Stage Dive Planning

[T-Dog]

I treat staged decocompression like an overhead situation and I'm back on the boat with at least 1/3 reserve in each of my tanks - including stages and deco bottle. I've never needed that reserve gas, but the day that I do need the reserve I know it will be there, and my buddies keep the same practice.

I think your original plan where you keep 1000psi in your stage is the correct decision for a 70min run time and 142ft3 gas consumed. If your not comfortable dealing with the extra cylinder on the day of the dive (ex: sea states) then have a back-up profile you & your buddy can do on just the double AL80s.

 

[Thalassamania]

I treat staged decocompression like an overhead situation and I'm back on the boat with at least 1/3 reserve in each of my tanks - including stages and deco bottle. I've never needed that reserve gas, but the day that I do need the reserve I know it will be there, and my buddies keep the same practice.

I think your original plan where you keep 1000psi in your stage is the correct decision for a 70min run time and 142ft3 gas consumed. If your not comfortable dealing with the extra cylinder on the day of the dive (ex: sea states) then have a back-up profile you & your buddy can do on just the double AL80s.

The rule of thirds only makes sense if you have a significant horizontal swim to make during which you might have to supply your buddy with air. Overgearing and overcomplicating creates dangers whilst solving nonexistant problems.

When you use a 12 pound sledge to try and kill a fly, the fly gets away and you put a hole in the wall.

 

[karstdvr]

The rule of thirds only makes sense if you have a significant horizontal swim to make during which you might have to supply your buddy with air. Overgearing and overcomplicating creates dangers whilst solving nonexistant problems.

.

I think rule of thirds makes sense even in an OW condition where currents etc can become an issue. I don't know of many situations where having too much gas resulted in an accident,but the converse I can think of many accidents.

 

[grazie42]

How hard is is really to have that 80-stage, seems to me it doesen´t add much complexity and if that´s what the OP wants for his peace of mind (or whatever), go ahead. I still think he´s within the "more gas is good"-window...

I´d propably keep the stage in reserve, you´re planning the dive with some conservatism in your consumtionrate (I assume) anyways, so even if something minor goes wrong you should still be able to get out on your BG. If your buddy has a problem, just hand him your stage and everyone is back to being comfy again...

ymmv...

 

[Thalassamania]

First of all, everyone has the right to go to hell in the hand basket of their choosing. That said: when you post a dive profile and ask for comments, you allegedly have in interest in what others think of the basket you’ve chosen, as well as imply an interest in what basket others might be choosing.

In my opinion, what we see here is a classic example of taking things too seriously. Now, don’t get me wrong, diving (especially decompression diving) is a serious business, but it is best approached from a minimalist prospective. I’m not talking DIR now, just common sense. A 110-foot wall dive does not benefit from the rule of thirds. Is more gas better than less gas? Sure, until carrying it weighs you down. I’m a scientist by training. I work from the extremes into the center to try and determine what’s best. I want to dive the lightest, easiest to handle rig that I can, commensurate with minimizing risk. Every pound that I add that does not yield an immediate and definable reduction in risk means that I’ve overshot the inflection point and I’m now adding risk rather than reducing it.

At one extreme is a single tank, I could possibly make the dive and do my deco on one tank, but I’d have to keep my SAC down below 0.4, I’d come up trying to pull a vacuum on the tank … not a good idea, and there’s no contingency for anything to go wrong.

At the other end of my spectrum is a pair of 130s with two 40s that I used for some deep mixed gas work, nah … even though more gas is better, this rig is too heavy and the increased hazards of just getting in and out of the water don’t make it worth while.

So I move up in gas volume till I find what I need, which I’ve calculated out to be 150 cubic feet of gas. Double 80s would fit the bill nicely and give me a slight pad. Do I want to take anything else? Maybe 40 cubes of pure oxygen? Maybe not?

As for the rule of thirds: It is designed to provide gas for a pair of divers to reach their exit point after one suffers a complete failure. That is not needed for a 40 min to 110 ft. wall dive, since at the point of failure the dive is called and the ascent is immediately begun. You can calculating your gas needs for an open water dive and pad the calculation appropriately as needed, this is very different from a dive in which a penetration is made and even the rule of thirds may not provide enough gas.

If you're going to take a minimalist approach, and grant that it provides increased safety, part of that approach is to not carry more gear and make more switches than you need to. Carrying a stage adds a switch and oes not offer any protection from currents; it just makes the effect of a current that much more pronounced.

There's nothing wrong with practicing and testing, but understand what you’re doing and when you’re doing it. To my mind this is a case of turning a stroll in the park into a Class-5 technical climb for no discernable reason.

 

[Rick Murchison]

Personally, I always like to leave at least a few hundred psi in the stage. In a pinch I could breathe from it for a few minutes while getting things sorted out, or could hand it off to a buddy so that we could make our ascent to the surface without being tethered to each other in the event we had been sharing air through the deco.

theskull

Ditto. To say nothing of reducing the chance of trashing a first stage.
Rick

 

[Rick Murchison]

A 110-foot wall dive does not benefit from the rule of thirds. ... I want to dive the lightest, easiest to handle rig that I can, ... Double 80s would fit the bill nicely

Yep.
In open water, so long as each of you you retain enough gas for you and your buddy to make a safe ascent and deco stops in the event of a "total gas failure," there just isn't any need to carry extra tanks. If you're really concerned about loss of your buddy and a "total gas failure" happening near the end of the dive simultaneously, rig your doubles as independents and keep enough gas in each for the ascent (Yeah, you might have to shave a minute or two off your bottom time...). 'Course if all those bad things happen at once there's probably a lightning bolt waiting for you at the surface anyway
Rick

 

[T-Dog]

In my opinion, what we see here is a classic example of taking things too seriously. Now, don’t get me wrong, diving (especially decompression diving) is a serious business, but it is best approached from a minimalist prospective. I’m not talking DIR now, just common sense. A 110-foot wall dive does not benefit from the rule of thirds. Is more gas better than less gas? Sure, until carrying it weighs you down. I’m a scientist by training. I work from the extremes into the center to try and determine what’s best. I want to dive the lightest, easiest to handle rig that I can, commensurate with minimizing risk. Every pound that I add that does not yield an immediate and definable reduction in risk means that I’ve overshot the inflection point and I’m now adding risk rather than reducing it.

At one extreme is a single tank, I could possibly make the dive and do my deco on one tank, but I’d have to keep my SAC down below 0.4, I’d come up trying to pull a vacuum on the tank … not a good idea, and there’s no contingency for anything to go wrong.

At the other end of my spectrum is a pair of 130s with two 40s that I used for some deep mixed gas work, nah … even though more gas is better, this rig is too heavy and the increased hazards of just getting in and out of the water don’t make it worth while.

So I move up in gas volume till I find what I need, which I’ve calculated out to be 150 cubic feet of gas. Double 80s would fit the bill nicely and give me a slight pad. Do I want to take anything else? Maybe 40 cubes of pure oxygen? Maybe not?

As for the rule of thirds: It is designed to provide gas for a pair of divers to reach their exit point after one suffers a complete failure. That is not needed for a 40 min to 110 ft. wall dive, since at the point of failure the dive is called and the ascent is immediately begun. You can calculating your gas needs for an open water dive and pad the calculation appropriately as needed, this is very different from a dive in which a penetration is made and even the rule of thirds may not provide enough gas.

If you're going to take a minimalist approach, and grant that it provides increased safety, part of that approach is to not carry more gear and make more switches than you need to. Carrying a stage adds a switch and oes not offer any protection from currents; it just makes the effect of a current that much more pronounced.

There's nothing wrong with practicing and testing, but understand what you’re doing and when you’re doing it. To my mind this is a case of turning a stroll in the park into a Class-5 technical climb for no discernable reason.

I understand the "less can be more" philosophy, and that every piece of equipment you take on a dive that you don't need becomes a liability - not an asset. However, decompression is an overhead situation, and the diver who made the original post calculated a 17 min "swim" prior to surfacing, based on 40min @ 110ft and 10 min @
80 ft. What if the "failure" (entanglement, loss of gas, lost buddy,...) occurs at the end of the dive do you really want to blow off a 6 min stop @ 20', and an 11 min stop @ 10'? I'd rather carry the gas, deal with the situation, and do my deco time.

Perhaps in your dive experience this is an acceptable risk, and you have developed commensurate gas reserve limits based on the amount of open water deco you have on a dive. I don't have that level of experience, so I'll carry the 1/3 gas reserve, which I'm accustomed to doing in caves or the open ocean. You reference a SAC of 0.4 cfm, while mine is 0.7 cfm in doubles. In your example of a dive requiring 150ft3 you're OK with 160ft3 of gas, but that 10ft3 reserve would last you roughly twice as long as it would me - I need a bigger reserve physiologically, not just because it violates my personal dive rules.

Perhaps the diver starting this thread has a higher SAC rate, and/or his buddy may have a higher SAC rate, or maybe like me they have lower tolerance to risk. Having only 10ft3 or 20ft3 of reserve on a dive requiring roughly 20 min of deco would tug on my mind much harder than the current tugging on my stage! It would probably increase my SAC rate, and decrease my dive time.

If a diver calculates a deco dive requires 140 ft3 of gas and they only take 160 ft3 of gas then he/she is treating it just like an open water dive with a gas reserve of ~15%.
(20ft3/140ft3 = 14%; 500psi/3000psi = 16% or ~13ft3 in an AL80). A 40ft3 stage for the dive profile originally posted is the minimum I would contemplate.

Also there is no way to gain experience diving stages for longer and/or deeper dives, except by going out and diving them and getting that experience. Perhaps this is an unstated goal of the dive plan. On the day of the dive, if the sea conditions or diver's condition indicate the stage would be more of a liability than an asset, then leave it on the boat and dive a pre-planned abbreviated dive schedule rather than pushing the gas limit or getting in water over your head.

 

[JimC]

I would breath the stage down to somewhere between 1000 and a 300. Your back gas has redundancy and is thus the safest gas you have. Use it last. I usually bring my stages down to the 300psi range to leave a bit in the jug for the fill monkey, gauge error and regulator.

 

[Thalassamania]

I understand the "less can be more" philosophy, and that every piece of equipment you take on a dive that you don't need becomes a liability - not an asset. However, decompression is an overhead situation, and the diver who made the original post calculated a 17 min "swim" prior to surfacing, based on 40min @ 110ft and 10 min @
80 ft. What if the "failure" (entanglement, loss of gas, lost buddy,...) occurs at the end of the dive do you really want to blow off a 6 min stop @ 20', and an 11 min stop @ 10'? I'd rather carry the gas, deal with the situation, and do my deco time. .

Decompression is not the same as rock or steel. The risk is bends vs. drowning. Regardless of the the length of the swim, air is only 110 feet away. If your tangled, untangle, if your gas is gone, use your buddy’s reserve, if your buddy’s gone, call the dive. Everyone knows those answers. If you and your buddy each breath 0.6SCFM and you each start the dive with 160 cubic feet of gas, you each have more than enough reserve to get the other home without blowing off anything.

Perhaps in your dive experience this is an acceptable risk, and you have developed commensurate gas reserve limits based on the amount of open water deco you have on a dive. I don't have that level of experience, so I'll carry the 1/3 gas reserve, which I'm accustomed to doing in caves or the open ocean. You reference a SAC of 0.4 cfm, while mine is 0.7 cfm in doubles. In your example of a dive requiring 150ft3 you're OK with 160ft3 of gas, but that 10ft3 reserve would last you roughly twice as long as it would me - I need a bigger reserve physiologically, not just because it violates my personal dive rules.

Read the posts, please. The dive, including all requisite reserves for my buddy and me would require 150 cubes. The extra ten, is … well … extra. The gas reserves for deco are as easy to calculate as the gas requirements for the dive, but you have to double them. I use a spreadsheet that has as inputs the depth, the two SAC rates, the time and depth of the stops. It’s really quite simple, it also figures for a deep stop, and an extra 3 min at 10., as well as extra gas to account for longer stops as a result of a one stop time or depth error. I can see no use for the rule of thirds on a wall dive. I submit that if you carefully think it through you will not find any use for it either. The only defense for using it is … it might come in handy in case … whatever. I can use that excuse for anything.

The calculation of 0.4 scfm was working the Bingo Air calculation backwards asking the question what would my SAC rate have to be if I were to use an 80. That’s clearly not a practical rate to use for this sort of calculation so that option was rejected. My actual SAC rate is above the 0.6 that was suggested for the dive, I was using 0.6 to keep the discussion from devolving into apples and oranges.

Perhaps the diver starting this thread has a higher SAC rate, and/or his buddy may have a higher SAC rate, or maybe like me they have lower tolerance to risk. Having only 10ft3 or 20ft3 of reserve on a dive requiring roughly 20 min of deco would tug on my mind much harder than the current tugging on my stage! It would probably increase my SAC rate, and decrease my dive time.

Then you would have to use a larger gas supply, the discussion was defined by the OP at 0.6 SCFM. What I am suggesting is the OP is, in fact, increasing the dive’s risks by packing more garbage than a sensible analysis would demonstrate is prudent.

If a diver calculates a deco dive requires 140 ft3 of gas and they only take 160 ft3 of gas then he/she is treating it just like an open water dive with a gas reserve of ~15%.
(20ft3/140ft3 = 14%; 500psi/3000psi = 16% or ~13ft3 in an AL80). A 40ft3 stage for the dive profile originally posted is the minimum I would contemplate.

With all due respect … read the damn posts before you type.

Also there is no way to gain experience diving stages for longer and/or deeper dives, except by going out and diving them and getting that experience. Perhaps this is an unstated goal of the dive plan. On the day of the dive, if the sea conditions or diver's condition indicate the stage would be more of a liability than an asset, then leave it on the boat and dive a pre-planned abbreviated dive schedule rather than pushing the gas limit or getting in water over your head.

Once again - With all due respect … read the damn posts before you type!