How deep/long before you start running 80's for deco?

[flareless]

When I started my Tech course I planned to use my 2 AL-80s as deco bottles. There are a couple of drawbacks;

- They're big (can also be a plus for bad gas planning though)
- They're VERY floaty at the end of the dive

The first point never struck me as a huge problem. That is until I was drifting down the St. Lawrence over the dynamited remains of the King at 175ft. My big 'ol 80, although slung close to my body, got me snagged on a protruding piece of wood as I assed over it. I got unsnagged quickly but made a mental note to obtain something smaller.

The floaty aspect was demonstrated during training. We were to bring deco bottles to practice with. A classmate brought an 80 with about 500 lbs of gas in it. Trying to keep the thing down was a nightmare. At one point we had to stick it under the deck of a small sunken boat at the practice site where we dive. When we pulled it out afterwards the thing nearly dragged him to the surface.

I now dive with a Faber 40 steel bottle. It's smaller, holds almost as much gas, negative at the end of a dive and there's practically zero chance some student will grab it thinking it's their tank. I plan to get a second bottle this spring.

One note; You might want to SERIOUSLY consider the rigging you plan to use on whichever bottle you chose. I favor the idea of NO METAL ON METAL CONNECTIONS. While some folks like to attach a stainless steel ring around the valve to attach the clip onto, I prefer the GUE rigging with nylon line. Consider my drift snagging. I had 2 options; push myself back against the current on the piece of wood and slip it free or cut the line on the top clip of my deco rigging, float free and hold my bottle in place. When your instructor is drifting away out of sight the last thing you want to be left with is only one option. Something to think about.

 

[Rick Inman]

We use whatever tank is best suited for the gas requirements.

But also, we decant our own gas (no booster), so 80's of 02 allow us to get the required gas into a tank at lower PSI's. 2000psi of O2 in an 80 will go a long way.

And an 80 with 2000psi will ride just perfectly on a leash.

Stage manipulation with (mostly) empty Al tanks takes a little practice, but they certainly shouldn't drag you to the surface, so we stay away from steel for deco/stage bottles.

 

[DA Aquamaster]

This is the method I prefer:

HOG stage rigging

You can spend several times more money and buy a kit from Dive Rite that uses webbing, but the end result is the same. I prefer to use clear tubing so that the rope remains visible and inspectable.

The floaty aspect was demonstrated during training. We were to bring deco bottles to practice with. A classmate brought an 80 with about 500 lbs of gas in it. Trying to keep the thing down was a nightmare. At one point we had to stick it under the deck of a small sunken boat at the practice site where we dive. When we pulled it out afterwards the thing nearly dragged him to the surface.

An AL 80 has about 4 lkbs of buoyancy when empty and a reg and stage kit adds at least a pound of negative buoyancy so at worst you are looking at 2.5 to 3 lbs of positive buoyancy with an empty AL 80 stage. That is not "drag you to the surface" buoyancy. In fact it is well within the range that can be accommodated with normal lung volume and screams of poor stage handling technique.

One way or the other you have to deal with the swing weight and deal with sudden changes if you take it off or put it on.

One drill Doc Intrepid had me try was to swim along and drop a stage and continue on a few inches off the bottom with no immediate change in wing or suit volume. You release just a bit of gas from the wing prior to the drop, maintain a bit more lung volume than normal to stay neutral, then maintain a bit less than normal to stay neutral after the drop. That will allow you, for example, to drop a full stage on a line over a silty bottom with minimum delay, fuss or task loading. Its well worth mastering.

As indicated above, an AL 80 with 2000 psi is near neutral, which also makes it handy for leaving as a stage in an over head environment assuming you use a third going in, a third going out and keep a third in reserve. In contrast however clipping off a near empty AL 80 however does end up putting 1-3 lbs of upward pull on the line, so where you leave it becomes very important. The same thing happens when it is attached to you but with proper stage rigging and a proper lenght of line on the lower clip, it should streamline fairly well and not be so far above you as to cause major problems. If it is too veritcal you need to look at the the geometry of your d-ring placement, the lenght of your boltsnaps and the lenght of the leash on the lower bolt snap.

Most steel tanks on the other hand end up being 2-3 pounds negative in that same empty with a reg attached condition and can cause over weighting issues early in the dive when you have full back gas as well as multiple steel stages or deco bottles. I tend to avoid steel deco bottles and stages whenever possible. The thing I hate the most is that they are either negative or very negative and end up hanging below me for the entire dive rather than streamling back and up slightly. Plus the overly negative when full trait is more likely to try to roll you to the side they are clipped to.

Another option is the Alumninum 72 that is now available. It is AL 80 in lenght but only 6.9" in diameter and best of all it is dead neutral at 500 psi with the average reg and stage kit attached. It should be ideal as a bailout for rebreather divers where a 40 is not enough and in some cases would make an excellent stage bottle for OC divers, especially in off shore deep diving applications where you would use all of it as bottom gas before switching to back gas and then keep the tank with you throughout the dive where the neutral when empty trait is ideal.

 

[battles2a5]

For Tech 1 (limited to 1 deco gas I believe) a 40 will get you a long way. Once I did normoxic and was using 2 bottles, I started using an 80 for 50% and a 40 for 100%. On most dives I could get away with a 40 for 50% until you start looking at lost gas scenarios. At that point, the 80 made more sense to me. Beyond that, your deco gasese/bottles may vary by dive plan. You should be able to cover all your bases w/ 2 40's and 2 80's (for the next few years anyway).

 

[Flightlead]

or when logistics demands it. I have four deco dives this weekend, 155' max depth and fairly short. deco for two of them on 50% only, the other two 50% and 100%. there is no easy access to 50% or o2 around the site. I'm taking two 80's of 50% and a 40 of 100%. don't need to take an 80 for any individual dive....but I'm not going to go buy a bunch of 40's just because.

 

[Flightlead]

When I started my Tech course I planned to use my 2 AL-80s as deco bottles. There are a couple of drawbacks;

- They're big (can also be a plus for bad gas planning though)
- They're VERY floaty at the end of the dive

The first point never struck me as a huge problem. That is until I was drifting down the St. Lawrence over the dynamited remains of the King at 175ft. My big 'ol 80, although slung close to my body, got me snagged on a protruding piece of wood as I assed over it. I got unsnagged quickly but made a mental note to obtain something smaller.

The floaty aspect was demonstrated during training. We were to bring deco bottles to practice with. A classmate brought an 80 with about 500 lbs of gas in it. Trying to keep the thing down was a nightmare. At one point we had to stick it under the deck of a small sunken boat at the practice site where we dive. When we pulled it out afterwards the thing nearly dragged him to the surface.

I now dive with a Faber 40 steel bottle. It's smaller, holds almost as much gas, negative at the end of a dive and there's practically zero chance some student will grab it thinking it's their tank. I plan to get a second bottle this spring.

One note; You might want to SERIOUSLY consider the rigging you plan to use on whichever bottle you chose. I favor the idea of NO METAL ON METAL CONNECTIONS. While some folks like to attach a stainless steel ring around the valve to attach the clip onto, I prefer the GUE rigging with nylon line. Consider my drift snagging. I had 2 options; push myself back against the current on the piece of wood and slip it free or cut the line on the top clip of my deco rigging, float free and hold my bottle in place. When your instructor is drifting away out of sight the last thing you want to be left with is only one option. Something to think about.

I have been using 2 80's throughout my training. I regularly drop and pick up two 80's at once w/o changing depth. It s/b no problem to manage the buoyancy changes if you anticipate it. I have no problem with an empty or near empty 80 on a hang or swimming. if you aren't using that bottle (if its empty you probably aren't ) I find it easiest to clip the top clip of the bottle to my hip d-ring and it just finds its own place.

 

[nilsdiver]

I think you need to look at minimum gas and your lost deco gas procedures. I believe that will drive tank size and deco gas selection to a degree. But I am still just at the single AL40 deco bottle level, so I haven't really looked at this issue myself.

 

[NorthWoodsDiver]

After cave training and personally seeing people loose deco gas or at least the abilty to use the deco gas they carry I have decided to always dive thirds on all gas including deco gases unless a certain situation calls for different protocol.

This means carrying larger tanks but usually I can stash them anyway. For this weekends dive I will be using an 2 40's of 40% and a 40 of 100% for a 215' multilevel mission orientated dive.

 

[willardj]

I like 80's for 2 reason's.
1. Being I do short Deco's 15min or less. An 80 with 80% or 100% will last me a hole weekend of diving on 1 fill.
2. I have plenty of them.

 

[Jimmer]

Thanks everybody for the replies, it gives me a lot to think about. I do have one of my 40's strung up with a Dive Rite stage kit, simply because my LDS set up my first one for me, but I'll definetly go and make my own kits for the rest. I managed to find a really good SS clip supplier very close to home as well.

So how common is it to not have access to a shop with a booster pump for O2 fills? I notice at least one person mentioned that as a reason for the 80. My LDS doesn't have on, but the shop I'm doing my tech training with does, as well as 2 other local shops that I could go get fills from after I'm certified.