Liquid in your tank

[Amber]

My brother-in-law keeps insisting that our scuba tanks contain a liquid form of air. How true is this statement? I don't see how this is possible and it has been an on-going argument for a while now. He says that is the only way a scuba tank can get lighter after a dive. Does the tank get lighter or does it just feel that way?

Thank you,
Amber

 

[Scubaroo]

You win the bet - it's just denser gas. Air has weight.

"At 48 F a cubic foot of normal pressure sea level air weighs almost exactly 1 1/4 ounces" - quote from http://www.overflite.com/thermo.html. I'm sure there's much more authorative sources of data, but that was in the first few search results on Google

 

[Amber]

I knew he was wrong. Now I have to go and gloat and argue some more
Amber

 

[joens]

that is very funny and your brother -in-law is mistaken
the tanks do indeed get lighter as they empty but that is because air actually weighs something itself . there is no liquid in a tank .unless someone got water into the tank in that case they have other problems.
joens

 

[MechDiver]

...sure would help with dry mouth though

 

[Amber]

That's what I said. I'm a diver and he is not so I KNEW they got lighter but he always insisted that air doesn't weigh anything.

 

[Scubaroo]

If air didn't weigh anything, there would be no atmosphere - earth has air because gravity keeps it in place! Ask him what he thinks "air pressure" is, and why the wind blows!!!

 

[stiebs]

He might be getting some high-school chemistry/physics a little confused.

Forget about "air" for the moment, and think about individual gases (eg, oxygen, nitrogen). In their normal, room temperature states, these basic chemical elements exist as a gas.

You have probably heard of liquid nitrogen, often used by chemistry teachers to dip stuff like fruit into, and then hit it with a hammer and watch it shatter. Like any other element, when a gas is cooled, it will turn into a liquid. Cool it even further, and it will turn into solid matter. These state-changing temperatures vary hugely between different chemical elements.

Where your brother-in-law may be getting confused, is that the temperature at which a gas will condense into a liquid is effected by ambient pressure. For example, water is a solid (ice) at 0C, or 32 F. It is a gas (steam) at 100C, or 212F. But only at 1atm. If you increase or decrease the pressure, you will adjust the freezing and boiling points of a substance.

Having said all that, and probably making buggerall sense in the process, air in a SCUBA tank is not compressed enough to be able to change its state at normal operating temperature.

Either that, or he's been watching The Abyss too much...

 

[DiverBuoy]

The purpose of having a strong inflexible container such as a scuba cylinder is for the purpose of cramming more and more gas (air) molecules into the container - as you do this the pressure increases (but the tank can handle it) the density (or number of air molecules) increases and proportionally the weight also increases. The gas (air) needs to be dry to prevent corrosion of the inside of the cylinder. We also leave enough gas (air) pressure in the cylinder even when storing it long-term to prevent moisture from entering the cylinder.

In short, we do EVERYTHING we can to prevent liquid from getting in the cylinder.

 

[scuba-punk]

does he play paintball? the only reason i ask, is that back in the day with the co2 systems, the co2 was actually liquid in the tank. that may be where he got the idea. the next time he rides in an airplane and his ears are squeezing on the landing, just tell him to click his heels together three times and say "air has no weight" over and over again...