Empty Oxygen Cylinders
Pipeline and Hazardous Materials Safety Administration · Sep 16, 1997 · Reference No. 97-0141
Interprets: 49 CFR § 173.29(b)(2)
Reggie’s quick take
Someone asked how completely empty oxygen cylinders must be purged to fall outside the Hazardous Materials Regulations. PHMSA said oxygen is a Division 2.2 gas and is covered only when cylinder pressure is at least 280 kPa (40.6 psia) at 20 C (see § 173.115(b)(1)). A cylinder with pressure below 280 kPa is not covered and counts as purged for § 173.29(b)(2)(ii).
Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.
This is in response to your telephone inquiry regarding empty cylinders which previously contained oxygen. You asked about the extent to which such empty cylinders needed to be purged (see 173.29(b)(2)(ii)) in order for them not be subject to the Hazardous Materials Regulations.
Oxygen is a Division 2.2 gas and as such is only subject to the regulations when the pressure in the container (cylinder) equals or exceeds 280 kpa (40.6 psia) at 20 C (see 173.115(b)(1)). Therefore, oxygen cylinders where the pressure has been reduced to less than 280 kPa are not subject to the regulations and are considered to have been purged to the extent necessary for the purposes of 173.29(b)(2)(ii).
A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.