Surface interval

[1RUSTYRIG]

well... a small surface interval is one in which you drink a beer

a good surface interval gives you time for a shot of whisky and a beer

a long surface interval is two beers and a whisky or two whiskys and a beer

You have it all wrong Andy...

A long surface interval is called "happy hour" dude...get it right!

 

[boulderjohn]

Because I'm aware of only one such table and it is way too liberal for me to consider it safe. The damned thing says you're clear on N2 after only 6 hours. The thing is scary!

Assuming of course, that you are talking about the PADI table, it does not actually say that.

In creating a dive table, the slowest compartment that can possibly affect the next dive is the one that determines the surface interval planning. Because the Navy divers are usually involved with longer first dives, often decompression dives, it makes sense for them to use the 120 minute compartment for planning. The 120 minute compartment washes out in 12 hours, but there is still N2 in the system. It is in the very slow tissues, though, that will not reasonably affect dive planning.

In creating its tables, PADI theorized (and used Doppler bubble observations to confirm) that by requiring a more conservative first dive than the Navy tables allow and by not allowing decompression diving, it made more sense to use the 60 minute compartment for planning. The 60 minute compartment washes out in 6 hours. Like the Navy tables, it assumes that the remaining N2 is in slower tissues that will not reasonably affect dive planning.

In summmary, the PADI table assumes that people who use it are recreational divers only. It says that the tissues that affect planning are free of N2 within 6 hours; it does not say the entire body is free of N2 after 6 hours. It is intended for recreational use. If you are diving outside of the limits of recreational diving, you should not use it.

 

[Walter]

I've been hearing that line for years, I didn't believe it the first time I heard it in the late '80s and I don't believe it now. I do agree with your last phrase.

 

[boulderjohn]

I've been hearing that line for years, I didn't believe it the first time I heard it in the late '80s and I don't believe it now. I do agree with your last phrase.

What is it you don't believe?

 

[sweatfrog]

I've been hearing that line for years, I didn't believe it the first time I heard it in the late '80s and I don't believe it now.

Wow, you really are on the cutting edge, since PADI didn't release the current version until 1998. The research they did was in Washington State. A long way from Florida.

 

[boulderjohn]

I've been hearing that line for years, I didn't believe it the first time I heard it in the late '80s and I don't believe it now. I do agree with your last phrase.

I am seriously interested in knowing what it is about what I wrote that you don't believe. Since just about everything I wrote is simple historical fact, if you are disputing the facts, you are saying that I misunderstood, was misinformed, am mistating, or lied.

I rather think you are disputing the theoretical basis for the difference in the tables, which would lie in one or more of three areas:

1. The decision to make the first dive more conservative
2. The decision not to allow decompression diving
3. The decision to use the 60 minute compartment as the basis for planning

Since you have said the table is not sufficiently conservative, my guess is that it is only the third item that bothers you.

Is that true? If so, why is it wrong to use the 60 minute compartment in recreational diving? Why is the 120 minute compartment more appropriate?

 

[Thalassamania]

You know, this whole table thing's gotten a bit strange. In the english speaking world there are only three fully tested sets of tables (and even those are not "fully" tested) U.S. Navy, DCIEM and Royal Navy. Everything else is, I believe, either a reformating or conservative rounding of one of those tables. There are a few with some scienfic basis Buhlman, Huggi and DSAT, but that are short on testing.

What we see now is a tower of babble where divers trained under different agency standards do not know where their tables came from, how they compare to other tables or even the base tables (U.S. Navy, DCIEM and Royal Navy) or know how to use their own or the base tables. Frankly it does speak well for the industry.

When I use tables I dive U.S. Navy with a one letter group safety factor, and that's that. When I use computers a try to make them simiarly conservative. And I breathe a LOT OF OXYGEN!

 

[Charlie99]

There are a few with some scienfic basis Buhlman, Huggi and DSAT, but that are short on testing.

Interesting. All of the references I've read say that DSAT did quite a bit of testing, including doppler monitoring, and REPETITIVE dives.

Huggi, USN/Workmann, PADI/DSAT, and Buhlman are really all the same dissolved gas algorithm, with very minor differences between the half times chosen and the dissolved N2 pressure limits for the various compartments. The naked, unpadded algorithms are much closer to each other than most computers are to the algorithm they supposedly implement --- i.e. the amount padding or extra conservatism of most computers is more significant than the differences between the various dissolved gas models.

Of all of the models you mentioned, only DCIEM is unique. It assumes serial loading of the compartments, all the other ones assume parallel. On the DCIEM tables, repetitive dive NDL limits are set as a fraction of the 1st dive NDL (i.e. one uses multiplication), rather than assuming a residual nitrogen level that must be subtracted from a 1st dive NDL to get a repetitive NDL. DCIEM is still a dissolved gas model though.

The other basic type of model is the dual phase, aka bubble model. VPM and RGBM are two examples.

 

[m3830431]

I was taught that if you surface for five minutes or less and then head back down, it's all counted as the same dive.

Most computers require 10 minutes.

 

[Thalassamania]

I'm not trying to say that one is better or worse than another, just that we're loosing the ability to communicate with each other on the altar of marketing.