LP 95 vs. HP 100

[Luis H]

PACKER ENGINEERING INC, 1950 North Washington Street, Naperville, IL 60566

The results of that test can be very misleading if you don’t know what you are looking at. You need to be aware that the fatigue test (and normal hydro test) is performed in the material elastic region just under its yield strength. The permanent expansion in a hydro test confirms that.

If the tank is pressurized just above the yield strength…it will work harden the material in very few cycles and the burst (ultimate strength) will be much lower.

If you think that all 3AA tanks behave just like that test, we would never see any 3AA cylinder fail a hydro test. Granted, proper hydro test failures of 3AA cylinders are rare.

 

[Luis H]

3. Again, a 3AA tank is a 3AA tank; same specifications.

So despite different dimensions, it's still structurally the same?

Yes the ratio of the diameter and wall thickness are designed to meet the same structural and stress requirements on all 3AA cylinders. A smaller diameter cylinder can have much thinner wall and it can be structurally equivalent…same wall stress.

 

[DA Aquamaster]

But.............if you don't feel comfortable with all that, don't dive them by any means!

I choose to dive HP tanks pretty much because I can get a legal fill anywhere while maintaining a normal safety margin and get the same volume I'd get with an LP tank of similar dimensions and weight.

I also don't get bent out of shape with divers who push their Lp tanks to 3500 psi, assuming they are propery inspected. But I also don't advocate or recommend pushing them past that. 3800-4000 psi strikes me as pushing the envelope a bit too far and in my experience when you push the envelope too far in any endeavor someone eventually gets bit.

 

[marchand]

For the record, I dive LP120's normally slow filled to 3700. occasionally I have them filled to 4000, but most of the time I'm diving with buddies who are using 95's so the extra gas isn't necessary. Just in case you were wondering a set of LP120's holds ~360cf of gas at 4000psi.

 

[PR_SMITH]

The results of that test can be very misleading if you don’t know what you are looking at. You need to be aware that the fatigue test (and normal hydro test) is performed in the material elastic region just under its yield strength. The permanent expansion in a hydro test confirms that.

If the tank is pressurized just above the yield strength…it will work harden the material in very few cycles and the burst (ultimate strength) will be much lower.

If you think that all 3AA tanks behave just like that test, we would never see any 3AA cylinder fail a hydro test. Granted, proper hydro test failures of 3AA cylinders are rare.

Ok, so let's apply this information to the tanks mentioned in Marchand's post #64:

For the record, I dive LP120's normally slow filled to 3700. occasionally I have them filled to 4000, but most of the time I'm diving with buddies who are using 95's so the extra gas isn't necessary. Just in case you were wondering a set of LP120's holds ~360cf of gas at 4000psi.

Is the 4000psi mentioned above the yield strength of an LP120?

If yes, is the tank getting work hardened, and therefore lowering the ultimate burst strength?

1- HP hoses are rated to ( I believe) 5000psi, 25% over even a 4k psi fill.
2- There's no more stress on a regulator with a LP tank at 3500/3600psi than a HP tank.
3- Most burst disk these days are available in the 5k psi range, or just plug them.
4- Didn't OMS state their tanks could withstand 10,000 fills to 4000psi? I know it was something around those lines....
5- "It hasn't happened yet" is a pretty solid argument when you have almost 20 years of overfilling without incident.

How does that mean it might not happen at 21 years, 22 years, whatever?

But.............if you don't feel comfortable with all that, don't dive them by any means!

DA Aquamaster summed this up nicely with alot of pretty solid points in post #37

 

[Luis H]

Quote:

Originally Posted by marchand
For the record, I dive LP120's normally slow filled to 3700. occasionally I have them filled to 4000, but most of the time I'm diving with buddies who are using 95's so the extra gas isn't necessary. Just in case you were wondering a set of LP120's holds ~360cf of gas at 4000psi.

Is the 4000psi mentioned above the yield strength of an LP120?

If yes, is the tank getting work hardened, and therefore lowering the ultimate burst strength?

The 4000 psi is the hydro test pressure (for a 2400 psi cylinder), which by design it “should” be just below the yield strength of the steel in a 3AA cylinder. This assumes a perfectly good cylinder that has not lost any material due to corrosion and the heat treatment has not been compromised in any way.

Even then, material science is not a precise science. We always talk about a yield point, but in reality it is a region. It is just convenient to publish one number for yield strength, but anyone that has taken any strength of material classes knows that there are tolerances on any of this values.

If you are familiar with hydro testing there is a allowable amount of permanent deformation. But the allowed is very small. Not every spot on the steel will yield at the same stress, it varies.

Then there is another big variable in this situation, the actual measurements. If you notice the test was performed with very precise instrumentation that were carefully specified. This is very important since they are so close to the yield strength.

If you have seen a hydro test facility you will notice they use a very precise pressure gauge that is calibrated on a regular basis.

The gauge in most dive shops (or the ones we all own) are on the other hand probably good to within a few percent of the reading. What I mean is that the so call 4000 psi is actually somewhere between 3800 and 4200 psi. I know some think their own unit are better, but when was the last time they calibrated it against a precise lab instrument.

Another variable is going to be temperature of the tank after it leaves the fill station.

Most of the time is probably ok as shown by the experience in Florida, but I would never buy a used steel tank from Florida or anywhere near it.

I surely don’t want to be around when someone pressurizes a cylinder to its hydro pressure using compressible gas.

There is a reason there is a 6 or 8 inch burst disc on the back of a hydro test chamber…I had to replace it once in our hydro test facility back 30 years ago. I was not there when the tank split, but I heard about it. We use water for pressure testing for a reason…

 

[DA Aquamaster]

You need to be aware that the fatigue test (and normal hydro test) is performed in the material elastic region just under its yield strength. The permanent expansion in a hydro test confirms that.

If the tank is pressurized just above the yield strength…it will work harden the material in very few cycles and the burst (ultimate strength) will be much lower.

I think that sums up for me better than anything else why I have such an aversion to pushing a tank to its test pressure on a regular basis.

When you actually do a hydro test or even observe one being done, that little bit of water that still remains above the zero mark when all is said and done that represents permanent expansion of the tank has a psychological impact when you understand what it really means.

I wonder if a requirement to observe a hydro test, and an explanation of the expansion and permanent expansion that occurs, before allowing divers to request overfills would reduce the number of divers who want them - or for that matter the number of fill operators who will give them.

And Luis is right, hydro test equipment gets calibrated every 6 months. In contrast the average dive shop fill guage is probably lucky to be accruate with in 1-2% in the middle of its range when it's new and most never get recalibrated. How accurate it really is at 4000 psi after a few years of use is any body's guess.

 

[PR_SMITH]

All this over-filling business sounds more and more like Russian Roulette!

 

[Luis H]

5- "It hasn't happened yet" is a pretty solid argument when you have almost 20 years of overfilling without incident.

But.............if you don't feel comfortable with all that, don't dive them by any means!

I very much admire risk taking. I for one, very much like a good adrenaline rush.

I don’t consider myself an adrenaline junkie, but I have been very active in some serious white water kayaking, some rock climbing, a bit of ice climbing, sky diving, flying gliders, alpine tree skiing, etc. I even rode a motorcycle for a while…and IMHO that is the most dangerous thing I have ever done since I had no control of other drivers.

Note: I don’t consider Scuba diving a high risk sport…just a nice relaxing sport…it is a different type of rush.

In all cases when taking risk I like to think that I was taking a very calculated risk and I learned as much about it as I could. What I find sad is when someone takes “unnecessary” risk unknowingly out of ignorance.

If there is not an option…by all means take the risk, but do learn what it is really involved. If there is an option…taking unnecessary risk it just has no point.


OBTW…20 years may sound like a long experience to some, but for a steel structural item (like a cylinder) it is not. Keep in mind that the cave diving population is very small and 20 years is not really that long for a cylinder. We have pressure cylinders in service that are over 100 years old.

Heck, I don’t normally dive a Scuba regulator or tank that is that young.


Just because it hasn’t happen…well keep in mind, there is “always” a first time…for anything.

 

[ucfdiver]

How does that mean it might not happen at 21 years, 22 years, whatever?

They might. HP tanks might blow up in 20 years as well. Just because it has a stamp that says xxxx psi doesn't mean that nothing bad can happen.

As I said, if you're not comfortable with 10's of thousands of cave fills being done without incident, then by all means DON'T DO IT, but blindly ignoring statistics seems a bit odd.

All this over-filling business sounds more and more like Russian Roulette!

Of course it does, you're ignoring statistical evidence that would state otherwise. I could say the same thing about HP tanks.....they might blow up in 20 years. Also are you aware that OMS has stated their tanks can withstand 10,000 fills to 4000psi?

I very much admire risk taking. I for one, very much like a good adrenaline rush.

I don’t consider myself an adrenaline junkie, but I have been very active in some serious white water kayaking, some rock climbing, a bit of ice climbing, sky diving, flying gliders, alpine tree skiing, etc. I even rode a motorcycle for a while…and IMHO that is the most dangerous thing I have ever done since I had no control of other drivers.

Note: I don’t consider Scuba diving a high risk sport…just a nice relaxing sport…it is a different type of rush.

In all cases when taking risk I like to think that I was taking a very calculated risk and I learned as much about it as I could. What I find sad is when someone takes “unnecessary” risk unknowingly out of ignorance.

If there is not an option…by all means take the risk, but do learn what it is really involved. If there is an option…taking unnecessary risk it just has no point.


OBTW…20 years may sound like a long experience to some, but for a steel structural item (like a cylinder) it is not. Keep in mind that the cave diving population is very small and 20 years is not really that long for a cylinder. We have pressure cylinders in service that are over 100 years old.

Heck, I don’t normally dive a Scuba regulator or tank that is that young.


Just because it hasn’t happen…well keep in mind, there is “always” a first time…for anything.

I'm not using 20 years to compare cylinder age. The higher the fill, the more metal fatigue, the less time it lasts. I'm sure there's no one getting cave fills that would tell you otherwise. So what? If my tank lasts 20 years, that's fine with me.

Are we taking this risk out of ignorance? Really? Marchand gave you several links to prove it's been researched.

Also, weren't LP faber tanks first imported into the US as 200bar tanks, which would be around 3200psi?