Is there a possibility of hydrogen ignition when filling a weather balloon? There's a guide here on how to seal a weather balloon after filling:
http://ukhas.org.uk/guides:sealing_the_balloon
http://ukhas.org.uk/_media/guides:neck8.jpg
Won't the static electricity from the duct tape cause the hydrogen gas to ignite (there's always a little air (oxygen) in the balloon before filling, plus, a very little amount can leak from the filler)?
 A: There are several ways that Hydrogen could ignite when filling a weather balloon. I definitely don't have many solid answers to give, but here are some some thoughts to consider.
Upper flammability limit
As Pierre pointed out, you can try to be above or below the flammability limit LFL/UFL. However even if you are above the flammability limit in balloon, filling a balloon with pure H2 is still a big risk. Think of it like this, if you fill the balloon with pure H2, then the balloon gas is technically above the UFL, but any leaking H2 will leak out into air, and be much more diluted. This leaking gas could easily be flammable and if ignites, it's likely going to burst your balloon and burn the remaining H2.
Lower Flammability Limit
LFL for H2 is 4%. If you were to fill your balloon with 96% Helium and 4% H2, you could well be on the safe side for sending the balloon up with slightly lower cost. Obviously this is going to do only very little to offset your costs sadly.
Ways that you could accidentally ignite the H2
Sparks
Sparks from electronics could potentially happen and ignite your H2. I don't know how likely this is (you'd have to find some electrical engineers to ask) but you'd want to try and make sure electronics and power sources are insulated.
Static Discharge
A spark from static electricity could definitely happen during filling and let things go down in flames. You'd have to look into the materials you're working with and see what your risks are. I think filling the balloon on a super dry day and while wearing fleece might not be a good idea. However I'm no expert on static electric discharge.
Flames
Someone smoking or lighting a flame near balloon could set off your H2. This is pretty easy to control at your launch site.
My team just launched our first weather balloon and only used helium. We're considering H2 for launch 2 but there are several risks we are worried about. I am imagining that we may proceed with an H2 filled balloon if we can devise a mechanism to fill the balloon at a distance and reduce the number of people in its immediate range.
Hope this helps!
A: The gas won't ignite as long as the concentration is higher than UEL/UFL (upper explosive/flammability limit). So as long as there is no hole, you should be fine.
That said, I wouldn't play with hydrogen if I can use Helium instead !
