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

[Jimmer]

Ok, so I'm jumping the gun a bit, since my Tech 1 class isn't until the spring, but I need a distraction here at work before I pull my hair out.

I have 2 al40's and 2 al80's that I've decided will be dedicated deco bottles. I'm going to O2 clean them and get them marked and stage rigged over the winter, and was wondering at what point you guys start using al80's? Obviously the 2 40's will be O2 and 70' bottles, but what about the 80's? Is anyone using an 80 for O2? Or should I mark them 70' and the other as a bottom stage?

I know I know, ask my instructor his opinion, but like I said I need a distraction and my deco class is on my mind lately.

Thanks,
Jim

 

[Blackwood]

No reason you can't temporarily modify tank markings.

I'd probably mark it up as a stage and use gray duct tape to cover the permanent markings if I wanted to use it for O2. Anything over about 35 minutes on the O2 bottle and I'd look at an 80.

 

[Hallmac]

Ok, so I'm jumping the gun a bit, , I was wondering at what point you guys start using al80's?

Thanks,
Jim

When the dive requires it.

Slinging more tank than you need for the sake of showing off is a good way to lose dive buddies. Proper dive planning includes gas management, & equipment requirements, If your dive profile requires more deco gas than a 40CF can handle that would be a good time,

 

[texdiveguy]

I persl. like al80's as deco-stage bottles, some don't. I like st72's also, some don't.....I even use an al50 for EAN50.

Gas planning is your main goal, that will answer your minimum size cylinder question.....persl. preference is the other part.

 

[nereas]

Once you get a copy of V-Planner, that will tell you gas volumes needed. Then you can decide what size of tanks to sling.

You also however need to consider a reserve as well, for your co-dependend buddy/buddies / CF (3 or more).

Not everyone in tech diving is 100% self-reliant.

For those who are self-sufficient, normally your self-rescue backup plan is to have enough gas to cover in case one or more of the other bottles fails. This you can compute on V-Planner by running alternative scenarios, such as "what if bottle #1 fails?" and "what if bottle #2 fails?" etc.

You will certainly learn the art of contingency planning as a tech-deco diver, when your life depends on your calculations.

 

[DA Aquamaster]

In general I want to have twice the deco gas that I plan on actually using.

A dive to 170 ft for 20 minutes results in 26 minutes of deco and requires 17 cu ft of 50% if I am using only 50% for deco (DPlan, deco SAC of .4 cfm).

In that case a 40 is still fine, but barely. Any deeper or longer, and I will probbaly want more deco gas along for either lost gas contingencies, lots of current with a higher SAC on deco, or to have the extra gas to get a buddy through deco.

However, if I go any deeper or longer and I am likely to want to add a second deco gas anyway.

For example, if I go 10 minutes longer at 170' with both 50% and 100%, I will have 50 minuts of deco and I will then be using 15 cu ft of 50% and 10 cu ft of 100% and two 40's will still work fine.

Equally important is the lost gas planning. If I have to do the dive on only one gas or the other, I will need 30 cu ft of 50% or 11 cu ft of 100% (and 23 cu ft of the reserve backgas), so the 40's are still adequate.

A bit deeper - 200 ft for 30 minutes on 18/45 for example - with 32% and 100% will have 58 minutes of deco using 27 cu ft of 32% and 16 cu ft of 100%. In that case a 40 is not enough for the 32% (27 is more than half of 40 and a lost gas profile would require 55 cu ft), but a 40 is still fine for the O2 (only 22 cu ft with a lost 32 % profile, and 59 cu ft of the reserve back gas). So I'd end up with an 80 with 32% and a 40 with 100%.

In short, an 80 does not really come into play for deco until you are doing fairly deep and long trimix dives where you will be using a 30% or 32% deco and or travel gas. Consequently I don't O2 clean the 80 but mark it as the 120 bottle and tape over it if i use it as a stage. If you are doing the GUE thing I am pretty sure it will end up being the 100 ft bottle.

Some divers however just bring one 80 to use on the whole trip for mild deco. Its less than optimum but cuts down on the tanks you take.

 

[mark99]

In most of the deeper or longer 2-gas deco profiles you'll find you use more of the 50% (as in the example above where you used 1.5X). Where the 80 comes in handy is in a typical 2 deco dive day where you'll likely find yourself running low on 50% if you only have a 40cf. So I'd suggest one 40 of 100% and one 80 of 50% for the longer dive days (i.e. maybe 2 deco dives), and a 40 of 100% and 40 of 50% for the shorter (i.e. maybe one deco dive), and you have the 40 of 100% which is adequate for most single gas profiles.

 

[nereas]

And I on the other hand recommend 4 40s and NO 80s ever.

I use:

TMX 20/40
TMX 30/30
EAN 50
100% O2

But I know a lot of divers who like 80s. To each his own.

 

[nadwidny]

Nobody cares what you recommend since it's all based on fantasy.

 

[nereas]

Nobody cares what you recommend since it's all based on fantasy.

I'll bet you smoke while you fill those O2 bottles as well, huh?