Deco Diving - Computer Help Needed (Cobra 3, or others!)

Good afternoon,

I just picked up a Cobra 3, and I am trying to understand the decompression mode. I have no intention of being on a deco dive anytime soon, but I would really like to understand these features. If you have a Cobra 3, great! If not, please see if you can answer my questions.

I know that when you stay down past your no-deco limit, the computer enters the deco mode. At that time, "ceiling," ""stop" and "asc time" is displayed. I have read the manual, and watched the Dive Computer Training videos, but still have questions:

(1) "Ceiling" indicates that, based on where you currently are, you must slowly assend up to slightly below the ceiling (usually 30 feet, right?) and hang out there for a deco stop. Apparently, this number could change once you are at the ceiling - but why would this number everf change?

(2) "Asc Time" means, I believe, the total time it will take you to complete the ENTIRE decompression process from where you are until you get to the surfance. Right? If that is the case, where is the number that tells you how long to stay at each deco stop? Shouldn't there be a countdown timer at each stop?

(3) I am also having difficulty generally understanding the differences between the floor and ceiling concepts with the Cobra 3. I get that the ceiling is where you need to hover near to decompress, but what is the floor? Suunto confusingly explains this as "The deepest depth at which the decompression stop time will not increase." What does that mean?

Thanks!

Simply,

Ceiling is a term for a depth that you must stay below
floor is a depth you must stay above

In deco mode you will be guided eventually to the surface, making one or more stops. These stops have a floor and a ceiling you must stay between for the amount of time the computer gives you. Once complete you will move (shallower) to the next stop or surface. If you go above the ceiling it will tell you to go back down, if you go below the floor you will increase the deco time as you are "on-gassing", taking on nitrogen.

Make sense?

decompression:

Simply,

Ceiling is a term for a depth that you must stay below
floor is a depth you must stay above

In deco mode you will be guided eventually to the surface, making one or more stops. These stops have a floor and a ceiling you must stay between for the amount of time the computer gives you. Once complete you will move (shallower) to the next stop or surface. If you go above the ceiling it will tell you to go back down, if you go below the floor you will increase the deco time as you are "on-gassing", taking on nitrogen.

Make sense?

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That makes sense. Have you (or anyone else on the forum reviewing this) ever used a Cobra 3? I would love to know the answer to questions above regarding what is displayed on the screen (and particularly question no. 2).

For most computers, you want to come up to the ceiling and wait. If you went into deco a little, the ceiling may be 15 ft. If you went into deco "a lot" then it may be at 30 feet (or more).

Once you are in deco, you probably want to come up at 30 feet per minute until you reach the ceiling. Stay just below the ceiling, depending on the depth of the ceiling, it will decrease (indicating it is time to do a shallowerer stop).

The ascent time is the entire time it takes from NOW until the surface, if you follow the schedule exactly. If you deviate from the schedule, then the ascent time may be a little longer.

I haven't used a cobra but I have (had) many Suunto's; Octopus2, Vyper, Vytec, geckko, D3. The other computers I've had are also similar with respect to deco mode.....essentially it tells you where to be and how long. You can increase your understanding of what's going on in deco by just a bit of googling for decompression theory. If you can understand "on gasing, off gasing, bubble mechanics, tissue compartments, gradients, haldanian/buhlman theories, O2 windows, ratio deco, minimum deco and more" you are on your way.........

First - bear in mind that most recreational diving computers provide 'emergency' deco info... a "get out of jail card" solution. Nothing more, nothing less.

Most importantly, and without appropriate training and knowledge, you won't be able to predict what the computer will do in any other circumstance other than a direct, immediate ascent according to the computer read-out. Even then, with varied dive profiles and multi-dive schedules, the results may not conform to vague expectations.

1) They display a ceiling. That is the maximum depth you can ascend to. If you exceed that depth (rise shallower) you stand an unreasonable risk of DCS. Computers normally indicate a range for optimal decompression/off-gassing - typically starting a couple of meters below the ceiling.

2) They display a floor. That is the maximum depth at which you will begin to off-gas. Below that, you will still accumulate deco. Above it, you should start reducing deco - but it is fluid and changes over time/depth. Off-gassing become more efficient as you near/reach your ceiling. Depending on your dive profile and residual nitrogen from previous dives, the indicated floor will vary, as will the rate at which you off-gas.

3) They provide an ascent time. This is the total time to surface. It's the only number that matters for most divers, because it allows them to cross-reference with their air supply.

There isn't a 'count-down' at each ceiling... you ascend to a given 'ceiling' until that ceiling raises. When the final ceiling disappears, you ascend to the surface - although some computers/algorithms give you a 3 min safety stop also. Most computers mandate an ascent rate of 8-10m per minute.

It's aimed to be simple and fool-proof because it is, as mentioned, a contingency resolution to get recreational divers to the surface with minimal chance of DCS, if they exceed an NDL. In that respect, simplicity is key and it doesn't assume any knowledge of more advanced (appropriate) decompression procedures.

In essence... you exceed an NDL... you immediately ascend at the indicated rate for your computer. You stop at, or just below, the given ceiling and wait until that ceiling expires. Then you ascend again at the given rate.

Please, help a french diver to understand why this subject is in the "technical diving specialties" room ?

So, based on the screen of the Cobra 3, if I accidentally enter deco mode and I am at 75 feet, and the computer displays a ceiling of 30 feet, and an ascent time of 10 minutes, and if it takes me two minutes to reach the ceiling of 30 feet, and my ascent time now says 8 minutes (because I just traveled up for two minutes), does that now mean that I have to stay at this ceiling of 30 feet for the 8 minutes (meaning that my ascent time becomes, or equals, my deco countdown time to stay at the ceiling for decompresion)?

DuboisP:

Please, help a french diver to understand why this subject is in the "technical diving specialties" room ?

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Deco = Technical. Don't you french people know that!

DitrasetMan:

So, based on the screen of the Cobra 3, if I accidentally enter deco mode and I am at 75 feet, and the computer displays a ceiling of 30 feet, and an ascent time of 10 minutes, and if it takes me two minutes to reach the ceiling of 30 feet, and my ascent time now says 8 minutes (because I just traveled up for two minutes), does that now mean that I have to stay at this ceiling of 30 feet for the 8 minutes (meaning that my ascent time becomes, or equals, my deco countdown time to stay at the ceiling for decompresion)?

Click to expand...

Someone has to know this...