Step 10: Hydrogen Production
Anyway, with the amounts of reactants previously mentioned, you should get enough gas to fill up your envelope completely -- maybe you need a little less, or a little more. If it's your first time, halve the number of ingredients and use a lot of water to get a feel for it before you go for the full amount. If you want to store the excess gas, be aware that any sort of container you put it in (hospital pouches, in my case), if it's not made of mylar or material of a similar quality, will leak relatively quickly and will not maintain the purity of the gas. In general, it's best to make only as much as you need.
When setting up your reaction, you should consider putting in some sort of buffer between the reaction vessel and the envelope -- mine is the pouch with the two tubes in it. If you start the reaction with the water line close to the spout, even at low temperatures, the sheer volume of bubbles produced will cause water to bubble into the tube leading from the spout. This is normal, and will not do any serious harm as long as you sufficiently separate your balloon from the reaction.
Never leave a reaction unattended, and check the temperature regularly -- if you let it get too hot, your envelope could be ruined.
Set up your reaction like in the two pictures: add the aluminum first, then water. At this point, attach your rubber tubing to the spout, and when you're ready, add the lye. Wait for about ten seconds to see how many bubbles form -- if the water level rises too much, just pour a bit out into your basin. Once you've got the level right, wipe the inside of the neck of the vessel with a rag to remove moisture (otherwise your plug will slip out) and then plug it with a rubber stopper. Use duct tape if necessary.
Within a few moments, you should feel/see a flow of gas from your tube.