The govorning regulation is DOT CFR 49, Subpart D, 173.115(b).
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Basically, if a tank is above 41psia, its a restricted HAZMAT, but
if its below 41psia, its completely unregulated.
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For international flight rules, the DOT referred me to International 
Civil Aviation Organization (ICAO) 1995-1996 edition, chapter 2 on 
page 2-2-1, which is pragmatically identical to the above.  This does
not guarentee that each country doesn't have their own rules, but is
a general guide.
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I have the complete text of DOT CFR 49, Subpart D, 173.115(b).
online at URL: <a href="http://www.webspan.net/~hummer/flyingpony.html">
http://www.webspan.net/~hummer/flyingpony.html </a>
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This page also contains a DOT 800# to verify if its still current.
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-hh ( H NED Huntzinger )