Decompression Theory and Various Algorithms

[Andy]

Hey all,

I'm currently in the process of teaching the new Accelerated Decompression Procedures course to some of the divers in my club.  However, one thing that the course is light on is the various deco algorithsms and theories that are out there.  It recommends using the new BSAC Oxygen Stop tables (start looking at some dives on these and they are scary! ) or a multi-gas computer.

I don't like teaching students to do something without giving them all the information they need to make an informed decision themselves.  I don't want to tell them to go out and buy a computer (if they are so inclined) without them being able to look at the various models and understand what they are doing.

So I'm looking for a book or downloadable article that explains the various methods of calculating deco.  I've got plenty of articles  that explain one method or another myself, but nothing tht does a comparison.

Anyone know of any out there?

thanks

Andy

[Stephen McMullan]

http://www.dive-tech.co.uk/deco%20for%20divers.htm

http://www.aquapress.co.uk/bookdetail.asp?id=221&cat=101

[LiamM]

Id agree with Stephen, Marks book is excellent. If you have nosey around marks website you'll also find a selection of papers that explain the differences between the alograthims.

its fairly easy to get your head round the thinking behing and processes that lead the various tables dissolved gas vs bubble model etc. His book does deal with the bsac 88 tables but not the new ones - afaik there is alot of debate about the background to the bsac tables and how they came about.

I think its good that you are going into this detail, if your students can gain enough knowledge to help them understand the implications of where each model begins your stops then it allows them to start adding in appropriate conservatism.

[Stephen McMullan]

I also like 'Deeper Into Diving' (also linked above) and of course Gurr's 'Technical Diving From The Bottom Up'. 'Deco For Divers' is a great book and is probably the best for talking specifically about deco but the other books cover a lot more about other aspects of deeper diving. People tend to get too wrapped up in algorithms, maths, which algorithm is the right one etc. to the detriment of other aspects of diving I find.

But if its specifically to fill in a hole on the BSAC ADP course then Deco For Divers is the one.

Andy, why haven't you read all these books yourself by the way?  

[Andy]

I own some of these myself including Deeper into Diving and Technical Diving from the Bottom Up.  In fact, I'm pretty sure I've lent them to you over the years!

Just didn't want to pass on a set of articles to the students as I know most them won't bother reading them.

Didn't know about Deco for Divers but a copy has been ordered this morning via Amazon.

Andy

[emmbee]

People tend to get too wrapped up in algorithms [...]

Me: keeping my head down while porting my Buhlmann planner to my shiny new android phone

[...] to the detriment of other aspects of diving I find.

Phew, all good then

Cheers,

Matthieu

[Stephen McMullan]

Actually not about you this time - one of my clubmates and Andy's students was on to me - apparently for his chosen depth, mix and bottom time - BSAC tables were giving him 7 mins of deco, Buhlmann was saying 16mins and V-planner 32 minutes (nah I don't believe that one either) and he was wondering which one to choose (deja-vu for the n-th time!). Of course I told him to add them all up and divide by 3 - McMullan algorithm! I'm suprised Andy hadn't taught them this.

[emmbee]

BSAC tables were giving him 7 mins of deco, Buhlmann was saying 16mins and V-planner 32 minutes

I think that sums up decompression "theory"...

Better to call them models than it theory, IMHO.

Of course I told him to add them all up and divide by 3 - McMullan algorithm!

Write a paper . This kind of hack is par for the course, along with Buhlmann Trimix a/b values and gradient factors. Among others.

Cheers,

Matthieu

[Divin'Dec]

All,
Is there a book:website that describes fully gue ratio deco that i could also pass on to students.

Could  a similar algorithm be applied to rebreather diving or is stewies method used by all?

Divindec

[Andy]

All,
Is there a book:website that describes fully gue ratio deco that i could also pass on to students.

Could  a similar algorithm be applied to rebreather diving or is stewies method used by all?

Divindec

Hang on, Stewie has a method???  I thought he just made it up on the day depending on how blue the water was, and what he had for breakfast that day.

[Stephen McMullan]

Its covered in Deco for Divers also.

There are articles:

http://teamfoxturd.blogspot.com/2007/04/tech1-ratio-deco-explained.html (recommended)
http://en.wikipedia.org/wiki/Ratio_decompression

I'm sure you could come up with something similar for CCR for certain mixes & set points but the question would be why would you bother?

My initial reaction to the prospect of doing maths in my head underwater was "fuck that shit" but once I got past that bias and had a go at reading the articles the next blocker was its an OC strategy where you're using the standard DIR gas selections. So no good for CCR constant p02 diving. Anyways I don't buy all the "the computer is your crutch" argument so I'm happy for the tools to do the work they are designed for. I do of course know a couple of lads on CCR whose deco plan is a slate and a couple of depth timers. Nothing wrong with that but thats different from calculating deco on the fly. Anyways perhaps Mike Griffin is the best person to comment on its use.

BTW - As far as I know only Stewie uses Stewie's method which depends on a VR3 by the way, his substantial experience and a couple of physiological indicators

[Divin'Dec]

hey,
mick covered ratio with me last time we dived and as i have a B.Sc in mathematics i did get it fine but i still cant wait for my delivery of my x1.
However having read deco for divers their appendix at the end does not cover it in enough detail. Mick do you know of any resource that ppl can share?

Dec

[Stephen McMullan]

Try the links off the wikipedia page above

[thumpr]

Andrew Georgitsis has a pdf on ratio deco. It used to be up on the 5thd website. There is no complex formula or magic. It is just a collection of observations and approximations based on the DIR standard gases.

At the end of the day it is just a way of remembering some approximations or rules of thumb. The principle is equally applicable to CCR diving. For example I know that 25 minutes at 60m is about 90 minutes in the water (15/50 and 1.3 setpoint). 25@70 is about 120min and 25@80 is about 160min. I am sure if you spent some time with the tables you could come up with a reasonable way of approximating the deco schedule. I suspect the GUE approach of breaking the deco into thirds would be reasonably close. The decompression models are an approximation anyway.

Having said all that I wouldn't jump into the water without looking at all the possibilities using a computer program and actually calculating the deco for my planned bottom time. I also carry a computer

AFAIK it is used as a training tool in GUE to get the divers to think about what deco they are doing and how it is structured. It is not used as a replacement for decoplanner (the GUE tool of choice). After doing some deco diving you do get an impression for what seems "right" in terms of runtime and shape. IMHO I think that this is all that is intended.

Mike

[Stewart Andrews]

BTW - As far as I know only Stewie uses Stewie's method which depends on a VR3 by the way, his substantial experience and a couple of physiological indicators

Good advice all round Stephen...
I'm off to bed now and as sure as hell am going to look for my 'physiological indicator'... (now where do I start)