"Cave Fills"

[The Chairman]

Besides your "feelings" can you provide *any* evidence?

It was covered in my material sciences class at UF. It's even given a blurb in wikipedia. Yield (engineering) - Wikipedia, the free encyclopedia Like gas laws, Hooke's law is an "ideal". That means while it's great to describe certain mechanisms it doesn't describe all the elements at play. Feel free to try to slap me down again. It's not my fault that you don't know this.

 

[cool_hardware52]

It was covered in my material sciences class at UF. It's even given a blurb in wikipedia. Yield (engineering) - Wikipedia, the free encyclopedia Like gas laws, Hooke's law is an "ideal". That means while it's great to describe certain mechanisms it doesn't describe all the elements at play. Feel free to try to slap me down again. It's not my fault that you don't know this.

Not sure why you continue to find it necessary to suggest I'm ignorant or don't understand the concept.

How do you reconcile that with your call for less hate?

Pete, that a theory was mentioned in college course doesn't make it *fact*

Even if this theory has been proven, which it has not, it does not prove that it is in play WRT scuba tanks.

You have yet to provide *any* evidence to support your contention.

You first made an absolute declarative statement regarding "work hardening" in post #3

Hey, I love the fact that my LP120s have 180 cf of air in them, but they've got to be work hardening as they are being pumped so high.

Then we learned that you don't understand the process

Tobin, I certainly don't understand the process,

Then we learned it's a "concept" that "suggests"

There is a concept of "True Elastic Limit" which suggests that small plastic deformations happen at relatively low pressures and are cumulative.

Then we learned your original absolute declarative statement is based on your feelings

but I feel that can account for hardening and fatigue over time.

I'll ask again, do you have *any* evidence to support your feelings?

The experiment I would design to investigate your theory of work hardening at hydro pressures in 3AA tanks would be as follows:

Get two new tanks with sequential serial numbers.

1) Keep one as a baseline.

2) Continuously cycle the other to the test pressure until failure

3) Do a metallurgical examination of the two tanks. Work hardening should be easy to identify if it occurred.

Why hasn't this already been done? I'd suggest because step two would be nearly infinite…….

IIRC OMS used to claim 10,000 cycles to 4K for their 2400 tanks *without* failure.

Let's assume it would take 50,000 cycle to generate a cycle. This is a total guess on my part.

If each fill empty cycle requires only 5 minutes one would need about 6 months around the clock to reach 50K cycles.

It also suggests that "routine" unexplained cylinder failure is infrequent.

Tobin

 

[Charles2]

Please stop your bickering before this turns into a split-fin vs. Air-Integration debate. At great personal risk, let me make a few comments:

A hydro is evaluating if the tank suffers permanent deformation at the test pressure.

-- Not quite - should have said "A hydro is evaluating if the tank suffers a permanent deformation of more than 10% of the elastic deformation after the release of the test pressure"

Permanent deformation does not require work hardening.

-- Not quite - should have said "Permanent deformation usually results in some work hardening"

a tank that passed a retest or hydro has been stretched, but not permanently deformed, i.e. it is able to return to or very close to it's original volume.

-- Not quite - Any material that has not returned to its original shape after being stretched is permanently deformed.

I certainly don't understand the process

-- Agreed

How does one "work harden" a material without plastic deformation?

-- Agreed - see comment above. "Permanent deformation usually results in some work hardening"

I do know that Hydro tests are evaluating work hardening in a tank

-- Not quite - but there may be some relationship.

I haven't met the engineer who can adequately describe the failure process for me.

-- I have.

A couple of additional facts:

• Aluminum alloys generally strain harden (work harden) at a greater rate than ferrous alloys. This alone would be reason enough for not "cave filling".
• The specifications for the manufacturing of compressed gas cylinders contain a sufficient factor of safety so that inadvertent, moderate overfilling will not present a significant hazard over a reasonable lifetime; however, continual overfilling to a pressure that is greater than 90% of the test pressure will certainly shorten the active life of the cylinder and at some point present a hazard to the owner.

 

[cool_hardware52]

A couple of additional facts:

• Aluminum alloys generally strain harden (work harden) at a greater rate than ferrous alloys. This alone would be reason enough for not "cave filling".

Agreed. Aluminum is a very different animal

• The specifications for the manufacturing of compressed gas cylinders contain a sufficient factor of safety so that inadvertent, moderate overfilling will not present a significant hazard over a reasonable lifetime; however, continual overfilling to a pressure that is greater than 90% of the test pressure will certainly shorten the active life of the cylinder and at some point present a hazard to the owner.

What is the test pressure for a LP 2400 psi tank with a "+" rating?

What is 90% of that value?

Tobin

 

[oldschoolto]

 

[cool_hardware52]

Would have been far more exciting if it was full of gas and not water, makes the point why retesters "hydro" tanks. My water jet produces pressures a bit over 60,000 psi, but a piping rupture, which is not uncommon, results in an immediate drop in pressure because water is only slightly compressible. The HP tubing typically fails just like the tank, it splits along the length of the tube resulting in a "fan" of water. Fortunately it's not very dangerous.

Remember this is an Aluminum cylinder not the 3AA Steel LP tanks central to this discussion.

Tobin

 

[RJP]

Typically cascading banks are filled to the end-state pressure, be that 3000, 3600 or other pressure.

I see them closer to 4,000 in Cave Country.

4,000 here in "wreck" country too.

If the bank was 3,000 PSI, you'd never be able to fill an AL80. If 3,600 you'd never be able to fill an HP tank.

 

[halocline]

The normal protocol for hydro is 5/3s of a tanks pressure.

2,400 is pushed to 4,000
3,000 is pushed to 5,000
3,440 is pushed to 5,750

The 5/3 of working pressure is only on 3AA tanks. The SP tanks (3442) are tested at 150% of working pressure. (5163 PSI) These are by far the most common HP scuba tanks. If they were tested at 5/3, they'd likely fail every time. Something to consider with cave fills; they are certainly well documented successful on the LP 3AA tanks. Not sure how I'd feel about a SP tank being filled to test pressure.

 

[The Chairman]

The SP tanks (3442) are tested at 150% of working pressure. (5163 PSI)

Good to know. I'm not a hydro tech, so I've only read the 5/3s from generalized literature.

 

[cool_hardware52]

The 5/3 of working pressure is only on 3AA tanks. The SP tanks (3442) are tested at 150% of working pressure. (5163 PSI) These are by far the most common HP scuba tanks. If they were tested at 5/3, they'd likely fail every time. Something to consider with cave fills; they are certainly well documented successful on the LP 3AA tanks. Not sure how I'd feel about a SP tank being filled to test pressure.

The "safety margin" on 3AA tanks is quite a bit wider than SP tanks. Unlike some here, I have not promoted, encouraged, celebrated or advocated overfilling *any* cylinder, but if I had to choose between a SP tank and a 3AA to fill beyond the rated pressure the choice would be easy.

Tobin