. So that splashes are not a problem.
An over kill would be to also have a separator "wall" below the electronics, also with small holes to allow pressure to "go through" but small enough to
not allow splashes to go upwards. There will also be a weight at the bottom so that the vehilce is vertically stabilized (air at the top, lead at the bottom).
Touching anything in the bottom of the sea or using a manipulator will be very dangerous to bring it upside down, so it will be strictly forbidden.
For now this is a disadvantage that I have accepted.

As you say, balancing the pressure does not balance the volume, but this is to our advantage!
Because as soon as air is released (EDIT: from the scuba tank into the hull) the pressure will be automatically balanced the same as before (equal to the ambient pressure at that depth).But since there is now more air in the hull than before (because we released some more), the new volume of the air will be larger, therefore the waterline will move down. Thats the whole idea! This is how real submarines work as well.

Yes the sea water is an issue. Its amazing how quickly non-stainless objects acquire rust, without even touching sea water.
This is something I am aware of. I do not know what to do about it. I HOPE I will find a way to get around that (e.g. "paint" everything with nail polish
or some other coating). Its one of the things that will be ignored for now

Thanks for your feedback, and no it is not negative at all, it is greatly appreciated,
Dimitris

[rossrov]

Hi bigbadbob. I enjoy reading your posts, and there are some good points you make in the one above. Not negative at all, just good advice.

Hi also dtrip. I think your idea really has some merit. Exactly the same concept as a SCUBA BC vest. A refinement on the open-bottomed or open vented hull would be to put all the electronics into a bag, and attach the bag to the camera window end of the electronics assembly. The cables and air supply and venting would enter/exit through this rigid window part. The window is also where you would attach the the rest of the ROV. Then, if you want, put the bag inside a rigid tube shell for streamlining, aesthetics and protection from rocks, fish teeth, thruster propellers etc. For safety, leave the end of the tube opposite the window open, so you wont have a potential bomb on the surface.

As bigbadbob says, volume is what sets the buoyancy. So if you lose control of the air on a descent, the ROV will sink faster and faster the deeper it goes.

Would be great to see you build your air ballast system and post a video of it working.

[dtrip]

Hi bigbadbob. I enjoy reading your posts, and there are some good points you make in the one above. Not negative at all, just good advice.

Hi also dtrip. I think your idea really has some merit. Exactly the same concept as a SCUBA BC vest. A refinement on the open-bottomed or open vented hull would be to put all the electronics into a bag, and attach the bag to the camera window end of the electronics assembly. The cables and air supply and venting would enter/exit through this rigid window part. The window is also where you would attach the the rest of the ROV. Then, if you want, put the bag inside a rigid tube shell for streamlining, aesthetics and protection from rocks, fish teeth, thruster propellers etc. For safety, leave the end of the tube opposite the window open, so you wont have a potential bomb on the surface.

As bigbadbob says, volume is what sets the buoyancy. So if you lose control of the air on a descent, the ROV will sink faster and faster the deeper it goes.

Would be great to see you build your air ballast system and post a video of it working.

Hello!
A bag, hmm ? Thats a good idea, but I need to think about it because the bag does not allow thrusters or headlights to be placed
upon it. Therefore in the long run a rigid hull would be required, but then the rigid hull can contain the air in the frist place, so the
bag becomes redundant. If you think otherwise, please advise.

A "runaway" decent would be fatal, thats for sure. But Im counting that the 200 bar scuba tank can release air fast enough so that it
can stop the descent. And there must be a way for the pilot to know where the waterline is. Either by sensors or by a camera looking
directly at the waterline or some gauge or something. And another thing is not to allow a lot of tether from the surface, so that a
runwaway descent can be stopped before it is too late.

Indeed, more and more I am convinced that the first step should be a demonstration of this air ballast system in a pool. No need for
thrusters, headlights, long tethers and sea water, at first. If nothing else, the community will be benefited by the results.

I ll do my best towards this direction.

[Bubbles]

How big scuba tank are you planning to use?
Scuba tanks are heavy.Any estimation about total weight of the system?

[dtrip]

How big scuba tank are you planning to use?
Scuba tanks are heavy.Any estimation about total weight of the system?

bubbles,
Scuba tank is not an issue, since those tanks have great pressure inside so they contain a lot of air. Even the smallest one will do.
Current estimations are as follows:

ROV volume = about 10 liters (so that I can work easily inside it)
ROV weight = about 9 kg (so that when you leave it on the water, almost all of it is in the water, so that little air needs to be released for it to dive)

scuba tank = 1 lt, 200 bar, cost about 115 Euros new, weight 1.7 kg.

EDIT =
This one: http://must-dive.gr/eshop/index.php?mai ... 41a089daa9
END EDIT

This contains 200 lt of air. At 90 meters depth (10 atm) the ROV must be filled with 9 (volume) x 10 (pressure) = 90 liters of air.
So there is enough air in the tank for the ROV to go 90 meters deep, surface, then dive again, the re-surface.
Since the typical "mission" will consist of going down there, do the job, come up, there is plenty of threshold.
Also, a 3 liter scuba (600 liters) can be used, for even more air.

In the initial tests (swimming pool, 3 meters deep), I will be using a fire-extinguisher, which can contain 10 bar (they work at 15-25 bars normally). I will pump this up with an aluminum 11 bar bicycle pump (its an average pressure for bike tires, they go as high as 25 bars). I am currently investigating how to "connect" the extinguisher to the valve and the pump (need some adaptors and looking for them).

bigbadbob,
sorry I did not understand what you said about pressure. It seems you warned me about a "runaway" dive effect. Im sorry, I thought you meant something else. Indeed as I said a runaway dive is possible. The pilot must be alert and slowly release air into the hull, while diving. But not too much so that the dive does not stop. Its tricky, but with the correct input about where the waterline is, it can be done. (dont ask me how, I dont know yet!) ---> suggestions welcome

As always, suggestions are more than welcome!


EDIT
PS : In the example above, Im not sure there will be enough pressure left in the tank to resurface a second time (it will require >10 bar left to counter the pressure and be able to "inflate" the hull). But anyway you get the idea, there's no problem with the weight/size of the scuba tank. Unless you want to send something really small, really deep. Then the scuba tanks needed will become comparable to size and weight of the ROV itself.
END EDIT

[dtrip]

update = Scratch the extinguisher idea. I ll get a normal scuba tank from the beginning.

[rossrov]

Hi again dtrip. The bag idea would allow:

1. The electronics to stay dry, even if the ROV is inverted
2. A much smaller air supply tank

Think of the bag as a big sausage skin with one end open. The open end has a circular window in it, a large hose clamp holds the window in place. The other details are in my previous post.

Depth holding ability could be achieved with two regulators sensing the water/air pressure, adding or venting air in the bag (or bell)

Ross

[dtrip]

I am also considering propulsion using air from the scuba tank.
Thats a serious consideration, because compressed air holds respectable amounts of energy.
YouTube "compressed air bike" (if you crazy inventors haven't already, that is!) and you'll see
what I mean.

Need to check wether its dangerous for any humans nearby (for their ears, perhaps its too loud).

Today I received the Arduino kit. Boy that's a small controller. Great kit though. Everything one needs
to begin. Im also trying to find a cheap used scuba tank before buying a new one. Next is to get my
hands on a relay and valves and patch up the basic control system (joystick into the arduino =>
controlling the relay => controlling the valves).

rossrov, I see, I will seriously think about it when the time comes, thank you!

[dtrip]

As for the runaway dive problem, I have thought a solution:

If the hull contains two floaters inside (e.g. bottles of water filled with polyurethane),

then if it starts sinking and the water level rising,

at some point the water level will reach the floaters and the vehicle will stop sinking any further.

It is so, isnt it ?