Jim, Gary - did you guys read the following (FAA Order 8000.40D)?
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgOrders.nsf/0/ed3ccb2b0c40bdad86256abf006f380c/$FILE/8000-40.pdf

1. It applies to Part 91 aircraft, among others (para 1)
2. It says the FAA Adminstrator has adopted "the rules and regulations
of DOT, RSPA, USCG, UL, MIL-SPEC, and applicable manufacturers as
acceptable methods for controlling the hydrostatic tests and life
limits of pressure cylinders" (para 4d)
3. It says "pressure cylinders used aboard aircraft should be
maintained under the same specifications prescribed by the appropriate
regulatory agency and manufacturers if no other requirements are
available." (para 4e)
4. It says "Cylinders which have reached their hydrostatic test due
date cannot be recharged or installed until hydrostatically tested."
(para 6e)

This seems pretty clear - even if your oxygen tank is non-DOT approved,
you have to follow the manufacturer's specs and you have to have it
hydrostatically tested in order to use it in your aircraft.

-John

jphoenix wrote:
I think you got it right Gary. Those 180 and 173 rules are all about
transportation in commerce. However, most IA's will still want to see a
hydro date if that bottle is in your glider during your annual.

Jim