You might want to read this also:

http://warmkessel.com/jr/flying/td/jd/13.jsp (from avweb)

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Advisory circular
A/C 43.13-1A
Section 3 Miscellaneous Equipment
DOT regulations require ICC or DOT 3AA designation cylinders to be
hydrostatic tested to 5/3 their working pressure, every 5 years.
Cylinders bearing designation ICCor DOT 3HT must be hydrostatic tested
to 5/3 their working pressure every 3 years, and retired from service
15 years or 4,380 filling cycles after date of manufacture, whichever
occurs first. ================================================== =====
=========================================
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can anyone direct me to FAR's concerning general aviation oxygen ,
specifically inspection of oxygen bottles, life limit of bottles etc.
Thanks