What's in a hydro?

[guruboy]

First off, disclaimer - I am not a trained technician.

Second, this discussion should be directed at steel tanks only.

So what does a hydro test actually mean for the scuba industry?

My understanding is that a hydro test checks the tank for structural weakness. If the tank has too much permanent expansion (PE), that is a sign of the tank weakening. Too much PE = condemn. No one wants exploding tanks.

However, if a tank that fails hydro is tested again, it might "pass" the next hydro.. and next.. and maybe so on forever.. So does failing hydro really mean the tank is weak?

And what about all those people doing cave fills? A cave fill might very well be close to the pressure used in a hydro test. Who is to say that the cave fills aren't stretching the tanks to "failure" levels and it just so happens that the PE from the official hydro test is low enough to pass? Cave country isn't exactly filled with stories of exploding tanks.

Let's not forget that if you own your own compressor, I don't think it's illegal to fill them without a hydro.

 

[BRT]

Now you have opened a can of worms.

 

[joe10540]

This should be interesting

 

[guruboy]

Some good reading on hydro testing.

Hydrostatic testing...how much can the tank expand and still pass?

 

[runsongas]

the hydro is testing for elastic deformation vs plastic deformation for the steel. if you have plastic deformation, the tank will exhibit a larger permanent expansion. it is unlikely that it would pass on a retest, unless if the tank was borderline on the first test. eg it fails at 10.5% but might test at 9.5% the second time. in those cases, the prudent option is to replace the tank as it no longer has the same margin of safety as when it was originally manufactured. Failing the hydro test does show the tank has weakened as it means the elastic limit and yield strength of the steel has decreased.

cave fills are generally only done with 3aa tanks which are the lower carbon 3aa alloy. low carbon steel has a lower Young's modulus than the higher carbon steel used in 3442+ psi tanks. the lower young's modulus means that for a given percentage increase in elastic deformation, a lower percentage increase in material stress is exhibited and therefore the material will reach the yield limit slower. a high carbon steel may still have a higher effective yield limit, but you cannot overfill it as much on a percentage basis before it will hit its yield limit. this is why performing the same cave fill on a special permit tank with a higher carbon alloy will prematurely stress it more. this is also the reason why the test pressure is lower for special permit tanks than for 3aa.

cave filling will not cause a weak tank to pass hydro erroneously. the weakened tank will still exhibit a permanent expansion above 10%. cave filling does have an effect on obtaining the plus rating as the REE value is fixed based on the original tank volume. a tank that has been gradually stretched out will exhibit a larger expansion that may be within 10% but not within the REE.

a cave fill does qualify as a roundout procedure for the tank if it is galvanized. the reason for the roundout procedure is that the galvanization is frequently out of round. this causes the stress to be distributed unevenly for the zinc coating which causes a larger deformation in order to reach a rounded shape. Because the zinc is bonded to the steel, it causes the steel to not retract correctly. this then causes a false failure as the tank then appears to have expanded more than allowed. a retest or prestretch will remove this behavior and most hydro testers will perform a retest before condemning a tank.

this video from xs scuba goes over the hot dip galvanization issue

 

[guruboy]

Some good info. Thanks.

the hydro is testing for elastic deformation vs plastic deformation for the steel. if you have plastic deformation, the tank will exhibit a larger permanent expansion. it is unlikely that it would pass on a retest, unless if the tank was borderline on the first test. eg it fails at 10.5% but might test at 9.5% the second time. in those cases, the prudent option is to replace the tank as it no longer has the same margin of safety as when it was originally manufactured. Failing the hydro test does show the tank has weakened as it means the elastic limit and yield strength of the steel has decreased.

I agree back to back high PE is a good indication of weakness.

What would you say if a hot dipped galvanized 3AA tank (LP72) failed with 14% PE but then had only 1.4% PE on a re-test?

 

[guruboy]

the reason for the roundout procedure is that the galvanization is frequently out of round. this causes the stress to be distributed unevenly for the zinc coating which causes a larger deformation in order to reach a rounded shape. Because the zinc is bonded to the steel, it causes the steel to not retract correctly. this then causes a false failure as the tank then appears to have expanded more than allowed.

How long does it take a tank to become "round" once at 90% test pressure and how long does that effect last?

 

[runsongas]

Some good info. Thanks.



I agree back to back high PE is a good indication of weakness.

What would you say if a hot dipped galvanized 3AA tank (LP72) failed with 14% PE but then had only 1.4% PE on a re-test?

depends if a prestretch was done. if not, then this is not unexpected behavior due to the coating and the true value is the one from the retest.

 

[runsongas]

How long does it take a tank to become "round" once at 90% test pressure and how long does that effect last?

less than a minute iirc. not sure on how long the effect lasts exactly as generally you wouldn't test the tank again for 5 years.

 

[tbone1004]

Some good info. Thanks.



I agree back to back high PE is a good indication of weakness.

What would you say if a hot dipped galvanized 3AA tank (LP72) failed with 14% PE but then had only 1.4% PE on a re-test?

it was probably tested wrong if it does that much. Could have been a minute leak in the QD or the tank wasn't fully filled with water etc etc.

Typically the round out and relax takes less than a minute, as does the entire hydro process for that matter. Pump it up to 90% of test pressure *my hydro shop usually pumps it up to full test pressure IIRC*, let it sit for a few seconds to record the data, let it relax back to 0psi, then send it back up. They will put it on a different QD if there seems to be some weird problem going on as sometimes they don't seat properly