I have had a lp 72 steel tank for the wife for 10 years. Manufactured in 1973, it has always passed hydro without any problem, and last time I had it tumbled and everthing was fine. This time it came back Fail Hydro from Sports Chalet. They said the Pct_Exp was 14.2% so yes I understand why it would fail. (10% is max) They X'd out everything on the tank rendering it useless. But I was curious as to how it failed so bad this time. So I took the tank to another dive shop and they sent it out for a second opinion hydro, just to see, this time it passed @ 9.5 % exp (barely passed) But it did pass, and I could have used the tank another 5 years. I asked Sports Chalet about it and they said the second test wasn't accurate because they didn't have the report from the first test to compare. Is this normal?
Hydro on older tank
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Chugwhump
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Chug
It could happen.
captain
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Could be they didn't do the round out procedure before the hydro called for by PST technical bulletin D 100.
I would say the test sounds accurate to me, pre-rounding or not.
If the first test showed permanent elastic expansion to be 14.2 % and a second test showed 9.5%, that cylinder is bad.
If, it was in good shape and only needed a rounding prior to testing there would have been a much smaller % on the second test. With it coming out only a half percent from failure the second time around I would feel ok about the first test.
Example, 1st test 14.2% and retest 1.25% then I would have been suspect.
If the first test showed permanent elastic expansion to be 14.2 % and a second test showed 9.5%, that cylinder is bad.
If, it was in good shape and only needed a rounding prior to testing there would have been a much smaller % on the second test. With it coming out only a half percent from failure the second time around I would feel ok about the first test.
Example, 1st test 14.2% and retest 1.25% then I would have been suspect.
captain
Contributor
Luis here on the board got 3 replacement 72's out of the tester that failed them because the round out wasn't done. I wouldn't automatically blow it off as a bad tank, old 72' rarely fail hydro if done properly.
I've seen a steel cylinder fail hydro because there was a leak at the valve and slight pressure creep during the hydro test. Once the slight leak was found and corrected by the hydro technician, the cylinder passed. (To see the pressure creep, they had to hold the cylinder at an elevated pressure and that's when they saw the pressure loss).
It is unlikely that your cylinder failed the first test if it passed the second test. Makes me wonder if there was an error in the testing process during the first hydro.
It is unlikely that your cylinder failed the first test if it passed the second test. Makes me wonder if there was an error in the testing process during the first hydro.
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oxyhacker
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Actually, when you do two tests in a row on a steel tank, the first test ends up acting as the "prestress" cycle for the 2nd one, increasing the odds that it will pass the 2nd one. So the results of the second test prove nothing about the first. Measuring expansion is not a repeatable, non-intrusive procedure like, say, measuring water capacity or weight, where you can measure over and over and get the same results each time. When you hydro test a tank you expand it, and it doesn't return all the way, so the tank is changed a little each time, and will as a result behave differently each retest. It is not at all uncommon for a tank that barely passes one year to pass with a good margin the next.
This is why I get nervous when I hear a requalifier or dive shop say "Well, the tank passed but just barely, so you really ought to play it safe and junk it". The DOT standards have a perfectly adequate safety margin built in so there is no need to add more, and anyhow a tank which almost fails one hydro may pass the next 5 easily.
Hydrotesters, BTW, use a special calibration tank to calibrate their tester. It's a normal cylinder which has been deliberately grossly overpressurized, so it will expand hardly at all at normal pressures.
So you really don't have any grounds to go after the shop (though it is always worth scrutinizing the test sheet to be sure they didn't do something stupid).
This is why I get nervous when I hear a requalifier or dive shop say "Well, the tank passed but just barely, so you really ought to play it safe and junk it". The DOT standards have a perfectly adequate safety margin built in so there is no need to add more, and anyhow a tank which almost fails one hydro may pass the next 5 easily.
Hydrotesters, BTW, use a special calibration tank to calibrate their tester. It's a normal cylinder which has been deliberately grossly overpressurized, so it will expand hardly at all at normal pressures.
So you really don't have any grounds to go after the shop (though it is always worth scrutinizing the test sheet to be sure they didn't do something stupid).
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I agree to most of it, however, by pre-stressing the cylinder the second test is generally much lower in PEE thus my comments above. Two back to back tests over or near the 10% indicates to me that the cylinder was weak.
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