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rogersides
06-10-2003, 13:49
Hi All,
I've been reading some papers about the RGBM and Haldane theories for calculating decomopression and no stop times etc. I'd like to find some information about the theory behind the BSAC tables as they don't seem to correspond to the haldane/buhlmann ones.

I think the BSAC tables were made by Dr Tom Hennessey, but searching on the web doesn't seem to give any information about how he calculated them. Does anyone know where i can find somebackground information, or can give me a simple explaination of his theory.

It's just for personal interest.

cheers, Roger
chris cherrington
06-10-2003, 14:22
Hi All,
I've been reading some papers about the RGBM and Haldane theories for calculating decomopression and no stop times etc. I'd like to find some information about the theory behind the BSAC tables as they don't seem to correspond to the haldane/buhlmann ones.

I think the BSAC tables were made by Dr Tom Hennessey, but searching on the web doesn't seem to give any information about how he calculated them. Does anyone know where i can find somebackground information, or can give me a simple explaination of his theory.

It's just for personal interest.

cheers, Roger

BSAC Tables are (I am fairly sure) based on the single tissue model from the Royal Navy - originally by Hempleman. You won't find much on this model as just about no-one uses it. It does not lend itself to computer based calculations and the Bulmann ZHL series are used by most software and dive computers. Therefore most research work with things like accelerated deco is Bulmann based. It is (I think) also why you don't get Equivalent Air Depth with BSAC Nitrox.
The attached link will tell you more about the various models used. There's not much on the web about the BSAC tables as only a few people use them and most technical divers use Bulmann and they form the bulk of Internet divers!!
Chris
Vic
06-10-2003, 15:51
> BSAC Tables are (I am fairly sure) based on the single tissue
> model from the Royal Navy - originally by Hempleman.

Very similar, AIUI.

The best (only?) reference I've seen on them is in Lippmann's "Deeper into Diving" - a bit old now, but still a worthwhile read.

Isn't it about time we got the technology behind BSAC '88s into the public domain? The system itself is fabulous - but the numbers are rather suspect in the light of current decompression thinking...

Vic.
Chris Edge
06-10-2003, 22:19
The papers on this subject have been published. One of the references is: Hennessy TR, Hempleman HV. An examination of the critical released gas volume concept in decompression sickness. Proceedings of the Royal Society of London Series B - Biological Sciences 197(1128):299-313;1977. The basis is the diffusion model in which gas is present on one side of an infinite sheet. The pressure of gas in the tissue is then given by the diffusion equation. The amount of gas is proportional to sqrt(Pt) where P is the pressure of the gas and t is the time over which diffusion takes place. It is this non-linearity in the model which prevents one from carrying out air equivalence modelling, unlike the Buhlmann tables.
Mick Penfare
07-10-2003, 14:43
The model was by Hempleman & Hennesey and was used to produce the RNPL tables. BSAC used a slightly modified version which was the BSAC/RNPL table, it had smaller time and depth increments. There is a description of the model (written by Hempleman IIRC) in an old edition of Bennett & Elliot.

Mick
steve swift
07-10-2003, 19:17
The model was by Hempleman & Hennesey and was used to produce the RNPL tables. BSAC used a slightly modified version which was the BSAC/RNPL table, it had smaller time and depth increments. There is a description of the model (written by Hempleman IIRC) in an old edition of Bennett & Elliot.


I had a quick check in my copy of B&E 2nd ed (1975) in which there is a chapter entitled Decompression Theory: British Practice, by Hempleman. No mention of Hennesey though. Did he appear later?

Steve
diverd
07-10-2003, 21:34
...Royal Navy (BSAC) tables were introduced in 1952 based on a single tissue model of decompression to control type-1 joint bends. The Royal Navy tables were revised in 1968 to incorporate a variable ratio of tissue out-gassing. Considered too conservative, they were again modified in 1972. In 1988 the manner of calculating repetitive dives was revised for BSAC use and included in the BSAC-88 tables.

My 2p's worth!
regards
diverd
Vic
08-10-2003, 01:58
>> BSAC Tables are (I am fairly sure) based on the single tissue
>> model from the Royal Navy - originally by Hempleman.
>
> Very similar, AIUI.

The above is total cobblers. My understanding was wrong.

Thanks to Tom Hennessy for pointing out the error!

Vic.
Chris Cherrington
08-10-2003, 10:11
BSAC Tables are (I am fairly sure) based on the single tissue model from the Royal Navy - originally by Hempleman. You won't find much on this model as just about no-one uses it. It does not lend itself to computer based calculations and the Bulmann ZHL series are used by most software and dive computers. Therefore most research work with things like accelerated deco is Bulmann based. It is (I think) also why you don't get Equivalent Air Depth with BSAC Nitrox.

Like Vic I have in my inbox a reply from the man himself - much respect!!

The model is not a slab but a three group model. I was wrong and like Vic I am happy to say so..

Come on BSAC you paid for these tables how about putting the theory up on the website so there is no confusion and misunderstanding (which seems to have come from Lippman BTW).

Chris
Mick Penfare
08-10-2003, 11:25
My appologies. I had a quick skim through my books and it looks like I'm mistaken. As you say, the chapter in B&E is by Hempleman and doesn't mention Hennesy. He does get a few mentions in the references at the end of the chapter and a couple in the refs of the chapter 'decompression theory and application' some of them in conjunction with Hempleman so perhaps that's where I remembered it from. From the references it looks like Hennessy was't involved until the 70's whereas the Hempleman model dates from the 50's.

Mick
diverd
08-10-2003, 22:17
Yep, me too! My research also stands corrected, thanks Tom.

I think this just goes to show how daft it all quite is, that really valuable information like this remains so difficult for the average diver, and Instructor even, to get their hands on. There's so much valuable information that the diving world at large is gagging to get their hands on, but which remains allusive to most. As dive courses appear to become scantier by the season, as commercialism becomes the order of the day, it's hardly surprising the amount of misinformation we are all left to argue about on forums just like this one. How rewarding to receive some new refreshing hard-core facts that seemed to be alluding most of us!


regards
Diverd
Philip Smith
10-10-2003, 19:21
How rewarding to receive some new refreshing hard-core facts that seemed to be alluding most of us!

and will continue to elude most of us and be the subject of ignorant speculation, unless Dr Hennessy would be kind enough to post his e-mail replies on the forum!

Philip Smith
janos
11-10-2003, 19:34
I've been reading some papers about the RGBM and Haldane theories for calculating decomopression and no stop times etc. I'd like to find some information about the theory behind the BSAC tables as they don't seem to correspond to the haldane/buhlmann ones.


On a slightly different note, I'm looking to start learning a bit more about the science behind decompression theory. Not for any practical reason,(my diving is nowhere near that deep and I'm quite happy using a computer / tables) but I've a physics background and am curious and would like to learn more.

So, what books can you recommend for me to start with?

Thanks in advance,
Janos
Chris Cherrington
12-10-2003, 19:44
On a slightly different note, I'm looking to start learning a bit more about the science behind decompression theory. Not for any practical reason,(my diving is nowhere near that deep and I'm quite happy using a computer / tables) but I've a physics background and am curious and would like to learn more.

So, what books can you recommend for me to start with?

Thanks in advance,
Janos

Try "The Decompression Matrix" by Bob Cole ISBN 0 9520934 2 1
My copy was 16 quid from Kent Diving. Very accessible and easy to understand, better than Lippman and with very up-to-date stuff. You will find loads on the web too, search on the big names like Bruce Wienke (RGBM).

Prepared to be confused..........

Chris
janos
17-10-2003, 12:44
Try "The Decompression Matrix" by Bob Cole ISBN 0 9520934 2 1
My copy was 16 quid from Kent Diving. Very accessible and easy to understand, better than Lippman and with very up-to-date stuff. You will find loads on the web too, search on the big names like Bruce Wienke (RGBM).

Many thanks, I've got a copy on order.

Prepared to be confused..........

I'm ready - my degree was in Physics, so I'm used to being confused and not understanding anything!!!

Cheers,
Janos