Tribes 1 Legendz

I want to put a compressed air tank or CO2 cartridge inside of a 40L roll top bag with a time delayed release valve so I can make the bag and whatever it's attached to float to the surface after a specified amount of time.

What size air tank or CO2 cartridge, and how much PSI would be required to fill the 40L bag with air?
Also, I need help figuring out the release valve idea... Like, should it be battery operated or could it simply be pneumatic?

The object I want to lift is either 3/8" or 1/2" galvanized chain, which weighs at most 2.53 lbs/foot. The average depth is 20 ft, so if the airbag were attached at the bitter end of the chain, it would need to displace around 50.6 lbs of salt water. Salt water weighs 8.6 lbs/gallon, and there's a little over 10 gallons in 40 liters, so a 40L bag should be able to lift like 90 lbs when full of air at sea-level density. Thus the weight of the contraption and any excess air pressure should be less than 39.4 lbs (round down to 39, cause I'm not sure of the bag's weight).

Also, if there's a way to remote trigger the valve rather than using a time delay, that would be better... Perhaps a microphone that listens for touch tones attached to the valve inside the waterproof bag, and an underwater speaker connected to an old touch tone phone in the boat?

ask dark to do some math for u

A diver i work with pointed out that i'd need twice as much air to fill the bag at the bottom of the harbor than on the surface, due to the pressure exerted by the depth. Unfortunately, this means, if you inflate the bag 100% at depth, it will pop before it reaches the surface... so i'll need a bag with a pressure relief valve, and twice as much compressed air as required to inflate the bag at sea level.

Inverse square law.

PV=nRT

Instead of a pressure relief valve, leave the bottom of the bag open (like a diving bell) to let the excess air out. Attach the chain to the bottom of the bag to keep needed air from leaving.

Or, instead of triggering a cartridge to fill the bag, fill it with air at the surface and attach ballast, then trigger the ballast release on the bottom. That way you could leave the bag sealed since the air will only expand to 1 atmosphere. Rocks are cheaper than CO2 cartridges and a mechanical string cutter is simpler than a pneumatic system.

My understanding is that a bag filled with air on the surface, then sank to the bottom with ballast, would compress to a non buoyant density. Thus twice as much air would be required to displace enough water to make the bag buoyant again, and as it floated up, would decompress until it popped.

Leaving the bottom open like a diving bell would work, except that i was relying on the air bag itself to protect the release mechanism from the corrosive and wet environment.

Detachable ballast is an idea i entertained, but decided against, as any moving parts outside the bag would be subjected to 6 months of salt water, freezing temperatures, mud buildup, sea weed and sea creature growth. Any one of which could seize up the mechanism.

If i were to utilize this idea, the buoy would be foam, attached to the bitter end of the light top chain, and the release mechanism would be a sacrificial linkage between both ends of the top chain. While the buoy would be buoyant enough to float the top chain, it would surely not be buoyant enough to float the heavier bottom chain, right?
Wrong. Smaller boats require smaller moorings. The lightest bottom chain is equal to the lightest top chain and the next lightest is as heavy as the heaviest top chain. So this system would only work for the heaviest moorings, which accounts for a small percentage of the field.

So i'm back to needing an inflatable bag with a pressure relief valve, an unknown number of CO2 cartridges, and some kind of programmable release mechanism. Although, i could forgo the pressure relief valve by using a bag that is twice as big as required, filling it halfway at bottom, which would decompress to completely full on the surface. However this would require either very precise maths or massively overcompensated thresholds.

A pressure relief valve would be easier. My only concern about that would be if water managed to find its way in that valve.