PVHO (pressure vessel for human occupancy) Rules call for 25% corrosion allowance, ABS has always accepted ASME PVHO Guidelines. Best Regards: Phillip Janca Chairman HyperTec, Inc. World Leader in Hyperbaric Technologies 800-218-3588 940-564-5600 940-564-5609 Fax <A HREF="http://www.hypertec.ws">http://www.hypertec.ws</A>