Weighing other gases

CO2 (carbon dioxide)
What you need:
  • Plastic pop bottle, with a valve stem in the cap, as described on the previous page.
  • Tire gauge for measuring pressure
  • Dry ice
Drop some chips of dry ice into the bottle (wear gloves! wear safety glasses!). The dry ice sublimates into CO2 gas and fill the bottle. If there is enough, it will completely flush out all air and there will be nothing but CO2 gas in the bottle. If you put the cap on, and measure the pressure with the tire gauge, you should see it rise slowly. I found that about 2 tablespoons worth is enough to reach 45 psi, bleed off all the overpressure, and reach 45 psi again. I kept the chips moving around on the inside, since I don't know if cooling the plastic to dry ice temperature will embrittle it and cause it to fail. It takes about 5-10 minutes for 2 tbs to sublimate under these conditions, which is sufficiently slow so you can easily do lots of pressure readings and keep things under control.

WARNING:DON'T LET THE PRESSURE RISE TOO MUCH! If the cap is on tight and there are still bits of dry ice in the bottle, always monitor the pressure with the tire gauge, and don't let it rise above say 50 psi (~3 atm). If the pressure rise above this, let some gas out of the bottle with the valve or by loosening the cap a little.

You should aim to have a pressure of about 45psi when all the dry ice is gone. Note that the CO2 gas is cold when it comes off the dry ice, and after the last chip has disappeared, the pressure should continue to rise a tad until the bottle has reached room temperature.

Since you have driven the original air out of the bottle, you have the equivalent of 8 liters of CO2 in your (2-liter) bottle, and the weight you now measure is that of 8 liters of CO2 minus 2 liters of air, which you measured in the previous experiment.

Helium
What you need:
  • Plastic pop bottle, with a valve stem in the cap, as described on the previous page.
  • Tire gauge for measuring pressure
  • Helium, at your local party store
Take your bottle to a party store where they sell helium balloons. Ask them to put some helium into the bottle, and have your tire gauge ready so you can stop when the pressure is 45 psi. Everything else is the same as for weighing air.
(I haven't done this yet - I have to go to the local party store to see what kind of hose might be needed to do this...)

Argon
Same as Helium: one of my son's young friends pointed out that you can go to a welding supply shop and get argon.
Last update 7 Oct 99 HvH
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