Choice of Cylinders

[jhelmuth]

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

Actually ~10.6 lbs

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.

My Seaquest 5/3 (Large I think), booties and gloves are ~12 lbs total at surface (seems about right) - don't know about the calculations or measurements part, but I'll assume about 5 (probably less, but not a big deal).

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.

Well, I think its maybe 15 lbs - but OK, I'll go with 20 at bottom is neutral - it is neutral, right (not bottom heavy)?
By the way, how much ditchable weight am I wearing?

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.

Does your wetsuit continue to become more buoyant as you surface? If yes - dosen't that make swiming up even easier as you surface? Can I ditch weight - or am I diving with no ditchable weight?

I'm 6'1 and 210 lbs. I don't throw a weight belt, but I do have a back-infaltion, weight-integrated BC that I add 12 lbs to if I dive a single tank to 100' weraing my Seaquest Isoflex 5/3. I can swim it all up without much too much effort, but that's because I can easily ditch 4lbs if I want. And if I were to dive doubles, I'd have to think about the right setup - but I think another 4.3-5.3 lbs won't kill me (especially if I use my head, think it through before I dive in, and practice it often enough that I react appropriately). I'll bet my life that I'll figure out the right setup, know the proceedures for an emergency and be practiced enough to execute it. Because I am betting my life on it.

Maybe this was not quite faily stated - but maybe so. I appologize if I'm wrong.

 

[Don Burke]

Actually ~10.6 lbs

That depends on your starting and ending pressure. 208 cubic feet of air weighs 208*.075 = 15.6 pounds. Using 80% of that gives you a swing of 12.48. Using 87.5% would give you a swing of 13.65. Divers often press these tanks well above rated pressure, so it could be a heck of a lot more.

My Seaquest 5/3 (Large I think), booties and gloves are ~12 lbs total at surface (seems about right) - don't know about the calculations or measurements part, but I'll assume about 5 (probably less, but not a big deal).

I would imagine neoprene foam varies from batch to batch. What I found was the buoyancy had a one pound positive offset and the rest varying with reciprocal of pressure. My guess is that the bubbles in the neoprene are at a slight pressure, which would account for the offset.

There are also Rubatex wetsuits that vary very little with pressure. I've never tried one since a trilam drysuit and a neoprene wetsuit cover everything I have come across so far.

Well, I think its maybe 15 lbs - but OK, I'll go with 20 at bottom is neutral - it is neutral, right (not bottom heavy)?

I don't understand the question. The 20 pounds is how negative you would be at 100 feet with full tanks if you were weighted for neutral at 10 feet with nearly empty tanks.

By the way, how much ditchable weight am I wearing?

That's part of the issue. The steel tanks reduce the amount of ditchable weight you're wearing. With some tanks, you wouldn't be wearing any ditchable weight at all.

Does your wetsuit continue to become more buoyant as you surface?

Yes

If yes - dosen't that make swiming up even easier as you surface?

Absolutely

Can I ditch weight - or am I diving with no ditchable weight?

That brings us back to the core of the discussion. You can't ditch weight that isn't ditchable. There are many ways to dive steel tanks in a wetsuit. Some are safe, some aren't.

I often dive a single steel 112 with a steel backplate. With a wetsuit, it's a pretty good setup. I haven't figured out what I consider a safe way to dive steel doubles with a wetsuit.

I tried the steel plate with the 112 in a pool with no exposure protection once. That's the only place I'd dive it that way.

I'm 6'1 and 210 lbs. I don't throw a weight belt, but I do have a back-infaltion, weight-integrated BC that I add 12 lbs to if I dive a single tank to 100' weraing my Seaquest Isoflex 5/3. I can swim it all up without much too much effort, but that's because I can easily ditch 4lbs if I want. And if I were to dive doubles, I'd have to think about the right setup - but I think another 4.3-5.3 lbs won't kill me (especially if I use my head, think it through before I dive in, and practice it often enough that I react appropriately). I'll bet my life that I'll figure out the right setup, know the proceedures for an emergency and be practiced enough to execute it. Because I am betting my life on it.

I always bet my life on my setup. That's what we are talking about. Dive what's safe.

Maybe this was not quite faily stated - but maybe so. I appologize if I'm wrong.

It appears you're right on the mark. You've already thought out the rig.

 

[jhelmuth]

Don,

The swing weight of the tanks was based on the buoyancy charicteristics I have for LP PST 104s (-5.3 lbs. full). And if divers press them to higher presures - that's their problem - I can't assume it. My bad if this does not accurately reflect you scenario.

The 15 lbs at bottom was the sum of the buoyancy chariteristics of the exposure gear and the tanks (10 + 5). The neutral question applies because you are neutral at 15' with 500 psi. So this reflects the lead you are wearing at the surface to counteract the bouyant wetsuit, full tanks and all the other gear to be able to sink at the begining of the dive. Since all the remaining gear remains constant, the "swing" is always the tank(s) at a given depth since ther are "breathed down". Properly weighted, this (plus any compression in the wetsuit - which seems to be ~ 7-11 lbs) is what the diver is swiming up.

Steel tanks reduce the ditchable weight, but shouldn't eliminate it.

The point is, if you don't have ditchable weight, then you haven't thought through the problem enough to realize that the tank choice (or some other component) is wrong. If you really don't have any ditchable weight, then go to Aluminum (or modify some other component - whatever is the smartest thing to do).
Tank selection is an intelligent choice. Blind blather of whats right/wrong without consideration to the whole is stupid. Generalizations based in fact and experience typically do point to the right answers (just not always).

 

[Don Burke]

Don,

The swing weight of the tanks was based on the buoyancy charicteristics I have for LP PST 104s (-5.3 lbs. full). And if divers press them to higher presures - that's their problem - I can't assume it. My bad if this does not accurately reflect you scenario.

I think you'll find the swing to be quite a bit more than that even when working within the manufacturer's recommendations. The tank tables are notorious for being wrong. Even very reliable companies have some bad data.

http://www.fifthd.com/e-education/productinfo/tanks.html

That table shows 3.17 pounds between empty and full for a PST 104, which defies the laws of physics.

The point is, if you don't have ditchable weight, then you haven't thought through the problem enough to realize that the tank choice (or some other component) is wrong. If you really don't have any ditchable weight, then go to Aluminum (or modify some other component - whatever is the smartest thing to do).
Tank selection is an intelligent choice. Blind blather of whats right/wrong without consideration to the whole is stupid. Generalizations based in fact and experience typically do point to the right answers (just not always).

Precisely. The statements that started this thread are an oversimplification of a couple of general rules of thumb.

 

[jhelmuth]

I think you'll find the swing to be quite a bit more than that even when working within the manufacturer's recommendations. The tank tables are notorious for being wrong. Even very reliable companies have some bad data.

http://www.fifthd.com/e-education/productinfo/tanks.html

That table shows 3.17 pounds between empty and full for a PST 104, which defies the laws of physics.Precisely. The statements that started this thread are an oversimplification of a couple of general rules of thumb.

Don,

I'm sorry but I disagree. The "weight" of the air itself is not the only part of the physics problem. What is very important to consider is the volume of water that it occupies. The buoyancy is relative to the actual weight of the water displaced.
I can't verify nor deny the manufactures statements of their buoyancy charicteristics, but I do not consider them notoriously bad. That is a claim that I assume you can backup with actual measurements and experience. I do not take retoric for fact.
Can you explain to me why it defies the laws of physics?

 

[SDAnderson]

Don,

I'm sorry but I disagree. The "weight" of the air itself is not the only part of the physics problem. What is very important to consider is the volume of water that it occupies. The buoyancy is relative to the actual weight of the water displaced.
I can't verify nor deny the manufactures statements of their buoyancy charicteristics, but I do not consider them notoriously bad. That is a claim that I assume you can backup with actual measurements and experience. I do not take retoric for fact.
Can you explain to me why it defies the laws of physics?

Technically, the weight of air is affected by the temperature, humidity and atmospheric density. That said, for an apples to apples comparison, a common reference point is 20C (about 70F) at 40% humidity and 1 ata. In those conditions, air weighs approximately 1.204 milligrams per cubic centimeter, or approximately .0755 pounds per cubic foot. Using this as a reference, I don't think it would have been unreasonable to base his assertion on standard assumptions but it might have been better had he stated he was doing so.

The discrepancies between the bouyancy characteristics reported by the manufacturers and what people experience in the real world seem to be attributable to three primary factors:

• What constitutes "full" and "empty." Some refer to full as the rated pressure, some use the + pressure. Some refer to empty as 500 psi, some refer to empty as, well, empty.
• Variances in the amount of metal used in an individual tank. There isn't a lot of volume in steel, so a little, more or less, can substantially impact the tank weight.
• Lying liars. Because so many folks use this number and so few actually hop in the water and check, there's an incentive to stretch the truth. Hard to believe, perhaps, but true.

When all is said and done, 104 cubic feet of air will weigh approximately 7.852 pounds. Of course, if you heat it to 250 F, you will reduce its weight by about 20%.

The tension evident in the discussion here, along with the apparent complexity, is what leads to simplifications like "No diving steel tanks in a wet suit." As with any generalization, it's not always right but it's often right and always safe - which ain't a bad place to be.

 

[LUBOLD8431]

Just to clarify, LP-104's are -12.6 when full, according to the new PST chart, which I believe to be much closer to the truth than older specs.

 

[quimby]

I'm sorry but I disagree. The "weight" of the air itself is not the only part of the physics problem. What is very important to consider is the volume of water that it occupies. The buoyancy is relative to the actual weight of the water displaced.

The volume displaced is constant, the delta is change in vol of air so the bouyancy change is the weight of the air.........

When all is said and done, 104 cubic feet of air will weigh approximately 7.852 pounds. Of course, if you heat it to 250 F, you will reduce its weight by about 20%.

But if we are talking about air in a tank the pressure is the only change and the moisture level should be close to non existant.

There are also Rubatex wetsuits that vary very little with pressure. I've never tried one since a trilam drysuit and a neoprene wetsuit cover everything I have come across so far.

True but even 231 doesnt make enough of a diff to matter.

The tension evident in the discussion here, along with the apparent complexity, is what leads to simplifications like "No diving steel tanks in a wet suit."

Absolutely, the point is as said before neut at 10ft with empties(500#) and difference in suit bouyancy at bottom and weight of air. Now dumpable weights, how long to dump air weight, and what could you swim up are pretty much the rest of the varibles. Won't even mention shooting a bag etc......
which is why I dont get excited over the generalization.

 

[jhelmuth]

The volume displaced is constant, the delta is change in vol of air so the bouyancy change is the weight of the air.........

You misunderstood. The volume "change" is relative to the buoyancy charicteristics of the tank. If I have 2 tanks - both hold an equal amount of air; and Tank A has a displacment volume of 1.0 cubic feet; while tank B has a displacement volume of 0.9 cubic feet - which is more buoyant?

I considered all other vairiables (temp, etc.) to be held to the same conditions (as a fair comparrison).

And I know these things can be "read" (interprited) in the wrong tone. I wanted Don to understand what I was talking about. He's not stupid or anything otherwise. I don't know the man to draw those types of conclusions. Nor do I think he was meaning any ill toward me (or anyone else). I felt that he wanted the information and I think if he looks back on his training, thinks through what I've said (and others comments too - I'm not God here), I think he'll see what is flawed in his thinking (at least as I understand that from his posts).

No harm or ill intended.

 

[quimby]

You misunderstood. The volume "change" is relative to the buoyancy charicteristics of the tank. If I have 2 tanks - both hold an equal amount of air; and Tank A has a displacment volume of 1.0 cubic feet; while tank B has a displacement volume of 0.9 cubic feet - which is more buoyant?

Now we get into displacement vs bouyancy. While I understand your point, it is meaningless to me without the tank weight etc. Again it boils (no pun intended) down to + or - bouyancy empty at 10ft. and weight of air, which is one of the changes during the dive and the other being suit compression or change. Ditchable weight vs fixed weight and can you exit with reasonable safety.
I also think this is a discussion maybe slightly derailed from the original topic, but I dont believe anyone is trying to or has been offended.