Ask A Scientist Chemistry Archive

Question:  Why doesn't the air in an air tank make a diver
float to the surface? ---Jeff

Answer 1:  Buoyancy ... obviously if the same volume of air was taken in a
balloon instead of in metal tanks it would be very difficult for the diver
to go below the surface.  The tanks are very heavy because they are made
of metal.  In water they "feel" lighter than they do in air because
of buoyancy.  They feel lighter by the weight of the water they displace.
That is, the weight of water that has the same volume as the tanks.
Now, when you put air in the tank it does not expand like a balloon
so the amount of water it displaces does not change -- only the
weight of the tank and the air it contains increases.  So the more
air you put in the tank the heavier it is and the more it makes the diver
sink.
An experiment a diver could do would be to take a plastic trash sack
to the bottom of a swimming pool (wouldn't want it to get loose in
a natural body of water!) and catch the bubbles coming from his
breathing apparatus in the plastic bag.  Since the air can now expand
it will displace more water than when it was compressed in the air
tank and will be more buoyant.  It won't take long til the trash
sack, as it fills with uncompressed air, pulls him to the surface.
 gregory r bradburn

Answer 2:  All absolutely true.
The analysis is somewhat complicated by the fact that
scuba divers usually end up having to wear lead weights
around their waist along with the tank to help them sink...
fat is more buoyant than water (which is why I float when
I scuba dive!).
However, if you drop a filled scuba tank over the side of the
boat (no diver), it sinks to the bottom.

prof topper