Calculating buoyancy of a canister light

[tchil01]

I bought a used canister light and I want to figure out what effect, if any, it will have on my weight belt setup. The light out of water weighs 14 lbs. The total air volume inside the canister is 191 cu.in. and the battery displaces 120 cu.in. which leaves me 71 cu.in. of air space. Can I figure out the lights buoyancy from this info, or is there some other way to calculate it?

Thanks
Ty

 

[nickjb]

All you need is the weight of the canister and the overall volume.

I'm converting units back and forth so this isn't exact.

If the total volume is 191 cu.in (plus a bit for wall thickness) then it should displace about 7 lbs of water

SO...

Up = 7 lb
Down = 14 lbs

Total (14-7) 7lb negative buoyancy

PS
The best way to work out the effective weight in the water is to weigh it in the water. Better to use a spring balance (fishing style) than the bathroom scales. No need to find out the volume at all.

 

[jonnythan]

The only things you need are out of water weight and total canister volume. If the canister is sealed with a battery inside, it doesn't matter how much air is there. If it's a cylinder, you can measure it and calculate the volume.

Say it has a diameter of 5 inches and is 14 inches high. Its volume is 5^2 * 3.14 / 4 * 14 = 274.89 cubic inches.

So we need to find out how much this volume of water weighs, and subtract it from 14 to find out its buoyancy.

Water is about 62.4 lbs per cubic foot, or 62.4 lbs/ 1728 in^3.

274.89 in^3 * 62.4 lbs / 1728 in^3 = 9.93 pounds (it will be slightly higher for seawater).

So, this canister would have a buoyancy of about negative four pounds underwater.

Also, you can just check with the manufacturer.. they should publish buoyancy characteristics

 

[jonnythan]

Keep in mind that the wall thickness can be significant.. if you have a canister of the dimensions in my previous post and it had 1/4" lexan for the wall, it would take up 36 cuibic inches more space than the internal volume.

 

[Rick Murchison]

Since my main concern with a piece of equipment is how it will effect my weighting, I use a balance bar hung over the water (in my case I use my swimming pool) - on one end of the bar I hang the object under the water (in this case, the canister light); on the other end I put lead weights in a bag under the water until it balances. The chief advantage of doing it this way is that it automatically eliminates any volume measurement errors, and automatically accounts for the buoyancy of the lead itself.
My own canister light, for example, is worth eight pounds of lead.
Rick

 

[Marlinspike]

My own canister light, for example, is worth eight pounds of lead.

8 lbs of lead at $2 per pound is $16. WOW...that is a cheap canister light!!!!!


Sorry, couldn't resist

:spank: Marlinspike

 

[Rick Murchison]

Originally posted by Marlinspike


8 lbs of lead at $2 per pound is $16. WOW...that is a cheap canister light!!!!!


Sorry, couldn't resist

:spank: Marlinspike

HHHhhaaarrr!
But ye're payin' way too much fer yer lead, lad...
Rick