Decompression and EAD

[Alan Ewart]

Back in November Tom Hennessey popped up & promised some new thoughts on decompression theory, tables and EAD. Anyone know what the status is on this. Been waiting patiently for 3 months. Tom??

[tristan green]

Well remembered, Alan.

This was posted by Tom on 9 December 2004:
:My forthcoming post (KL permitting) will discuss my perception of some of the pertinent issues.
:
:Thoughts on EAD will follow shortly.

I'm looking forward to seeing this when Tom has time to post it.

Regards,
Tristan

ps - I hope that KL isn't sitting on it :o)

[Andy Wade]

Well remembered, Alan.

This was posted by Tom on 9 December 2004:
:My forthcoming post (KL permitting) will discuss my perception of some of the pertinent issues.
:
:Thoughts on EAD will follow shortly.

I'm looking forward to seeing this when Tom has time to post it.

Regards,
Tristan

ps - I hope that KL isn't sitting on it :o)

LOL. I have this image of Keith sitting on a post.
He's certainly not a 'fence sitter' anyway!

;-)

.

[Nigel Hewitt]

Back in November Tom Hennessey popped up & promised some new thoughts on decompression theory, tables and EAD. Anyone know what the status is on this. Been waiting patiently for 3 months.

It isn't going to happen is it?
I was hoping to hear something about EAD as it is virtually assumed by other people. If there is a hole in EAD is there a hole in END too?

And then, last night, we were doing students. The SD question about how long before you can ascend 200m on the drive home. The TO was showing them how to use the tables while I poked at my calculator. 200m in air is the equivalent of 28cms in water. An hour of 'stops' to ascend just over a foot? But what if I accidentally did all my stops one foot too deep? BSAC 88s worry me at times and I'd like to know what I am missing.

[Chris Cherrington]

It isn't going to happen is it?

Lets start on our own Nigel.

Here's a kick off.

Chris

[Nigel Hewitt]

Lets start on our own Nigel.

Here's a kick off.

Wooo. A 25 year old paper debunking EAD.
Can we get more than the abstract? I couldn't find an obvious way...

[Philip Smith]

Back in November Tom Hennessey popped up & promised some new thoughts on decompression theory, tables and EAD. Anyone know what the status is on this. Been waiting patiently for 3 months.

Dr Hennessy published some thoughts on nitrox and equivalent air depth in 1991. Since the article may not be widely available (reference at foot of message), here are some quotes from it that provide some insight to his thinking at the time:

"The decompression tables developed by the NOAA are similar to the standard repetitive air diving tables used by recreational divers. However, it is important to appreciate that the EAD principle can be used for a single, no-stop dive only and it cannot be applied to decompression stop dives, to repeat dives or multi-level dives.
"If the dive contains stops the rate of elimination of nitrogen from the tissues changes, and gas also comes out of solution. It is not possible, therefore, to relate the dive to the equivalent air depth because the ensuing volume of gas depends on the absolute pressure which is not the same as the equivalent air depth. Similarly, the actual suface interval following a EAD-derived nitrox dive cannot be properly used to plan a second EAD-derived nitrox dive because air rather than nitrox is breathed during this important period."

"The rate of elimination of tissue nitrogen depends on the gradient between the inspired and dissolved partial pressures of nitrogen in tissue in addition to the absolute pressure per se once a gas phase forms at some point of the decompression. Hence, to achieve self-consistent predictions it is necessary that the decompression model should include a gas phase so that the absolute pressure may be allowed to exert an influence on the gas elimination process. However, with the exception of the BSAC'88 model, decompression models of current tables do not have this property and the models should not be used to predict dives in other situations. Hence, with the exception of single no-stop dives, the tables associated with such models should be used only for diving from the surface on air and returning to the surface where air is again breathed. For diving at altitude, flying after diving, multi-level diving and decompression-stop diving on different mixtures of nitrox, the conventional models should be separately re-calibrated for each case."

The article includes a table comparing no-stop times on USN and BSAC88 tables using Nitrox I (32%) calculated using EAD. It seems from the quotes above that in relation to anything other than single no-stop dives, he thought that the EAD concept was invalid for not only his own model, but for the others available at the time too.

Reference:
Hennessy, T.R. (1991) Nitrox (Airox) -- a future recreational diving gas? Underwater Technology, 17(1): 28-31.

Philip Smith

[Chris Cherrington]

:=Lets start on our own Nigel.
:=
:=Here's a kick off.

Wooo. A 25 year old paper debunking EAD.
Can we get more than the abstract? I couldn't find an obvious way...

More of the same here.

Chris

[janos]

:=Similarly, the actual suface interval following a EAD-derived nitrox dive cannot be properly used to plan a second EAD-derived nitrox dive because air rather than nitrox is breathed during this important period."

I don't understand this.

I thought the principal behind EAD was that a dive with a ppN2 of (say) 4 bar produced the same decompression requirements irrespective of the gas you were breathing for the dive. It doesn't make any assumptions about what you're breathing on the surface. Which confuses me.

I take the point about EAD on deco-dives though. But this means that if you use EAD on a Nitrox deco dive, then you're being too conservative, as you're going to be on a richer mix on your 6m stop then the model thinks you are.

But I can't see this making more than a few seconds difference for a typical deco dive.

Laters,
Janos

[Mark Powell]

Reference:
Hennessy, T.R. (1991) Nitrox (Airox) -- a future recreational diving gas? Underwater Technology, 17(1): 28-31.

Philip Smith

Philip would it be possible to get a copy of this paper? I'd be happy to pay any costs for copying, postage, etc.

You can contact me at
mark at dive-tech.co.uk

[Khaled Alwassia]

I goggled the reference and came up with the following link.

<a href="http://www.cisatlantic.com/trimix/AQUAcorps/survive/enaxwkshp.htm" >http://www.cisatlantic.com/trimix/AQUAcorps/survive/enaxwkshp.htm</a>

Khaled

[Chris Cherrington]

I goggled the reference and came up with the following link.

http://www.cisatlantic.com/trimix/AQUAcorps/survive/enaxwkshp.htm

Khaled

cheers Khaled. V. interesting (esp. the PO2 stuff!!!)

Chris

[Martin]

It isn't going to happen is it?
Probably not. I'd be very interested to hear what Tom has to say.

And then, last night, we were doing students. The SD question about how long before you can ascend 200m on the drive home. The TO was showing them how to use the tables while I poked at my calculator. 200m in air is the equivalent of 28cms in water. An hour of 'stops' to ascend just over a foot? But what if I accidentally did all my stops one foot too deep? BSAC 88s worry me at times and I'd like to know what I am missing.

It's all just a conspiracy so that you ahve to stay and extra hour and put some more beer money into the local economy.