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Those Crazy Cave Divers! or You Put HOW MUCH Air in There!?!


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#1 duganalexzander

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Posted 17 April 2011 - 08:38 PM

Hey all,

I'm beginning my transition into sidemount this summer, while planning on starting technical diving next summer. In my research, I've discovered that many cave divers purchase Low Pressure steel tanks strip out the LP valves, replace them with High Pressure versions and then Fill a cylinder rated for 2400 psi to 4000-5000 psi as they need every extra bit of air they can get.

When I first heard it, I thought the diver I was talking to was a nut doing something stupid. But, after several emails with various cave divers I've discovered that this is actually commonplace.

Honestly, filling a tank to twice the rated service pressure seems crazy to me, but everyone is doing it. So I'd like to know why. I'll leave the following questions for all of you.


Do you know someone who does this?

Why is it 'ok' to fill a low pressure tank that high?

What cylinder sizes are commonly used, and how much air can you cram in there?

Why (in detail) is it better to used overfilled LP Steels than a HP Steel tank? [I was planning on diving with a pair of HP 130's in sidemount, but i've been told repeatedly to use overfilled LP's because of 'better buoyancy'. I'd like this elaborated on if anyone is up to the challenge.]

#2 shadragon

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Posted 18 April 2011 - 06:43 AM

As far as the LP question is concerned, I do not know.

While I appreciate your enthusiasm, I do think you are putting the cart before the horse as far as your gear and training goes. Sidemounting and tech training should come before purchasing and especially using the gear. That training will answer 90% of your questions and you can make an authorative decision on what gear or setup works best for you.

I have a pair of doubles sitting against the wall of my local dive shop. It is all set up for deep diving with shiny new deco tables, lift bag, reel, knife and other gear clipped off on the harness. That is where it will stay until my Advanced Nitrox and Deco Procedures course this summer. There is no doubt in my mind that I could put the doubles on right now and go to 140 feet or deeper, but I am waiting until I have a qualified instructor beside me and full team of safety divers within close proximity.

Last, I know several cave divers and not one of them is 'crazy'. They are the most level headed and safety conscious divers I have ever seen. Their training is exemplary and their standards incredibly high. They do nothing without a very good reason and they are never cavalier about their safety.

All the best in your development as a diver.
Remember, email is an inefficient communications forum. You may not read things the way it was intended. Give people the benefit of the doubt before firing back... Especially if it is ME...! ;)

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#3 ScubaPunk

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Posted 18 April 2011 - 10:03 AM

Ok Simon, in all fairness he never said he was going to go out and purchase any gear. Although I don't see anything wrong with it if he does. It would be nice to start the training with ones own gear rather than rental stuff. It sounds like a good question to me, (and a bit crazy from a non tech diver point of view). I don't think dugan was calling Cave divers crazy, but only saying the process of overfilling a tank sounds crazy. I'm sure it is a common practice that has been tested out to be safe, but it is a curious question. Perhaps some of the tech divers here at SD will be able to enlighten us.



#4 shadragon

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Posted 18 April 2011 - 12:56 PM

Ok Simon, in all fairness he never said he was going to go out and purchase any gear.

He said this in his first post "I'm beginning my transition into sidemount this summer, while planning on starting technical diving next summer." Sounds like he is starting to use sidemount gear (which is technical in my mind) a year before taking the courses to learn how. That is all I was referring too. I don't know of any shops that rent side mount gear so I assumed he was buying. Am sure he will correct me if I am wrong. Been known to happen on very rare occasions. hehehe :D
Remember, email is an inefficient communications forum. You may not read things the way it was intended. Give people the benefit of the doubt before firing back... Especially if it is ME...! ;)

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#5 duganalexzander

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Posted 19 April 2011 - 02:33 AM

Oh not a problem, I've been known to have my foot in my mouth on more than one occasion. Especially in public forums.

With regards to "Crazy Cave Divers", I meant crazy in an endearing manner. (Perhaps I should have illustrated that above) I'm very impressed by their training, thoroughness, and dedication to what they love doing. But like I said before, filling a tank to twice the rated service pressure caught my by surprise.

Up until recently, I thought the limiting factor regarding the service pressure of LP Steel tanks was the structural integrity of the cylinder itself, not the valve. HP Steel tanks weigh more, and I believe have thicker walls, which led me to believe that it was to preserve structural integrity. Honestly I'm not sure if I would overfill a low pressure tank without first taking one to the mechanical engineering building to run a stress test.

Also, to clarify, I've no intention of playing with gear which i'm not trained to use. With my luck and track record, it wouldn't work out well. That being said, I've been told that model x will be more than sufficient for my needs only to find out that I'm expected to upgrade to model Y before doing something more advanced. Thus if I can foresee expected necessities now, I can likely get my gear purchase right the first time round.

Finally, I haven't started to use sidemount gear yet. However I do not consider a basic sidemount rig to be technical in nature. As the Wench pointed out on one of my other threads, PADI offers basic sidemount classes as part of Advanced Open Water Training. Now my local dive shop doesn't offer any sidemount training, however the instructor/owner has been supportive of my choice and has put me into contact with a couple local divers who do use sidemount. Both of which have been helpful, and both offered to let me test out their rigs to see if I liked it. Now, the dive shop has a nice big swimming pool I can use to putz around in and adjust and readjust to my hearts content. After everything is perfect in the pool, my buoyancy is great, I can do valve drills, unclip and reclip my tanks, then I'll grab an experienced buddy (either one of the afore mentioned cave divers, or my instructor) and test it out in the lake.

To clarify, I see learning to use a sidemount rig effectively as a prerequisite for anything above Adv. Nitrox. If i'm doing a decompression course, I want to have every action with my rig be second nature before I take the class.

And thats my 2psi. Feel free to disagree, though i'd like it if you provided reasons why.

Thanks for your input.

-Will

#6 ScubaPunk

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Posted 19 April 2011 - 08:05 AM

Ok Simon, in all fairness he never said he was going to go out and purchase any gear.

He said this in his first post "I'm beginning my transition into sidemount this summer, while planning on starting technical diving next summer." Sounds like he is starting to use sidemount gear (which is technical in my mind) a year before taking the courses to learn how. That is all I was referring too. I don't know of any shops that rent side mount gear so I assumed he was buying. Am sure he will correct me if I am wrong. Been known to happen on very rare occasions. hehehe :D

I stand corrected.:P

#7 WreckWench

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Posted 19 April 2011 - 08:35 AM

In the early days diving nitrox was EXTREMELY technical. Now not only do divers consider it SOP but there is a movement to even teach nitrox while teaching open water. (Personally I hope that movement does not gain a lot of popularity but it illustrates what we see as technical one day is common diving practice/gear the next day.)

Having said that...diving sidemount is NOT that technically challenging nor is the gear that technically challenging. You do need training but diving sidemount is far easier than diving doubles back mount and is basically the same as diving 2 stage bottles clipped off rather than in pony bottle configuration.

Again you need basic training to learn how to band your tanks, switch from tank to tank and clip them off to you and your sidemount adaptor and bp & wing.

The basic sidemount set up is either a fully integrated sidemount system offered by most manufacturers or a back plate and wing and a sidemount adaptor offered by OMS and possibly others. The advantage to the latter set up is that when you are not diving sidemount your backplate and wing will let you dive in any conditions including travel conditions (that is all I dive) and it gives you the most flexiblity in my opinion to dive the full range of options. Buy the right bp/w and you'll be able to dive single tanks or back mounted doubles. (Yep...let SD help you get the right gear at a better than right price via our partners including DeepSeaSupply & OMS).

By a stand alone side mount 'system' and that is all you can dive with it which means most people will now need two setups or kits.

So for Will my suggestion is to dive backplate and wing first to get used to that. Adding the sidemount adaptor is easy, inexpensive, and will not squeeze you to death like some systems that transfer the air inflation the wrong way or taco out destroying trim. Once you are ready for the class it can be done in a weekend. You can drive to Lake Jocassee in Salem SC (just below the NC stateline) and stay with me and OTWDiver can do your instruction and you can try out several different sidemount systems including OMS as part of your training. Then you can make a better educated decision on the gear you want for yourself. By June OTWDiver will be able to certify you under 3 different agencies or all three in one class. No other instructor will be able to do that when he has finished his NAUI crossover. So choose TDI, PADI and/or NAUI and he can do your training.

And this offer is good for anyone that wants to move into sidemount! We are going to be offering sidemount on the Roatan trip this November and will look at more options for training at say Dutch, Lake Jocassee, and other venues in the US. All we need is 6 people in the class and a place for OTWDiver to stay and he will come to you. That is when he is not flying to Europe and Asia to teach the very same classes to instructors who will then start teaching them to students. We are on the tip of a very large trend in diving. :cool1:

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#8 lv2dive70

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Posted 04 May 2011 - 06:27 AM

I'm late to this thread but here's my 2 cents... I've been diving doubles since November, I bought LP95's and dive in N FL as well as have a LDS with a generous filling philosophy. The thing about LP tanks is that the main advantage is realized only if you have a relationship with a LDS that will overfill or if you are going to be doing a lot of diving in cave country (where the practice is common). All that being said - there is a lot more to choosing tanks than the HP/LP choice:

I'm having trouble trimming out my LP95's bc they are short and I end up head heavy. Not to say I can't trim them out, it just takes more conscious effort I could be expending elsewhere. Also, to keep them on my back in anywhere near a reasonable place, my crotch strap is so short it's bordering on ridiculous. I've had two (actually three) instructors suggest I get longer tanks. Now that has nothing to do with LP/HP but it is THE most important consideration for me. So I've narrowed down my options to 1) keeping my LP95's, 2) getting LP85's, 3) getting HP100's, or 4) getting AL80's (doubtful just bc of capacity issues but I want to look at it.)

in thinking about replacing my LP95's this is what I am looking at:
  • Ability to trim out in the water - next steps are to dive the HP100's (I've already dove LP85's and AL80's) and evaluate impact on trim etc.
  • Review buoyancy characteristics when full and near empty. Can I swim up when full with failed wing? Do I need to add v-weight to manage positive buoyancy at the end of the dive (for AL80's)? etc.
  • Finalize the math the different tanks (LP95's overfilled to x, y, z, LP85's overfilled to x, y, z, HP100's and AL80's) and get the deltas on paper to look at in black and white in terms of total Ft Cu.

All that to say... you may want to dive with several tanks before making a decision. I can't say I regret buying my LP95's because I've gotten quite a few dives out of them, but I'm not sure they are the right tank for me - and I'm only five months in figuring that out. A not inexpensive lesson to learn!
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#9 Landlocked Dive Nut

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Posted 04 May 2011 - 06:54 AM

I still don't know why these low pressure tanks are over-filled so far beyond their rating. If they can safely handle that fill, why aren't they rated higher? And why do those doing the fill feel no liability in over-filling to that degree?
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#10 shadragon

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Posted 04 May 2011 - 11:50 AM

I believe I quoted this article for another question a while back, but here it is again for reference.

Lots of good info on tanks and their construction. Pay attention to the part about replacing burst disks. :)
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#11 lv2dive70

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Posted 04 May 2011 - 07:50 PM

I believe I quoted this article for another question a while back, but here it is again for reference.

Lots of good info on tanks and their construction. Pay attention to the part about replacing burst disks. :)

'
OK -appreciate the link. I have seen this before and think it has a lot of good information. Glad to have the direct link to it again! OTOH - I have a hard time buying into this concept of no LP tanks when I'm diving in cave country every other weekend. :respect:

To your point - I saw the statement regarding burst disks and I would like to pay attention :teeth: but I'm not clear what is meant, perhaps you or someone could help me understand: "Irrespective of the risks, overfilling is also a logistics hassle because most fill stations will not overfill and it requires changing the burst disks in the valve"

Edited by Kate P, 04 May 2011 - 07:51 PM.

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#12 shadragon

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Posted 05 May 2011 - 07:27 AM

Burst disks are the safety valve that will blow (theoretically) before the tank. That way you get a mad rush of air relieving pressure before hitting the tank rupture pressure.

So if you swap out the burst disks for higher rated ones you run the risk of a tank rupture if you over pressurize past the rating of the tank as the burst disk will not fail by design. Overriding the built in safeties is not a very safe thing to do in my book.


Remember, email is an inefficient communications forum. You may not read things the way it was intended. Give people the benefit of the doubt before firing back... Especially if it is ME...! ;)

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#13 duganalexzander

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Posted 09 May 2011 - 07:14 PM

So if you swap out the burst disks for higher rated ones you run the risk of a tank rupture if you over pressurize past the rating of the tank as the burst disk will not fail by design. Overriding the built in safeties is not a very safe thing to do in my book.


Posted Image

Yet it is still a very common practice, so there must be something to it. If it was that unsafe, wouldn't there be an increased number of accidents/fatalities?

#14 grim reefer

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Posted 21 December 2011 - 06:33 PM

Burst disks are the safety valve that will blow (theoretically) before the tank. That way you get a mad rush of air relieving pressure before hitting the tank rupture pressure.

So if you swap out the burst disks for higher rated ones you run the risk of a tank rupture if you over pressurize past the rating of the tank as the burst disk will not fail by design. Overriding the built in safeties is not a very safe thing to do in my book.



I'm one of those crazy cave divers.

There is no danger of a LP steel tank without a defect rupturing at "cave fill" pressures (3600-4000psi) which happen to be below the hydrostatic test pressure (4400 psi for LP tanks if I recall). Repeated overfilling can possibly reduce the life of the tanks but I'm not even sure about that. You are effectively doing a hydro test with every fill. I guess it depends on the design of the tank. It is however important to keep them visualed and in hydro since rusty steel is not very friendly under high pressure, but this would be true under normal pressure (2640 psi is pretty high when you think about it).

All that being said, I personally use HP tanks and run them no more than about 10% over rated pressure, I do it because I think the tanks might last longer and sometimes I can't get cave fills where I live. It's not because I fear the tanks exploding.

Relatively safe overfilling only applies to steel tanks. Steel has a large elastic range and much more benign failure modes than Aluminum. I would not overfill an Aluminum tank.
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#15 WreckWench

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Posted 22 December 2011 - 06:11 AM

Thank you GrimReefer...great reply and it was nice to see an old friend...er....thread again!

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