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Those Crazy Cave Divers! or You Put HOW MUCH Air in There!?!


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#16

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Posted 22 December 2011 - 10:56 PM

Burst disks are the safety valve that will blow (theoretically) before the tank. That way you get a mad rush of air relieving pressure before hitting the tank rupture pressure.

So if you swap out the burst disks for higher rated ones you run the risk of a tank rupture if you over pressurize past the rating of the tank as the burst disk will not fail by design. Overriding the built in safeties is not a very safe thing to do in my book.



A tank can still fail even with a burst disk if you pressurize the tank to hydrostatic pressure multiple times. The concept of a hydrostatic pressure test is to take the material to near yield conditions TO SEE IF IT EXPLODES. This generally creates a hysteresis effect where the material does not go back to its original shape as soon as the pressure is relieved. Even getting close to yield can create a similar condition. While this in itself is not necessarily bad, bringing it back to yield afterwards can be since it can create a plastic condition (permanent deformation). Once the plastic condition sets in then any further filling to exactly the same pressure will not create any further permanent (plastic) set BUT any overfill beyond that will. That means if the tanks even heats up in the sun, it will overpressurize and plastically set even more. This cycle will continue until the ultimate stress is reached in the material and the tank ruptures. The strain required to go from yield to ultimate failure is very short for these materials. If this material state is ever reached then the tank will be very sensitive to the upper pressure. Badly calibrated fill gauges can be enough to do more harm to the material. Eventually, the tank can explode if it ever reaches this plastic condition even with a burst disk. This is why tanks have ratings. 10% overfill does not create a problem since significant margin remains but routinely taking it to hydrostatic pressures mostly likely will. Whether it lasts one year or ten years depends on the quality of the material, however, we have found through nondestructive testing and thousands of empirical tests that small voids may not be a problem until plastic set begins and microcracks reach that void. At that point it becomes unstable and goes all at once. The crack might take hundreds of hours to reach a microvoid but it when it does the whole tank will unzip at one time. So in short, you can get away with doing cave fills many times but that doesn't make it safe. It's kind of like driving drunk, first 10 times nothing happens so you think its fine but then that last time doesn't go so well.

Granted this will happen much sooner with aluminum tanks due to the lower yield strength but it can still happen with steel tanks.

We have this problem on airplane structures where we often times design parts to plastically deform at limit load (maximum load ever expected). This is to save weight and we know the part will never see a load greater than limit so we can get away with it but if the load does go slightly above then the part typically catastrophically fails within very few cycles. I run these models quite often and see it first hand. It doesn't matter if its an aluminum bulkhead or a steel fastener. They fail quickly when they exceed the load that made them go plastic. This is highly dependent on material quality and the maintenance. We don't even parts to rub on each other since even small scratches are crack starters at stresses well below yield. I am not sure how much control tank manufacturers have over materials and also any dents and scratches will greatly reduce life if the tanks are routinely filled to hydrostatic pressure.

Routinely taking a tank to hydrostatic pressure is insane IMHO, however 10% overfills are generally safe since there is significant margin left and you should never be approaching a yield state in the material with a mere 10% overfill.











#17 shadragon

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Posted 23 December 2011 - 06:19 AM

however 10% overfills are generally safe since there is significant margin left and you should never be approaching a yield state in the material with a mere 10% overfill.

By design the 10% overfill is only for tanks rated for it. As I remember from my DM course materials there is a + symbol by the pressure rating (usually on steel tanks) to allow 10% overfill until the first hydro. After that it is the rated pressure only. I had a pair of steel 120's in Canada rated at 3442 with a + symbol. So the shop could fill to 3780'ish. Lots of air, but the shop told me to keep them out of the sun just to be safe.

I do know what you are saying as I still call a 3300 PSI fill on an 3000 psi rated AL80 "a good fill". However, it does take its toll on the tank long term. :)



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#18 lv2dive70

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Posted 23 December 2011 - 06:43 AM

Lots of crazy people in cave country (including me, in reference to this topic at least :teeth: ). I pulled this from another thread, couldn't find original source (even after looking for it), but apparently OMS used to have in their sales literature "Guaranteed for 10,000 Fills at 4000psi"... it was literally on the paperwork for LP85's. There is a difference between the pressure the DOT approves the tanks for, and the pressure that the tanks can handle safely for several years without failing.
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#19

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Posted 23 December 2011 - 09:30 AM

Lots of crazy people in cave country (including me, in reference to this topic at least :teeth: ). I pulled this from another thread, couldn't find original source (even after looking for it), but apparently OMS used to have in their sales literature "Guaranteed for 10,000 Fills at 4000psi"... it was literally on the paperwork for LP85's. There is a difference between the pressure the DOT approves the tanks for, and the pressure that the tanks can handle safely for several years without failing.


Feel free to quote paperwork that is no longer in existence. Statements like this are usually pulled from marketing for a reason. I'll rely on my Master degree in Mechanical Engineering and years of experience as a structural engineer over marketing literature that was for some reason pulled from the market.

And what would that difference be? Please provide a stress strain curve correlated to DOT requirements in your answer.

#20 WreckWench

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Posted 23 December 2011 - 09:33 AM

Lots of crazy people in cave country (including me, in reference to this topic at least :teeth: ). I pulled this from another thread, couldn't find original source (even after looking for it), but apparently OMS used to have in their sales literature "Guaranteed for 10,000 Fills at 4000psi"... it was literally on the paperwork for LP85's. There is a difference between the pressure the DOT approves the tanks for, and the pressure that the tanks can handle safely for several years without failing.


Feel free to quote paperwork that is no longer in existence. Statements like this are usually pulled from marketing for a reason. I'll rely on my Master degree in Mechanical Engineering and years of experience as a structural engineer over marketing literature that was for some reason pulled from the market.

And what would that difference be? Please provide a stress strain curve correlated to DOT requirements in your answer.


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#21 grim reefer

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Posted 23 December 2011 - 09:49 AM

Burst disks are the safety valve that will blow (theoretically) before the tank. That way you get a mad rush of air relieving pressure before hitting the tank rupture pressure.

So if you swap out the burst disks for higher rated ones you run the risk of a tank rupture if you over pressurize past the rating of the tank as the burst disk will not fail by design. Overriding the built in safeties is not a very safe thing to do in my book.



The concept of a hydrostatic pressure test is to take the material to near yield conditions TO SEE IF IT EXPLODES.



Are you sure about that? I thought the purpose of the test is to see whether the tank can still safely hold its rated pressure. This is done by measuring the deformation of the tank when pressurized to 5/3 of the the DOT rated pressure. The hydrostatic pressure may or may not be anywhere near the pressure required to cause the material to yield (as Kate P pointed out). So a hydro test is not really trying to see if a tank explodes, but whether it deforms too much under load which would be a warning sign that the integrity of the tank had been compromised.

You clearly have a materials background of some sort. I do too but I don't have a specific background in pressure containment vessels, so everything I say here is just based on engineering "common sense."

Your comments below about hysteresis and latent defects are the basis for my preferring to run high pressure tanks with marginal overfill. Since I cannot know the safety margin in the design of the low pressure tank it's impossible for me to know how close to yield I am taking the tanks when I am overfilling them. The closer and more often, the more likely for plasticity to set in. At that point the tank is toast.

From the tensile strength testing that I did (too many years ago) most steel alloys once past yield stress have a rather large plastic range, while aluminum alloys, once past yield had a much lower strain to fracture. If I recall, Aluminum would tend to "neck" and then fail catastrophically.

Also, FWIW, the number of load cycles seen by a scuba tank is likely much less than the number of cycles encountered by parts in the aviation or automobile industry and there is not nearly as much benefit (weight savings, cost savings) in taking the material to the ege of it's capability. As, the old engineering maxim goes- "when in doubt, make it stout".

This generally creates a hysteresis effect where the material does not go back to its original shape as soon as the pressure is relieved. Even getting close to yield can create a similar condition. While this in itself is not necessarily bad, bringing it back to yield afterwards can be since it can create a plastic condition (permanent deformation). Once the plastic condition sets in then any further filling to exactly the same pressure will not create any further permanent (plastic) set BUT any overfill beyond that will. That means if the tanks even heats up in the sun, it will overpressurize and plastically set even more. This cycle will continue until the ultimate stress is reached in the material and the tank ruptures. The strain required to go from yield to ultimate failure is very short for these materials. If this material state is ever reached then the tank will be very sensitive to the upper pressure. Badly calibrated fill gauges can be enough to do more harm to the material. Eventually, the tank can explode if it ever reaches this plastic condition even with a burst disk. This is why tanks have ratings. 10% overfill does not create a problem since significant margin remains but routinely taking it to hydrostatic pressures mostly likely will. Whether it lasts one year or ten years depends on the quality of the material, however, we have found through nondestructive testing and thousands of empirical tests that small voids may not be a problem until plastic set begins and microcracks reach that void. At that point it becomes unstable and goes all at once. The crack might take hundreds of hours to reach a microvoid but it when it does the whole tank will unzip at one time. So in short, you can get away with doing cave fills many times but that doesn't make it safe. It's kind of like driving drunk, first 10 times nothing happens so you think its fine but then that last time doesn't go so well.

Granted this will happen much sooner with aluminum tanks due to the lower yield strength but it can still happen with steel tanks.

We have this problem on airplane structures where we often times design parts to plastically deform at limit load (maximum load ever expected). This is to save weight and we know the part will never see a load greater than limit so we can get away with it but if the load does go slightly above then the part typically catastrophically fails within very few cycles. I run these models quite often and see it first hand. It doesn't matter if its an aluminum bulkhead or a steel fastener. They fail quickly when they exceed the load that made them go plastic. This is highly dependent on material quality and the maintenance. We don't even parts to rub on each other since even small scratches are crack starters at stresses well below yield. I am not sure how much control tank manufacturers have over materials and also any dents and scratches will greatly reduce life if the tanks are routinely filled to hydrostatic pressure.

Routinely taking a tank to hydrostatic pressure is insane IMHO, however 10% overfills are generally safe since there is significant margin left and you should never be approaching a yield state in the material with a mere 10% overfill.


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#22 lv2dive70

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Posted 23 December 2011 - 03:27 PM

Lots of crazy people in cave country (including me, in reference to this topic at least :teeth: ). I pulled this from another thread, couldn't find original source (even after looking for it), but apparently OMS used to have in their sales literature "Guaranteed for 10,000 Fills at 4000psi"... it was literally on the paperwork for LP85's. There is a difference between the pressure the DOT approves the tanks for, and the pressure that the tanks can handle safely for several years without failing.


Feel free to quote paperwork that is no longer in existence. Statements like this are usually pulled from marketing for a reason. I'll rely on my Master degree in Mechanical Engineering and years of experience as a structural engineer over marketing literature that was for some reason pulled from the market.

And what would that difference be? Please provide a stress strain curve correlated to DOT requirements in your answer.


LOL,,, why so defensive? What difference does this make to you, you don't dive in N FL where this practice is common?

Anyway, for me, I'll take decades of experience of dozens of cave divers i trust, over academic theory as described by one person who doesn't cave dive, any day of the week and twice on Sunday. Scott, you know as well i do that i don't know what the h*ll a "stress strain curve correlated to DOT rqmts" is? :blink: . My masters is in business, not engineering. OH WAIT :idea: but I do happen to know a - literal - rocket scientist - and a literal - nuclear power plant manager - who dive tanks that are cave filled. Does that mean it *is* OK? :lmao:

Welcome to Singledivers grimreefer, we don't usually have quite this much fun! :hiya: Thanks for the better technical explanation than I could have hoped to thought about providing. :respect: :respect:

At the end of the end of the day, to each his own. If you've got your own compressor, you've probably got enough knowledge/experience to make your own choices. If you are getting gas fills at a shop, they are probably making your choices for you, unless you are in cave country, in which case if you DON'T want an overfill, you will most like have to tell them. The only other thing to watch out for is some tank monkeys don't recognize LP tanks as such, fill them to 3500, and when the owner does not realize this has happened and leaves the tanks in the car in the summer, obviously the tanks can overheat and the burst disks will blow (happened to a friend of mine last summer - but the tanks were FINE after she replaced the burst disks :teeth: )
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#23 grim reefer

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Posted 23 December 2011 - 04:42 PM

I have no problem with a shop that won't over fill, or divers that don't want to over fill. Rated pressure is rated pressure, after all.

But 50 years of field experience in North Florida have shown that in-hydro steel tanks exploding due to cave fills is just not a real world problem. Dirty Oxygen bottles on the other hand, do occasionally explode and have actually killed people. I find the laissez-faire treatment of Oxygen fills at some of those same fill stations to be alarming and I always stand at a distance and out of sight when I see someone filling an O2 bottle. I keep my O2 bottles clean and get them filled at a place that fills O2 inside a blast containment chamber. So I might not be as crazy as I thought :)

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#24 duganalexzander

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Posted 31 December 2011 - 02:57 PM

I love you guys.

#25 duganalexzander

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Posted 31 December 2011 - 02:58 PM

I love you guys.


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#26 KY_BOB

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Posted 29 January 2012 - 09:40 AM

I've never pushed the limits to 4000psi in a LP tank but I routinely have my double LP95's filled to 3600. I've been told that the main difference in LP and HP cylinders is the DOT stamping that makes it legal to transport a pressured gas under high pressure. So I may be breaking the law by hauling overfilled tanks but the metal is the same as HP cylinders. The only difference in what the manufacturer has to pay for that DOT stamping.


FWIW: I have a set of 1973 LP 72's (2250psi rating) that I fill to 2800-3000. Those tanks do not have the amount of metal in them that modern cylinders do. I'm probably stressing the hell out of the metal in them. When they fail hydro, I guess I'll quit using them and call it money well spent.

#27 KY_BOB

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Posted 29 January 2012 - 12:14 PM

however 10% overfills are generally safe since there is significant margin left and you should never be approaching a yield state in the material with a mere 10% overfill.

By design the 10% overfill is only for tanks rated for it. As I remember from my DM course materials there is a + symbol by the pressure rating (usually on steel tanks) to allow 10% overfill until the first hydro. After that it is the rated pressure only. I had a pair of steel 120's in Canada rated at 3442 with a + symbol. So the shop could fill to 3780'ish. Lots of air, but the shop told me to keep them out of the sun just to be safe.

I do know what you are saying as I still call a 3300 PSI fill on an 3000 psi rated AL80 "a good fill". However, it does take its toll on the tank long term. :)




There is no such thing as a "+" on a HP cylinder or an AL in the US, they are only on LP steel cylinders. That allows a 2400psi cylinder to be legally filled to 2640. Those of us that take them above that are doing it at our own risk.



...and for the burst disk thing, they are either replaced with 5250psi disks or plugged.

#28 NJBerserker

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Posted 22 April 2017 - 11:51 PM

Correct me if I'm wrong, but arent OMS/Faber LP steel tanks rated for 4,000psi outside of the US?


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