Cave Fills on LP tanks

[kelemvor]

How much overfill are you comfortable with for HP tanks? Why?

I don't overfill HP or Aluminum tanks. The former because AFAIK most fill stations can't or won't provide much more than the HP is rated for. Maybe 3600 on a 3442, so little difference as to be negligible imo.


Aluminum, I think it's just too risky. Those things do fail sometimes from what I've read.

That's not entirely true because the early "cave fills" were much less ambitious than today's "cave fills." The evidence for the safety of filling an LP steel to 3600 hot and letting it cool to 3200 or whatever does not go back 30 years, because that wasn't routinely done 30 years ago.



I'm a big fan of bringing more gas. I do it by using cylinders with a higher rated capacity.



The available data on cylinder ruptures is very thin because such events are rare. But it is not true that most of the risk is during fill, either in terms of the number of ruptures or the number of fatalities. Piecing together what anecdotal reports I can find, I conclude that about 25% of the risk is during the fill, and most of the balance of the risk is while handling the tank between the conclusion of filling and the beginning of the dive.

Interesting points, and I agree that bringing bigger tanks is the better solution. However, outside of those weird heiser 150's or bringing a fill station bank bottle, I don't think it gets too much bigger than what I've got. My overfilled LP108's get me more gas than an HP120 would.

If only rebreathers required less meticulous care and feeding this would be a moot point... maybe someday.

 

[BRT]

That's not entirely true because the early "cave fills" were much less ambitious than today's "cave fills." The evidence for the safety of filling an LP steel to 3600 hot and letting it cool to 3200 or whatever does not go back 30 years, because that wasn't routinely done 30 years ago.



I'm a big fan of bringing more gas. I do it by using cylinders with a higher rated capacity.



The available data on cylinder ruptures is very thin because such events are rare. But it is not true that most of the risk is during fill, either in terms of the number of ruptures or the number of fatalities. Piecing together what anecdotal reports I can find, I conclude that about 25% of the risk is during the fill, and most of the balance of the risk is while handling the tank between the conclusion of filling and the beginning of the dive.

Can you show some incidents of LP steels that were in hydro and viz rupturing? If not, where do you get your risk?

 

[PfcAJ]

That's not entirely true because the early "cave fills" were much less ambitious than today's "cave fills." The evidence for the safety of filling an LP steel to 3600 hot and letting it cool to 3200 or whatever does not go back 30 years, because that wasn't routinely done 30 years ago.



I'm a big fan of bringing more gas. I do it by using cylinders with a higher rated capacity.



The available data on cylinder ruptures is very thin because such events are rare. But it is not true that most of the risk is during fill, either in terms of the number of ruptures or the number of fatalities. Piecing together what anecdotal reports I can find, I conclude that about 25% of the risk is during the fill, and most of the balance of the risk is while handling the tank between the conclusion of filling and the beginning of the dive.

You think people weren’t overfilling tanks in the late 80s and early 90s?

 

[victorzamora]

You think people weren’t overfilling tanks in the late 80s and early 90s?

I'm surprised you didn't comment on "letting it cool to 3200psi". I'd be pretty pissed if I got a fill that short.

 

[2airishuman]

Interesting points, and I agree that bringing bigger tanks is the better solution. However, outside of those weird heiser 150's or bringing a fill station bank bottle, I don't think it gets too much bigger than what I've got. My overfilled LP108's get me more gas than an HP120 would.

HP130s are readily available and are essentially the same dimensions and weight as your LP108s. They are made with higher strength steel. Why not use them?

HP149 cylinders are also available, though most people consider them too large to be practical.

All the 8" diameter 3AA cylinders that have been widely used for cave diving are available in equivalent size shape and weight in an HP configuration today:

LP95 <==> HP117
LP104/LP108 <==> HP130
LP120/LP121 <==> HP149

I looked up comparisons in PST/Worthington literature a while ago. Faber specifications are very similar:

LP95 8" x 23.7" x 41.9# <==> X8-119 8" x 23.9" x 42.5# ... 95 cf at 2610 PSI
LP108 (W), LP104 (PST) 8" x 26.5" x 45.95# <==> X8-130 8" x 25.8 x 44.7# ... 108 cf at 2844 PSI
LP121 (W), LP120 (PST) 8" x 29.2" x 49.9# <==> Worthington did not spec out a 149 but the PST E8-149 was very close 8" x 29.37" x 47.5# (not clear if any E8-149 were ever successfully manufactured)

As for LP85s, well, I understand that some people like them for the way they trim out or for the smaller diameter for sidemount, but if your goal is just to bring more air, why not use an HP100?

 

[Caveeagle]

You think people weren’t overfilling tanks in the late 80s and early 90s?

How about the 70s? One of my favorite sets of doubles are BOD '71 104s I got from a cave diver who's name appears on a lot of the early survey maps in N.FL.

I am certain these tanks have seen plenty of action over the years and just passed Hydro middle of last year.

And to respond to a couple. Comments here.. I don't promote over fills for new divers. But I'm also not going to hold my comments on thread where this is directly on topic.

If the mods. Perceive a safety risk for the general public, it's well within their rights to move or remove it.

 

[2airishuman]

You think people weren’t overfilling tanks in the late 80s and early 90s?

You were there, you tell me. Other people who were there have told me that the typical pressure of a "cave fill" was lower then.

 

[kelemvor]

HP130s are readily available and are essentially the same dimensions and weight as your LP108s. They are made with higher strength steel. Why not use them?

HP130 (130@3442) filled to 3600psi = 135cuft of gas
LP108 (108@2640) filled to 3600psi = 147cuft of gas

I'd be carrying less gas with the HP130.

Am I doing the math wrong or are you suggesting a higher overfill pressure on the HP130?

 

[LiteHedded]

You were there, you tell me. Other people who were there have told me that the typical pressure of a "cave fill" was lower then.

HA! he's so old

 

[LiteHedded]

HP130 (130@3442) filled to 3600psi = 135cuft of gas
LP108 (108@2640) filled to 3600psi = 147cuft of gas

I'd be carrying less gas with the HP130.

Am I doing the math wrong or are you suggesting a higher overfill pressure on the HP130?

the hp130s are really more similar to the pst 104s than faber 108s