Difference between pistons and diaphragms in High pressure tanks

[adm3745]

Hello,

I don't want to start an argument on overfilling..but sometimes for a long dive I run my LP Steels to 3800-4000. Any cave diver in Florida will understand why. Is it more harmfull to a diaphram regulator at these high pressures than it would be to a Piston regulator or is there no difference. Im currently on a Mk25 and really am unaware to how its hurting the Reg if at all. I justify it by saying its only above 3400psi for 10-20 minutes. Would a Diaphram be more susceptible to failure after repeating exposure to say 3800psi? (For DIN connections).

Thanks,

Adam

[Hallmac]

Hello,

I don't want to start an argument on overfilling..but sometimes for a long dive I run my LP Steels to 3800-4000. Any cave diver in Florida will understand why. Is it more harmfull to a diaphram regulator at these high pressures than it would be to a Piston regulator or is there no difference. Im currently on a Mk25 and really am unaware to how its hurting the Reg if at all. I justify it by saying its only above 3400psi for 10-20 minutes. Would a Diaphram be more susceptible to failure after repeating exposure to say 3800psi? (For DIN connections).

Thanks,

Adam

Well since you phrased your question as you did..... what does it matter?

You can't be smart one way and silly the other. The "cave fill" has gotten out of hand, the guys that started it had a fantastic understanding of what they were doing, knew why they chose the practice, and had service policies in place, including destroying the cylinders when they reached a certain amount of fills. Unfortunately the safety protocols and knowledge did not follow the basic principal. NOW, it is just a cheap way of taking more air along when your SAC is sick. Most who follow the practice have no knowledge of what they are doing. (OK, I have made my moral and ethical speech)


The diaphragm regulator will be cheaper to replace internal parts, Pistons generally run a bit higher in parts cost.

Most yokes are rated to 3750psi service with failures becoming common at 4800psi. As to the internals, most of todays regulators will handle 3800 psi.

[rjack321]

Any CE rated 300bar DIN reg is fine to even higher European fill pressures (like, 300 bar lol!). I wouldn't worry about the reg. But 4000psi in an LP tank is pushing even the boundaries of historical cave filling practices. Get a bigger tank, bring doubles, or both.

[halocline]

You often read that high pressure is tough on the 1st stage seat but I don't think that's actually true; the seat seals with IP, which determines how much force is acting on the seat. As long as the higher pressure gradient does not raise IP, there should be no effect on the seat.

What will be subject to more wear and tear are o-rings that separate supply pressure from ambient. In your MK25 that's the HP piston o-ring. It's protected from extrusion by bushings, but if you're worried about it you could get some .010 90 duro polyurethane o-rings. I use them in my MK5s and 10s, but those regs don't have the bushings.

The closest thing in a diaphragm reg would be the balance chamber o-rings, they're usually much smaller, and sealing supply from IP, so it's slightly less of a pressure gradient.

[adm3745]

Thanks guys...Hallmac, I've accepted that I may be degrading the life of my cylinder, but if I were seriously hurting my Mk25, I would probably scale back and just dive doubles...I dive LP 108's so I'd like to avoid the doubles as much as possible. As an engineer, I know the forces at work on the cylinder and that its "safe", but havn't gotten around to examining all the parts and seats in different regulators. As soon as I can afford the $400 HP tanks I'll prob upgrade to the HP 130's since their size and capacity is almost on par with the LP 108 tanks (something like 10 cf different).

[Hallmac]

I am glad to hear that you have the education back ground that allows you to understand the forces and physics of the practice.

As an Aerospace engineering student being able to predict the failure of the cylinder based upon the extended pressure could come in handy, depending upon your future projects. Aircraft have a number of metal fatigue inspections required due to stress.

For the record, you are degrading the life of the cylinder. The use of the words "may be" means that the outcome is unknown, unforeseeable, or debated. With metal that is not the case. It has a known fracture/fail point. When discussing DOT/CTC approved cylinders, that point has been demonstrated, recorded, and published before the cylinder is released. Metal is not self healing and does not like being stretched.

Sorry, I am just being a brat....well, that is not what others call me, but you get the drift...

Instead of doubles or dropping a load of cash on 130's, you could always sling a 40cf. Off the top of my head, lp 108 to 3750psi contains around 124cf of air. You might have to correct me there. If I am close, filling to specs and adding a 40cf pony would give you more air, redundancy, greater safety factor, and increase the coolness factor in your diving without the bulk and increased drag associated with doubles.

It would also relieve any worries about over stressing your equipment.

Being in Florida you should be able to find a second reg and a deco bottle at every garage sale you see.

At least consider it, and stay safe.

[adm3745]

You are right that metal is not self healing and I believe that even Hydro pressures (in this case 4400 I think) are just pushing the lower limits for yield strength. I usually keep these tanks at around 3300. I only will do 3800 once every couple months (If I really need the gas) so I feel safe in knowing that the Hydro tests will remain accurate and I do not have to throw away my tank because it's been deformed.

Anyway,

I already have it in the budget to get an AL 40 soon-but I was planning on using it for EAN 50/100. If I were to use it as an addition to my back gas, is it safe practice to plan that extra 40 cf into a gas plan? With doubles you can isolate a leak, but If I followed rule of thirds with the 40 included and a problem occured, I could be slightly short on air. I feel like I would either have to plan the 40 as contingency gas, and not include it in my 1/3 Travel so the true gains would be minimalized. I say this only because if I'm overfilling the 108 its because i'm going to be deep and always feel safer with more air. If I can carry an extra 50cf with no additional equipment (overfill), it makes me feel more comfortable. But again its less than occasionally I fill to these pressures.

The 108 at 3750psi holds 154 cf ideally. I like to plan with True capacity (Van der Waals).
On Air its about 145 cf, and 32% it holds around 148 cf.

Curse of the degree program, I like exact answers.

Thanks for the advice and concerns. Constructive criticism helps in many ways. Everytime I read an overfilling argument It reminds me to not get complacent with it.

[DA Aquamaster]

Basically you are talking about using an AL 40 as a stage. The thing to consider is that you still want at least a 1/3 reserve of your TOTAL gas on the dive in reserve in your back gas tanks.

So for example if you have 260 in back gas plus a single 40 cu ft stage for a total of 300, your end of the dive reserve is 100 cu ft / 1200 psi (assuming a 3600 psi fill). Consequently you'd figure your turn pressure accordingly to reflect 40 cu ft from the stage and 60 cu ft from the back gas, not the usual 86 cu ft, and you would turn at 3200 psi rather than 2400 psi.

I don't like exact figures and "exact" answers in cave diving as there are a couple near insurmountable issue with significant figures. 1) your SPG is probably accurate plus or minus 200 psi and worse it's accuracy will vary across its range. 2) Tanks are mass produced on really big equipment and exact volume will vary from tank to tank, even tanks of the same brand and size. So crunch away all you want with Van Der Waals equations but you are still measuring with a micrometer, marking with chalk and cutting with an axe.

Consequently, I prefer to use ideal gas laws, stay with simple ratios and calculations and just round everything up or down as needed to add some conservatism at each step.

From a rule of thirds perspective you are correct that an AL 40 would not do much as it would only add a total of 13 more cu ft for penetration.

Another issue to consider is that when you start adding a stage, even an AL 40, you are increasing your penetration and the distance and time you will need to exit in case there is a problem, so the bar starts to get higher once you actually start using the extra gas.

Another option I use in 2 person teams is to carry a buddy bottle with the additional "third" of reserve gas that a third person would have added to the team. My usual buddy and I have good SACs and good swimming speeds and due to the prentrations we could achieve on thirds our full cave instructor suggested the buddy bottle approach as a way to reduce the risk of a total gas loss failure near max penetration with a 2 person team.

[adm3745]

I like the idea of the buddy bottle. Invariably I think that I should just stop trying to stretch single tank dives, and just do the doubles for the long/deep ones. I'm hoping that

Also, the only reason I continue to use Van der Walls is because it proves that I actually have less gas. If the equations went the other way I would not use them. It is just one way that I know I've added another level of conservatism.

Lastly, the math you did for turn pressure with the stage...They way you worded it you'd have to breath the whole 40 cf bottle on the start of penetration right? If you were to hold on to it or leave it on a line and breath it at the end of the dive, would you have to plan for if that tank failed and effectively just plan the dive for your back gas?

[rjack321]

The hydro test pressure of a lp108 is 4000psi, basically what you are filling it to. As long as you are cool with it probably only lasting 10 or 15 years vs. 80+yrs then I really wouldn't get tooo worked up about 3800psi fills.

But I can't believe you are talking 1/3rds, bringing an al40 as a bottom stage and all these schenangins for recreational dives. Talk about convulted and complicated.

You can get a pair for double 95s used in FL for $500 or maybe even a little less - that is cheap. A set of double 72s for probably even less. Still too expensive?? Then maybe more advanced diving at the recreational margins isn't for you.