no SI

[redacted]

Sorry if the question was not asked correctly. Please reread the question. I was not looking for a rehash of what tables may or may not tell you to do. The question was 'what do you thinkof he rationale" of dive operators not to allow divers to do a longer SI if thats what they wish to do. Since irregardless of what table may of may not say the greatest volume change in a bubbler is the last 33 feet to the surface and more importantly the last 15 feet. Dirty dog no matter how many times you reread the question do not ever reply to any post I put forward. The fact that your friend can blindly read a dive table is more proof that divers blindly follow with no thought as to what is going on behind the tables or what they read. Your mean spirited, stupid kneejerk reaction saddens me that I came here to have an open dialogue and this is what I get.

So what does the relative changes in pressure in shallow water (less than 33 ft) have to do with the duration of surface intervals?

I normally expect an hour or more SI to be planned by a dive op unless it is discussed and changed before starting a multi-dive sequence. I would have no problem with a shortened SI before a shallow (25 ft +/-) subsequent dive. But part of my preparation would be an extra cautious ascent and rest stop on the preceding dive.

 

[Thalassamania]

A surface interval has two unrelated purposes, one may be to allow time to offgas and reduce your residual nitrogen load, the other, and likely more important, is provide an interval of when you have waited long enough that defines a specified compartment as the controlling compartment. That way you can made a second dive rather than an extension to the first dive.

 

[Dirty-Dog]

Sorry if the question was not asked correctly. Please reread the question. I was not looking for a rehash of what tables may or may not tell you to do. The question was 'what do you thinkof he rationale" of dive operators not to allow divers to do a longer SI if thats what they wish to do.

It's their boat and their schedule. They're not under any obligation to let you sit on the boat for a long UNNECESSARY SI. An SI required to make the subsequent dive safe is an entirely different issue, of course.

Since irregardless of what table may of may not say the greatest volume change in a bubbler is the last 33 feet to the surface and more importantly the last 15 feet.

When did the issue change to ascent rates? This has nothing to do with how long an SI needs to be.

Dirty dog no matter how many times you reread the question do not ever reply to any post I put forward. The fact that your friend can blindly read a dive table is more proof that divers blindly follow with no thought as to what is going on behind the tables or what they read. Your mean spirited, stupid kneejerk reaction saddens me that I came here to have an open dialogue and this is what I get.

It was not intended to be mean spirited, nor was it a kneejerk. I'm happy to appologise, since that's how you read it. It was, as others pointed out, intended as "this is important, if you can't figure your SI needs, then learn how". And I would encourage you to fix your profile, since it's likely that others will also be confused by a professional asking very basic questions.

 

[bleeb]

Since this is the deco science forum, it's worth throwing the issue of microbubbles out for discussion.

Don't most tables and agencies recommend a minimum one hour SI for recreational NDL diving? Going shorter might not increase risk in any way that's actually measurable, but weak or strong, there must be some rationale for that time.

Along those lines, recommendations are not uncommon that bounce diving soon after a dive isn't a great idea because of the potential for bubble pumping. How long would you have to stay down during the second dive for bubbles to dissipate? For most shallow dives, wouldn't you be more likely to be going up and down by a larger percentage pressure change, exacerbating bubble pumping? OTOH, the second dive could be analogous to deco diving protocols with provisions for redescending for missed deco stops when asymptomatic? However, presumably the risk probabilities for mandatory decompression diving is considerably different.

IIRC, Doppler ultrasound studies have found a peak in the number of detectable bubbles somewhere around 50 minutes after surfacing. How much of an argument is this against shortened SIs?

 

[boulderjohn]

Don't most tables and agencies recommend a minimum one hour SI for recreational NDL diving? Going shorter might not increase risk in any way that's actually measurable, but weak or strong, there must be some rationale for that time.

PADI makes no such recommendation for normal diving. It only calls for a one hour surface interval if the diver is doing three or more dives in a day and at least one of them reaches the W or X pressure group, which is surprisingly hard to do. As I mentioned earlier, in the knowledge review problems students do while learning the tables, there are correct answers under an hour, including one that is only 4 minutes (imperial) or 0 minutes (metric).

The PADI tables were the first that were based on research that examined bubble formation during diving.

 

[Thalassamania]

... The PADI tables were the first that were based on research that examined bubble formation during diving.

I believe that Spenser's tables predated Ray Rodger's and Spenser's tables where specifically know as the "no-bubble tables." I think Ray used Spenser's work as a basis for his.

 

[boulderjohn]

I believe that Spenser's tables predated Ray Rodger's and Spenser's tables where specifically know as the "no-bubble tables." I think Ray used Spenser's work as a basis for his.

You are probably right. I know Doppler Bubble Imaging was used for the PADI tables, and I thought they were the first.

 

[Jim Lapenta]

Doing dives like that where the depths are less than 40 feet I would have no problem with. As long as it was my plan to follow theirs. If I did not like the plan I have three choices. Not do the second dive. Stay out for a longer SI and then end up doing a shorter second dive. Or finding another op to dive with. Option three would likely be my choice and that would have been determined before I even got on the boat or frankly before arriving in the Keys. Dive planning does not start when you get on the boat. It starts when you decide to dive. From that point on every aspect should be part of the basic plan. Including calling the op ahead of time and finding out what their policies are. But since many classes today don't teach planning period other than be back on the boat with 500 psi and follow the DM stuff like this is to be expected. Sadly.

It seems that in many cases the ops policies are blamed for people's bad or questionable experiences. Unless the op is holding a gun to one's head and saying "get in the friggin water" one is not under ANY obligation to make any dives at all. This is another area where the industry could be made better. Stop teaching divers that any one but themselves is responsible for them. From the time you make the decision to dive til the time you get home. Take some responsibility for yourself and use your own mind and judgment when it comes to diving. If that makes someone uncomfortable better to take up tennis or golf and stay out of the water.

 

[MauiScubaSteve]

On Turkey Day, I made the following series of EAN33 dives (trip report will be in the Hawaii 'Ohana this weekend);

Dive 1, to find a tie up for kayaks - 63' max depth, 43 minutes (this was a "working" dive, the last half towing two kayaks ~200 yards into the wind from 60'-30' depth).

SI - 10 minutes

Dive 2, scooter dive, 87' max depth, 69 minutes (went into mandatory at 43 minutes, never more than 5 minute stop required).

SI - 23 minutes

Dive 2 actually ended at 61 minutes (above). Those extra 8 minutes are repositioning the kayak to the mooring I could not find on Dive 1, that I did find at the end of Dive 2. The computer shows Dive 3 as 18' max depth for 0 minutes.

SI - 7 minutes (I'm guessing there was 30 minutes between Dive 2 and Dive 4, which was really Dive 3 of 3 dives)

Dive 4, scooter dive, towing kayaks back to shore, 30' max depth, 29 minutes.

I had "planned" to make two approximately 60 minute scooter dives. I had "planned" to tie up kayaks to the mooring by freediving. I had "planned" to kayak back to shore after 2 dives. Within 20 minutes of launching the plan changed, and I was drifting in the wind considering what would be the best "new" plan.

 

[tridacna]

Since irregardless of what table may of may not say the greatest...

"IRREGARDLESS"

Now that's precious. I've heard it but seldom seen it in writing.