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SESK: Stuff Everyone Should Know · sesk.info/epa-dot-never-liquid-full

DOT rules for liquefied gas cylinders say the liquid must never do what at temperatures up to 131 F?

It may not completely fill the cylinder.

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  1. Fall below half of the cylinder
  2. Completely fill the cylinder ✔
  3. Touch the cylinder valve

Why: A liquid-full cylinder has no room for the liquid to expand when it warms. DOT requires a vapor space to remain. This is the idea behind the 80 percent fill rule.

Huh, didn't know that: DOT lists a few exceptions to this rule, including carbon dioxide and nitrous oxide.

The dad joke
A full cylinder has no sense of humor. It will pop at the first warm joke.
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References

Answer: Completely fill the cylinder. A liquid-full cylinder has no room for the liquid to expand when it warms. DOT requires a vapor space to remain. This is the idea behind the 80 percent fill rule.

  1. 49 CFR 173.304 Filling of cylinders with liquefied compressed gases (Cornell LII) www.law.cornell.edu/cfr/text/49/173.304
    the liquid portion of a liquefied gas may not completely fill the packaging
    Checked 2026-10-10.

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