Decompression

[ianfirmin]

After a recent, rather bizarre, "my computer is better than your computer" thread I realised I did not know enough about this. Having done a bit of research I now realise that no one knows enough about this. So I've put some thoughts together and would invite comments. A bit long I'm afraid...

Thoughts on the reliability of decompression tables and algorithms and simple ascent gas planning:

From what I can glean the following statements apply:

Decompression algorithms are models based on various concepts where the constants in the algorithm equations are adjusted to a best fit with observed instances of DCI.

It has been difficult to obtain meaningful real life DCI vs. profile data for the following reasons.

Human testing is curtailed if the humans involved shows signs of DCI. Human susceptibility varies widely. Both within individuals and due to other difficult to measure variables such as temperature, hydration, work load etc. The largest groups of data consist of the numerous dives performed where no DCI was observed.

Thus, the “best fits” that provide the constants in the algorithm equations have a large probability of error.

Modern research indicates there are two main models for avoiding DCI, the Haldanian model and the bubble model. The Haldanian model (and its variants such as Bhulmann) work on the principle of limiting the differential gas pressure between absorbed inert gases in the tissues and blood and ambient. The bubble model is based on the prevention of bubbles of inert gas in the tissues and blood. Modern algorithms such as VPM and its variants such as RGBM strive to accommodate both of there models. The problem is that the two models have differently shaped ascent profiles.

The Haldanian model attempts to maximize the pressure differential beneath a limiting amount to maximize the excretion of absorbed inert gases. Above this limiting amount the risk of DCI increases rapidly.

The bubble model attempts to minimize bubble growth by keeping the pressure differential low between dissolved inert gases in the tissues and bubbles in the tissues.

Ascending reduces the ambient pressure and therefore increases the pressure differential both in the bubble and the Haldanian models. This is why saturation diving is so successful. The dangerous ascent phase is a small proportion of the dive time.

In both models the optimum ascent rate is a continuously changing one and the best implementation of both models would be this continuously changing ascent rate being limited by the factor (bubble or Haldanian) most prevalent at the time. Every dive is effectively a decompression dive. People have suffered DCI in 3m deep swimming pools. The so called No Decompression Limit is simply a point where the specified ascent rate is enough to decompress. No one would recommend popping up to the surface after a significant dive at depth even though official decompression time is nil.

Because it is very difficult for divers to accurately gauge ascent rates, especially continuously varying ones, the models are expressed as a fixed ascent rate with stops are varying depths for varying times.

All of this is imposed onto a fog of history. The currently accepted ascent rates such as 18m per minute or even 10m per minute are arbitrary.

“Deep Stops” originated from an observation from Dr. Pyle that he felt better after dives where he made these deep stops. They corroborated some theories at the time such as Hill’s thermodynamic theory and they make sense within the modern bubble theories.

The most recent research indicates that the current bubble models are poor and that deep stops can be counter productive for shorter deep dives. The penalty of the additional inert gas loading outweighs the benefits from bubble reduction.

However, the fact remains that these theories and models and just that; theories and models fitted to statistically inconclusive data. How do we use these theories and models to our best advantage?

I subscribe to Mr. Hewitt’s philosophy of using you computer as a road map for the fastest way to the surface having a reasonable chance of avoiding DCI. But why should we always take the fastest route? Subject to boredom, cold etc. If we have the gas why not pad out the shallow stops. Nitrox left in the tanks after the dive? Breathe it. You are still decompressing on the surface and a higher O2 mix will help that more than atmospheric air.

On a personal note I have noticed I get a real feeling of reverse squeeze in my head on shallow (<20m) ascents at 10m / minute. What’s happening in my body? I’ve recently made a conscious effort to have a very slow ascent from 20m. <5m/minute and try and make the ascent from 3m to the surface take at least a minute. I feel much happier for it.

So in order to have a good chance of not getting bent we know our ascent profile. The important thing now is to know how much gas you need to make that safe ascent. It’s fairly easy to calculate but it’s something I have not done until now.

The sort of diving I do is rarely below 30m and usually on 7 litre or 10 litre twins. What I’m looking for is a feel for the numbers and an idea of the bounding values. First we make an arbritary (but conservative) assumption of a SAC rate = 25 l/min and assume no deco. Let’s say a fast ascent is 10m/min with a 3 minute safety stop at 4.5m. A slow ascent is 5m/min with a 5 minute safety stop at 4.5m. I’ll illustrate the pressure drop for twin 7’s (14 litre capacity, Van de Waal ignored).

From 40m a fast ascent requires 409 litres (29bar) and a slow ascent requires 781 litres (56bar)

From 30m a fast ascent requires 296 litres (21bar) and a slow ascent requires 556 litres (40bar)

So, now we are at the surface, how much air do we need. To really NEED your regulator at the surface you are in one hell of a sea state. Certainly such as sea state that you would not have started the dive in it. 125 litres (9 bar) gives you 5 minutes which should be more than enough. Thus 70 bar in a twin 7 set gives more than enough for one person to make a slow ascent from 40m with a reasonable factor of safety and 30 bar gives you a safe ascent from 30m.

As an aside, one illustration of the dangers of blindly applying models is the concept of narcosis. This fits nicely with the lipid solubility of the gas. Nitrous Oxide (Laughing Gas) is very soluble and gives effective narcosis at low pp’s. Helium is very insoluble and much less narcotic, even at high pp’s. Nitrogen is, let’s say, mediumly narcotic. Noticably narcotic at higher pp’s. Aaaah, people say, that means Nitrox, with elevated fractions of Oxygen, is just as narcotic as air because the lipid solubility of Oxygen is similar to Nitrogen. What little research that has been done indicates Nitrox is less narcotic than air. O2 is not an inert gas, it’s metabolized in the tissues and it’s effective pp in the tissues is lower. The so called “Oxygen Window”. Blindly applying the lipid solubility criteria to the gases we breathe is not enough.

Comments welcome.

[Garf]

Originally Posted by ianfirmin

[font=Arial][size=3]
The sort of diving I do is rarely below 30m and usually on 7 litre or 10 litre twins. What I’m looking for is a feel for the numbers and an idea of the bounding values. First we make an arbritary (but conservative) assumption of a SAC rate = 25 l/min and assume no deco. Let’s say a fast ascent is 10m/min with a 3 minute safety stop at 4.5m. A slow ascent is 5m/min with a 5 minute safety stop at 4.5m. I’ll illustrate the pressure drop for twin 7’s (14 litre capacity, Van de Waal ignored).

From 40m a fast ascent requires 409 litres (29bar) and a slow ascent requires 781 litres (56bar)

From 30m a fast ascent requires 296 litres (21bar) and a slow ascent requires 556 litres (40bar)

So, now we are at the surface, how much air do we need. To really NEED your regulator at the surface you are in one hell of a sea state. Certainly such as sea state that you would not have started the dive in it. 125 litres (9 bar) gives you 5 minutes which should be more than enough. Thus 70 bar in a twin 7 set gives more than enough for one person to make a slow ascent from 40m with a reasonable factor of safety and 30 bar gives you a safe ascent from 30m.[/FONT

Comments welcome.

Ok, now double it because either you or your buddy has had a problem resulting in an OOG, which initiated the ascent. How are you going to decide which one of you gets to breathe?

We work on 30 litres per minute required for the ascent. and THEN we double it to 60 becuase we always carry enough gas to get a teammate out of the poo. Working on this assumption, our minimum gas from 30 metres in twin 232 7s would be about 1200 litres or 85 bar. And that's 30 metres, never mind 40

[ianfirmin]

Quite right too. Punch any numbers you want into it. The numbers I gave were minimum and used to illustrate the difference between a "slow" and a "fast" ascent. All figures (including ascent rates and safety stop times) were arbitrary.

[Mark Chase]

Originally Posted by Garf

Ok, now double it because either you or your buddy has had a problem resulting in an OOG, which initiated the ascent. How are you going to decide which one of you gets to breathe?

But obviously the diver has an independent reserve gas supply or has calculated his reserves based on the need to isolate the twin set, so this is not an issue, right?


On the subject of dive computers, its fairly simple. If your planing on doing decompression diving you need a suitable computer. That would be something like a VR3, Cochran, Shearwater, Abyss Explorer etc etc.

All of these will do reasonable decompression profiles but some have a range of deco options that need to be understood first so they are NOT just plug and play.

Decompression computers are a long way from being a fast rout out of the water. My decompression computer would be a lot slower out than a dive planned using a PADI recreational dive planner, at its upper limits. Its slower than Ratio Deco and its massively slower than Buhlman Tables (something like BSAC 88s)

SO a couple of key points here:

1: your premise that computers offer the fastest way to the surface is wayyyyyy off

2: Sunnto Vipers and Aladdin Pros are NOT decompression computers and frankly the Vytec isn't very good at all but at least it makes an effort to be one.

3: No one should undertake a decompression dive without some basic knowledge of deco theory.

4: The alternatives to computers are Tables which need understanding and Ratio Deco which needs comparison with tables as its levels of aggression vary massively across a range of diving.


I suppose it is fair to say anyone buying a proper deco computer can whack it on a very conservative setting and just dive it. However i would still advise against it.

ATB

Mark Chase

[ianfirmin]

To me, every dive is a decompression dive and no one should dive without some knowledge of decompression theory. This may only have to be suitable ascent rates or it may be mixed gas theory, stages, extended range etc... or anything in between.

Diving a square profiles using tables may well give you the shortest dive time. It's your choice. Make it up as you go along, you can do what you want. I'm nearly 55 years old and most of these tables were written to give not too much DCI in very fit young men (selected from the US Navy et al.)

Time moves on and the BSAC (88) tables and similar have been superceded by better things. Surveys like Gilliam's show there there were NO instances of DCI in many thousands of dives for users with computers who used them correctly. People using tables correctly DID have a few instances of DCI. What I said was that a computer was a "road map for the fastest way to the surface having a reasonable chance of avoiding DCI" and I'll stick with that.

You seem interested in this "my computer is better than your computer business". Assuming you mean "decompression diving" to be diving that requires a substantial amount of stop time then should you really be using a computer at all? Gas management and gas planning is so crucial to this sort of diving. This should be well worked out before the dive. What happens when your "decompression computer" tells you to stop for 30 minutes and you only have 20 minutes of gas left?

Finally, I wasn't really discussing diving that requires long decompression times or different makes of computer. I was mainly trying to make the point that decompression schedules are glorified guesses that seem to work well for most people most of the time. If you don't need to get up quickly then don't. If you do need to get up quickly then have the knowledge to minimise your chances of DCI.

[Garf]

Originally Posted by ianfirmin

You seem interested in this "my computer is better than your computer business". Assuming you mean "decompression diving" to be diving that requires a substantial amount of stop time then should you really be using a computer at all? Gas management and gas planning is so crucial to this sort of diving. This should be well worked out before the dive. What happens when your "decompression computer" tells you to stop for 30 minutes and you only have 20 minutes of gas left?

What happens when your tables say you have 30 mins of stops to do and you only have 20 minutes of gas. I fail to see the point you are making.

As for "should you be using a computer at all", apart from those DIR nutcases, the vast, vast majority of decompression divers (dives requiring mandatory decompression stops as opposed to safety stops) use computers to guide them through the ascent. This does not mean they are neglecting to plan their dive, or utilising appropriate gas management.

Just so you know Mark chase reads decompression theory in the bath, and when he can't sleep at night he counts OTUs, so you might to tread carefully when debating deco with him

[Garf]

Originally Posted by Mark Chase

But obviously the diver has an independent reserve gas supply or has calculated his reserves based on the need to isolate the twin set, so this is not an issue, right?

unless the isolator fails, smart arse

[tootricky]

Originally Posted by ianfirmin

What little research that has been done indicates Nitrox is less narcotic than air. O2 is not an inert gas, it’s metabolized in the tissues and it’s effective pp in the tissues is lower.

I'd be interested to see your sources for this. It was my understanding that research had been inconclusive and that evidence in favour of nitrox being less narcotic than air is purely anecdotal.

Cheers/Nic

[ianfirmin]

Originally Posted by Garf

What happens when your tables say you have 30 mins of stops to do and you only have 20 minutes of gas. I fail to see the point you are making.

The point that it may be a bad idea to "plan" a deco dive with a computer as you are doing it.

Originally Posted by Garf

the vast, vast majority of decompression divers (dives requiring mandatory decompression stops as opposed to safety stops) use computers to guide them through the ascent. This does not mean they are neglecting to plan their dive, or utilising appropriate gas management.

I'm sure they do. Be silly not to.

Originally Posted by Garf

Just so you know Mark chase reads decompression theory in the bath, and when he can't sleep at night he counts OTUs, so you might to tread carefully when debating deco with him

What are ya? Chimp or wimp?

[Garf]

oh goody, the old "is oxygen narcotic" debate.

My take on this is that I think I have read everything i can get my hands on about the subject and the evidence appears to be inconclusive. So rather than ending up the statistic that disproves the theory I'll take the safe route and assume they are both as narcotic as each other. The lipid solubility model suggests Oxygen is even MORE narcotic than Nitrogen, so I'd be interested to see these sources too.