12 Hours or 24 for full nitrogen off gasing?

[Karlbiederman]

I am Certified NAUI and was taught that after 24 hours it is not a repetative dive. However I have been helping a friend of mine who is an Instuctor Evaluator for MDEA teach some open water classes and according to their books they only wait 12 hours for a dive not to be repetative. That is a 12 hour difference and seems like a big descrepancy. I am not promoting one way or another Just wondering what you go by for your dive tables?

 

[DBailey]

Well, here are a few of my observation...

On the tables, in group "A" of the surface interval, you still have a top end. Why have that top end? If you go past it (typically 3:00 or 6:00 hours) is it no longer a repetitive dive? For example, if I waited 7:30 after my last dive and group "A" is 3:45 to 6:00, my SI doens't match anything on the table so I should be off-gassed.

On my computer, a repetitive dive is anything that occurs within the "no-fly" time. If I do two morning dives and a night dive, the following morning is considered a repetitive dive.

In the end as long as you plan your dive based on your residual nitrogen, repetitive dives will take care of themselves. It is the no-fly time that (18 hr or 24 hr) that is more important.

 

[SKBRDVR1]

I think the DAN studies that were conducted concluded that it was "greater" than 12 hours but less than 24. I've seen 18 hours used!!!

 

[oly5050user]

I am Certified NAUI and was taught that after 24 hours it is not a repetative dive. However I have been helping a friend of mine who is an Instuctor Evaluator for MDEA teach some open water classes and according to their books they only wait 12 hours for a dive not to be repetative. That is a 12 hour difference and seems like a big descrepancy. I am not promoting one way or another Just wondering what you go by for your dive tables?

depends on what the controlling tissue group is used for the table designed..
US Navy table used 120 minute group compartment as control group so theirs is a 12 hr to be considered "washed out" of residual nitrogen..(based on 6 half times)
PADI used 60 minute group compartment as control and thus requires a 6 hr time period(again based on 6 half times).I believe NAUI used the navy table so if they do your instructor is mistaken with a 24 hr. surface interval..MDEA most likely uses navy table with 120 min. tissue compartments.
For more info on half times look it up in padi's encly. of diving..

 

[DiveMaven]

I know what DAN says, etc., but for me if my computer hasn't had time to turn off then I consider it a repetitive dive and plan accordingly.

 

[Walter]

The PADI RDP clears you in 6 hours. The US Navy tables clear you in 12. DCIEM (and I think BSAC) clear you in 18. NAUI clears you in 24. NAUI merely took the US Navy tables, reduced the NDLs and added 12 hours to the A repetitive group. I've seen studies that suggest we aren't really clear for 48+ hours after diving.

The definition of a repetitive dive has always been a dive that takes place between 10 minutes and 12 hours after a previous dive. Most agencies have not altered this definition.

 

[rockjock3]

The PADI RDP clears you in 6 hours. The US Navy tables clear you in 12. DCIEM (and I think BSAC) clear you in 18. NAUI clears you in 24. NAUI merely took the US Navy tables, reduced the NDLs and added 12 hours to the A repetitive group. I've seen studies that suggest we aren't really clear for 48+ hours after diving.

The definition of a repetitive dive has always been a dive that takes place between 10 minutes and 12 hours after a previous dive. Most agencies have not altered this definition.

PADI is one that has. A dive is counted as the same dive if you surface and go back down within 5 minutes. So any SI over 5 minutes and less than 12 hours is a repetitive dive for PADI.

 

[divnamite]

Hehehehe, 5 minute surface break, that gives me enough time to break wind to off gas.

 

[oly5050user]

PADI is one that has. A dive is counted as the same dive if you surface and go back down within 5 minutes. So any SI over 5 minutes and less than 12 hours is a repetitive dive for PADI.

wrong on this..any dive within 6 hr period is considered a repetitive dive on a padi rdp..controlling tissue compartment is a 60 min compartment..based on this half time theory after 6 cycles(6x60 min=6 hrs)you are desaturated enough to consider that your nitrogen level as low enough to start fresh again..Theoretically you really never get rid of all absorbed nitrogen due to half time theory,it keeps on getting lower and lower but never reaches zero..of course when you get into rdp's "special rules "there are other factors taken in also to be more conservative..

 

[DA Aquamaster]

Half times are like Xeno's arrow. You can keep halving the distance from the point of the arrow to the target infinitely and theoretically never reach the target, but obviously the arrow does get there.

Deco theory is just that - theory. Compartment half times are only approximations of how tissue theoretically reacts to increases and decreases in partial pressure. How many compartments and how short or long the shortest and longest compartments should be varies from model to model. The passage of six half times should get the compartment down to about 1.5% of it's full capacity but it is still not "empty" and, like Xeno's arrow, it theoretically never will be completely "0" again.

So numbers of compartments, compartment half times and washout times all come down to whatever the developer of the model thinks is good enough based on what ever assumptions and conditions he or she is using and staying within those assumptions is what keeps you safe using the table.

One thing to consider then is that many tables (or computers for that matter) are not designed to be used for multi-level diving over multiple consecutive dive days and the left over bits in those theoretical compartments could theoretically bite you toward the end of your week long 2 or 3 dive per day dive vacation.

Personally, a table with a 6 hour out makes me very nervous, mostly because I grew up with US Navy based tables with a 12 hour out, but also because a 6 hour out lets you do potentially stupid things like make 3 dives that day, knock off at 3 pm and come back out for a night dive at 9 pm and not have to consider it a "repetetive" dive. What ever you use, understand what the assumptions and limitations are and play it much more conservatively in situations where those assumptions or limitations are stretched or even exceeded.