Dive Tables vs. Computer Algos - repetitive rec dives

[boulderjohn]

@boulderjohn

I use rule of 120 for first dive, then assuming a 2 hour SiT, then I use rule of 80 for dive 2 if for whatever reason I don't have computers or tables with me.

So what do you do on a dive boat that has a first dive followed by a one hour surface interval followed by a second dive followed by a return to port? Your plan would have you doing your second dive in the marina, where you will have to deal with shallow depths and lots of boat traffic.

 

[tbone1004]

@boulderjohn I wouldn't ever get on a boat without at least a set of tables, usually a pair of computers so I don't have to worry about it. The only times that I'll follow those "rules" are if I'm diving in a quarry, lake, or some other benign environment where I know going in that I'm not going to go into deco and couldn't be bothered to grab a spare computer *usually if someone wants to dive my computer to try it out or whatever*. I don't use that on any "real" dives.

@dmaziuk the discussion regarding those compartments is interesting. So the 60 minute compartments with DSAT doesn't say that there aren't slower compartments, just that they don't accumulate fast enough to matter within recreational no decompression limits. Which is probably true. Now they will obviously have nitrogen "in them" and it will accumulate over time, but will it ever get to a point that that is the "leading compartment" to put you into a no deco time.
*not a PADI guy*, but if they say don't dive every third day, then those slow compartments theoretically build up enough over those two days, that they want them to clear out over a long SiT. Haven't run the numbers, not going to bother because I don't care if I go into deco, but the point is that IF you are diving truly square profiles to the T with PADI tables, there isn't a computer sold today that I am aware of that won't stop you before the dive tables.

 

[boulderjohn]

*not a PADI guy*, but if they say don't dive every third day,

Where do they say that?

 

[tbone1004]

Where do they say that?

@dmaziuk said the authors of DSAT said that, I've never seen/heard of that before

 

[boulderjohn]

@dmaziuk said the authors of DSAT said that, I've never seen/heard of that before

Neither have I. I have certainly never said that to a student.

 

[boulderjohn]

Well theoretically authors of DSAT recommended that you don't dive every third day

I have never heard this. Can you provide a link?

Theoretically the slowest "tissue compartment" in DSAT in 60 minutes.

No, the slowest tissue compartment in DSAT is not 60 minutes. They chose the 60 minute compartment to guide surface intervals on dives completed within the limits of the table. They do not deny the existence of those other compartments, and they are considered in all the planning.

When you dive to the limits of the tables on repetitive dives, they invoke special rules which have the effect of having a slower compartment guide the dives. These are the WX and YZ rules. When doing multiple dives that bring you to those pressure groups on any of those dives, you follow those special rules. If you reach the W or X pressure group, you must have a minimum of 1 hour surface interval on all subsequent dives. If you reach Y or Z, your minimum surface interval is 3 hours.

 

[dmaziuk]

I have never heard this. Can you provide a link?

Hamilton, Rogers, and Powell. (1994) Development and validation of no-stop decompression procedures for recreational diving: the DSAT recreational dive planner @ http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/4228/DSAT_1994.pdf?sequence=1 has as teh last sentence of "I. ABSTRACT":

Even so, based on this evidence and suggestions of other experts, we recommend limiting the number of full-time dives per day to 3 or at most 4, and suggest including a day with reduced level of diving (or none) every 2 or 3 days.

OP: speaking of "testing" tables and computer algorithms, "Decompression testing and statistics" on p.13 seems particularly relevant, though the entire paper is well worth reading.

 

[boulderjohn]

Hamilton, Rogers, and Powell. (1994) Development and validation of no-stop decompression procedures for recreational diving: the DSAT recreational dive planner @ http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/4228/DSAT_1994.pdf?sequence=1 has as teh last sentence of ...

Thanks. I had never seen nor heard of this before. I have certainly never heard it said in instruction.

 

[dmaziuk]

No, the slowest tissue compartment in DSAT is not 60 minutes. They chose the 60 minute compartment to guide surface intervals on dives completed within the limits of the table. They do not deny the existence of those other compartments, and they are considered in all the planning.

Correct, I was wrong to say the slowest compartment it "has" is 60 minutes. Figure 6 illustrates it rather nicely: 2-hour compartment gets to M-value only by day 6, and 8-hour one: never. So the slowest compartment that "matters" is 60 minutes -- as long as you don't go over 6 4-dive days.

When you dive to the limits of the tables on repetitive dives, they invoke special rules which have the effect of having a slower compartment guide the dives. These are the WX and YZ rules. When doing multiple dives that bring you to those pressure groups on any of those dives, you follow those special rules. If you reach the W or X pressure group, you must have a minimum of 1 hour surface interval on all subsequent dives. If you reach Y or Z, your minimum surface interval is 3 hours.

It's in C.2 on p. 51: they saw noticeable difference in doppler bubbles between groups X-Z and K-W, however, they go on to say that 6 dives/day would produce much higher doppler scores than either. Hence the special rules, but if you look at the table there, they're for <= 4 tanks/day (and you might want to apply X-Z rule to group W as well).

 

[Jay]

No. The PADI tables have not changed since they were first created. The 25 minutes at 100 feet was (and still is) part of the Navy tables and those that are derived from them. If you look at post #11 from @tbone1004, you will see ...

Yes, I did read and understand his post, as well as yours, but I did have it dead-set in my mind that there had been alterations. I'm unable to locate my actual PADI tables from the year when Bananarama's Venus was #1 but I'm fairly* certain this is them courtesy of the internet: (and just to be clear, when I said PADI tables in my earlier comment, I meant PADI produced/branded ones - irrespective of their source)

[*meaning no flames]