buoyancy of a propane tank buoy.

[NorthWoodsDiver]

I will be placing a modified propane tank in a local pit as a underwater marker buoy. I emptied the tank, flushed with water for 1 hr, purged with argon gas, welded on some rings and a proper anchor line attachment point and removed the normal tank handle and valve. I then painted it john deer yellow.

Anyway to make my job easier I wanted to find out how much I would need to sink it or make it close to neutral and how much I will need for anchor weight since it wont be directly attached to any structure.

The tank is the standard "20lb" size found on a bbq grill or heater and its weight is very close to original weight with brass valve.

Thanks for the assistance

 

[SenseiPSK]

If I'm doing the math right it should be around 20lbs positive, but that's depending on how accurate the numbers are I looked up on the internet for the volume and weight of the cylinder. So in theory you would need a bit over 20lbs to keep it down, but it might take some experimenting to find the right weight, but hopefully that's at least a decent starting point.

Chris

 

[rjack321]

More like 40lbs or so to make it neutral. Its over 5 gallons water capacity and not very heavy.

 

[NorthWoodsDiver]

Well I guess I will take along everything I have for weight and hook on an anchor or something too. I figured 20 to 40LBS just based on the buoy that is in another location, you can tug on it and it barely moves.

 

[vshearer]

I wanna see a tank explode...underwater

 

[TheWetRookie]

I wanna see a tank explode...underwater

No, NO , NO, don't go there, we don't need another milliion page thread on exploding tanks

 

[scubafool]

I wanna see a tank explode...underwater

Somebody ban this user.

 

[SenseiPSK]

Any word on how much it actually needed?

 

[amascuba]

You need to know how much water that the thing is going to displace along with how buoyant it already is to answer that questions. Look up Archimedes Principle.

 

[Charlie99]

You need to know how much water that the thing is going to displace along with how buoyant it already is to answer that questions. Look up Archimedes Principle.

Alternatively, you look at the stampings on the cylinder.

For example, the 20# tank I just looked at has WC 47.6 and TW 16.6. That means that when filled with water will hold 47.6 pounds of water. An air filled tank immersed in water will have that buoyancy from the volume displaced by air. The Tare Weight of 16.6 pounds is how much the steel of the tank weighs.

Steel has a fairly high but finite specific gravity, so it doesn't have a full 16.6 pounds of negative buoyancy from the steel. I figure about 14.6 pounds.

The net difference of 47.6-14.6 = 33 pounds is how much lead you would need to sink an standard propane tank filled with air.

And the cylinder probably won't be filled quite all the way with air, so the actual buoyancy might be a couple of pounds less.
===============================

OK Northwoods Diver, how much did it really take???? And how close to full of air did you have it?