Accidental Deco

[boulderjohn]

You should not be using a recreational computer to plan for unplanned decompression. You should always plan to have plenty of gas for a dive, but if you think there is a chance you will exceed NDLs by 10 minutes, then there is something seriously wrong with your planning.

Every recreational computer I know says very clearly in its manual that its decompression algorithm is deigned for unplanned deco only and should not be used for planned decompression diving.

 

[chrpai]

Totally understood. Maybe I'm not saying it right.... if you are the type of diver that somehow got yourself past NDL by 10 minutes... I've never quiet understood the value of your computer now calculating your stop times as you'd first have to bring enough gas to be able to do those stops. Not that you'd plan to exceed NDL by 10 minutes but you'd have to have planned enough to have that much gas in reserves. I'm guessing the person who did this no longer has 1/3 of his single 80 left and probably doesn't have a 40 slung at his side.

 

[tamas970]

That's right, for planning/reserve sizing you need a PC/table, that handles deco. However, you don't have to do it all the time, since you are not planning a deco dive without certification, just preparing for certain contingencies. Or take a look at this: Ascending From A Dive (ascent rates, deep safety/deco stops & time limits)
It says an ascent from 130Ft requires 1-2min deep stops in about every 10Ft and a longer stop at 15Ft. According to their algorythm, you are on the safe side if you calculate with 30min longer dive time = you need the corresponding reserve (and also means, that the typical 6-20CuFt ponies are useless for deco-ascent). Of course, the algorythm of the dive computer might be different, in an "oh s#" scenario I'd follow my computer and extend the last two stops as far as my air lasts.

Every recreational computer I've owned that can calculate "oh s***" stops has no way of planning those stops. So how would you go about calculating how much more air is needed? You might use some dive planning software to get an idea but the computer isn't going to tell you. Right?

 

[Mr Carcharodon]

My el-cheapo Subgear XP10 (cheapest computer I could find) can calculate for deco stops. If someone dives 30+m/100+Ft deep, get a dive computer and get familiar with deco procedures. E.g. calculate, how much more air is needed in case of a 10min NDL overdue at 40m/133Ft. Some research on reasonable reserves/nitrox pony can enhance your safety a lot while deep-diving within rec. limits. (Anyway, NDL is not hard science, even if observed, additional safety never harms.)

Every recreational computer I've owned that can calculate "oh s***" stops has no way of planning those stops. So how would you go about calculating how much more air is needed? You might use some dive planning software to get an idea but the computer isn't going to tell you. Right?

A ratio deco course would provide the framework to do the deco planning. Gas planning is the other side of the coin. Of course for RD to work you have to maintain awareness on your dive which should avoid accidents in the first place. The other advantage of RD is being able to update you plan real time which would occur if you overstayed.

 

[chrpai]

I thought I read somewhere that RD is for standard gases (Trimix) only. No?

 

[dumpsterDiver]

That's right, for planning/reserve sizing you need a PC/table, that handles deco. However, you don't have to do it all the time, since you are not planning a deco dive without certification, just preparing for certain contingencies. Or take a look at this: Ascending From A Dive (ascent rates, deep safety/deco stops & time limits)
It says an ascent from 130Ft requires 1-2min deep stops in about every 10Ft and a longer stop at 15Ft. According to their algorythm, you are on the safe side if you calculate with 30min longer dive time = you need the corresponding reserve (and also means, that the typical 6-20CuFt ponies are useless for deco-ascent). Of course, the algorythm of the dive computer might be different, in an "oh s#" scenario I'd follow my computer and extend the last two stops as far as my air lasts.

Can you explain again why a 20 cu-ft bottle is useless for deco? I'm confused?

 

[flots am]

Totally understood. Maybe I'm not saying it right.... if you are the type of diver that somehow got yourself past NDL by 10 minutes... I've never quiet understood the value of your computer now calculating your stop times as you'd first have to bring enough gas to be able to do those stops. Not that you'd plan to exceed NDL by 10 minutes but you'd have to have planned enough to have that much gas in reserves. I'm guessing the person who did this no longer has 1/3 of his single 80 left and probably doesn't have a 40 slung at his side.

The "oh sh**!" stops on a computer are usually pretty shallow. If you're diving with a buddy, you can show him your computer and get him to bring down more gas for you. On a boat dive, there's always someone who came back with a half a tank.

In any case, you do what you can do. Stay down and clear the stops, or stay as long as you can and get on O2 on the surface.

flots.

 

[Mr Carcharodon]

I thought I read somewhere that RD is for standard gases (Trimix) only. No?

EAN32 is a standard mix.

 

[boulderjohn]

A ratio deco course would provide the framework to do the deco planning.

I thought I read somewhere that RD is for standard gases (Trimix) only. No?

EAN32 is a standard mix.

Perhaps there is a terminology problem here.

In a Ratio Deco class, you can learn a NDL system usually called Minimum Deco. It is a system for calculating NDLs and the appropriate ascent. In the context of this discussion, it would probably be more accurate to use the term Minimum Deco than Ratio Deco.

but it is for EANx 32, so it won't do much good for someone diving air or another nitrox mix.

 

[rjack321]

but it is for EANx 32, so it won't do much good for someone diving air or another nitrox mix.

You can use ratio deco with any mix(es). In fact the original rules were based on air with EAD applied.

Its a good online course, you can find it on the UTD website under online learning.