On the use of hydrogen and helium

There are a few surprising things about hydrogen and helium and a few obvious ones. This assumes you will use a cylinder from a welding supply store. The hose for filling can be anything. I use the acetylene line and torch head of an old welding set as it has valves and is handy. The threads on the tank and regulator for hydrogen are left handed as any combustible gas is. That makes the acetylene line a good match.

Purchase a regulator for the cylinder. It is not sensible to use the valve on the cylinder as the pressure is very high and you will have no control.

The helium available from party balloon stores is commonly a diluted mixture and will not work well. Make sure you know if you are getting pure helium. Do not mix hydrogen and helium in one balloon unless you know if the helium is pure. If it is diluted, it is most likely with air which is free. If that is the case, you may be mixing hydrogen with oxygen and that will not end well.

The behavior of these two gasses surprises most of us. We expect gases to cool when expanded and we have a very high pressure gas. This effect is the Joule Thompson effect and applies to gases being released through a valve. For hydrogen or helium, it is basically zero. There will be no change in temperature when filling so no need to wait for the balloon to come to room temperature. It will already be there.

When the balloon is ascending, a different thing happens. The gas expands but cools as the expansion takes some of the energy out of the gas. The cooling is the adiabatic lapse rate. The lapse rates of hydrogen and helium during the ascent of the balloon are greater than the lapse rate of the atmosphere. The balloon will get cooler faster than the air around it. This is not important in a clear sky but it does matter if the balloon were to go into a cloud. It will be cooler than the cloud and there will be condensation that will increase the weight of the balloon. Given that water droplets will stick to the balloon and condensation will also occur, things can go very wrong. Try to have a clear sky.