Thread: Brown's gas??
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Posted to rec.crafts.metalworking
Andy Dingley
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Default Brown's gas??

On Thu, 08 Dec 2005 14:52:46 -0800, Tim Wescott

- There's some babble about a "cool flame". This is simply ridiculous.
The idea that it "radiates at 129C" is particularly silly. It's a
hydrogen-oxygen flame - they're hot, that's how they work. It's not
magic though, nor are normal rules of physics or chemistry suspended for
this magic gimmick.

That may be the one (partially) true claim, actually.

None of their claim, as I paraphrased it, is at all true. I don't just
shake words out of a Scrabble bag, I use them very carefully.

Hydrogen remains largely transparent as it burns, so it emits very
little light.

Indeed. But although it radiates _little_ light, the spectrum of that
which it does radiate follows the same Planck distribution as for
glowing soot.

There is no competent way in which you can equate the dim light of a
hydrogen flame with "radiation at 129C"

So if you looked at
the flame with an infrared pyrometer it would show up as not much warmer
than it's surroundings.

That's a fault with infrared bolometers. Better modern pyrometers (such
as those used for measuring gas temperatures) don't measure total
fluxes, they measure the ratios between fluxes at a number of different
wavelengths. Older ones (disappearing wire etc.) simply used a target
of a known heated material (usually a lump of firebrick) and looked for
the peak emission wavelength / colour.

bright yellow flames get that way because soot is black,

"Black body" radiation has _nothing_ to do with the fact that cold soot
looks black to human vision. Red ochre paint is a good simulation of a
black body radiator at gas flame temperatures and it will return to
looking just the same red colour when it's cool.