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Chemistry 101.
Heat up any element and you force the electrons into a higher "orbit" or energy state. Eventually, that electron will
fall back to its natural state, emitting a photon of a characteristic wavelength in the process. White light is a
combination of all the available wavelengths combined. Split out all the wavelength and you get a rainbow. If the
light comes from burning a particular set of elements, that rainbow will exhibit "bands" or bright stripes of one
particular color (ie, wavelength). Every element on the periodic table can be identified by a particular set of bands
that are more accurate than finger prints. Astronomers use this phenomena to accurately identify what elements are
burning in a distant start.
Carbon has a distinctive fingerprint that is very different from the iron, nickel and chromium in a glowing exhaust.
See page 3 of
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CGAQFjAG&url=http%3A%2F%2Fwww.tempe.mi.cnr.it%2Fzizak%2Ftutorial%2Fcairol06-flame-emission.pdf&ei=t-QbUMenOczWiAKR6oGYAg&usg=AFQjCNHah9qiwVWEvkBeVo1YXXaTxnpWgA&sig2=_nZhVPA8PD3CiUNUe4Rn7A
To build a reliable flame detector, you'd build a sensor that would respond to wavelengths of a particular band that is
characteristic of carbon, and then add an IR detector that just confirms there is a lot of heat present. The article
even suggests that it would be possible to tell the difference between an atmospheric gas flame and a welding flame,
because of the nitrogen in the air.
Dave wrote:
I would say that the glowing red headers wouldn't set EITHER off. The
NEWER ones are less likely to be fooled by a welders arc.. remember,
these devices are meant to be used in refineries as fire detection
triggers to deluge sprinkler systems..... welding and other hot work
happens in that setting.
I remember at a fly in at Pecan Grove about 8 years ago or so, someone
was presenting about the detector and we were ALL surprised when in the
middle of the presentation the detector "detected".... and we all
scratched our heads until I noticed a propane radiant heater with a
sunflower style burner in the corner 40 feet away was our source.
The detectors use UV spectrum for flame/combustion detection. They use
IR for crosscheck/validation, not primary detection. Thats my laymans
understanding of it.
Dave
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