Choice of Cylinders

[rab]

...I get that fixed and:
2. I jump back in the water, descend to the bottom and shred my inflator hose on some wreckage. With the drysuit, I just use the suit for buoyancy control and abort the dive. With the wetsuit, I will probably be able to swim up the rig and ditch weight on the surface, although I might have to ditch weight to ascend.

If it's the hose that's failed, just use the oral inflation option. Duh! To have a BC failure, you'd rupture the bladder or loose a valve. (Even with a valve falure you might be able to maintain a posture that kept enough air in the BC if you started out properly weighted.)

-Rob

 

[Jerryg]

I've done my research and my decision was a PST E7-119. This is based on my body composition(fat) and the desire to be slightly positive with a 3mm shorty, BC, and full tank. That allows me to always have some ditchable weight that will achieve positive bouyancy at any point in the dive. Obviously the tank is not optimal for all conditions but covers the range of diving that I intend to do. The equations around buoyancy, ditchable weight, changes in buoyancy during dives etc etc etc need to be understood by you before you purchase the tank. Good luck in your decision.

 

[Don Burke]

If it's the hose that's failed, just use the oral inflation option. Duh!

Your vocabulary is showing. I was referring to the corrugated hose on the BC.

To have a BC failure, you'd rupture the bladder or loose a valve. (Even with a valve falure you might be able to maintain a posture that kept enough air in the BC if you started out properly weighted.)

Try that sometime. Being about eight or nine pounds heavy (that's about where you'd be with double 80s and a wetsuit) makes it pretty much impossible.

 

[MartiniTime]

Many who have already posted above me have probably said it better than I will, but none the less I'll add my two cents.

Even if you were swimming up with 5 KG / 11 pounds of dead weight, which it is apparent you are not - who cares? That is very little weight.

Everyone on here who is a Dive Master or has been a lifeguard had to tread with 10 lbs out of the water for two minutes holding it with your hands, sans any gear, including fins. Usually in fresh water.

Swimming up and additional 10 lbs with fins is a nominal task at at best. My 105Lb wife can do it, and thats 10% of her body weight. When you add bouyancy from all the other gear you are wearing, it's a fractional amount of weight.

Its a non issue. Essentially your choice of tanks is a mute point.

What should be stated is - you need to be able to swim up from your maximum depth with your current gear config. You can help mitigate this buy having redundant bouyancy with either a dry suit, or a double bladder wing. Thats it.

 

[Don Burke]

Don, you gotta reference for that? I want to read up on it a bit.

Here's is a little discussion about compressibility vs pressure in real gasses. 6.22 is the title of interest.

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/06.html

This is a direct link to the graph
http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/FG06_22.JPG

I find this one a little easier to digest. He used steam as his example, but the principles apply.

http://www.eng.vt.edu/fluids/msc/dense/dense.htm

These dense gasses can do some rather liquid-like things.

For the life of me, I can't find where the pressure I quoted comes from, probably dive shop lore. I can personally verify the change in compressibility is significant at 4500psig from my submarine expreience and have no problem believing that the low 3000s would be where the curve starts to depart from the ideal. I don't own any high pressure dive gear, so I haven't messed with that region in years.

 

[Diver0001]

Here's is a little discussion about compressibility vs pressure in real gasses. 6.22 is the title of interest.

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/06.html

This is a direct link to the graph
http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/FG06_22.JPG

I find this one a little easier to digest. He used steam as his example, but the principles apply.

http://www.eng.vt.edu/fluids/msc/dense/dense.htm

These dense gasses can do some rather liquid-like things.

For the life of me, I can't find where the pressure I quoted comes from, probably dive shop lore. I can personally verify the change in compressibility is significant at 4500psig from my submarine expreience and have no problem believing that the low 3000s would be where the curve starts to depart from the ideal. I don't own any high pressure dive gear, so I haven't messed with that region in years.

Thanks!

R..

 

[Soggy]

As far as I know, according to DIR, diving single steels with thick wetsuits is considered "acceptable," but double steels are a no-no. A set of double 104s on your back with a wetsuit and you ain't getting to the surface anytime soon if your wing fails.

 

[Don Burke]

Swimming up and additional 10 lbs with fins is a nominal task at at best. My 105Lb wife can do it, and thats 10% of her body weight. When you add bouyancy from all the other gear you are wearing, it's a fractional amount of weight.

A set of 104s would be about 13 pounds heavier full than at 500 psig.

My Bare 5/3 XL wetsuit plus hood, gloves, and booties is about 16 pounds positive at the surface. Some measurements I made at 10 and 15 feet lead me to believe that combination would be about 5 pounds positive at 100 feet.

That's about 20 pounds negative at the bottom. That 20 pounds is applied to a diver that is neutral, as opposed to 10 pounds applied to a swimmer who is slightly positive.

I'm 6'1 and about 200 pounds and I throw a 24 pound weight belt onto a pier to get out of the water at least a couple of times a month. I wouldn't bet my life on being able to swim that 20 pounds or so up. I'd put a few hundred dollars on it, but not my life.

 

[MartiniTime]

I was refering to a single 104, not doubles. None the less, its still not that hard unless they are very full.

Two Steel 108's, (in this case OMS) full, are negative 16 lbs. So at 1500 or so psi, they are at 8 lbs negative approx., that is zero sum, with no positive for wetsuit, or any gear. Thats 10 total with an aluminum plate, or 14 with a steel plate.

I can put on my aluminum plate, with no wing and double OMS 108's (mostly empty) and swim the length of the pool underwater with jet fins, on one breath with little trouble.

 

[Don Burke]

I was refering to a single 104, not doubles. None the less, its still not that hard unless they are very full.

Full tanks is exactly the condition I prefer to plan for.

Two Steel 108's, (in this case OMS) full, are negative 16 lbs. So at 1500 or so psi, they are at 8 lbs negative approx., that is zero sum, with no positive for wetsuit, or any gear. Thats 10 total with an aluminum plate, or 14 with a steel plate.

You generally don't get to choose your tank pressure for an incident.

I can put on my aluminum plate, with no wing and double OMS 108's (mostly empty) and swim the length of the pool underwater with jet fins, on one breath with little trouble.

That would be 16 pounds minus about 12 pounds of gas, plus 2 pounds of plate for a total of 6 pounds. I can see how that wouldn't be much of an issue with full lungs.