OMG, my SI was less than 60 minutes!

[doctormike]

So let's talk bubble pumping (but keep it simple based upon the thread being in the basic forum).

Do the tables account for this? Which (if any) computer algorithms account for this?

The manual for my very old Uwatec Aladin discusses simulation of microbubble formation while in surface mode. They do not explain what this means, but it does seem to imply that the ZH-L8 ADT algorithm from way back in the early 90's understands and accounts for shortish surface intervals?

I think that this is the reason why Suuntos have a reputation as being conservative. They penalize for sawtooth profiles (i.e. multiple ascents). This was discussed in a recent thread by Andy, right?

 

[KeithG]

[Q

I think that this is the reason why Suuntos have a reputation as being conservative. They penalize for sawtooth profiles (i.e. multiple ascents). This was discussed in a recent thread by Andy, right?

Penalize or "account for"?

I used the word "penalize" in my first post since I have been led to believe that planned NDLs at 59 minutes SI are significantly shorter than planned NDLs at 60 minutes SI - the diofferences are more than 1 more minute of off gassing - I could be wrong. I am not sure what happens in that magic minute just before the hour.

The manuals for my Uwatec Aladins discuss seasaw diving and microbubbles. So I assume the ADT part of the ZH-L8 ADT algorithm is either "accounting for" or "penalizing" these behaviours. I have not noticed any jump in NDL in the planning mode around the 59 minute mark, but that does not mean it is not there.

 

[doctormike]

Well, we are always guessing about dive computer that uses a proprietary algorithm. But I think that's just a semantic difference, right? The computer reduces the NDLs based on something other than just depth, mix and time. It's like conservatism. If you set conservatism high is it penalizing you? Or adjusting the NDLs? Not really sure I see the difference.

 

[sea_ledford]

There isn't anything that magically happens at 60 minutes, but boundaries need to be drawn somewhere, otherwise all the tables would be 15 pages long. It's easy to remember 1 hour minimum.

Computers can keep track of surface intervals in much smaller increment, minutes at least if not seconds. Trying to compare the tracking abilities of computers vs tables isn't even a worthwhile conversation except as a selling point for computers.

I recall with PADI there had to be a 10 minute surface interval between training dives to count them as two dives. So you could surface, chat for a few minutes, go back down to 30 feet and get 2 dives done at once. But you couldn't come up to the surface and go straight back down and count that as two dives. Again, boundaries have to be drawn somewhere.

-Chris

 

[KeithG]

There isn't anything that magically happens at 60 minutes, but boundaries need to be drawn somewhere, otherwise all the tables would be 15 pages long.

Actually the PADI tables I found online handle SI's down to 2 minutes AND less - the tables are already big enough.

As you said, there is nothing magic about 60 minutes in the tables themselves - hence this thread. As posted above, the PADI tables have special directions that introduce both 1 hour and 3 hour SI's, under certain conditions. Many people seem to be aligned to the 1 hour SI rule,.

No-one yet has claimed any knowledge of the 3 hour rule...

 

[dmaziuk]

Actually the PADI tables I found online handle SI's down to 2 minutes AND less - the tables are already big enough.

As you said, there is nothing magic about 60 minutes in the tables themselves - hence this thread. As posted above, the PADI tables have special directions that introduce both 1 hour and 3 hour SI's, under certain conditions. Many people seem to be aligned to the 1 hour SI rule,.

No-one yet has claimed any knowledge of the 3 hour rule...

We always take the magic 3 hour SI for lunch: less and the lunch gets rushed, more and you can't squeeze in a 4th dive before dinner. Also, I believe in PADI tables SI of around 1 hour takes your pressure group halfway down, which may or may not make it easier to guesstimate your schedule.

Last dive trip the op was doing "40 minute" SIs -- in quotes because by the time everyone got back in the water it was probably over 50 minutes. That was on shorter dives, under an hour and mostly around 50 minutes, generally into 20m-ish depths with more square-ish profiles. That trip my cressi leonardo was showing NDLs of less than 99 much more often than usual.

 

[sea_ledford]

Actually the PADI tables I found online handle SI's down to 2 minutes AND less - the tables are already big enough.

As you said, there is nothing magic about 60 minutes in the tables themselves - hence this thread. As posted above, the PADI tables have special directions that introduce both 1 hour and 3 hour SI's, under certain conditions. Many people seem to be aligned to the 1 hour SI rule,.

No-one yet has claimed any knowledge of the 3 hour rule...

Right, but they aren't in 2 minute increments. And I was referring to training dives, which almost always have special rules related to liability and not physiology. The SDI tables I have at hand, which are slightly modified Navy tables, have minimum SI interval of 10 min across the board, any less than that and you are on the same dive. Repet groups go up at 40-50 minute intervals for the most part. So at an hour you have gone up 2 levels (ie K to I), not just one.

The 1 hour also works for logistics of recreational boat diving. It gives the crew time to swap tanks, provide food and drink, chat about how awesome the dive was and for the boat to move to a new site. All without making it a 12 hour day on the water.

No one knows about the 3 hour rule because no one really dives tables any more and probably don't do much more than lip service to them in training. They probably don't know anything about exceptional exposure corrections either.

I usually have about a 5 hour SI on most of the diving I do, so I don't spend too much time thinking about them anymore

-Chris

 

[AdivingBel]

We always take the magic 3 hour SI for lunch: less and the lunch gets rushed, more and you can't squeeze in a 4th dive before dinner.

Sounds like my Bonaire/Curacao schedule.

 

[boulderjohn]

I recall with PADI there had to be a 10 minute surface interval between training dives to count them as two dives.

There has not been such a rule for PADI since I have been a professional. Perhaps there was one before that. In the current manual, the requirements for training dives are extensive, and they can be found on pages 26-28 in the instructor manual.

No-one yet has claimed any knowledge of the 3 hour rule...

No one knows about the 3 hour rule because no one really dives tables any more and probably don't do much more than lip service to them in training. They probably don't know anything about exceptional exposure corrections either.

The reasons for the 1 hour rule and the 3 hour rule, which are for repetitive dives (at least 3) that include reaching the very highest pressure groups, are well known.

The research leading to the PADI RDP was extensive, and it was published. Remember that the reason for it was due the fact that the Navy tables, which were really all anyone had then, were really not suited to recreational diving. They used the 120 minute compartment to govern surface intervals. That decision was pretty arbitrary; in fact, the 120 compartment was created in theory when that decision was made--earlier tables did not even include it in their thinking. Navy dives tended to be very long, and they usually only had one per day. Recreational dives tended to be much shorter, and they usually wanted to do 2 a day.

The research indicated that for the vast majority of recreational dives, the 40 minute compartment could safely govern surface intervals. As the dives got longer, the 40 minute compartment became less suitable. they decided to go with the 60 minute compartment to govern the surface intervals, and the table is based on that decision. However, when you do repetitive dives at the edge of NDLs, the 60 minute compartment becomes more on the edge. For that reason, they increased the required surface intervals for those dives in order to be conservative. They made two blanket rules rather than make another set of tables for those circumstances.

Remember that the 1 hour and 3 hour rules only exist for a minimum of 3 dives that get near the NDLs. They are irrelevant for a 2-tank dive.

 

[Bob DBF]

The research leading to the PADI RDP was extensive, and it was published. Remember that the reason for it was due the fact that the Navy tables, which were really all anyone had then, were really not suited to recreational diving. They used the 120 minute compartment to govern surface intervals. That decision was pretty arbitrary; in fact, the 120 compartment was created in theory when that decision was made--earlier tables did not even include it in their thinking. Navy dives tended to be very long, and they usually only had one per day. Recreational dives tended to be much shorter, and they usually wanted to do 2 a day.

John, I agree that the PADI research made a great step forward with the RDP for recreational diving however, catagorizing the Navy diving procedures as arbitrary is way off the mark. The decisions that were made were a combination of Haldane's research and the actual effects of the dives on the divers that made them. The US Navy has been at the forefront of diving research since they formed the NEDU in 1912. Their procedures are made to keep their divers safe while performing the tasks that are necessary for the Navy.

One shouldn't label the best practices for one type of diving as arbitrary, when compared with the best practices of another.


Bob