Place of dive tables in modern diving (Split from the basic thread)

[Eric Sedletzky]

Even going by the RDP, 7 minutes of "deco" is unwarranted for a 40 minute dive to 20m.

A dive to 20m would be counted as a dive to 70’.
40 minutes is right at the max NDL at 70’, so according to the RDP a direct ascent at max 60 fpm and a safety stop at 15’ for three minutes is sufficient. I don’t know where you get seven minutes, eight minutes is the next number after 1-5 minutes of overstay. Are we looking at the same RDP, or is there a later version I don’t know about?

 

[dmaziuk]

Actually, I’m looking at my PADI RDP right now. For others who have never seen a PADI table, here goes:
...
2. A safety stop is required any time a diver comes with three pressure groups of a no-decompression limit, and or a dive to 100’ or greater.

Yep. PADI Recreational Dive Planner lets you plan no-stop dives where a stop is required. Also if you're in pressure group A on your first dive ever, you can go to 40 metres for 9 minutes, however, if you're in a pressure group A after 6 hours SI, you can only go to 40 metres for 7 minutes.

An excellent teaching tool indeed, as long as your students are OK with 9 = 7 and no means yes.

 

[boulderjohn]

Yep. PADI Recreational Dive Planner lets you plan no-stop dives where a stop is required. Also if you're in pressure group A on your first dive ever, you can go to 40 metres for 9 minutes, however, if you're in a pressure group A after 6 hours SI, you can only go to 40 metres for 7 minutes.

An excellent teaching tool indeed, as long as your students are OK with 9 = 7 and no means yes.

The only way you can be in pressure group A "on your first dive ever" is if you ascended to altitude from a lower elevation. In that case, you must treat your first dive as if it were a second dive and add pressure groups depending upon altitude changes.

A diver is not in ANY pressure group for the first dive otherwise. Being in Pressure Group A is NOT the same as doing a first dive.

 

[tursiops]

Also if you're in pressure group A on your first dive ever, you can go to 40 metres for 9 minutes,

This is certainly NOT on the RDP! Have you mistyped something?

The only way you can be in pressure group A "on your first dive ever" is if you ascended to altitude from a lower elevation. In that case, you must treat your first dive as if it were a second dive and add pressure groups depending upon altitude changes.

However, there is NO pressure group that would give you 9 minutes at 40m; the lowest pressure group is A, and that would only allow 7 minutes. So, yes, altitude procedures would give you an artificial pressure group (and thus Residual Nitrogen Time) for your first dive, but one adds TWO pressure groups for each 1000ft of altitude, so even for a 1000ft-altitude dive, you'd start in PG B, which would only give 5 mins NDL. If you waited 47 minutes before doing that first dive at altitude, you'd drop to PG A, and thus back to 7 mins NDL. But never 9 mins NDL.


ADDED AS AN AFTERTHOUGHT:
This demonstrably reinforces the concept that tables are hard for people to use....

SECOND AFTERTHOUGHT:
The metric PADI RDP shows 9 mins NDL for 40m, not 10 mins for 130 ft, even though 40m and 130 ft are often (sloppily) used interchangeably. However, Pressure Group A does not even exist on the metric table for 40m; you use PG B, which is 5 mins RNT, so you'd only have 9-5=4 mins NDL at 40m.

 

[Dan_P]

A dive to 20m would be counted as a dive to 70’.
40 minutes is right at the max NDL at 70’, so according to the RDP a direct ascent at max 60 fpm and a safety stop at 15’ for three minutes is sufficient. I don’t know where you get seven minutes, eight minutes is the next number after 1-5 minutes of overstay. Are we looking at the same RDP, or is there a later version I don’t know about?

You're exactly right and that's my point

The study in question [1] did much more stop time than called for, and further, tested deep stops deeper/longer total than called for in Min Deco, which is what caught my attention.

If I did a 20m 40 minute dive on air, I'd probably do 4 minutes total, including travel time (10m/min), averaging the deep stops separate at 6m.

[1] Schellart N et al. Bubble formation after a 20m dive: deep stop versus shallow stop decompression profiles. Aviat Space Environ Med 2008; 79:488 – 94.

 

[tursiops]

You're exactly right and that's my point

The study in question [1] did much more stop time than called for, and further, tested deep stops deeper/longer total than called for in Min Deco, which is what caught my attention.

If I did a 20m 40 minute dive on air, I'd probably do 4 minutes total, including travel time (10m/min), averaging the deep stops separate at 6m.

[1] Schellart N et al. Bubble formation after a 20m dive: deep stop versus shallow stop decompression profiles. Aviat Space Environ Med 2008; 79:488 – 94.

I'm not sure you read the Schellart paper carefully. It says: "Then, at 10 msw/min an ascent was made either to 10 msw (DSD) or to 4 msw (SSD). DSD comprised a stop at 10 msw during 4 min (including ascent time, as also holds for the shallow stop), and a stop at 4 msw during 3 min. For SSD a 7-min stop was performed at 4 msw. Thus, irrespective of the type of dive performed, total ascent time was 7 min 24 s."
The deep-stop (DSD) divers took one minute to get from 20m to 10m, then stayed for 3 mins, then went to 3m for 3mins. Total ascent time 7 mins 24s. The shallow-stop divers (SSD) just ascended to 3m and waited out the same total ascent time, so they had a longer safety stop. This was consistent with the goal of comparing two groups of divers who spent the same total amount of time getting from the bottom to the surface, but spending more of the total time deep. Is this not what their hypothesis was?

 

[Dan_P]

I'm not sure you read the Schellart paper carefully. It says: "Then, at 10 msw/min an ascent was made either to 10 msw (DSD) or to 4 msw (SSD). DSD comprised a stop at 10 msw during 4 min (including ascent time, as also holds for the shallow stop), and a stop at 4 msw during 3 min. For SSD a 7-min stop was performed at 4 msw. Thus, irrespective of the type of dive performed, total ascent time was 7 min 24 s."
The deep-stop (DSD) divers took one minute to get from 20m to 10m, then stayed for 3 mins, then went to 3m for 3mins. Total ascent time 7 mins 24s. The shallow-stop divers (SSD) just ascended to 3m and waited out the same total ascent time, so they had a longer safety stop. This was consistent with the goal of comparing two groups of divers who spent the same total amount of time getting from the bottom to the surface, but spending more of the total time deep. Is this not what their hypothesis was?

If you need to do 4 minutes total ascend and instead do 7 minutes, your "overstay" means relatively increased gas loading compared to off-gassing.
That increased gas loading would in this case be either at 4m, or part 4m and part 10m.

I feel it stands to reason that extending stops beyond what's called for by the bottom portion of the dive, will always be disadvantageous, in particular to the divers stopping deeper - in my logic, it would make more sense to test the actual ascends called for by the major overall schools of thought in diving (recreational):

Ascend at 10m/min (possibly run a third group doing 18m/min ascends) to 5m for a 3-minute stop, or
Ascend at 10m/min to half depth, then 1 minute stops in 3m-intervals to the surface.

 

[dmaziuk]

The metric PADI RDP shows 9 mins NDL for 40m, not 10 mins for 130 ft, even though 40m and 130 ft are often (sloppily) used interchangeably. However, Pressure Group A does not even exist on the metric table for 40m; you use PG B, which is 5 mins RNT, so you'd only have 9-5=4 mins NDL at 40m.

This demonstrably reinforces the concept that tables are hard for people to use....

Show me the schedule, using PADI RDP, that lets me dive to 40 metres for 9 minutes, and then dive to 40 metres for 9 minutes at some later point. It should be doable, right? When? The best I can offer is 24 hours and that number is not in the tables.

 

[tursiops]

Show me the schedule, using PADI RDP, that lets me dive to 40 metres for 9 minutes, and then dive to 40 metres for 9 minutes at some later point. It should be doable, right? When? The best I can offer is 24 hours and that number is not in the tables.

Why do you keep talking about a second dive? And are you talking about 40m or 130 ft?

 

[tursiops]

Show me the schedule, using PADI RDP, that lets me dive to 40 metres for 9 minutes, and then dive to 40 metres for 9 minutes at some later point. It should be doable, right? When? The best I can offer is 24 hours and that number is not in the tables.

There is no mystery with this. The metric tables show 9 mins at 40m, exiting in PG G. After 4h42min SI, you can do it again. That's right off the table, side 1, clear as day. What is the problem?