Depth in multiple dives

[okvij]

I know the rule for multiple dives - start with the deepest dive and continue in descending order of depth. I have asked several time for the reasoning behind this and got several different answers of which I couldn't make sense and don't even remember.

Can anyone explain the rationale here? Thanks.

 

[KenGordon]

I know the rule for multiple dives - start with the deepest dive and continue in descending order of depth. I have asked several time for the reasoning behind this and got several different answers of which I couldn't make sense and don't even remember.

Can anyone explain the rationale here? Thanks.

Your NDL will be relatively longer for later dives in the sequence that way, so more time underwater overall.

It was also a bit overstated as a hard rule in the past. You can do a shallow dive followed by a deeper dive, you just need to plan for the deeper dive being shorter that way.

The reason you get poor answers is there is not a good answer and people don’t like admitting ‘because that is what my instructor told me’.

 

[boulderjohn]

A 2001 workshop was convened to answer that very question. In that workshop, it was determined that the very first mention of this was a suggestion (not a rule) in the 1972 PADI OW manual. No one at PADI knew who had made that suggestion or why it was made. Over the years, the suggestion eventually became a rule, still with no reason. The workshop determined that there was no reason for the rule in recreational diving, although they did bow to participant Bruce Weinke's argument that it did make some sense in technical diving.

Common sense will tell you the most likely reason for the rule with near certainty. IN those days, diving was guided by the US Navy tables. Those tables called for very long surface intervals to begin with. It made a huge difference (as it does with all tables), though, whether you did the deepest dive first. If you did the deepest dive first, the surface interval before a planned second dive was much less than if you did the challower dive first.

In short, it is not a matter of safety. If you have had enough surface interval to do your planned second dive, you can do that planned second dive safely, even if it is deeper than the first. If the second dive is deeper, you will just have to wait a bit longer to do it and get the full time length you intend.

 

[Lake Hickory Scuba]

Hopefully this will help explain it for you.

 

[tbone1004]

basically what was said above. Physiologically? Not a whole lot of reason to go one way or another. as @boulderjohn said above though, if you follow USN dive tables or any variants of *NAUI, old PADI, etc*, then you are going to be in for a world of hurt trying to do a reverse profile as the USN tables were designed for one long dive per 24hr period not repetitive diving.

 

[АлександрД]

I suspect, that main reason for this rule - is economical reason
Many recreational diving companies, who arrange diving tours has standard schedule:
Dive for 40-50 min, lunch, another dive for 40-50 min.
40-50 min - it is average diving time for "standard spherical diver" to have enough pleasure and stay safe (coldness, NDL, weakness and other reasons), and left 50 bar in tank at the end.
For this schedule is better to make second dive more shallow to keep same diving time as for the first dive.

 

[Jamesdclxxv]

Hopefully this will help explain it for you.

Is this a limitation of diving with tables or do computers give reduced ndls for reversed profiles too?

 

[hhmoore]

I believe they would...but, depending on the type of dive, not to the same extent. Most of my diving is from shore; so, while I may go to a depth of 60 ft, only a small portion of my dive is at that depth. The chart creates a square profile, using max depth and total time; whereas the computer recalculates when you change depth (increasing NDL as your depth decreases).
If you were boat diving, however, and spending your time at the planned depth, the computer won't be as different from the tables.

 

[tbone1004]

Is this a limitation of diving with tables or do computers give reduced ndls for reversed profiles too?

that's a bit complicated.

So using Deco Planner on 50/80 with EAN32
100ft gives you 17 mins of NDL
1 hour sit
80ft for 33 mins with NDL

There is no difference in NDL if you reverse those profiles *with deco planner it always gives at least a 1 min stop at 10ft, so I muscled it until it showed 2 mins at 10ft and that was my "NDL".

If I use VPM+3
100ft for 22 mins
1 hour sit
80 for 32

reversed
80 for 36
1 hour sit
100 for 17

So with Buhlmann at least with those profiles, there is no benefit. With VPM you get essentially the same total dive time *53 vs 54 mins* but it is distributed differently depending on where you want to prioritize your dive time

 

[boulderjohn]

I suspect, that main reason for this rule - is economical reason
Many recreational diving companies, who arrange diving tours has standard schedule:
Dive for 40-50 min, lunch, another dive for 40-50 min.
40-50 min - it is average diving time for "standard spherical diver" to have enough pleasure and stay safe (coldness, NDL, weakness and other reasons), and left 50 bar in tank at the end.
For this schedule is better to make second dive more shallow to keep same diving time as for the first dive.

I know that there aren't too many people diving tables these days, but there are some--or at least that is almost what some people say in recent ScubaBoard posts. What those posts actually have said is that the divers have memorized the first dive limits of the tables (either US Navy or any of the others, like SSI or NAUI, that follow the US Navy tables) and use them for their dives. In case any such person is reading, I would like to repeat a warning I have posted several times before.

As was said earlier, the US Navy tables were made with the assumption that the divers would be doing one dive a day. Their choice of the controlling compartment for repetitive dives (120 minute) was a decision made without a lot of study, becaues it usually didn't matter. When it created the PADI RDP in the early 1980s, DSAT's research determined that the 40 minute compartment could control repetitive dives most of the time, but they eventually decided to use the 60 minute compartment. They also cut short the first dive limits. The goal was to create a manageable surface interval for 2-tank dives, and they succeeded. Surface intervals became much, much shorter. Modern dive schedules were made possible, and the typical surface intervals noted above are consistent with the DSAT (PADI) algorithm.

Therefore, if you are using the first dive limits of the US Navy tables (rather than DSAT) and then repeating those first dive limits after a DSAT-consistent surface interval, you are NOT following the US Navy tables. You are falling considerably short of the required surface interval for your second dive. If you are doing a deeper second dive, then you are seriously violating those tables.