compensating for payload

[scubersteve]

So if I were to outfit a rov with a manipulator or dredge or some such thing and retrieved something that weighed two pounds or so,
how would I compensate the buoyancy once I got it on board? Normally the buoyancy trim is a very exacting adjustment and picking up two pounds of payload throws that right out the window.
Coming up with enough vertical thrust to power it to the surface seems like a tall order too...

So what approach would you use?

[a_shorething]

Lift bag? Drop ballast?

You could set up a lift bag hooked up to a small C02 canister and inflate it by opening a valve maybe?

I've read about people setting up ballast in such a way that if there is a loss of communication or something for a long period of time, an electromagnet would be de-endergized and the ballast would be released. It would need to be enough to offset your proposed payload, but if you know ahead of time, might be able to do it.

[rossrov]

Hi Steve. Posted this a while back - you may have seen it already viewtopic.php?f=13&t=1156#p6077

Ross

[FJM]

I like the idea of the lift bag. Remember that as the payload rises, the pressure will decrease and the air volume will increase, so it will rise faster and faster. If you have to put a lot of air/CO2 into the bag to make it rise, I assume as the volume exceeds the bag capacity it would just escape (assuming the bottom is open)?

For the drop ballast idea, I assume you would use rocks or something environmentally friendly? Or possibly have a light, strong line tied to it so the ballast can be retrieved? I have heard of drop ballast being used as an emergency system - if power is lost, the electromagnet is de-energized, dropping the weight and making the ROV positively buoyant. Retrieval would not be a consideration as it's an emergency measure.

If you know ahead of time, how about attaching a line with a hook (grappling hook or carabiner) to the manipulator and hook it to the object. The manipulator lets go and then you simply pull the payload up.

Fred

[duncan]

I'm on a ship doing ROV dives right now. The ROV goes as deep as 6k, so they have a couple sandbags attached to a hook on the bottom of the ROV so that it sinks faster. Then when it's time to come up they can hit a switch and release the sandbags then rise faster.

Like someone said, you'd want the offset weight to be the right amount to give you neutral buoyancy so it might be finnicky, but I reckon if the commercial ROV guys do it then it's probably the best/most tried and tested way

[Zaibach]

I think I'll throw my vote in with the lift bags and drop weights.
Lift bags for bringing more up and drop weights for "something has gone terribly wrong". At least small scale.

I remember seeing somewhere, I want to say youtube, that someone was playing with inner tubes for lift bags. They had essentially taken some PVC tubes, drilled them like swiss cheese and sealed the ends with the mechanism and inner tubes inside. One valve allowed gas to flow in inflating the tube and making their rig more buoyant the other along with water pressure allowed gas to escape. I don't think I would rely on them to achieve neutral buoyancy but if I were hauling something off the bottom they would be useful.

If you weren't changing your depth too much you could probably adjust them fairly accurately to regain some degree of neutral buoyancy at depth after picking up a load. I would suspect the external pressure changing the displacement of the tubes with variation in depth would probably require constant correction and burn through any gas stores fairly quickly. Also have no clue on what depth/external pressure would render an inner tube based system completely worthless but its probably well outside hobby level.