LP 95 vs. HP 100

[Scott M]

And I was happy getting just under 3600 in my HP100's.

 

[stairman]

I got a sweet fill of 4200 once in my lp95's.We got to talking and...well my burst discs are doubled up 5250's so no worries.Thats alot of pressure on my Atomics though.I feel certain that a neck ring would perforate before the steel ruptured.I still stay below 3400psi unless Im heading to the water.

 

[PR_SMITH]

I got a sweet fill of 4200 once in my lp95's.We got to talking and...well my burst discs are doubled up 5250's so no worries.Thats alot of pressure on my Atomics though.I feel certain that a neck ring would perforate before the steel ruptured.I still stay below 3400psi unless Im heading to the water.

There is a lot of talk here about overfills; this is the first mention of the effects on the first stage.
Weight considerations aside, is it not easier on the equipment to use LP tanks, rather than push everything towards the limits? Not to mention the ease of getting consistently full fills? Especially on boat fills?

 

[dannobee]

There is a lot of talk here about overfills; this is the first mention of the effects on the first stage.
Weight considerations aside, is it not easier on the equipment to use LP tanks, rather than push everything towards the limits? Not to mention the ease of getting consistently full fills? Especially on boat fills?

Your last sentence brings up an interesting point that rarely gets addressed. I don't know about Jersey, but here on the other coast, getting the boats to fill to 3000psi is difficult enough, 3500 in nearly impossible. Having the LP95 allows you to put a label on the tank saying "fill to 3000 psi," guaranteeing a decent fill (it'll cool down to 26-2700psi).

 

[Jimmer]

Your last sentence brings up an interesting point that rarely gets addressed. I don't know about Jersey, but here on the other coast, getting the boats to fill to 3000psi is difficult enough, 3500 in nearly impossible. Having the LP95 allows you to put a label on the tank saying "fill to 3000 psi," guaranteeing a decent fill (it'll cool down to 26-2700psi).

So why not dive Hp119's or Hp130's, and when you get underfilled, you still are diving with the same, or more gas as an Lp95 filled to that pressure, but you aren't pressing the safety factor of the tanks, and you can still get 3500PSI fills at the shops that refuse to overfill Lp tanks.

 

[Dive-aholic]

Way back when a cave fill was 2900-3000 psi - about 120%-125% of the service pressure, then it was 3400, 3500 then 3600ish. Now you are indicating it has creeped al the way up to 4000?

You do of course realize that 4000 psi is the test pressure for a 2400 psi tank? There is over filling then there is "crazy" then there is "friggin nuts". Carrying a tank around in a state of perpetual hydro test is pretty firmly in the "friggin nuts" category.

I understand the "everyone does it and no one has gotten blown up yet" argument, and is all great fun - until someone dies to make the point that the prudence got left behind long ago.

I have this interesting mental image of an overfilled LP 95 detonating at the start of a dive with a resulting chain reaction of similarly overfilled LP 95's sympathetically detonating through the whole team...

They do make larger tanks you know...

I'm not sure where marchand is getting fills, but where I'm at, 3700 is about the norm. It's a nice slow fill that cools down to 3600. I've been to a few cave shops and haven't come across any that regularly fill to 4000.

 

[marchand]

Cave East and Amigos normally. I heard that Wayne at Amigos has filled tanks to 4500ish psi. Of course that could of been a nice slow fill that sat in the back of a truck in the sun during the summer.

 

[Stu S.]

I got a sweet fill of 4200 once in my lp95's.We got to talking and...well my burst discs are doubled up 5250's so no worries.Thats alot of pressure on my Atomics though.I feel certain that a neck ring would perforate before the steel ruptured.I still stay below 3400psi unless Im heading to the water.

I'm sure the dive shop owner thought I would be pleased.

I opened the valve and the gauge went up to 3950. More gas than I expected. Within seconds, the HP hose expanded near the first stage and... bang!

There are other things that could have failed. If a burst disc or o-ring had failed, it would have done so in the dive shop, instead of giving me a surprize on that boat.

 

[marchand]

I'm surprised a HP hose would fail at such a low pressure, all of mine are rated to 5000psi. How old was that hose?

 

[DA Aquamaster]

I got a sweet fill of 4200 once in my lp95's.We got to talking and...well my burst discs are doubled up 5250's so no worries.Thats alot of pressure on my Atomics though.I feel certain that a neck ring would perforate before the steel ruptured.I still stay below 3400psi unless Im heading to the water.

While calculating thirds in the water, 3600 PSI is a good number. Obviously, in order to get that in 70 degree water, you need slightly more when leaving the fill station. That's pretty much standard in cave country. However, if I'm doing a bigger dive, I'll ask for the max, which at most shops is 3900. In those cases, I'm normally headed straight to the dive anyway, though there may be a lunch break in there somewhere.

Since violating thirds is one of the five risk factors for cave deaths and tank explosion is not, the need for volume far outweighs the fear of tank failure.

That would correspond with my experience with HP tanks with a 3800 ish fill that cools to 3600.

I agree the math is easier at 3600. But then again, if I have 3400 or 3500, I round down to the first number divisible by 3, which would be 3300 and turn the dive after 1100 psi is used (in this case the turn pressure is 2400 psi.) that is technically a bit less than thirds, but its on the safe side and as you indicate, violating thirds is one of the big 5 reasons people die.

You are however incorrect. People do not die because they violate thirds, they die because of poor gas planning and/or failure to turn the dive when they are supposed to.

Thirds is often not adequate, but it is often over relied on as a generic planning figure. Having bigger thirds is nice, but it does not address the real issue. Having more gas along does not reduce that risk as the risk is in the gas planning, not in the volume.

It's a lot like saying that a car with a 40 gallon tank is twice as safe as a car with a 20 gallon gas tank for a driver who runs out of gas a lot. The bigger gas tank does not solve the underlying problem that the driver forgets to check the gauge and get more gas when needed. It may reduce the frequency of running out of gas on a given route, but it does not solve the problem.

I also agree with you that tank explosion is not a big killer, but I guarentee the first time a massively overfilled LP 95 one blows up in a dive shop, a parking lot or in a cave or spring, people will die, more people will get sued and rules will get enforced. Will it happen today, next year, within the next decade? Who knows.

But it is safe bet statistically that if the cave community continues to push tanks to their full test limit with zero safety margin, eventually a tank with an inclusion or other pre or post certification defect is going to explode under that kind of no margin for error pressure.

There are also other issues and practices that come into play.

1. Many of the regs I see in use are regs that are known to have issues with high pressure o-ring pinching at pressures in excess of 3500 psi. In most cases the leaks starts out slow - just a few bubbles out the ambient chamber or a blown seal in a sealed chamber reg. But the potential for a much larger leak is there and if you dive two regs with simiolar service life and history. it is not uncommon to expect them both to fail around the same time. That screws with the idea of having reliable equipment and potentially reduces your redundancy.

2. Doubling burst discs can be exceptionally problematic. No burst disc assemblies are actually designed to be doubled so you have issues with limited thread engagement and you have issues with the disc being stressed outside the design limits as it has neither the flexibility nor the ability to be properly torqued and washered as intended. I've seen doubled burst discs blow and I'd argue they are FAR more likely to do so than a properly rated single disc. If you have to have 4000 psi pressure, use a burst disc designed for 4350 psi tanks - they are available and are much safer than doubling the discs.

3. Cave divers are exceptionally safe and many will have a fit over a thing like a 2" fastex quick release buckle. But in stark contrast, the community as a whole seems to take a total departure from common sense on the issue of tank fills in the quest for a little more gas to allow a bit more penetration. As a result they invite a whole cascade of potential failure issues (tank, burst disc, regulator, HP hoses, etc) that are 100% preventable as they have the option of just using a larger set of doubles and/or stage bottles.

I can't quite decide if number 3 is due to a collective ego thing, huberis, ignorance, or the fact that it has not yet shown up in accident analysis as a leading cause of death in cave diving - although I'd love to do statistical research on reg and burst disc failures to see if their is a correlation between fill pressure and risk of failure.

Either way, beyond certain limits that I personally think we have already passed at about 3500 psi, the safest way to address the need for more gas volume is to use larger tanks and/or stage bottles rather than to pump tanks to their test limits. Some day that practice is literally going to blow up in someone's face. If I were sure it would just be the offending diver who got killed or mained I'd be ok with that - kinda like I am ok with base jumping where the only potential victim is the guy who chose to jump. But the odds are that it will be an innocent bystander and potentially even a child who ends up injured or killed and I have no tolerance for that.

I also don't see the need. For any LP tank you can find a properly rated HP steel tank that will carry nearly the same volume at a 3500-3800 psi pressure in a similar sized package. Other than already owning one and other than the whole cave community thinking it is ok to overfill LP tanks to 3500-4000 psi because everyone does it and no one has died (yet) there is just not much reason to go the LP route, especially if you are buying a new tank. They may be slightly cheaper, but given the costs of cave diving the extra $50 for an HP tank tank is minimal

If anyone can enlighten me with a solid argument, other than the seriously flawed and rather ignorant "it has not happened - yet" argument, why any cave diver should be ok with a fill to a tank's test limit, despite the potential risks and consequences, I am all ears.