Need opinions on LDS purchase

[JimC]

Well, I don't carry a pony bottle for starters.. but..

If your carrying it to give you a redundant 1st stage, get an h-valve.
If your carrying it to have more gas, get a bigger tank, get doubles and/or carry stages.


If your ever in a situation where you need a pony bottle, its very likely that you and your buddy are both low on gas and you'll end up shareing the sucker at some point.

You don't need a pony in a gas falure.. you have a buddy for that, and your planing your dive so he has the gas you need.. I hope. If your planing a dive in which you REQUIRE the pony in the event of a failure, your not diveing with a pony bottle, your diveing with a tinny stage bottle. A stage bottle that liekly dousn't have redundancy (read buddy carrying one)

I wonl't comment on solo diving.

 

[SM Diver]

JimC once bubbled...

since your OW training didn't include free acents or any reasonable boyancy controll).

I've had training in buoyancy control, and am quite good at it now. I did get training during the BOW cert class, and followed up with PADI's PPB class. However, I just would feel more comfortable with a weighted ascent line.

There are added risks to this dive, not the least of whice is having a potentially large fish on my stringer, and handling my spear gun. So, I'd like to have some way to latch my fish and gun to the bottom of the line, at the mushroom weight. Yes, it's dangerous, but I plan on starting my ascent at 1200, and hopefully will not just have shot a fish (in which case he'll be coming up underneath me on the dangling shaft).

I'm diving air, as I am not Nitrox certified yet. I am going to do this very conservatively. I am going to the grotto tomorrow with buddy to do a 100 foot dive to work on skills some more. I'll admit right now I don't have nearly the knowledge that you guys do, but I rely on redunant computers (Cobra on the console, Vyper on the wrist), and I am going to hang my 13cf pony tank.

Ravie: Thanks for your comments. I'll probably be down only 12 minutes. Not a lot of time to find the tournament winner, but if we jug that ledge right from the surface, anything is possible.

 

[SM Diver]

Jim, you've obviously given a lot of thought to SAC rates. Like I said, I rely on my COBRA, which is conservative, and does give me profiles based on the respiratory rate on previous dives. Can you recommend a specialty class that will enable me to better understand SAC, and do the calculations? That is one thing they definitely don't teach in BOW.

Thanks.

 

[madmole]

To Jim C

You've just demonstarted the VERY reason for a pony or extra stage cylinder. its a completely separate piece of kit (this is NOT meant as a flame)

Isolated twins, Y or H valves all have failure modes that can compromise ALL of the gas in the unit, (Isolator failure, neck seak failure). Admitedly very rare and unlikely.

But a pony is only as safe as the calculations you do to ensure it contains adequate gas for the bail out. Who needs 2 DV's on it. each buddy should be carrying his own gas and a secondary source, or you can resort to good old sharing if youve really screwed up.

Personally everyone I dive with in the UK carries their own gas and bail out/reserve. Wouldn't dive with someone stupid enough to expect me to get them out of a hole (although I would). Exception to this is of course novices and trainees, but we encourage them to also plan, and carry bail out.

 

[JimC]

Yes, twins and h valve has falure modes that do comprimse all of the gas in the unit. They do however both solve the more likely to fail first stage, which is the intent of them.

In the unlikely even of a neck o-ring going, you have a budy, and you abort the dive. If your diveing in a way that you can't do that relying on your bailout, then its not a bailout. Its a stage at that point.

Nothing wrong with stage bottles, as long as you treat them as such and are trained to handle them. Many people don't. I see buddy after buddy dive in to deep waters, surfacing with almost no air in there tanks. These people typicaly are diveing with one pony between them. They have no backup on ther reserve.

My issue is that the idea of a pony bottle is flawed. Its ether unneccisary, its a stage or your solo.

----

SAC rade is your consumption of gas, as mesured at the surface (1 atm). I use cubic feet, since all my stuff is imperial.

(60 min dive @ 33fsw, on an 80cf tank)
To figure your SAC rate for a dive, first you have to get your DAC (SAC at depth) and cf/psi for your tank.

DAC = (gas in - gas out) * convertion to cf / time
DAC = (3000psi in - 500psi out) * (80cf/3000psi) / 60min
DAC = 2500psi * 80cf/3000psi / 60min
DAC = 1.11 cf/min

So, during your dive, you averaged 1.11cf/min gas use. Next figure your average depth. You can get this off your computer most likely.

SAC = DAC / abs depth in atm
SAC = 1.11 cf/min * ((33fsw/33fsw) +1)
SAC = 0.55 cf/min

Keep track of that for your dives and you will be able to tell a good deal about the dive. How hard you worked, how stresed you were, etc. You also get a good idea of what your SAC rate is. You can then use that to figure out how long you will have on various dives. After a while you will even beable to get a rough estimate of remaining time based on your PSG and depth.

Your AI Computer does most of this all for you. I got this stuff in my BOW, and don't know any specific classes for it. It should be covered in BOW, as your gas is your life. Knowing how long it will last is.. important.

 

[techdiver2us]

Some of you guys scare me. My argon bottle is bigger than 6 cu. ft. ..... the 500 psi .... to stop - - Rule of thirds. If I finish my dive with less than 1000 psi I would conclude the dive was either poorly planned or poorly executed. Pony I always carried was a 30 cu. ft.

 

[MikeS]

When I was going to go wreck diving in NC where many of the wrecks are below 100 fsw I decided I wanted a pony bottle, strictly for bailout. Your buddy is an alternate air source but let’s face it things can and do go wrong, what if you run out of air looking for your lost buddy? Above 60 feet a controlled swimming ascent is a realistic option but below sixty feet it’s not for several reasons the most important of which is that it is not safe or practical to practice. I don’t want to get into the discussion about a pony, versus H-valves, versus doubles as it has been beat to death in other threads, other than to say in my situation I think a pony is the best option.

So then I started trying to figure out what size pony to get. There are a wide range of options from a 3 cubic foot spare air to stage slinging an AL 80, and plenty of hype and a plethora of opinions and antidotes. The funniest comment I heard was that “a Spare Air has just enough air for you to see your life flash before your eyes.” With the help of others on the board with much more experience than I, I set out to calculate how much air I needed.


I used an Excel Spreadsheet to calculate the amount of air required, I cannot attach a .exe file to the board but if you are interested PM an address and I’ll email it. You have to make some assumptions the most significant in terms of air consumption is the air consumption rate. Additionally, you have to determine a profile, I settled on the following:

Two minutes at depth to switch to the pony regulator and turn on the tank valve (turned off after initial pressure check to prevent gas loss due to free flow) and figure out and rectify the problem that caused my primary tank/regulator to stop working.

Ascent at 30 feet per second to a safety stop at 15 feet.

Safety stop at 15 feet for three minutes.

Ascent at 30 feet per second to the surface.

Here are the gas requirements for a few scenarios, feel free to play with the spreadsheet and come up with your own. My typical SAC is consistently in the 0.6 to 0.7 range but I used higher values based on the fact that this would be a stressful situation and adrenalin would likely kick in.

From 115 fsw with SAC 1.0 cubic feet = 23.8 cubic feet
From 115 fsw with SAC 1.2 cubic feet = 28.6 cubic feet
From 99 fsw with SAC 1.0 cubic feet = 20.6 cubic feet
From 99 fsw with SAC 1.2 cubic feet = 24.7 cubic feet

Based on these numbers I went with a 30 cubic foot pony.


Mike

 

[madmole]

Whats funny for me is that 25 years ago I was doing dives to that depth on a 40 cuft MAIN tank!!!! That was considered a normal size tank then!!!!.

Progress is wonderful. At least you couldn't ever get to NST on it.

We didn't carry any redundant gas then and would just share with our buddy if there was a problem . twin hose sharing, ah those were the days! Ignorance is bliss

 

[SM Diver]

Jim, thanks for explaining that. You got me on the right track now.