[The following is an excerpt from a letter to Allen Meece]

[Updated 4 September 2002]

A good reference on the explosive limits (flammability limits) of gases is E. Meyers _Chemistry of Hazardous Materials_ c. 1989 from Prentice-Hall. It's a chemistry textbook for firefighters, and is full of information on what will burn and how to stop it.

I'm afraid I can't oblige you by providing specific references on hydrogen's flammability limits in air at 50mb. As far as I can determine, either no one has ever investigated the matter experimentally (an unlikely claim, given the existence of data down to 750mb, but one which appears in more than one source on the internet) or there are no common applications for that data to justify re-publishing it. If exact figures exist online, I can't find them.

That said, I realize that my earlier statement, that hydrogen is within its explosive limits over the entire altitude range of the platform, is misleading.

First, it's too inclusive. The gas inside the lift cells is >90% hydrogen by volume, which is above hydrogen's upper explosive limit (75% by volume at 1ATM). Given a spark, there is not enough oxygen inside the cells to sustain a flame inside the cell, even at sea level. For the hydrogen to burn, the flame has to start outside the cell where oxygen is available. This means that any hydrogen burning experienced at the platform is likely to begin at a rip, vent, or orifice where the hydrogen can get out and mix with the outside air.

Second, I can't support it with exact data at the extreme lower end of the VBP's pressure range. I know from anecdotes that hydrogen combustion is possible at similar pressures. For example, older model mass spectrometers used hydrogen instead of helium as their carrier gas at relatively low pressures (<100mb) and had problems with combustion inside the chamber even at operating pressure. So, I “know” it will still ignite basically because I know the oral traditions of my former professors.

The explosive limits of hydrogen are relatively constant between 0.75 and 2 ATM, and although it has to vanish eventually, the flammability range of hydrogen is quite large compared to other gases (from 4% to 75% as compared to, say, methane at 2% to 9%). This means that even if the range for combustion is dramatically restricted at low pressures, there should still be conditions in which ignition can occur. Reduced temperature doesn't dramatically affect the flammability range because it just raises the initial energy required for ignition without dramatically reducing the total potential energy available from combustion. The flammability range of hydrogen at 50mb is bound to be smaller than hydrogen at sea level, but so is the flammability range of methane at sea level, and methane is not generally regarded as inert. Lacking data to the contrary, it is logical to assume that a hydrogen/air mixture exists that will burn at 50mb.

But at this point I'm unable to say exactly what that mixture is. To do so will require a more extensive literature search. I'll see what I can do, but I'm sure it will require a trip to the local university library. I could be very wrong about this, but I don't think so.

For now, I assume that there's a fire hazard all the way up.

CME