Wassup with Cave Fills?

[tbone1004]

While we're asking the cave fill questions:

I understand the somewhat accepted practice of routinely overfilling LP steel tanks, but what's the thinking on overfilling HP steels? Especially tanks like the PST 3500's which are relatively thin? Are HP steels rated with the same conservatism as LP steels or less?

what we call "HP" tanks are built to different engineering standards vs. the 3aa or 3al standards. The 3aa and 3al standards call out a 5/3x test:working pressure ratio vs. a 3/2 ratio for what we call "hp" tanks whether they are 3442 or 3500 as they are built under special permits from the DOT that are unique to each manufacturer with regards to the alloys used. All of them have to pass cycle counts to test pressure in the thousands or to working pressure in the tens of thousands.

The practicality of overfilling HP tanks though is where it is often not worth it because of a combination of Z-factor, cost of running the compressor *it costs more to full to 5000psi in electricity than it does to 3000*, and the fact that the majority of high pressure banks are at 4500psi as a function of the SCBA industry until very recently when they've gone to 6000psi compressors.

When we look at cave fills where we want something easily divided by 3, banks that are filled to 4500psi, tanks that are rated to at least 10,000 cycles to test pressure, and factoring in p1t1=p2t2 for the tanks cooling down then filling to 4000psi hot and cooling to 3600psi is about as convenient as it gets.

It should be noted that the fill pressures are directly proportional to temperature and if we are staying within the letter of the law, so long as the tank cools to its working pressure when it is stabilized at ~70F, then it is not an overfill. This is a fun argument when ignorant dive shop owners will set a regulator to 3000psi and constantly short fill AL80's because they don't understand that they can in fact fill them to 3500psi so long as they cool to 3000, but that's a separate issue.

I wonder if they stop at 3000 because that is all their compressor (optimized for AL80s) will do?

I'm not aware of any dive shop sized compressors made in the last ~40 years that are incapable of filling to at least 4000psi comfortably. The compressors are designed for the SCBA filling and have used 4500psi banks for a very long time. Prior to that with low pressure banks the pumps may have stopped at 3000psi or so but anything made in most of our lifetimes are designed to fill 4500psi banks. If they flat fill everything to 3k it is usually done out of ignorance/laziness with not adjusting the regulator and has nothing to do with the compressor itself.

 

[tursiops]

I'm not aware of any dive shop sized compressors made in the last ~40 years that are incapable of filling to at least 4000psi comfortably. The compressors are designed for the SCBA filling and have used 4500psi banks for a very long time. Prior to that with low pressure banks the pumps may have stopped at 3000psi or so but anything made in most of our lifetimes are designed to fill 4500psi banks. If they flat fill everything to 3k it is usually done out of ignorance/laziness with not adjusting the regulator and has nothing to do with the compressor itself.

Thank you. You just explained what I meant by "optimized."

 

[NW Dive Dawg]

So why are cave fills not available for open water, NDL divers? For the most part, because they really aren't needed. If you are staying within NDLs, getting that extra gas shouldn't do you a whole lot of good.

I get that John and it makes sense...... but as long as some shops turn a "blind eye" are OK overfilling steel tanks then why shouldn't all of them do it. I think we're all generally happy when we get a "nice" fill. Who doesn't want extra gas on any dive! For the cave folks.......do these shops charge extra for these "overfills"...

I guess the other component here might be that OW operators want divers back on the boat because time is money. Also, they probably don't want inattentive "occasional" divers getting into to deco trouble..

Anyway, I was just curious. As for the question about HP steels, when I was teaching and managing a shop we would regularly fill HP Steels to 4K on a hot fill so that they would cool down to 3,600 - 3,700..

 

[tursiops]

do these shops charge extra for these "overfills"

Yes...because most charge by the cubic foot.

 

[Marie13]

Yes...because most charge by the cubic foot.

Depends on where you’re getting your fills. Where I’ve gotten fills in cave country, yes, by cft. Midwest? Flat fee for air/nitrox.

 

[NDonahue]

This thread is going to be like the script of a Monty Python movie:

/BEGIN LOOP

FL Diver: ”We‘re careful divers who don’t encourage the normalization of deviance”

Other Divers: “But isn’t overfilling an example of the normalization of deviance?”

FL Diver: “No, because we’ve been doing this for a long time, so we think it’s OK”

Other divers: “But isn’t that the definition of the normalization of deviance?”

/END LOOP

Repeat Loop Ad Nauseum…

 

[Lorenzoid]

Other Divers: “But isn’t overfilling an example of the normalization of deviance?”

FL Diver: “No, because we’ve been doing this for a long time, so we think it’s OK”

Could it be that the "deviance" is the act of gathering data, and when enough data has been gathered, it's not normalization of deviance but rather persuasive evidence that the norm was incorrect to begin with? For example, if there were no incidents in 100 overfilled tanks, maybe that could be normalization of deviance, but if there were no incidents in 100,000 overfilled tanks, maybe it's okay to draw a new conclusion from that about where the safety norm actually lies?

 

[berndo]

... then why shouldn't all of them do it.

He answered it in the post you quoted. You don't need it for OW dives.

BTW: Maybe it's not that smart to talk publically about what shops overfil tanks. Cave fills are done in other places than FL too and people appreciate when you don't run around advertising it publically.

 

[rob.mwpropane]

Idk what's up with everywhere else.. but here's what works for me.

HP - 4000 (cools to 3600)
LP - 36-3700 (cools to 3300)
Al - 33-3400 (cools to 3000)

I'll fill less if it's a more benign dive, but these are my max's that I'm comfortable with and work well.

 

[tursiops]

This thread is going to be like the script of a Monty Python movie:

/BEGIN LOOP

FL Diver: ”We‘re careful divers who don’t encourage the normalization of deviance”

Other Divers: “But isn’t overfilling an example of the normalization of deviance?”

FL Diver: “No, because we’ve been doing this for a long time, so we think it’s OK”

Other divers: “But isn’t that the definition of the normalization of deviance?”

/END LOOP

Repeat Loop Ad Nauseum…

Good point.

Could it be that the "deviance" is the act of gathering data, and when enough data has been gathered, it's not normalization of deviance but rather persuasive evidence that the norm was incorrect to begin with? For example, if there were no incidents in 100 overfilled tanks, maybe that could be normalization of deviance, but if there were no incidents in 100,000 overfilled tanks, maybe it's okay to draw a new conclusion from that about where the safety norm actually lies?

Better point.