Reverse Profiling

[rathernotbeknown]

Hi Guys,
I am a fairly regular poster on this forum with a different account however for a varity of reasons I would rather pose this question anonymously.

How do people feel about doing reverse profile days e.g. doing a couple of shallow no deco dives, for example a warm up dive(s), followed by a deep 'main' dive? as apposed to doing the traditional deepest/longest first?

I realise this is often the case while teaching say the final 2/3 lessons of OD but how about doing recreational days like this?

does it significantly increase risk?

Thanks all

[Nigel Hewitt]

This gets discussed a lot.
There is some fool symposium often quoted that listened to a lot of papers that suggested there was absolutely no difference between deepest first and deepest second and then came up with some plucked out of the air recommendations which basically looked like they bottled out of saying it was OK and fudged it.

Frankly a 'sort out' splash could make a deep dive far safer.

I personally judge every dive on it's merits. I know if I have done a moderate dive first I will be well gassed up already but my computers will track that. Doing deeper dive second will mean I will probably spend more total time that day dangling on a string than deepest dive first but often you can't arrange it that way.

Time and tide wait for no man and you either want to do the dive or you don't.

[craigwarren]

This gets discussed a lot.
but my computers will track that
...
Time and tide wait for no man and you either want to do the dive or you don't.

I agree with Nigel.

Tide times are what they are, sometimes the slack is too early to be possible so you take an afternoon slack, which means either a very late end to the day or a reverse profile.

When doing a reverse profile the first dive can be an enjoyable shallow drift which doesn't last very long.

My Computer will keep track of the requirements for the deep dive... but I would take a long surface interval.

[bythesea]

IMO reverse profiling is not an issue if the dives are within approx 10 metres of each other, a 20m followed by a 50m is not such a bright idea mainly as the first dive will compromise your times on the second.

I think the main concern in the days of yore with RP was working out the times, with the advent of computers this got a lot easier....


I am more interested in why you think this to be so controversial you had to set up a new account and why the mods here allow multiple sign ins....

[Edward]

I am more interested in why you think this to be so controversial you had to set up a new account and why the mods here allow multiple sign ins....

It doesn't take much to hide a second (or more) general user registration. If the OP hadn't declared it I wouldn't have any reason to check. BSAC Member registrations are different.

Not something I'm particularly concerned about.

Edward

[Andy (treerat)]

IMO reverse profiling is not an issue if the dives are within approx 10 metres of each other, a 20m followed by a 50m is not such a bright idea mainly as the first dive will compromise your times on the second.



Agreed

[SUNDODGER]

Being a scaredy cat, notwithstanding having had years of 'just say NO to reverse profile diving' rammed down my throat - no thanks!

I take 2 computers on my dives (OK both elderly, but who cares) and a lot of common sense. Surely the projected bottom time for the second dive would not be worth the effort?

As I recall, all these teeny weeny bubbles that are leaping around your body after the first dive to say 20m are still there (obviously depending on SI) when you do your second dive to say 40m - only they get made smaller therefore they can get into places in your body/brain other bubbles can't, which is not good when you come back up and they begin expanding!
Re-read your physiology notes, especially tissue absorption rates/times and decide if it is worth the risk.

There are old divers and bold divers, but no old bold divers. Yeah, I know, rubbish!!

[Nigel Hewitt]

I think the problem with reverse profiles is that it contradicts the assumptions that go into collapsing a multi compartment tissue model into a single code letter for the surface interval. Think about it. It is obviously impossible.

Stay on the same computers and dive the same model extended over both dives and you have as good as you can get assuming you do all the staying hydrated stuff. If you must use tables play by their rules and stay on the boat for the second dive. We'll tell you how good it was when we get back.

[Andy (treerat)]

The question is too wide - what definition of reverse are we talking? What m difference?


1st dive to 25m, 2nd to 26?

1st dive 15 m for 20 mins, gradual slope up to the surface.
2nd dive, 20 m for 20 minutes, gradual slope up to the surface.

I would have thought it's as safe as a 40 m dive where you have to rise up to 30 m before dropping back down to 35 before you surface.


-

[Nigel Hewitt]

The question is too wide - what definition of reverse are we talking? What m difference?
I don't think there is any justification to the 'small increases' don't matter position. That's just dodging the question.

Obviously (to me) a 6 meter splash for ten minutes then a 25 meter dive after lunch is OK as is 20 meters then 22 meters.

Let's say 38 meters for twenty five minutes in the morning so you're into stops with the prospect of 60 meters in the afternoon with a two hour plus run time. I would do it if I thought the dives would be interesting enough. If you wouldn't... why not?

[bythesea]

As I recall, all these teeny weeny bubbles that are leaping around your body after the first dive to say 20m are still there (obviously depending on SI) when you do your second dive to say 40m - only they get made smaller therefore they can get into places in your body/brain other bubbles can't, which is not good when you come back up and they begin expanding!


Are the old bubbles going to compress more than the new bubbles and are the old bubbles going to expand more than the new bubbles on the way up?

[PeteM]

Are the old bubbles going to compress more than the new bubbles and are the old bubbles going to expand more than the new bubbles on the way up?

No but bubbles need a nucleus to form on and micro bubbles can form that nucleus. Micro bubbles are mainly formed near the surface but take some time to redissolve. Going into a deep dive having already created micro bubbles from an earlier shallow dive may therefore increase your risk. How much that increased risk is I have no idea and personally would not worry about it

[bythesea]

I was told that too Pete during my basic training but other than a mention in a lecture have never seen any further evidence that it is a worry.... so I don't....worry that is...

[Nigel Hewitt]

Microbubbles in the blood aren't much of a problem as they won't survive one trip through the lungs. The microbubble problem is microbubbles in non-flowing tissue. They would take more time.

As Pete points out however microbubbles are not in themselves a problem because they are tiny but only if they for a centre for a larger bubble to form. It is the large bubbles, blocking things up so they can't clear, that are the problem. However your body is full of stuff that will provide nucleation centres for bubble growth if they want to form. Microbubbles tend not to form in stationary fluids because they have a very high pressure core and usually require cavitation or such to generate them. The actual heart valves are the suspects here.

I gather the smart money in deco theory is that it takes two minutes for most of your blood to have done the circuit from your extremities to get through the lungs to gas off so a two minute stop flushes your blood down to inhaled gas levels and that normally cleans microbubbles. I just don't see microbubbles persisting between dives. To stay a bubble needs to be big enough to block a blood vessel so the tissue it is in is no longer perfused with nice clean 'surface' blood and the bubble is not dissolved away. Once a bubble is big enough to block things up it is self perpetuating. The cure is recompression so it becomes small again and frees up so the lungs can flush it out.

Obviously you aren't going to do the second dive, deeper or shallower, if you are showing signs of being bent but if it's not apparent yet will it worsen things? Actually I think not. I doubt that the second dive would constitute a recompression treatment but I see no reason it would leave you in a worse state than doing the dives in opposite order.

(Sorry. That wasn't supposed to be that long...)

[MattS]

Microbubbles in the blood aren't much of a problem as they won't survive one trip through the lungs. The microbubble problem is microbubbles in non-flowing tissue. They would take more time.I thought the point of them being micro-bubbles was that they do not behave as bubbles and while they are micro-bubbles they may pass through the lung bed unimpeded.

As Pete points out however microbubbles are not in themselves a problem because they are tiny but only if they for a centre for a larger bubble to form.From what I remember, the forces involved in decompression are not adequate to cause a bubble to instantaneously burst into existence (pun intended). Therefore, the theory goes, that all DCI inducing bubbles grow from micro-bubbles (i.e. bubble nuclei). Hence, the theory goes, every micro-bubble has the potential to be a problem.

Microbubbles tend not to form in stationary fluids because they have a very high pressure core and usually require cavitation or such to generate them. The actual heart valves are the suspects here.Not entirely accurate. Cavitation is only one method of bubble nucleation. There are others. Place a candle in a beaker of stationary soda water and bubbles will form around it. Bubble nucleation occurring at the interface between whetting and non-whetting materials; similar to the interface between blood and the blood vessels for instance. Of course if we want to be pedantic, which we probably need to be, no fluid is ever truly stationary.

*Allegedly* bubble nuclei have a propensity for forming between moving materials of different viscosities; for instance between blood and the blood vessels. If you had a strong enough microscope what you would see is one material 'tearing' away from the other, the surfaces parting irregularly. The irregular separation causing minute areas of lowered pressure. At this microscopic level the forces involved are enough for a surface tension to form between the liquid and the tiny pocket of low pressure. i.e a bubble nuclei, bubble seed or micro-bubble. The surface tension around this 'not really a bubble' is relatively massive though.

The *fool symposium* you speak of was attended by some of the leading lights in decompression bubble modelling. They were not prepared to condone reverse profiles as the models they were studying indicated reverse profiles could be a problem.

I gather the smart money in deco theory is that it takes two minutes for most of your blood to have done the circuit from your extremities to get through the lungs to gas off so a two minute stop flushes your blood down to inhaled gas levels and that normally cleans microbubbles.There is somewhere between 60,000 to 100,000 miles of blood carrying capillaries in the average body, so the 2 minute explanation is a bit simplistic.

After 2 minutes, a 'large proportion' of blood will have circulated the 'major blood vessels' a couple times (more than 2 and less than 5). The theory goes, that the lowering of pressure between arterial and venous vessels encourages bubble growth to occur in the larger veins. A two minute deep stop being sufficient for these bubbles to become trapped within the lung bed. The pause in the ascent reduces the density of new bubbles forming due to ambient reducing, allowing the bubbles that have formed time to reach the lungs, lodge and resolve. Density being an important factor in coalescence, whereby two small bubbles, or presumably micro-bubbles, in close proximity touch and become one larger bubble.

I just don't see microbubbles persisting between dives.That is the million dollar question. But if they do, as the leading lights in decompression bubble modelling that attended the *fool symposium* seem to think, reversing profiles by significant pressure differentials is not a great idea. If half of what they think they have learned since the late 1980s turns out to be half correct, reversing profiles after a decompression dive is really not a great idea. Further, BSACs own Dr Tom Hennessy was sufficiently convinced that free phase gas persisted that he engineered the 88s repeat dive tables accordingly. i.e. They get very conservative very quickly. Now while I don't agree with everything the venerable Dr Hennessy has to say, I can not argue with his credentials as a renown physiologist and decompression scientist.

Once a bubble is big enough to block things up it is self perpetuating. The cure is recompression so it becomes small again and frees up so the lungs can flush it out.Once a bubble is big enough to be called a bubble there is a pretty good understanding of how it will behave. The sticky bit being that a SCUBA dive has insufficient spare energy to push a surface tension into existence. That being the case, the hypothesis leads towards a bubble existing in an embryonic state prior to the dive commencing.

I doubt that the second dive would constitute a recompression treatment but I see no reason it would leave you in a worse state than doing the dives in opposite order.You don't see a problem with inert tension within leading and fast tissues, reaching a higher pressure than when the micro-bubble formed then? The dissolved tension diffusing into the bubble nuclei, so lowering the surface tension and making it easier for the micro-bubble to grow into a bubble.

(Sorry. That wasn't supposed to be that long...)Difficult to consider the subject without getting long and detailed. I admit one of my motivations for posting is the number of 'brief' it don't matter posts.

Sure, if you mainly carry out decompression using Hi-FO2s and/or spend most of the dive breathing trimix, it might not matter. The parameters for single mix air or Nitrox diving are rather different though. Purely anecdotal, but when I hear of tales of, for instance, divers with a history of problem free, long, deep, decompression diving, getting bent after breathing Nitrox for 25 mins at 20m in Vobster and the only thing you can point at is the reverse profile, it makes me think there might be something in it.

As far as I am concerned, the subject is sufficiently complicated, diverse and muddy that I am more than happy to regurgitate the findings of the *fool symposium* when asked by people that are not quite so interested in the detail as I am - or was to be honest. Reverse profiles are probably fine within a 12m differential, where no dive in the day requires mandatory decompression stops. Beyond that no one really knows but the experts are sufficiently worried not to recommend it.

[rathernotbeknown]

Hi All,
Nigel, sorry did have a quick scan through the forum but couldn’t see any related posts. Even after (and before!) the current lot of replies I would agree with you especially about the 'sort out' splash.
@bythesea, thanks for input, have a look below as I have laid out what would end up happening on the day if it were to go ahead. The reason for wanting to set up a separate account is 2 fold, the answers I have been given in club appear to have been aimed at point scoring over other members and not offering a constructive discussion on the subject so didn’t want to that to spread to here, I was also lead to think this was a stupid question and I should know the answer to such a simple question; as Sundoger said '.....having had years of 'just say NO to reverse profile diving' rammed down my throat....' and again not wanting that to spread here as many of the club are members of this forum! probs going to be leaving said club but don’t want to burn bridges or offend as that is not my aim.

Thanks to the mods/powers that be for allowing this through if multiple registrations are a problem (just have this and my main one!) However this account will be closed once this thread is finished. I got the idea from the YD forums (sorry!!) where you can e-mail the mods with a post in their 'it happened to me' bit and they will post it for you

To put into context with regards to depths am talking about doing 2x 10-15m dives (no more than 20-30 min bottom time for each) with a student then after lunch later that afternoon doing a 35-40m dive with a friend, all likely to be on air poss with a 40-50% stage for the end of the deeper dive, my computer will only accept one mix per dive so this would be untracked (but only beneficial).
In this situation is using a nitrox back gas throughout the day likely to be beneficial (more so than if a 'normal' profile used with nitrox)?

Cheers for your help/comments with this guys =)

[Ron MacRae]

In this situation is using a nitrox back gas throughout the day likely to be beneficial (more so than if a 'normal' profile used with nitrox)?

It might be and it shouldn't do any harm so why not.
I'm not a medical guy so this opinion is worth what you paid for it.
When I do training, which tends to be a bit up and down, I normally use 40%.

In fact for all diving I do I use as rich a mix as possible & a 50% pony for deco.
I get signifficantly better no deco times than others on the boat and over a long weekend this can mount up.

For the small %age ammount it adds to the expense of the day why not?

I must admit that the more I read on deco theories the less sure I am about it.:confused:

Ron.

[NickMcV]

Having read through the comments, I broadly agree....
1, Given that I am doing two dives in a day - On tables I get a better answer if I do the deep dive first. On computer - well, it depends on a lot of things.
2, I'd prefer to do a deep dive first while I'm fresh.
3, I WOULDN'T, as is advocated by some 'professional' guides in the Red Sea purposely bounce to a depth below what I wanted to do in the afternoon just so the profiles lined up nicely on the log sheet.

I don't think there is really an absolute answer to this - there is possibly a slightly higher risk of DCI with the 'reverse' profile, but if your dive is planned to minimise this, and you don't have an unfortunately accelerated ascent, I don't see a big problem.

But I'm prepared to be proved wrong!

[rathernotbeknown]

Hi All.
Thanks for the responses, I think this account has served its purpose so will now be removed

[MikeTickle]

Mark Powel talked about this at the dive show last year - quoting a workshop in 1999.

From a bit of web searching I foudn this link:
http://www.scubadiving.com/training/basic-skills/2006/10/deepest-dive-first-not-anymore

Which concludes:

Key Findings of the "Reverse Dive Profiles Workshop"

* Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.

# Reverse dive profiles are being performed in recreational, scientific, commercial and military diving.
# The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any specific diving experience that indicates an increased risk of DCS.
# No convincing evidence was presented in the workshop that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.
# The attendees found no reason for diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 meters (130 feet) and depth differentials less than 12 meters (40 feet).
Proceedings of "Reverse Dive Profiles Workshop"

The complete text can be ordered from:

American Academy of Underwater Sciences (AAUS), 430 Nahant Rd., Nahant, MA 01980-1696; tel: (781) 581-7370, ext. 334; fax: (781) 581-6076; e-mail: aaus@neu.edu; web: www.aaus.org.

Undersea and Hyperbaric Medical Society (UHMS), 10531 Metropolitan Ave., Kensington, MD 20895; tel: (301) 942-2980; fax: (301) 942-7804; e-mail: uhms@uhms.org; web: www.uhms.org.

[David Cowan]

Hi,

I think the findings of the Reverse Dive Profile (RDP) Workshop (of 29-30 October 1999) are controversial and should be treated with appropriate caution until further research has better quantified the risks of reserve dive profiles. My personal observation is that no diving organisation (such as BSAC or PADI et al) has appeared to change their advice on RDP post-workshop.

An article entitled ‘Reverse Dive Profiles’ by Dr David Sawatzky in Dive Log Australasia Issue 223 (February 2007) (reprinted from DIVER Magazine, Canada) cited the following 2 papers whose research appears to suggest that reverse dive profiles are more hazardous.

PAPER No. 1 - McInnes, S., Edmonds, C. and Bennett, M.H., ‘The relative safety of forward and reverse diving profiles’ at http://archive.rubicon-foundation.org/4059 (paper assessed 21/09/2010). The abstract for this paper states that ‘Our findings suggest that multi-level and repetitive dives performed in the established FDP manner are less hazardous than those performed in the reverse profile mode, at least for the exposures we chose. We believe the recommendations of the workshop should be re-examined.’

PAPER No. 2 - St Leger Dowse, M., Gunby, A.’ Moncad, R., Fife, C.E. and Bryson, P.J., ‘A Prospective Observational Study of Reverse Diving Profiles (RDP) in UK Female Recreational Divers’ at http://archive.rubicon-foundation.org/1326 (abstract assessed 21/09/2010). The abstract for this paper states that ‘statistical analysis indicates symptoms rates are higher for RDP than non-RDP dives, but not significantly’.

[Richard Whitcombe]

Reverse profiles are pretty much part of the job here outside courses. Training standards from some agencies prevent them so it's not done in those circumstances but i have no issue doing them when not teaching for myself.

[David Cowan]

Proceedings of "Reverse Dive Profiles Workshop"

The complete text can be ordered from:

American Academy of Underwater Sciences (AAUS), 430 Nahant Rd., Nahant, MA 01980-1696; tel: (781) 581-7370, ext. 334; fax: (781) 581-6076; e-mail: aaus@neu.edu; web: www.aaus.org.

Undersea and Hyperbaric Medical Society (UHMS), 10531 Metropolitan Ave., Kensington, MD 20895; tel: (301) 942-2980; fax: (301) 942-7804; e-mail: uhms@uhms.org; web: www.uhms.org.

Or downloaded for no cost from http://archive.rubicon-foundation.org/dspace/handle/123456789/4244

[MattDuke]

Over the years things change and idea's adapt. I normally take a balanced review of the facts and theories available, allign these concepts with my own knowlege and experience, and then make a judgement.

In the case of reverse profiles I have absolutly no problem with the "Concept", but the vast majority of sports diving is done differently, so we don't have a great deal of emphirical evidence to go on.

Therefore I put this in the "Wait for the pionners to hurt themselves fist" pile.

My understanding of the BSAC is that you shall dive safely following well documented rules and reccomendations. The positive side is that you should have a good quality control of divers, based on an extensive history. The negative side is that you maybe a bit slow to act.

I don't think that there is much to gain by being "Bleeding Edge" in this respect, and would not urge BSAC sports divers break this golden rule for a while yet. Let someone esles do the testing! I would fight for my last breath to get the BSAC to include Hog looping in their training. I would do the same to get BSAC not to act too quickly in changing proven decompression best practices. That's just me!

[Richard Whitcombe]

I would do the same to get BSAC not to act too quickly in changing proven decompression best practices.

You mean like their 1-man stance with "1.44 partial pressure is fine for 32% nitrox because we say so" ?


That said i don't think anyone would ever accuse BSAC of being "bleeding edge"!