[Closed] Hydrogen, Helium, Balloons & Fire Question

Think about what combustion actually is...

http://en.wikipedia.org/wiki/Flammability_limit

Hydrogen burns at 4% in air and explodes at 17% anything above these proportions in your helium mixture will still burn / explode (although not quite as readily, the helium will interfere with the combustion to an extent) there's still plenty of air around outside the balloon.

The helium wont stop the hydrogen and the oxygen from finding one another. You'd need a barrier of unmixed helium between the hydrogen and the air. So a balloon of hydrogen inside a balloon of helium, if both burst simultaniously theoretically helium would shield the hydrogen until they mixed

I use argon when I welding to blow a shroud of inert gas around the weld to stop oxygen getting to it, but you need to do just that, keep blowing pure argon on the site continuously to keep other gases away. If i mixed argon and oxygen and blew that on the weld the argon would be doing nowt to stop the oxygen getting involved

You are Billy Bastard the Evil Clown and I claim my free circus tickets!

Hm but - imagine the mixed gasses as an evenly spaced array of molecules - surely an Oxygen molecule and a Hydrogen molecule need to be adjacent to react? I'm thinking of calculating critical densities.

The molecules are moving about pretty rapidly though, once they are mixed they'll be in and out of proximity very frequently. And we're talking about a burn rather than a bang, theres time for things to find each other

Would the aeroplane take off from the conveyor belt if it was filled with enough hydrogen then, or would it being so light mean that the wheels wouldn't have enough grip?

I think Oli's on the right lines here. So, using his number:
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If we assume air is 89% nitrogen and 21% oxygen for ease of maths, ignoring the other stuff in it, then a 4% hydrogen mix would be 86% N, 20% O and 4% H?
So if we introduce helium into that, it would displace some of the hydgrogen. At, for example 50/50 the new mix would be 86% N, 20% O 2% H and 2% He, which means that the proportion of H is now below 4% and won't burn.
To get the hydrogen up to 4% again at 50/50 mixture we would have 84% N, 18% O 4% H and 4% He, is the proportion of oxygen now too low for the hydrogen to ignite at 4%?
Does that make sense?
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Druid, if you fill the plane with helium (or hydrogen) then it would be ligter than an air-filled plane and so would be more likely to take off at any given speed. Whether it could reach the required air-speed on a conveyor belt is a matter of some discussion, if the conveyor belt speed increased to always be equal and opposite to the wheel speed then it would never achieve any air-speed at all and so would not take off. I beleive there are a few here who disagree with that.

What a strange thread.

Druid, if you fill the plane with helium (or hydrogen) then it would be ligter than an air-filled plane and so would be more likely to take off at any given speed

But then it would crash because all the passengers and crew will have died of asphyxia. If it was helium the the black box flight recorder would be hillllllllarious though.

Is that why they have the drop-down oxygen masks? Does the expired CO2 get taken away by the mask somewhere or is it expelled into the cabin/cockpit, and if so how does this alteration in the composition of the helium/CO2 mixture affect the weight of the plane? Is the oxygen for the masks already on board in cylinders somewhere, ie. no net increase in weight, or is brought in from the air outside the plane somehow?
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[EDIT] Oli, is that 4% by mass or volume? And if anyone else who knows what they are talking about wants to contribute can we please assume 1 atmosphere and 20 degrees c.