View Full Version : RGBM model vs PADI vs BSAC
Dr1Gonzo
30-06-2008, 13:54
The Suunto dive computers seem to use the RGBM model for decompression.
How different is this to the PADI tables or BSAC table models?
Seems pointless to have a dive computer working on a different decompression model from what you were trained to use?
Michael Purcell
30-06-2008, 14:11
The BSAC tables are based on a square profile.
The computer will allow you to multi-level dive.
Run a simulation on your computer and work it out on the BSAC tables and you will find on deeper dives/exposures the 88 tables can be quite aggressive.
Most recreational diving is not a square profile and benefits from a computer to maximize your enjoyment.
So why waste your time learning tables?
First, it gives you a better understanding of the implications of on-gasing/off-gassing. Second it gives you an alternative. Third it is not black-box voodoo. Fourth, should you progress to more aggressive diving, understanding tables, regardless of the variety is a must. If you understand tables early on you can transfer the knowledge to other tables quite easily later on.
Dr1Gonzo
30-06-2008, 14:19
It's not that I don't want to use tables, I will always learn the manual mehtods before using a computer as it's the backup.
What I mean is that if a computer uses different decompression models to the tables then the timings are going to be out.
Say you use a computer with RGBM but your bussy only uses dive tables, you have to go with his method?
Also, you can never compare...
With RGBM do you still make a safety stop at 5m?
yes they will be different unless you happen to do a true square dive profile even if they use the same mathmatical algorithme to model the nitrogen on and off gassing methods.
you know that the tables assumes a square proffle
as the computor calculates the on and offgassing continiusly not based upon a single time/depth based input range.
as to the tables/computor issue.
if i dive on a computor and my buddy does not then he has to use the table values i do not. simple as that realy i have seen buddy pairs diving off one computor (and once to my shame i was forced into it at a padi dive centre prior to getttingmy own) but i personaly despise the practice especialy as the non computor wearing partner was normaly 5-8 meters deeper than the one wearing it thus the deeper diver had no idea where they lay, it does happen.
QUOTE
Most recreational diving is not a square profile and benefits from a computer to maximize your enjoyment.
no it doesnt maximise your enjoyment it maximises potential no deco time for the dives and calcualtes the N2 ongassing more accuratly and in realtime as opposed to a single input forumat of a table which has to allow massive reducancy
What are you advicating;
That the computer manufactures use specific tables (i.e. BSAC 88 or PADI RDP)?
That the Diver training agengies adopt manufactuers tables?
Remember none of the dive computer manufactures use a standard published table (with prehaps the exception of the VR3). To some degree they are all paded & have there own corrections. Some even compensate for changes in water temperature & diver breathing rate.
Even with a VR3, which allows you to print a table prior to the dive, you seldom have the same information on the VR3 as that in the pre printed runtime. Because the computer calculates in real time your actual profile.
Mixing hard tables & computers is difficult. If you do repeat dives it gets progressively harder. On recreational dives I use a table to give me a guideline, which I can check the computer against. On technical dives, generally even though I have a computer I run to a hard table, which is generally more punitive than the computer.
BSAC88 tables are easy to use & understand. On deeper dives with extensive decompression they are somewhat agressive when compared to other tables. Generally, by the time people are doing this diving they have learnt significantly more about decompression & are using other methods to plan there dives.
Remember that the tables in the dive computers are constantly changing. The table that Suunto used in the Solution, is nothing like the table in a Vytec DS!
Gareth
Here's an interesting thing- Suunto don't use RGBM just licence the name. They are Buhlmann computers. The DS bit just adds Pyle stops and turns off the 3 min safety stop, something you can do yourself in your head with a standard Suunto.
You will also find that they penalise you if you sit out your deco at 6m. If you go to 5m instead they don't. Why? I would guess it's cos the PADI RDP has a final 5m stop.
BSAC tables are based on stuff that isn't in the public domain but (IIRC from my deco theory course) it's based on a combination of perfusion (a la Buhlmann) and diffusion theory with a few bubbles thrown in for good measure- quite sophisticated really and belies the "1988" name (which really really needs to go).
Keep an eye out for the release of a book by Mark Powell this year called "Decompression Theory and Physiology". It's very, very good and should be on your christmas list.
If you are using tables then a deeper understanding of what is going on really helps. Then you realise that if you simply go ahead and have a guess and make something up, you probably won't be far wrong. It's all a bit depressing and the tables out there are really based on a whole pile of guesswork, fudge factors and a bit of empirical testing.
Nigel Hewitt
30-06-2008, 14:52
The Suunto dive computers seem to use the RGBM model for decompression.
How different is this to the PADI tables or BSAC table models?
Seems pointless to have a dive computer working on a different decompression model from what you were trained to use?
If you can tell us which is the 'right' formula for working out decompression we'd all be very relived to know because it could save us endless hours debating things. :rolleyes:
I dive with two computers and they don't agree but I can run my dive so I keep them both happy and also do it slower which, I believe, is safer still.
Sadly deco isn't an exact science. Most of us know somebody who has had an 'undeserved' bend, that is they obeyed the tables or the computer and still got bent. Bends don't do training and they definitely don't read tables so they get it wrong occasionally I suppose.
ChristianG
01-07-2008, 13:26
Remember none of the dive computer manufactures use a standard published table (with prehaps the exception of the VR3). To some degree they are all paded & have there own corrections. Some even compensate for changes in water temperature & diver breathing rate.
Diver breathing rate? How do they do that? Or is it that you can now load your gas supply into the thing?
If you are using tables then a deeper understanding of what is going on really helps. Then you realise that if you simply go ahead and have a guess and make something up, you probably won't be far wrong. It's all a bit depressing and the tables out there are really based on a whole pile of guesswork, fudge factors and a bit of empirical testing.
Exactly. As my sig says.
Diver breathing rate? How do they do that? Or is it that you can now load your gas supply into the thing?
Air integrated units. I can't remember which ones. They may not be available now.
I use a Suunto & have used various Suunto's since I started diving (Companion originally, EON, Vyper & Vytec), now I have a Vision on the CCR. If I had the cash (which I don't), & the price was right I would probably get a VR3 as well.
(The EON was a mistake, it was air integrated & a console.)
I like the Suunto's and am happy with their algorithm, display & operation. This means I don't tend to pay much attention to what is currently available.
Gareth
ChristianG
01-07-2008, 14:07
Air integrated units. I can't remember which ones. They may not be available now.
I know that. The trouble with all AI units (that I know of) is that they work on pressure rather than volume of gas. Unless you can punch the latter into the unit they can only calculate according to the pressure of gas remaining, but not volume.
Consider diving such a unit on a 232 bar 10 litre to a particular depth. Now do exactly the same dive but using two manifolded 232 bar 10 litre cylinders. The unit will now assume that your gas use has halved simply because the pressure of use has halved.
It's a common misconception among divers. No computer can know your actual breathing rate without knowing your volume of gas, and that requires input to the computer by the diver. If that doesn't happen it's an impossibility.
Ben Panter
01-07-2008, 14:35
Christian,
Could be that they register against 'normal' breathing rate, by examining the fraction change in the measured pressure gradient? It remains to be seen as to whether that is, in fact, a useful indicator, but it would certainly give an indication of extertion over and above the normal background. No use if you spend the whole dive fighting against a drift, obviously...
Ben
I know that. The trouble with all AI units (that I know of) is that they work on pressure rather than volume of gas. Unless you can punch the latter into the unit they can only calculate according to the pressure of gas remaining, but not volume.
Consider diving such a unit on a 232 bar 10 litre to a particular depth. Now do exactly the same dive but using two manifolded 232 bar 10 litre cylinders. The unit will now assume that your gas use has halved simply because the pressure of use has halved.
It's a common misconception among divers. No computer can know your actual breathing rate without knowing your volume of gas, and that requires input to the computer by the diver. If that doesn't happen it's an impossibility.
Christian
I thought they calculated on the basis of speed of pressure drop at any given depth. i.e. 5bar/min pressure drop at 20m, then a change to 20bar/min pressure drop at 20m would indicate a significant increase in breathing/stress.
In truth I didn't resarch this much because I am not a fan of Air Integrated computers.
Any way I fear we are diverging from the OP? I am still not sure what the original post was suggesting?
Gareth
ChristianG
01-07-2008, 15:26
I thought they calculated on the basis of speed of pressure drop at any given depth. i.e. 5bar/min pressure drop at 20m, then a change to 20bar/min pressure drop at 20m would indicate a significant increase in breathing/stress.
Yup. But what they can't tell you is remaining volume of gas. You referred to breathing rate which is directly related to gas volume, which they can't give you because they don't know the gas volume and calculate pretty well as you describe above.
Any way I fear we are diverging from the OP? I am still not sure what the original post was suggesting?
1) No, not according to the subject matter.
2) Yup, just a tad confusing.
The fact of the matter remains that all computers will have their differences, sometimes even the same model computer, leave alone trying to compare them to manual tables.
All these things are built, one way or another, by we hooman beans, unfortunately hooman beans are fallible.
Keith Littlebury
01-07-2008, 17:12
The Suunto dive computers seem to use the RGBM model for decompression.
How different is this to the PADI tables or BSAC table models?
Seems pointless to have a dive computer working on a different decompression model from what you were trained to use?
Here is my simple understanding of it.
The BSAC tables are based on Haldanes original tables so very old and very restrictive. Designed for navy divers.
PADI tables were redone more recently and particularly for recreational divers to be more conservative and allow more repetitive dives.
RGBM? I've been meaning to find out that myself because, as you say, it's different to the tables we've learnt giving different ascent rates and repetitive dives etc and it's used by my suunto computer. Presumably its also conservative.
The only way to find out for sure is get the book explaining RGBM. Also the PADI Encyclopaedia of Recreational Diving is v informative.
I did start reading up on the whole decompression thing myself and there's a point where it just gets too involved and complicated and I thought Sod It i'll just go and dive.
If you find out i'd love to know.:)
The Suunto dive computers seem to use the RGBM model for decompression.
How different is this to the PADI tables or BSAC table models?
RGBM is very different to the decompression models used by both BSAC and PADI (and most others). It might be considered *fractionally* closer to the BSAC '88 model - but not in any meaningful way.
Seems pointless to have a dive computer working on a different decompression model from what you were trained to use?
Well, you're going to get that. Even if you trained on the Buhlmann algorithm that most computers use, you're still likely to use something ike ZHL-16B, whereas your computer will use ZHL-16C.
But TBH it really doesn't matter; unless you really *want* to understand the theory behind decompression shemes, they're pretty much "black magic" anyway - you pick one, and you do what it ways, whether you understand the maths or not.
Vic.
ChristianG
01-07-2008, 18:07
Could be that they register against 'normal' breathing rate, by examining the fraction change in the measured pressure gradient? It remains to be seen as to whether that is, in fact, a useful indicator, but it would certainly give an indication of extertion over and above the normal background. No use if you spend the whole dive fighting against a drift, obviously...
Ben,
You make a point but, then again, you make a point. :)
Fact of the matter is that nothing will get you a proper breathing rate, I do not deny that a computer could do it and easily, provided that machine knows the gas volume, preferably as the Water Content. If it does not have that information it cannot calculate it. Simple as that.
As you say, computers go by pressure drop against depth which is fine in some circumstances. It just doesn't work for other applications.
Let me, oh dear, we're about to get into a spot of bother, add something to this apropos <It just doesn't work for other applications>.
So how can these digital marvels calculate nitrogen absorption when they don't know gas intake?
Maybe we should take that one to an entirely separate thread?
What I mean is that if a computer uses different decompression models to the tables then the timings are going to be out.
Only if you swap systems mid-sequence. If you don't understand exactly what you're doing, you just shouldn't do that.
Say you use a computer with RGBM but your bussy only uses dive tables, you have to go with his method?
*Always* use the more conservative approach.
Vic.
if i dive on a computor and my buddy does not then he has to use the table values i do not.
You what?
Are you implying running your decompression schedules independently of each other? If so, then you haven't *got* a buddy...
it ... calcualtes the N2 ongassing more accuratly
Does it? That would be interesting...
and in realtime as opposed to a single input forumat of a table which has to allow massive reducancy
Computers also involve massive redundancy. For the triangle-shape profiles you frequently see on holiday dives, you're likely to get more time in the water from a computer dive. But it is not true in the general case that computers give you more time.
Vic.
BSAC tables are based on stuff that isn't in the public domain but (IIRC from my deco theory course) it's based on a combination of perfusion (a la Buhlmann) and diffusion theory with a few bubbles thrown in for good measure
*Most* of the algorithm is in the public domain - but the coefficients are not (unless someone can tell me differently!)
The model contains two type of tissue, and models both dissolved gas tension and gaseous phase N2 in each. There is then a diffusion matrix to model the interchange of gas between those elements- and I don't have the coefficients to that matrix. So it is a true bubble model - if somewhat simpler than the likes of VPM et al.
quite sophisticated really and belies the "1988" name (which really really needs to go).
It's a very sophisticated piece of work. Where it falls down is in the numbers used to generate the tables 20 years ago - 15m/min is *way* too fast for my taste, and running a slower ascent penalises me hideously. I would *love* to see a BSAC '09 set of tables...
It's all a bit depressing and the tables out there are really based on a whole pile of guesswork, fudge factors and a bit of empirical testing.
..And then we talk about "accurate" modelling :-)
Vic.
The BSAC tables are based on Haldanes original tables so very old and very restrictive. Designed for navy divers.
No, they aren't.
RGBM? I've been meaning to find out that myself because, as you say, it's different to the tables we've learnt giving different ascent rates and repetitive dives etc and it's used by my suunto computer.
RGBM (and VPM, from which Wienke originally claimed his work was derived) are interested in controlling the size of bubbles generated. The upshot of that is that the deepest parts of the ascent are as important as the shallow parts, because keeping bubble diameters small increases the internal pressure (per Laplace's Law), and therefore increases decompression efficiency. This inevitably leads to deeper stops.
Presumably its also conservative.
That depends on the parameters you put into the model.
The only way to find out for sure is get the book explaining RGBM.
Wienke's book is not cheap - but all the VPM stuff is freely available, and doesn't differ from RGBM that much. It'll give you plenty to get an empirical understanding of what's going on.
Also the PADI Encyclopaedia of Recreational Diving is v informative.
Does it cover bubble models? It didn't the last time I looked, but that was many moons ago...
I did start reading up on the whole decompression thing myself and there's a point where it just gets too involved and complicated and I thought Sod It i'll just go and dive.
Deco theory is something you do, or you do not do. There is no point studying it in a half-hearted way, because you will get all the mathematical confusion without any of the understanding.
Vic.
Here is my simple understanding of it.
The BSAC tables are based on Haldanes original tables so very old and very restrictive. Designed for navy divers.
PADI tables were redone more recently and particularly for recreational divers to be more conservative and allow more repetitive dives.
RGBM? I've been meaning to find out that myself because, as you say, it's different to the tables we've learnt giving different ascent rates and repetitive dives etc and it's used by my suunto computer. Presumably its also conservative.
The only way to find out for sure is get the book explaining RGBM. Also the PADI Encyclopaedia of Recreational Diving is v informative.
I did start reading up on the whole decompression thing myself and there's a point where it just gets too involved and complicated and I thought Sod It i'll just go and dive.
If you find out i'd love to know.:)
Err No.
BSAC88 uses a completely different model to Haldane, based on Hempleman if memory serves me correctly.
Haladane theory uses multiple tissue compartments & on gases & off gases at the same rate for each compartment.
Hempleman use a single slab approach & off gasing is 1.5 times slower than ongasing. The Hempleman tables where the RNLP/BSAC tables
The Hempleman theory was the base line for Hennessey when he created the BSAC88's (Hennessey having worked with Hempleman).
Hennesssey tables are different in that there is a new unique table to use for each gas loading calculation (seven in all A-G). Unlike other tables that calculate a residual Nitrogen loading & apply this the to the table in combination with the repeat dive to give the decompression requirement.
The RDP (PADI tables) is based on the Haldanian system utilising multiple tissue compartments. On gasing & off gasing are symmetrical, unusually the off gasing is based on a 60 minute controlling tissue (rather than the US Navy 120m controlling tissue (Halftime))
As with all of decompression tables, they where design to operate under specific criteria deviating from these criteria make the table invalid (i.e. the RDP is a no stop table limited to 40m)
RGBM (Reduced Gradient Bubble Model) is a much newer concept incorporating bubble mechanics & the critical volume model.
See Vics comments above.
NOTE I am not a decompression physiologist & have a very limited understanding of decompression theory. I believe that Vic among others on this site have a much better understanding of decompression theory than I.
Gareth
BSAC88 uses a completely different model to Haldane, based on Hempleman if memory serves me correctly.
No!
This was first postulated in Lippmann's book, I believe. It's a story that's been repeated loads of times by loads of people - including me.
And it's completely wrong.
Although Hennessy worked with Hempleman on the 1972 RNPL tables (which are based on the slab model), the '88s are not derived from that model.
As with all of decompression tables, they where design to operate under specific criteria deviating from these criteria make the table invalid (i.e. the RDP is a no stop table limited to 40m)
There are other aspects as well - like how "dive time" is defined. The RDP measures to the time you leave bottom - and then assumes an ascent *at* 18m/min, not more and not less. Thus most dives on the RDP are not actually conducted according to the model...
Vic.
No!
This was first postulated in Lippmann's book, I believe. It's a story that's been repeated loads of times by loads of people - including me.
And it's completely wrong.
Although Hennessy worked with Hempleman on the 1972 RNPL tables (which are based on the slab model), the '88s are not derived from that model.
Vic.
Vic
Can you give me reference to this. I tend to use Lippmannn's book, I'll correct the information for myself if you can give me colabarating information, not that I am doubting you. You've done far more research on this subject than I.
Cheers
Gareth
Here is my simple understanding of it.
The BSAC tables are based on Haldanes original tables so very old and very restrictive. Designed for navy divers.
PADI tables were redone more recently and particularly for recreational divers to be more conservative and allow more repetitive dives.
This is not true. I did a side by side comparison on YD
RGBM? I've been meaning to find out that myself because, as you say, it's different to the tables we've learnt giving different ascent rates and repetitive dives etc and it's used by my suunto computer. Presumably its also conservative.
The only way to find out for sure is get the book explaining RGBM. Also the PADI Encyclopaedia of Recreational Diving is v informative.
I did start reading up on the whole decompression thing myself and there's a point where it just gets too involved and complicated and I thought Sod It i'll just go and dive.
I think for small (<10mins) of deco then it makes very little difference which model you choose. For greater amounts of deco, I wouldn't use any of the three methods you mention as I think they all stop too shallow.
Janos
R.Burgess
01-07-2008, 21:55
I've known a number of divers who have had to give up diving after getting bent. Two of them were experienced divers, strangely enough the dives they got bent on, were less physically demanding than dives I had known them to do safely on earlier occasions. At the time of the separate incidents they were diving well within the limits of their dive computors. For all the theories, I think all we can say is; the amount of time needed to decompress, is a guess based on averages, which may sometimes be unsuitable for 1 in 3 or 4 of us. :eek:
The buhlmann table and BSAC 88 in their current form are non user friendly. I think the adoption has resulted in many members not having a clue about simple rule of thumb no-stop sport diver dive planning, and has resulted in them being dependant on computors.
For all the theories, I think all we can say is; the amount of time needed to decompress, is a guess based on averages, which may sometimes be unsuitable for 1 in 3 or 4 of us. :eek:
I think 1 in 3 or 1 in 4 is a bit alarmist! Hundreds of thousands of dives are done each year in the UK, and most serious bends are reported in the BSAC Incident Report.
Janos
Can you give me reference to this.
Hennessy published his model in a paper : "Modelling human exposure to altered pressure
environments" in Environmental Ergonomics (eds Mekjavik, IB, Banister, EW, Morrison, JB,). That was in 1988. The paper is rather hard to find - I've only got a copy after Tom Hennessy kindly sent it to me when he pulled me up for claiming his was a slab model :-)
I tend to use Lippmannn's book
And so you should - it's excellent. It is not, unfortunately, flawless :-(
I'll correct the information for myself if you can give me colabarating information
The only copy of Hennessy's paper I've got is a paper copy (I don't think it's available digitally, although I did find the abstract available online), and I'm not even completely sure what I've done with that. But google for the paper name above & read the abstract - that'll give you sufficient evidence that the '88s are not based on the slab model, even if it doesn't give you much info on the model they do use...
Vic.
Ben,
You make a point but, then again, you make a point. :)
Fact of the matter is that nothing will get you a proper breathing rate, I do not deny that a computer could do it and easily, provided that machine knows the gas volume, preferably as the Water Content. If it does not have that information it cannot calculate it. Simple as that.
Breathing rate is breaths per minute. It's differnet to SAC. I would have thought breathing rate would be easy to calculate by looking at any spikes in tank pressure. And SAC would be easy to calculate by asking the user to enter tank size. Don't most AI computers do this?
Janos
ChristianG
02-07-2008, 09:32
This was first postulated in Lippmann's book, I believe. It's a story that's been repeated loads of times by loads of people - including me.
And it's completely wrong.
Although Hennessy worked with Hempleman on the 1972 RNPL tables (which are based on the slab model), the '88s are not derived from that model.
So I laboriously hauled out my copy of "Deeper Into Diving" (John Lippmann, 1st edition - July 1990).
Lippman says (Page 325):
Hennessy initially decided to base the new tables on the same decompression model as the RNPL/BS-AC Table since the model on which that table is based has been tried and tested over a number of years. However, since the RNPL/BS-AC Table has not really had the facility to be used and, hence, tested over a series of three or four dives per day, Hennessy had to first ensure that the model could be safely extended to cover these multiple diving situations.
So, if Lippmann postulated that the BSAC '88 Tables were modeled on Hempleman he didn't do so in this book. I vaguely remember that this was a more in depth continuation of "The Essentials of Deeper Diving" by the same author and I have no idea whether he made the allegation in that one. I doubt it though given that "Essentials" was probably written well before the BSAC '88 Tables saw the light of day.
Keith Littlebury
02-07-2008, 09:43
oops! apologies for giving duff info.:(
but thanks for correcting it :)
Nigel Hewitt
02-07-2008, 11:00
It is a bit alarmist. The 1:3 or 1:4 people I was referring to, have a PFO. Whilst people with PFO are more subsceptable to DCI's there are many people with PFO's who are diving without incident.We keep hearing this statistic.
It tends to make me assume that most PFOs are harmless diving or even that when the tests on deco models were done PFOs were factored in as a lot of the guinea pigs had them.
R.Burgess
02-07-2008, 11:05
I think 1 in 3 or 1 in 4 is a bit alarmist! Hundreds of thousands of dives are done each year in the UK, and most serious bends are reported in the BSAC Incident Report.
Janos
1:3 or 1:4 of us is expected to have a PFO. :eek: :D
So I laboriously hauled out my copy of "Deeper Into Diving" (John Lippmann, 1st edition - July 1990).
Lippman says (Page 325):
So, if Lippmann postulated that the BSAC '88 Tables were modeled on Hempleman he didn't do so in this book. I vaguely remember that this was a more in depth continuation of "The Essentials of Deeper Diving" by the same author and I have no idea whether he made the allegation in that one. I doubt it though given that "Essentials" was probably written well before the BSAC '88 Tables saw the light of day.
Christen
Check what Lippman says about RNLP tables created by Hempleman. - page 303.
Gareth
R.Burgess
02-07-2008, 11:18
Like you say, they could be factored in. Although I think its enough to conclude that for all the scientific theories, without the guinea pigs, we may as well just be guessing. :D
So I laboriously hauled out my copy of "Deeper Into Diving" (John Lippmann, 1st edition - July 1990).
Hennessy initially decided to base the new tables on the same decompression model as the RNPL/BS-AC Table since the model on which that table is based has been tried and tested over a number of years. However, since the RNPL/BS-AC Table has not really had the facility to be used and, hence, tested over a series of three or four dives per day, Hennessy had to first ensure that the model could be safely extended to cover these multiple diving situations.
So from this we get :-
Hennessy initially decided to base the new tables on the same decompression model as the RNPL/BS-AC Table
Hennessy had to first ensure that the model could be safely extended to cover these multiple diving situations
Nowhere does it say that Hennessy changed his mind about using the slab model - just that he realised it needed more testing.
So, if Lippmann postulated that the BSAC '88 Tables were modeled on Hempleman he didn't do so in this book.
He implied it in the section you quoted.
Vic.
1:3 or 1:4 of us is expected to have a PFO. :eek: :D
You also need to ask the question what size PFO. Small PFOs, which are by far the most common, are not likely to cause a bend. There was a very interesting talk at the DOC on this a few year backs.
My local chamber tech told me that PFOs are now 'sexy'. Much easier to blame your bend on a PFO than being dehydrated, doing a sawtooth profile, diving for three or four days on the trot etc etc.
Janos
Nigel Hewitt
03-07-2008, 08:45
My local chamber tech told me that PFOs are now 'sexy'. Much easier to blame your bend on a PFO than being dehydrated, doing a sawtooth profile, diving for three or four days on the trot etc etc.
Grief I'd rather have something I could fix without an inside job on my heart.
A really bad 'undeserved' bend that I couldn't account for would make even me reconsider diving.
If I knew what I'd done wrong I'd 'learn my lesson' and just be more careful in the future.
It tends to make me assume that most PFOs are harmless diving or even that when the tests on deco models were done PFOs were factored in as a lot of the guinea pigs had them.Decompression algorithms are validated statistically. The algorithm is used to produce sample profiles which are compared to a database of real World profiles and out pops the number of bends observed within the sample profiles. If the number is sufficiently low the model is acceptable. In the general scheme of things a sample of the bends within the profile database will have resulted from undiagnosed PFOs, so indeed PFOs and all other ailments are factored into the validation. The only organisations with profile databases large enough for useful validation are Navy's and commercial diving companies. The statistics are self selected to an extent and sport divers are advised not to completely trust tables because they are designed for young fit sailors.
Unfortunately the Navy's and commercial diving companies lost interest in scuba decompression just about the time Drysuits, Nitrox and Trimix entered the sport scuba diving equation. Sport diving profiles have changed significantly since the late 80s. The newest algorithms are mainly produced by part time computer programmers based on theoretical speculation and are not formally validated. Mind you, according to Lippmann, the 88s were never formally validated either.
PFOs are a very specific anatomical anomaly. A decompression model has no chance of saving you from one. Simply sneezing during or after a dive can produce a massive bend in someone with a PFO. A sneeze will pressurises the blood vessels causing a back pressure on the heart. The back pressure encourages the PFO to open and pumps inert rich blood directly into the arterial blood supply. As this inert rich blood travels further from the heart the blood pressure falls encouraging a bubble to form, grow and lodge. Breathing Nitrox and spending a few extra minutes at 6m ain't going to make a great deal of difference.
JP Imbert's (Comex, IANTD France) hypothesis pertaining to deep stops in extended range diving works on a similar (but less dramatic) principle. His idea is that after an extended range dive at 30m+ and a long ascent to 9m or 6m, significant numbers of bubbles are trapped in the lungs overwhelming the 'filter'. Micro-bubbles (not really bubbles) do not get trapped at the lungs, because the filter bed is 'full' of larger bubbles, so they are shunted into the arterial blood, exposed to more inert and falling blood pressure during the journey back to the lungs. The deep stop is thought to give provide a pause in bubble growth for the filter to clear - like screwing the top back on the coke bottle and letting the foam subside before it overflows.
We keep hearing this statistic.
It tends to make me assume that most PFOs are harmless diving or even that when the tests on deco models were done PFOs were factored in as a lot of the guinea pigs had them.
Dived with a woman for 10 years+ 800+ dives, inc cave etc, no problems. One day she did 2 dives well within tables and computer, and shallow compared to what she had done before. 1 hour later showed signs and symptoms of vestibular and skin bends. Helicoptered out. Chamber sorted her out.
Tests showed a 10mm PFO. A perfect tunnel according to the surgeon who fixed it.
Fine now.
So shows that you can get away with it for a considerable amount of time.
Regs
Gary(A)
Nigel Hewitt
03-07-2008, 12:00
Dived with a woman for 10 years+ 800+ dives
Tests showed a 10mm PFO. A perfect tunnel according to the surgeon who fixed it.So a 10mm 'perfect tunnel' doesn't give trouble for 800+ dives.
There has got to be another factor here. The risk is a PFO plus X.
If an item of kit didn't break for 800 dives I would think it very safe.
Grief I'd rather have something I could fix without an inside job on my heart.
A really bad 'undeserved' bend that I couldn't account for would make even me reconsider diving.
If I knew what I'd done wrong I'd 'learn my lesson' and just be more careful in the future.
I'm the same. But the psychology of bends is very interesting and I think most people don't like to admit making a mistake. I've observed different reactions to what people perceive as "good bends" (ie so-called undeserved) and "bad bends" (where the diver has made a mistake).
Janos
PS - It reminds me a bit of the Brass Eye episode on Aids...
So a 10mm 'perfect tunnel' doesn't give trouble for 800+ dives.Ordinarily the heart will shunt left to right, as blood pressure is higher on the left side. The blood in the left side is oxygen rich and inert poor. Blood that shunts left to right just gets pumped to the lungs where any inert (and it will not be much above ambient) is offgassed as usual.
There has got to be another factor here. The risk is a PFO plus X.
If an item of kit didn't break for 800 dives I would think it very safe.Yes, there probably will be an 'event' which causes enough pressure on the right side of the heart to shunt oxygen poor, inert rich blood on it's way to the lungs, over to the left side so bypassing the lungs. A sneeze, a valsalva, heavy exertion, one or all of those in combination with some otherwise insignificant event, take your pick.
As I understand it of course. I am not a doctor...but this bloke (http://www.scuba-doc.com/pfo.htm) is.
Nigel Hewitt
03-07-2008, 13:48
A sneeze, a valsalva, heavy exertion, one or all of those in combination with some otherwise insignificant event, take your pick.Interesting. All things that are rare on an ascent.
Hence the infrequency of occurrence.
Thanks for the link.
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