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Q:
How do I calculate the amount(volume) of helium I would need to keep an object of X weight afloat in water?
- Greg Sandy
Salem,MA
A:
Hi Greg,
According to Archimedes' Principle, the buoyant force on an object
is equal to the weight of the water it displaces. Subtract off the
weight of the material used to displace the water -- it is pulled down
by gravity. Water has a density of 1 kg per liter. Helium's density is
very small compared to that of water. Plain old air has a density very
small compared with that of water and is easier to come by than helium,
although helium is less dense even than air and therefore will make
things float better.
So... you just find the mass of the object you want to float in
kg, and that tells you how many liters of water you need to displace
with something that weighs much less than water. Be sure to add in the
mass of the boat or balloon or tank or whatever it is you are using to
contain the helium (and also the mass of the helium, which likely can
be ignored here because it is small). It is good to make the floatation
device bigger than needed -- boats usually extend above the water --
they aren't completely submerged. But not so much extra that the helium
will cause the object of weight X to float away in the air. This is one
other main reason it is more convenient to use air for floatation in
water -- you will not lose your floatation device watching it float
away in the sky when you take off your object of weight X.
Tom
(published on 10/22/2007)
Follow-up on this answer.
