tank size and pressure

[JackD342]

I'm trying to do some research on tanks, if I'm ever in the place to buy some. I'm interested in high pressure steel 133s, the weight of large tanks doesn't bother me. The information I've found on one website for Faber tanks says "these are the same as Faber low pressure 108, just in high pressure". Does anyone know if that means that for a 133 tank it's just plus rated for now, and eventually will be de-rated to low pressure (i.e. 108)? Thanks for any help

edit: website where I found my info
133 Cubic Foot - High Pressure Faber Cylinder

In case this thread has not actually answered your original question(s):

1. That "same as" statement was made by a retailer and not by the manufacturer or the distributor. Take it with a grain of salt. Maybe the intent is "virtually the same" or "darned close to the same."
2. The specs on the Blue Steel (distributor) site do show the same exterior dimensions and the same internal volume for those two models, but a 1.7 pound difference in weight. So they are NOT the exact same tank just with a different stamp/rating.
3. No such thing as "eventually de-rated." Closest issue to that description is if you can't (or don't) get the "+" rating performed and stamped at a future hydro, then a shop that (rightfully) fills according to marked service pressures will not fill to the +10% pressure. Another hydro would be required to re-instate the "+" rating.

 

[BRT]

absolutely it does. at the same time there are relative time proven safe risks to take. that still does not make it right, only statistically safe to date, and that could change in a heart beat.

so goes the battle with the word intent and logic.

On the other hand if 5 overfilled steel tanks blew up tomorrow the odds would still be really good.

 

[victorzamora]

I had not read this thread, until just now, but perhaps I am wrong or someone else is. It's my understand an OMS LP108 is rated 108 cuft. at 2400 and 112 cuft. at 2640. You'll often see both volumes listed which makes it confusing.I believe this is not common as most LP tank volumes are rated at the plus pressure.

So marketing jargon and half-accurate math is how we get to the numbers we're at....and, unfortunately, none of it is clear. However, OMS sells the Faber LP108 as an LP112. I believe that's because they use "technical atmospheres" of 14.2psi instead of 14.7psi (half-accurate math, again).

Typical understandings in cave country are that LP tanks are rated at their volumes at 2640psi. So an LP108 is 108ft3 at 2640psi. Discounting van der Waal forces, that would give you 147ft3 at 3600psi. Unfortunately, it's likely to be closer to 135ft3 (depending on what constants you believe and how you do your math) as there is a fall off from linearity as pressures increase.

 

[2airishuman]

Your pushing too hard for a cause that does not impact you. Not sure why the push back - I read your posts and mostly your points seem reasonable.

The reason I push back is that overfilling is being presented as a safe, routine, appropriate, expected, and perhaps even necessary practice to relatively inexperienced recreational divers who are asking for general advice on cylinder choices. I don't agree and believe it is irresponsible to frame the issue in that fashion. I believe there is enough risk in overfilling to decrease the overall safety of recreational diving. People ask me why and I state the case. That's where we are.

As for you and other divers who have already made your own decisions in this area and disagree with me, that's fine. I'm not trying to change the way you dive or the way you fill your cylinders. In the same light I'm not going to tell you to use a helmet, wear your seatbelt, put on a PFD, or quit smoking.

 

[NAM001]

If there had been an accident or incident in the cave community we would know. Other more individualistic groups less so.



I'm going to push back there and say that:

• There is such a thing as an HP120. I have 7 of them. My average purchase price was under $200 each. Or you can buy new ones, DGX has them in stock for $399 which includes a VIP and a nice valve.
  
• If you don't like HP120s because they are too tall, well, there are HP117s; or if 120 cf isn't enough you can get an HP130.
• There is such a thing as an HP149, DRIS' web site says they have new ones, used ones come up for sale regularly

Now there are people who say that there favorite pet LPxx trims out just perfectly for them but that the HPyy of similar dimensions just won't but the reality is that if you want more gas, there are options out there that work for nearly all divers that will give you more gas without resorting to an overfill.



Well, using that line of reasoning, if the odds of a cylinder rupture are one in a million, look at what it does to dive safety.

According to DAN there are an average of 4.7 fatalities per million dives. Most people on SB believe that their own risk is lower because the 4.7 figure includes some number of medicals, perhaps as many as 50%, and includes fatalities in various high-risk groups such as people making their first 20 dives after certification, people making dives far outside accepted norms and practices, and so on. So for you and me and the OP and most other people on SB, the chances of any one dive ending in a fatality are probably closer to one in a million.

Now, if the odds of your cylinder rupturing are also one in a million, you've just doubled your wife's chances of collecting on your life insurance policy.

Which is my point. One in a million is not good enough because it's a still a relatively large avoidable risk in an activity that is largely very safe. One in ten million would be enough that there would be threads raised at A&I and a hue and cry for improvement. But we don't know, because there aren't that many overfills.

Your point is well taken. I dont think over fills are as uncommon as you may think. Most shops do not have trained tank monkeys. they fill everything to 3k which results in lp's being over filled and hp's underfilled. Now with the degree of overfill that is really being talked about,,, I believe your comment is probably true. What I am about to say next is probably true for many including my self. When organizations go overboard to be over protective by trying to achieve ZERO accident records, they loose credibility. This happened wiht the decision to require all tanks to be O2 cleaned if containing 23.5% O2. That is really stupid when the basis is that it could be achieved by PP blending which would initially pose a % greater than 60%. I know of no body that believes the 23.% rule is logically responsible and it is ultimately ignored by most. We never had pro POT laws until the economy went south and the states decided that taxes were more important than health and safety. How many illogical decisions are made not for the public interest but for sake of political votes are made. No nothing should be able to justify complying with regulations, but we have a nation that is taught to exploit the regs for personal gain. There is no other way to rationalize the act of blocking a highway and calling it protected speech. when credibility is lost compliance soon follows and thepeople decide for themselves as to what is right and wrong. When it comes to over filling ,, those with need to get extra gas need nothing more than to see that (not quoting real numbers) if in 10 years 4 lp tanks ruptured and 3 have been from band rustingdds are in favor that over protection is in play. It doesnt have to be rational ,,, Its human nature.

 

[NAM001]

On the other hand if 5 overfilled steel tanks blew up tomorrow the odds would still be really good.

You are absolutely right. then an investigation by PSI will happen and it may turn out that all tanks were perhaps worthington lp 104's made in a 6 month window. One only lhas to look at Luxfer for the reality of that happening.

 

[dberry]

Just out of curiosity, aren't there tanks that get filled to 300 bar (4350 psi) in Europe? Are they a very different breed of tank, or is it just a matter of different regulatory agencies?

I was under the impression that US DOT regs are more restrictive than across the big pond. Is that relevant in all this discussion about the safety of overfills?

 

[JackD342]

Just out of curiosity, aren't there tanks that get filled to 300 bar (4350 psi) in Europe? Are they a very different breed of tank, or is it just a matter of different regulatory agencies?

I was under the impression that US DOT regs are more restrictive than across the big pond. Is that relevant in all this discussion about the safety of overfills?

I am also curious to hear a recap (probably for the umpteenth time) of the specs and the fill pressures in Europe and beyond.

And, there is a 4350 available in the US, but capacity at service pressure us still just 106 cf.
Luxfer Limited 106 Composite Cylinder — XS Scuba

 

[2airishuman]

Just out of curiosity, aren't there tanks that get filled to 300 bar (4350 psi) in Europe? Are they a very different breed of tank, or is it just a matter of different regulatory agencies?

I was under the impression that US DOT regs are more restrictive than across the big pond. Is that relevant in all this discussion about the safety of overfills?

They are made under a different regulatory scheme but are roughly similar in design and safety factor to the special permit HP cylinders sold in the USA. The main difference is a thicker wall. 300 bar cylinders are negatively buoyant, for example, one spec I found listed a 12l 300 bar as being -7 pounds when empty, and have a capacity of around 120 cf. In contrast a 3442 PSI 120cf cylinder is typically neutral to -3 pounds empty depending on the manufacturer and where the wall thickness happens to fall within the production tolerances.

I believe a few 300 bar fiberglass-reinforced aluminum cylinders have been sold in the USA, but they never caught on. The PST/Worthington/Faber special permits specify a maximum pressure of 3500 PSI, so if Faber (for example) wanted to sell a 300 bar cylinder in the USA, they would have to get a new special permit, which could be costly. The Faber and Worthington special permits are essentially the same as the original PST one (I have a list of the differences somewhere), so all the regulatory heavy lifting to get the original permit approved was done by PST back in 1987. (The Asahi special permit is also very similar except for specifying spun construction).

Any given cylinder wall material can be made thicker to produce a more negatively buoyant cylinder with a higher maximum pressure. Most divers prefer cylinders that have slight negative buoyancy, which is why most 3AA steels are rated 3AA-2400. Throughout scuba history there have been other pressures used with different wall thicknesses, from 3AA1800 (very floaty) to 3AA-3130 (very negative), with the old LP72s (3AA2250) being neutral when empty. This same technique can be applied to the more modern, heat treated alloy steels which is exactly what is going on with the 300 bar cylinders.

 

[2airishuman]

(crossposted)

I am also curious to hear a recap (probably for the umpteenth time) of the specs and the fill pressures in Europe and beyond.

And, there is a 4350 available in the US, but capacity at service pressure us still just 106 cf.
Luxfer Limited 106 Composite Cylinder — XS Scuba

I believe they are no longer actually in production.