This may give us a clue as to what changed...... (from LP Gas Magazine)....
DOT cylinder requalification rule to impact propane marketers
January 19, 2017 By
Megan Smalley
The
Department of Transportation (DOT) finalized a rule last year that affects the time frame for the requalification of propane cylinders and is set to take effect Jan. 23, according to the
National Propane Gas Association (NPGA).
The new rule impacts anyone who requalifies, fills, transports or prepares shipments of DOT cylinders. The changes were part of a
Pipeline and Hazardous Materials Safety Administration (PHMSA) rulemaking titled
HM-233F, which was published as a final rule on Jan. 21, 2016. HM-233F incorporated about 100 special permits into the federal hazardous materials regulations, NPGA says. The changes modify
49 CFR 180.209e of the hazardous materials regulations.
The rule reduces the initial requalification period for DOT cylinders from 12 years to 10 years from the date of manufacture. It reduces the period from 12 years to 10 years for cylinders requalified using volumetric expansion testing. However, the requalification period for DOT cylinders following a proof pressure test increases from seven years to 10 years. The five-year visual inspection method for cylinder requalification does not change.
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Link to source
(I'm not sure that's the whole story but it's likely the basic idea--- DOT rules changes)