What year is the tank?
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I think I would take a used pressure tank to a test facility to verify its condition and ability to perform its assigned duties.
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I lean toward simplicity.
I go with five-gallon twenty-pounders for several reasons.
Here are just a couple:
a -- they are easily re-filled without manoeuvring the entire vehicle at the station
b -- they are re-certified by the supplier each time a fiver is exchanged.
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https://frangergas.com/how-to-recert...d%20are%20safe
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(If I am reading this right...)
Apparently, your cert is good for twelve years from date of manufacture.
Apparently, after the first twelve, your tank needs a re-cert every half-decade.
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With fivers, the test and certification is included in the exchange price.