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# Weight calculation in water

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## Weight calculation in water

(OP)
Hi, I am trying to find weight of a electronics bottle (a aluminum housing of 9.915 inch OD and 35.7 inch long enclosing all electronics required of a underwater project)in water.

To achieve this I did following things.

1. Calculated expelled water weight. I will get this by multiplying sea water volume with sea water density(0.0372 pound/cubic inch).I've taken expelled water volume same as outer volume of ebottle itself(2806 cubic inch)(not considering inside hallow volume).

2. Then I subtracted the expelled water weight from weight of ebottle in air(105 lbs) to get weight of ebottle in water.

But the problem here is I am not getting right answer.

may be because I used large volume (by not considering hallow part inside) for the given weight, the weight of expelled water resulted more than expected. Infact that was almost same as weight of ebottle in air(104.4 lbs).

So the final answer I got is like ebottle weighing 0.5 lbs in water, which is not correct.

when I took hallow part of housing into consideration, I am getting 1206 cubic inches, with which I am getting weight of object in water around 60 pounds, which sounds right.

My question here is where did made mistake in my calculation??

It would be great help if somebody advise me where I did wrong.

Thanks,
mkp.

Replies continue below

### RE: Weight calculation in water

From the info you gave, here's what I come up with:

Displaced water volume:  2806 cu in = 1.624 cu ft
Water weight:  .0372 lb/cu in = 64.3 lb/cu ft
Buoyant force:  displaced volume x water weight = 104.4 lb
Weight of object in air:  105 lb
Weight of object under water: Weight in air - buoyant force=
105 - 104.4 = 0.6 lb.

This means that is at almost neutral buoyancy and will slowly and gently sink if let go under water.  It looks like your first answer is indeed correct.

If there is a leak and the interior fills up with water, then it will weigh more under water (displace less).  Given the 1206 cu in displaced volume, you are correct in saying that it would then weigh about 60 lbs.

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