Deepest Dive First, no exceptions?

[The Chairman]

I (for once) agree with Chairman.

Good for you! A broken clock is right twice a day! (Sorry, couldn't resist)

 

[scubadada]

No evidence to support increased risk of DCS with reverse profile dives DAN Europe - Reverse profiles

 

[tbone1004]

@txgoose
no idea what the actual NDL's are on the computer, but we'll use tables since that's where the discussion came out. NAUI says you get 130 mins on your first dive to 40ft, you aren't going to hit that.

with a great SAC rate of 0.4cfm *unattainable by most divers, especially new divers*, your DAC is going to be 0.9cfm. That first dive you would need a tank of 144cf to hit your NDL. Realistically you're going to have a SAC rate of 0.6-0.8cfm, which means you'll need a tank 200-300cf. That ain't gonna happen.

If you're diving AL80's, you'll get an absolute max of about 90mins on that tank, realistically about 50 minutes in real world diving. 50 minutes at 40ft, with instant turnover gives you 33 minutes MDT on the air tables. You'll break NDL before you hit gas limits on dive 2 since it is a reverse profile, has a less than 45 minute surface interval, and assumes a truly square profile. You are going to have longer than a 45 minute surface interval, that I can all but guarantee which ups your NDL to 42 mins, and from the time you surface to the time you descent, if it's 90 minutes, you're going to hit air limits on your 50ft dive before you hit NDL.
Diving a computer? basically not possible for most new divers to hit NDL's at shallower than 50ft depths with 2 dives per day assuming a SAC rate of 0.6cfm.

NAUI tables are abridged Navy tables. Doing a 40ft dive to 130min which is the NDL, followed by a 2:05 SIT gives you a 33 minute dive to 50ft on the next dive. Total bottom time of 163 mins with 33 mins at 50ft. If you reverse this, you get 80 minutes at 50ft, then 69 minutes at 40ft. Total of 149 minutes of bottom time, but you better balance the time at the deeper depths. In real world, you'll get about 50 minutes at 50ft on an AL80, and about 57 minutes at 40ft. In this case with a 2 hour SIT, you're going to hit gas limits in both circumstances but you end as an I diver by doing 50 then 40, vs a J diver by doing reverse profile. By the tables that is more nitrogen loading. In real world application? At those depths it's a wash. More exaggerated the deeper you go and the bigger the gap between the two depths

 

[Diver0001]

In our training we have been taught that a diver should always do their deepest dive first with subsequent dives shallower. How critical is this in shallower depths?

There was a symposium a number of years ago in which decompression scientists discussed the issue of "reverse profiles" at length. To make a long story short, they collectively decided that if the difference in depth in a reverse profile situation is not more than 40ft/12m that in the context of recreational diving they would not expect to see negative side effects.

I think shurite7 may be referencing the same symposium but he remembered the conclusions inaccurately.

R..

 

[shurite7]

There was a symposium a number of years ago in which decompression scientists discussed the issue of "reverse profiles" at length. To make a long story short, they collectively decided that if the difference in depth in a reverse profile situation is not more than 40ft/12m that in the context of recreational diving they would not expect to see negative side effects.

I think shurite7 may be referencing the same symposium but he remembered the conclusions inaccurately.

R..

This is the report I was referring to. It has been several years since I've read it, and stand corrected when it comes to the 40ft difference. I had forgotten about that part.

 

[dmaziuk]

And it promotes some less safe behavior. We encountered a DM who became concerned for our safety when he discovered that we only went to 40 feet on our morning dive before the 60 foot afternoon dive. He advised us that in the future we should do a quick bounce down to 70 feet in the morning so that we would be safe to go to 60 in the afternoon.

Interestingly enough, someone's recently mentioned in one of those computer threads, that VPM will set the bubble "crushing" radius on that 70' bounce dive and give you a different NDL for the afternoon 60' -- different from what you'd get without.

 

[boulderjohn]

here are some results of the workshop that was referenced a couple times already:

• The very first mention of doing the deepest dive first was a SUGGESTION in the 1972 PADI OW manual. PADI was part of the workshop, and they had no idea who made the suggestion or why.
• In subsequent PADI manuals, the suggestion eventually turned into a rule. PADI did not know why.
• Bruce Weinke, the creator of the RGBM algorithm, insisted that it was still a good idea for technical dive because of bubble crushing, and the workshop accepted that recommendation when it said there was no reason the deepest recreational dive had to be done first.

Although no one can tell for sure why PADI made that first suggestion in 1972, common sense says it had to do with the problems they were having with 2-tank dives at resorts (and other places) using the US Navy tables. The US Navy tables used the 120 minute compartment to guide surface intervals, and people were sitting out of the water between dives for a l o n g time. Someone obviously noticed that if you are planning 2 dives to 2 different depths, the required surface interval between those dives would be significantly shorter.

I don't have the old Navy tables with me, so I will give an example from the PADI tables, which did not exist in 1972 and which use the 60 minute compartment to guide surface intervals. Let's say we want to do 2 dives: 1) 90 feet for 21 minutes and 2) 60 feet for 40 minutes. If I do the deeper dive first, I have to be out of the water for at least 1:05 before starting the second dive. that's pretty reasonable. If I do the shallower dive first, I would have to be out of the water for 2:31. Remember that those are PADI numbers--with the Navy tables, the times would have been much longer.

Even doing it with the tables, if you have been out of the water long enough to do the second dive that you want to do, then there is no reason you can't do the dive.

 

[txgoose]

@txgoose
no idea what the actual NDL's are on the computer, but we'll use tables since that's where the discussion came out. NAUI says you get 130 mins on your first dive to 40ft, you aren't going to hit that.

with a great SAC rate of 0.4cfm *unattainable by most divers, especially new divers*, your DAC is going to be 0.9cfm. That first dive you would need a tank of 144cf to hit your NDL. Realistically you're going to have a SAC rate of 0.6-0.8cfm, which means you'll need a tank 200-300cf. That ain't gonna happen.

Based on five whole dives with the oldest (our current big breather) I figure we'll get 40 mins max on our AL80s. And as you said, that wouldn't be a square profile either. He doesn't seem to be nervous breathing, he just burns through it by: sprinting, turning to check on his buddies (me and the youngest, so far) and then repeat.

I appreciate your write up. I am reading all of the posts. Not because I need it this weekend (I now know) but because I do like understanding what I am getting into as our skills/comfort/depths increase.

 

[Boaty McBoatface]

If I understood the discussion, it seems like one argument for keeping the deepest first 'rule' would be that the rule is still relevant if anyone on the boat happens to be diving tables. In a sense, if you upend the rule you're almost mandating computers. Computers are great, but their cost is a barrier to adopting the sport. Moreover, tables are the backup plan for the many people who have but one computer.

 

[boulderjohn]

If I understood the discussion, it seems like one argument for keeping the deepest first 'rule' would be that the rule is still relevant if anyone on the boat happens to be diving tables. In a sense, if you upend the rule you're almost mandating computers. Computers are great, but their cost is a barrier to adopting the sport. Moreover, tables are the backup plan for the many people who have but one computer.

No, even for a diver using tables, there is no prohibition to doing the deeper dive first--if your surface interval allows you to do the dive you intend to do, then you can do it.

As far as mandating computers goes...

When I was a brand new diver doing my first post certification dives, I whipped out my tables after my first DM-led multi-level dive in Cozumel. I discovered that the dive I had just done exceeded the table limits by a huge amount. People watching me laughed and told me my tables would make a decent Frisbee. That episode, about 20 years ago, remains the only time I have ever seen a diver (me) attempt to use tables on a recreational dive outside of training.

Many dive operations do indeed require their divers to use computers--the first time I encountered this was more than 10 years ago.