Diving after travelling over a hill/mountain

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BlueTrin

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I am doing a BSAC training and saw that they have tables not only for diving at altitude but for travelling across a mountain for example.

Is there such a functionality in dive computers ?

I am not talking about diving at altitude I am talking about pre dive changes of altitude for example if you go from one beach to another but have to get across a mountain.
 
Most computer manuals have an altitude recommendation that is the same as "flying". The computer will have a timer when it has been dived and that shows "fly" after 12/24hrs.

There is huge controversy over a reduction in ambient pressure through altitude (real or artificial) and what is OK and what is not. I don't think the BSAC 88s are in any way exempt from that controversy.

Dive computers do not generally have any inbuilt feature to monitor ambient post dive (the Seiko models do) Whether any "warn" you by bleeping or something I have no idea. My old Seiko (Apeks Quantum) once showed three mountains when we crossed into Italy from France but it did not bleep. There is no facility as far as I am aware on any computer to show to what altitude one can go - I doubt the algorithm would be able to do so. The Seiko pictogram was for altitude diving.
 
"Diving after flying does not in itself create a decompression problem since flying leaves you with less inert gas in your tissues than would be there had you stayed at ground level unless you land at high altitude. Because of this, there are no set guidelines for when to make your first dive."
Ask DAN: Flying & Diving

In other words, going to altitude after diving is the only time it matters.

for example if you go from one beach to another but have to get across a mountain.

If you are NOT diving at the first beach, then the mountain doesn't matter.
 
Yea agree with both of you, I am not sure why flying or going over a mountain before diving would be an issue.

However that raises another question: BSAC tables have an altitude change that is safe after diving using your exit tissue code.

I don’t think any diving computer tells you when you would be safe or even if you are safe to travel over a mountain after diving by inputting the max altitude that you’d be travelling at ?
 
... BSAC tables have an altitude change that is safe after diving using your exit tissue code.

I don’t think any diving computer tells you when you would be safe or even if you are safe to travel over a mountain after diving by inputting the max altitude that you’d be travelling at ?

The BSAC 88s are widely regarded as out of date and to be ignored. Post dive reduction in ambient pressure is controversial to say the least. Some DAN studies suggest that you should wait 24 hours before reducing ambient pressure to any great degree (IIRC it is about 300m?) Many divers just hold the last stop a bit longer or use O2 and fly without any ill effects.

Most dive tables are based on AA Buhlmann. The 88s are grounded in RNPL and Hennesy's subsequent personal work for the BSAC.

You would need to ask Hennesy if the RNPL did any work on this subject and if so whether he used the data for the 88s or if it is simply an extrapolation on the mathematical algorithm of the tables.

Buhlmann's work on decompression is generally agreed to be definitive, but there is no such agreement on post dive reduction in ambient pressure. Therefore the tables and algorithms based on that (and later models like VPN) do not predict the post dive reduction that is allowed.

Please don't use the word "safe".
Decompression models are not "safe" any more than DAN's 24 hour recommendation is. If you wish to completely avoid decompression illness I can keep you safe - do not dive in the first place :wink:
 
The BSAC 88s are widely regarded as out of date and to be ignored. Post dive reduction in ambient pressure is controversial to say the least. Some DAN studies suggest that you should wait 24 hours before reducing ambient pressure to any great degree (IIRC it is about 300m?) Many divers just hold the last stop a bit longer or use O2 and fly without any ill effects.

Most dive tables are based on AA Buhlmann. The 88s are grounded in RNPL and Hennesy's subsequent personal work for the BSAC.

You would need to ask Hennesy if the RNPL did any work on this subject and if so whether he used the data for the 88s or if it is simply an extrapolation on the mathematical algorithm of the tables.

Buhlmann's work on decompression is generally agreed to be definitive, but there is no such agreement on post dive reduction in ambient pressure. Therefore the tables and algorithms based on that (and later models like VPN) do not predict the post dive reduction that is allowed.

Please don't use the word "safe".
Decompression models are not "safe" any more than DAN's 24 hour recommendation is. If you wish to completely avoid decompression illness I can keep you safe - do not dive in the first place :wink:
The whole post makes sense however I respectfully disagree with your last paragraph, unless you always talk in absolutes, one has to agree that safe means ‘safe enough to be commonly used with reasonably low risk. (I don’t like the BSAC tables myself, I just happen to have noticed they have tried to take in account changes in altitudes so it made me curious)

So what’s the answer to how can I know how much of a danger if on my return journey I have to drive through a hill/mountain (which isn’t so unusual) ?

I saw as wel the DAN answer is that it is not safe, and they effectively mention 300m. I think the same article says that altitude is a common factor in many accidents.

So if you drive over a hill/mountain, you are in uncharted territory with not enough statistical data to validate any reasonable model.

Your post is very well written and very informative!
 
The BSAC 88s are widely regarded as out of date and to be ignored.
...
Please don't use the word "safe". Decompression models are not "safe" any more than DAN's 24 hour recommendation is. If you wish to completely avoid decompression illness I can keep you safe - do not dive in the first place :wink:
Just because the 88 tables have been around a bit doesn’t make them any worse or better than any other set of decompression tables. To my knowledge human physiology hasn’t changed in the last 30 years, but it’s became the thing to have a big at anything BSAC.

I agree with your last statement though.
 
Those tables are about the only thing you will find. However the altitude part is largely superfluous, they don't have them in the Nitrox or OxStop versions.

There are non obvious effects, for example if start from an A, go up a hill, so are on a B at the top, but on return you stay on a B.

Unless you are really diving the tables, really all you can do is plan the dive as though at whatever altitude you will get to on the trip. Or leave plenty of time.

I tell people not to worry too much and to just make sure they revise the rest well so they can afford to get the table questions wrong. :)
 
It also depends on what kind of altitude differences are you talking about, a mountain can be 200 meters high or 2000 meters high, it really depends.

I regularly travel to the red sea to dive and on my way back home there is about 500 meters altitude difference , neither i or any other diver who goes there have ever had problems with it, every dive club there tells you its no problem.

How ever, it is a long slow climb, stretching for about an hour. so the increase is done slowly, a steep incline on a different mountain may be more serious, there isn't really too much data to go on here.

As a general idea, its best to avoid increases in altitude after a dive.
 
Just because the 88 tables have been around a bit doesn’t make them any worse or better than any other set of decompression tables. To my knowledge human physiology hasn’t changed in the last 30 years, but it’s became the thing to have a big at anything BSAC.

Yes, it is sad that Tom (Hennesy) did a job as was asked of him and it is now used as some sort of stick to beat BSAC for other reasons. The 88s were generous on the first dive and conservative on the second reflecting the thinking of the time.

The evolution of decompression theory now tends towards multi dive days and I think it is fair to say that this is at least some of the basis of criticism of the 88s. it is unfair in that (to the best of my knowledge) Buhlmann did not concentrate on multi dive recreational profiles as his priority.

Over the last 30 years the other big change is that deeper and more complex dive profiles have become relatively commonplace. Often called "technical" diving. It is a fact that this has not been accompanied by any increase in death or DCI so somewhere the theory must hold up. Various ideas have come and gone, bubble models and deep stops. But the fact is the majority of dives are still 40m or less and with short if any decompression. Often called "recreational" diving.

I believe PADI have abandoned their RDP altogether now as a published table? Such is the reliance on computers, nearly all of which are running on one or another of the Zurich algorithms. The advantages of a personal dive computer are such that there are models now readily available for helium based mixes and long decompressions. I am unaware of any that have the altitude feature as discussed available (but I am not a kit junkie so don't take my word for it). If there were a need (market) for it I am sure they would exist.
 
https://www.shearwater.com/products/teric/

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