Surface Interval

[String]

Thats one advantage of BSAC tables for anyone desperate enough to use them, a time vs tissue code graph.

 

[ShakaZulu]

I don't know enough about that to answer your question, ask some of the TECH dewds on this board. Mech Diver or Lamont, knows there stuff........I do believe increasing you SI will lower your PG, thus allowing a longer NDL on the second dive, but I don't know the ratio's.

 

[Xanthro]

Right, but what I was really wondering is if something like this is the case:

"After X minutes of surface interval, staying on the surface any longer than that doesn't give you much incremental bottom time on the next dive".

For example, does, say, the first hour of SI give you the biggest bang for the buck in terms of NDL for then next dive, or is it a linear function, where 20% more SI gives you 20% more NDL.

Look at your tables, get your pressure group, then look at the surface intervals to the right. It's group by pressure group.

The actual best SI will depend on depth and time.

Xanthro

 

[northcoast_diver]

Since there really is no "right" amount of surface interval, what do most people shoot for? I know the longer the better, but do most folks shoot for a particular duration before heading down again? I'm wondering if there's a duration that gives you a bigger bang for the buck, or is it a linear absorption? I guess I could look at my tables...

For NDL diving I plan on 1 hour surface intervals. That gives me time to rehydrate, pee, have a sugary snack, and change tanks.

For deco diving I plan on 3 hour surface intervals. This is Dr. BRW's current RGBM standard.

 

[northcoast_diver]

I don't know enough about that to answer your question, ask some of the TECH dewds on this board. Mech Diver or Lamont, knows there stuff........I do believe increasing you SI will lower your PG, thus allowing a longer NDL on the second dive, but I don't know the ratio's.

Shaka is exactly right. For planning multiple repetitive dives, longer surface intervals will give you longer NDLs.

Here are the extremes.

For a 10 minute or less S/I, most tables and software do not give you any credit for any surface interval time, and it is as if your repetitive dives are each a part of one continuous dive. Hence you are working off one original NLD time.

For a 12 hour surface interval, most tables and software clear your theoretical compartments completely. Some tables and software use 24 hours. Some even longer.

For anything in between 10 mins and 12 hours, the curve is exponential, with most of the benefit occurring during the first few hours of S/I time.

 

[Don Burke]

Right, but what I was really wondering is if something like this is the case:

"After X minutes of surface interval, staying on the surface any longer than that doesn't give you much incremental bottom time on the next dive".

For example, does, say, the first hour of SI give you the biggest bang for the buck in terms of NDL for then next dive, or is it a linear function, where 20% more SI gives you 20% more NDL.

On the PADI tables, three hours will take you from group Z to group A.

The first part of the surface interval has more offgassing due to higher differentials, although the curve does not have all that much curve to it.

There is no "sweet spot" that I can find.

 

[northcoast_diver]

On the PADI tables, three hours will take you from group Z to group A.

The first part of the surface interval has more offgassing due to higher differentials, although the curve does not have all that much curve to it.

There is no "sweet spot" that I can find.

So, Mr. Burke, do you feel "safe" when using the PADI tables?

I think I would prefer the USN tables instead. NAUI's tables are USN, but with shorter NDLs.

 

[voop]

Well, I do not really "shoot for" anything specific, SI-wise: whatever nitrogen loading I have in my system gets factored in to the plan for subsequent dives and amounts simply to (i) longer deco and thus (ii) more gas requirements. Neither are really a problem.

If you really want to push SI's down (or your margin up), breathing pure O2 on your SI helps offgassing -- tables and procedures to factor in the effect of this exists. (NOTE: don't do this without appropriate training, you can get seriously hurt!)

There's nothing really to "shoot for". N2 (or He-) loading is just a fact of diving which we must deal with.

 

[Viscya]

...
If you really want to push SI's down (or your margin up), breathing pure O2 on your SI helps offgassing -- tables and procedures to factor in the effect of this exists. (NOTE: don't do this without appropriate training, you can get seriously hurt!) ...

Yeh, Lets go for several hours of O2 exposure.


I shoot for 1:30 on recreational dives 80-120fsw. After that you are reaching diminishing returns. If you are going deep and have to account for O2 exposure also 2:00 is a good minimum.

 

[voop]

Yeh, Lets go for several hours of O2 exposure.


I shoot for 1:30 on recreational dives 80-120fsw. After that you are reaching diminishing returns. If you are going deep and have to account for O2 exposure also 2:00 is a good minimum.

Hey, hence the disclaimer of "don't do this without training" It ain't black-and-white, and in some situations, SI-O2 can make a healthy difference when used correctly. Of course, in other situations, it's just begging for problems.....