Why deeper in the morning?

[wombat]

Dear Doctor D:
Just joined the board, and figured I'd start out with a question for you or anybody else with an answer. I was taught, and have heard endlessly, that when you are doing multiple dives in a day, you should always do your deepest dive first, then shallower on subsequent dives. I've never heard a good explanation why (most often the response is the old "they haven't tested it any other way" standby). Is there a physiological reason this pattern is safer?
I am Curious Wombat

 

[jbd]

Dear Doctor D:
Just joined the board, and figured I'd start out with a question for you or anybody else with an answer. I was taught, and have heard endlessly, that when you are doing multiple dives in a day, you should always do your deepest dive first, then shallower on subsequent dives. I've never heard a good explanation why (most often the response is the old "they haven't tested it any other way" standby). Is there a physiological reason this pattern is safer?
I am Curious Wombat

Do search here on scuba board for reverse profiles.

 

[Diver0001]

Dear Doctor D:
Just joined the board, and figured I'd start out with a question for you or anybody else with an answer. I was taught, and have heard endlessly, that when you are doing multiple dives in a day, you should always do your deepest dive first, then shallower on subsequent dives. I've never heard a good explanation why (most often the response is the old "they haven't tested it any other way" standby). Is there a physiological reason this pattern is safer?
I am Curious Wombat

I'm sure Dr. Deco will give you more about the physiology of this so I'll fill in the side I'm more sure of.

In the big picture this idea is supposed to limit the risk of getting DCS on repetitive dives. Recent research is starting to show that unless the reverse profile has a difference of at least (iirc) 40ft, then this worry about DCS on reverse profiles isn't warranted.

That said, I still believe it's good advice for a couple of reasons.

1) The divers studied are by-and-large skilled and experienced technical divers. They make good plans, make nice profiles and control their ascent rates very carefully. A cross-section sport diver might not have that level of control.

2) The studies are inconclusive and none of it is backed up by laboratory research; it's all empirical. They, *do* show, however, an increased rate of DCS as a result of certain types of reverse profiles so what ever is happening is real. So my thinking is that while it's not really as reckless as you may have been lead to believe, you should account for this principle in your planning with extra safety stops, slow controlled ascents, etc.

3) Deep dives get shorter bottom times. If you do a deep dive first you get your maximum bottom time. Regardless of the deco theory, this is still the best way to get the most blub-blub-blub for your dollar as it were.

R..

 

[wombat]

Diver0001 - I hadn't thought about number three; good point!

 

[jonnythan]

Just to prove it to yourself, do two 60 foot and one 90 foot dives with a 1.5 hour surface interval between each, and see which pattern maximizes your bottom time.

 

[padiscubapro]

Just to prove it to yourself, do two 60 foot and one 90 foot dives with a 1.5 hour surface interval between each, and see which pattern maximizes your bottom time.

not a good example..
Tables have to make compromises,may of them use the 60 minute compartment for the surface interval, a shallow then deep gets cut short this way where as a DC that tracks all compartments during the SI (not all of them do) will give closer allowable runtimes..

Using the proper gas for the depths is also important since shallow dives are controlled by the slower compartments and they tolerate less overpressure gradient..

I do alot of deep diver training, and when I am running classes its NORMAL for each sucessive dive (usually 2 a day) to push deeper, these are deco dives so the proper gas is definately used..

They way I run my classes the first dive is usually about 15m(50ft)-20m(66ft) shallower than the second dive..
This way over a series of 4 dives I can get the diver from 50m(165ft) up to 100m(~330ft)

If the student has never done training with me the first dive will generally be a shallow dive of around 18m(60fsw) to evaluate skills followed by the first shallow deep dive of around 50m (thats a 30m/ 100fsw difference).

 

[Rick Murchison]

not a good example..
Tables have to make compromises,may of them use the 60 minute compartment for the surface interval, a shallow then deep gets cut short this way where as a DC that tracks all compartments during the SI (not all of them do) will give closer allowable runtimes..

However... every recreational dive computer I've checked still penalizes the reverse profile over the deepest first profile - not as dramatically as the tables, true, but the penalty is still there.
Bottom line - for the recreational diver not wanting to deal with deco stops or diving other than air or Nitrox, you'll get more time looking at the pretty fishies if you do your deepest dive first.
Rick

 

[padiscubapro]

However... every recreational dive computer I've checked still penalizes the reverse profile over the deepest first profile - not as dramatically as the tables, true, but the penalty is still there.
Bottom line - for the recreational diver not wanting to deal with deco stops or diving other than air or Nitrox, you'll get more time looking at the pretty fishies if you do your deepest dive first.
Rick

The worst offenders are the DC that fold RGBM (limits) over buhlman (SUUNTO, Mares ect) .. since they can't track bubble growth they really have to cut back the times.

The cochran computers seem to do a good job at handling reverse profiles as does other higher end computers like the Explorer and VR3 (which are designed for decompression diving).

 

[teksimple]

Dear Doctor D:
Is there a physiological reason this pattern is safer?
I am Curious Wombat

So far, no one answered your question, Wombat. But yes, there IS a physiological reason. "Because your computer tells you so" is not a physiological reason. To put it simply, by doing your deepest dive first, you crush the tiny always-present bubbles in your system that--according to the dual phase bubble model theories--are the root cause of DCS in the first place.

How the hell did they figure that out, you ask? Well, the tiny bubble theory has its roots in empirical data. Some of it came from Hawaiian diving fishermen who seemed to defy the normal tables, sometimes diving to near 300 feet on their first dive, and then blowing a half dozen scuba tanks on progressively shallower dives, all in one day. Later research using gel models confirmed the empirical data by revealing that the bubble nuclei--not just the dissolved gas--play an important role in the development of DCS symptoms. By doing a short, deeper dive first you crush the bubble nuclei and reduce the formation of bubbles from the nuclei "seeds".

There are a lot of hyperbaric research articles on the topic available in various medical and scientific journals, or you could read Bruce Wienke's "RGBM Diving in Depth", for example.

 

[Diver0001]

. By doing a short, deeper dive first you crush the bubble nuclei and reduce the formation of bubbles from the nuclei "seeds".

.

Thanks for this post. Question about the above. Do you mean the crushing results in a reduction of numbers or a reduction in size.

R..