High Capacity Tanks - which one?

[roakey]

rickc,

Several of us need answers to the first reply to this note to give you an accurate response.

But in the meantime, about cylinder buoyancy (pardon me while I shamelessly round numbers in the following text):

Cylinders change buoyancy about 1 pound for every 13 cubic feet of air or Nitrox pumped into them (Trimix is different and I’ll ignore it). On another board we settled on the term “swing weight” for this change in buoyancy. So for instance, an AL80 has an 80/13 = 6 pound “swing weight” from full to empty.

This 1 pound for every 13 cubic feet is simple physics. Nothing, not cylinder material, pressure, water salinity or anything else can change it. 1 pound for every 13 cubic feet of gas, period (also true above the water as well!).

So, during a dive with an AL80, assuming you drain the cylinder you’ll be 6 pounds more buoyant at the end of the dive than when you started.

So though you can’t change the swing weight of a cylinder, you CAN change the starting and ending buoyancy of a cylinder due to the cylinder’s volume (this is “external” volume, not the “internal under pressure” volume). This is why an AL80 may swing from –2 to +4 pounds from full to empty (note the 6 pound delta) and an HP80, for instance, may swing from –7 to –1. Again, the same 6 pound delta despite the high pressure and the different cylinder material.

In the example above, note that the HP80 terminates with buoyancy of –1, and the AL80 with buoyancy of +4. This would mean that switching from an AL80 to an HP80 you could drop five pounds from your weight belt.

Looking at an HP100, its swing weight is 100/13 = 8 pounds. Its empty buoyancy is 0, so you can drop 4 pounds from your weight belt. Interestingly, the PST HP100 weighs (in air) about the same as an AL80, so at the start of the dive you’re two pounds lighter as you gear up at the site! Lost Yooper is on the money when he states that (for example at 99 feet) that extra 20 CF of air only amounts to maybe 5 to 10 minutes, not much.

An aside about weight and lift: You determine your weighting needs at the MOST buoyant point in the dive: Shallow with an [almost] empty cylinder. In terms of lift (BC/wing capacity), you base it at the LEAST buoyant point in the dive: Deep with a full cylinder.

This is why the dual 100 pound lift bladders are, let me be polite, “A poor choice”. Taking the wildest configuration (and a poor one at that) out of thin air you’ve got a diver in a wetsuit diving twin 120s. If they’re weighted so they’re neutral when empty near the surface, you can take a WAG at what the BC/wing’s lift should be. 240 CF / 13 lbs/CF = 18lbs swing weight. Let’s say it takes 20 lbs to sink the wetsuit at the surface and that the suit looses ALL of it’s buoyancy at depth as its crushed (it doesn’t lose ALL its buoyancy but for argument’s sake we’ll say it does). So to make the diver neutral, you’ll need 18 + 20 = 38 lbs of lift. Anything beyond that is simply extra lift. Extra is a good thing, but only maybe 10, 15 pounds extra is all that’s required so a 55 lb lift BC/wing would be sufficient. You might even be able to get by with a 45 lb lift wing. Many folks point out that this rig weighs over a hundred pounds when you’re walking around in it, so of course you need dual 100 lb lift bladders! Well, no, because who cares what the rig weighs out of the water, the wings are only used IN the water, where the rig, properly balanced, will weigh nothing when the cylinders are empty.

Please note that diving dual 120s with a wetsuit is a HUGE no-no, I only used that as an example.

So please get back to us with the answers to the first reply, as well as the excellent question that LY asked: Are you having problems with excessive gas consumption? Because if so, they’re many cheap (read: no cost) solutions to that problem that don’t require going out and buying expensive cylinders.

Roak

 

[Scottri]

I dive with LP Steel 80's and 95's. I love em! I can get them filled anywhere and they last forever. I don't have to pay for the Visual Plus inspections. I dive in cold water so the less weight I carry in my BC the better. Aluminum requires me to carry about 5 extra pounds in my BC pockets.


Scott

 

[rickc]

I am a novice diver with 34 dives in the last four months since certification. Yes, I am having trouble with air consumption, or I would not be investigating this option to start with. I am always the first person to run low on group dives, and I am tired of worrying about it.

I assume that my air consumption will improve over time. However, right now, the key factor for me is that another 20 cf of air means that I can worry a little less about air and enjoy the dive a bit more.

Rick.

 

[Arduous]

Hi Rick:

Your breathing rate should settle down in time. As your experience and confidence grows the anxiety and unrest you may feel while diving will fade. We are all different and the amount of time and dives it takes for one to settle in varies greatly. Don’t rush it, dive well with in your abilities and have fun, the rest will come in do time. One way to gain added confidence is to further your dive education by taking advanced courses. The air supply thing is not a big deal. Don’t get hung up on this macho thing that all the great divers use less air than me so I must not be as good as them. This is just trash that will interfere with how you enjoy your dive. I know many divers that I consider great divers, certainly better than myself, but some of them use far more air than I do. Don’t worry about it. Dive a lot, have fun and be safe and before you know it, you’ll be the one answering all the questions

Arduous

 

[roakey]

Originally posted by rickc
Yes, I am having trouble with air consumption, or I would not be investigating this option to start with.

Rick,

There are other reasons to go with high capacity cylinders, too.

Now, typing this entire thing over again because I hit some mystery character sequence that erased my previous reply. Grrrrrrrrrrr.

We still don’t have answers to the base questions, but forging ahead assuming you’re diving wet, assuming you think you’ll not go technical and assuming you can get 3500 psi fills from your LDS and on a boat, I’d go with an HP100. You’re right, steels cost one heck of a lot more, but their buoyancy characteristics are far superior to ALs. The only reason ALs are so popular is cost. They certainly don’t work as well in the water as steels. A steel will also move your center of gravity (buoyancy?) up your body, flattening you out in the water, making you more streamlined and you’ll breathe less air moving about.

Plus you’ll drop four pounds from your belt right from the get-go.

As Arduous stated, your air consumption will improve in time. Waiting is cheaper.

Roak

 

[Warhammer]

Take Arduous's advice, don't worry about always being the first one to run out of air. It's no big deal. I recently got a friend of mine interested in diving, convinced him to certify and we've made numerous dives together since. He always runs out of air way before I do, but so what. One of the reasons is that I dive HP100s, not so much for more air, but for the reasons roakey mentioned, and the other reason being that I just breathe less than does he. But I really couldn't care less if I need to surface a little early because he's low on air. It's all about having fun, not who uses the least amount of air. The instructor at the shop I do business with can do 2 dives to my 1 on the same amount of air, big deal, just means that I have to switch tanks more than he does.

Now if your considering purchasing tanks, then give steels a look see for sure. But for the reasons mentioned by roakey, not just because you just need more air. If all you want is more air, then the AL100s are the cheapest way to go, but your back will pay big time on those long walks on the shore.

 

[Lost Yooper]

Let me say that I don't necessarily disagree about getting a bigger tank as I'm a fan of having lots of air. I'm not a particular fan of HP tanks, and I would probably recommend a LP PST 104 to use as a single tank. However, I'd like to target in on the real problem -- poor air consumption.

Ok, you're a new, cold water, wet suit diver who happens to be male, right (men tend to use more air than women)? In time, you can assume everything in this equation will change except the being male part (hopefully ). I'll try to list some of the problems you're are experiencing and some solutions as I know them. Hopefully, others will chime in here and add what I'll miss.

Factors important to air consumption: streamlined swimming position, stream lined gear, proper weighting, and warmth.

New Diver:

New divers are about the most ungraceful sight you'll ever see underwater . Don't sweat it, it will come around. It is important that you remain horizontal while swimming. This is odd for a while because it means tilting your head back so you can see. This is compounded with having a 7mil hood on and tucked inside your suit. It's something that has to be practiced, but is very important to being streamlined. Keep your arms in tight and don't use them for propulsion -- fins only. The less water you're pushing the less energy is required, and thus, the less air is required.

New divers tend to be way overweighted. The primary problem with being overwighted is that it requires your BC to be inflated more than it has to be -- less streamlined. If you're wearing a jacket style BC, then you're already at a disadvantage, so proper weighting is critical. Being overwieghted may also push your lower body down thus making a horizontal swimming position difficult -- less streamlined.

Cold water wet suit diving is a bad combo, IMO. If you're cold, your body uses more energy which increases air consumption. The best option is buying a premium shell dry suit (or semi dry, ick!) and spare yourself the agony. Since this is quite likely not an option, you're kind of stuck. Make sure you eat prior to diving (long term energy foods -- protein, nuts, etc). Food will keep you warm.

Lastly, for me anyway, take a look at your gear (in the water). Make sure you don't have junk dangling, and your hoses are routed nicely and streamlined.

If you want to see dramatic improvements in air consumption, swim perfectly horizontal; don't flog around with your arms; keep your buddy directly to your side, so you don't have to continuously look back; weight yourself correctly; eat prior to diving; consider getting a good dry suit; get a back inflation BC (particulary a harness, BP, and wing); and finally ..... DIVE, DIVE, DIVE!

There you have it ... some of it anyway.

Good luck.

Mike

 

[buff]

Burning a lot of air? Just tell the other divers: Muscle needs more O2 than fat.
Then run.

Mike

 

[jobowker]

I dive the balmy waters of new england with a 7 mil wetsuit and an aluminum 80. It comes down to money. If I can get the money set aside, I will upgrade to a spiffy new dry suit, environmentally sealed reg with a DIN fitting, and HP 100's. The dry suit will let me stay warmer (and use less air) and the HP 100 will be smaller than my 80, lighter out of the water, heavier in the water, and give me a little extra air. Since I curently run low on air before temperature or NDL, a little extra would be nice. But the upgades mentioned will run me about $2,000 in total. In the meantime, I found that I can cut my consumption down in a number of ways. First, on those chilly dives, I bring a thermos of warm water and pour in down my wetsuit before jumping in. It's a better way to warm up your wetsuit. Second, when I first get in the water, I stay at the surface for a minute or two until my breathing calms down. Third, since it's not a race, I fin a little slower so that my breathing rate stays in check. Fourth, as mentioned above by Yooper, keep your body streamlined and don't use your hands. That last part I had trouble with. I talk with my hands too, and it just didn't feel right at first keeping them by my side. When going for lobsters, I suck air because even though you are shallow you are moving rocks, chasing lobsters, spinning around, etc, so I want to conserve as much as possible for the rest of the dive. I'm not an expert by any means, but I did find improvement by the methods mentioned above.

 

[Seahorse]

I've just joined your board and have used a lot of info & links to determine which tanks I'll buy so firstly Thanks for that. I'm going for steel.
-"GREAT" I hear half of you cry and given the level and intensity of debate the other half will probably be saying something like "what a Loser" .Anyway what I cant decide on is size 80 / 100 or 120.
A site I looked up gave the following stats for these tanks
80Cu Ft BAR228 PSI 3500
100Cu Ft BAR228 PSI 3500
120Cu Ft BAR228 PSI 3500
My question is this. Is the only desernable difference between these 3 tanks A/ their physical size and B/ their Weight and Buoyancy or have I missed something because they appear to all hold the same amount of air which doesn't seem right.