Inadvertent Altitude

[Letz]

I was just having a conversation regarding potential altitude issues when going to and returning from a dive site, specifically in the Lakes, to be greeted by Richard at Wraysbury advising NOT to return a certain way back from Brighton for that very reason

I did take a route to Brighton recently which avoided the traffic jams in the town and, of course, there is Brighton Hill, but I'm not sure it was the route Richard was mentioning.

Now, BSAC have quite a lot of info regarding diving BEFORE and AFTER level changes in their '88 tables, including precautions to take, altering the current tissue code, where a level change has occurred BEFORE a dive
I definitely need to revisit this as I recall finding it quite convoluted

So I guess the points I'm raising here are :

1. What do we understand regarding travelling 'up hill and down dale' to a dive site BEFORE the dive

and

2. just what precautions should we be taking when returning home from a dive if it transpires that part of the journey could in fact take us up, or down, a level?

Oh, AND 3. Does anyone know a simple way of calculating the CTCs/appropriate info which is a bit more straightforward than the BSAC '88 tables?



Those who have done a course with Martyn Farr in the Silica Mines will probably remember going 'up hill and down dale' but the gradient wasn't great, even if it felt like it and, personally, my dives were shallow and short

[Keith Whittaker]

Can't really answer the questions but I remember similar issues being raised during my SD course in Plymouth regarding driving home across Dartmoor.

[GLOC]

Don't dive BSAC 88's

I can't see anywhere in the SE of the UK where this would case an issue.

1mb equals 30ft or 10m of air pressure. So if the highest point is a 1000ft/330m (which I doubt), then you are talking about a pressure difference of .033 of a bar or .3m difference in water pressure. TBH I can't see that making a difference by the time you have dekitted, loaded the stuff in the car and driven off.

Indeed, in the Lakes after a dive*, I can't see it making a major difference either because you don't go rocketing up to top of Snake pass immediately after the dive and there is a SI interval and you are driving slowly up the hill.

Just my £0.02

* by which I mean driving over Snake pass after a dive and not conducting a dive in the Lakes. TBH I wouldn't be bothered about diving in the Tarns either, just make sure you add a little to the stops and make sure you make a slow ascent!

[Snash]

Have a quick gander at this thread for some furhter info/ ponderings on it.

It is an issue we need to take account of when planning dives especially in Scotland - eg the Rondo if last dive of a Lochaline trip etc. The easist way to deal with it is scheduling your diving accordingly

Denial - Nope!!

Cheers

Simon

[Garf]

I've never understood this. As GLOC says, the pressure differential in air is so much smaller than it is under the water you are at most talking about the equivelent of a couple of metres. Even then you have had a long surface interval before "ascending" further, and then an incredibly slow "ascent" that probably works out at about a metre per hour!

I just don't get this post dive altitude stuff.

[Adrian Kelland]

Originally Posted by GLOC

Don't dive BSAC 88's

I can't see anywhere in the SE of the UK where this would case an issue.

1mb equals 30ft or 10m of air pressure. So if the highest point is a 1000ft/330m (which I doubt), then you are talking about a pressure difference of .033of a bar or .3m difference in water pressure. TBH I can't see that making a difference by the time you have dekitted, loaded the stuff in the car and driven off.

I don't have the 88s to hand, but I have a similar attitude in the south. Add to the time for sorting out boats, banter and maybe a drink and it can easily be a couple of hours since the end of the dive.

I might have a quick gander at the altitude bits if I was not familiar with an area.

VR3 handled the Alps pretty well, although would not switch on automatically during shallow dives <3-4m.

But surely the issue is the air pressure, not the difference in water pressure. .33 drop in air pressure is a lot if the calcs are based on a exit at 1 bar. Over to someone else please...

Adrian

[Letz]

Originally Posted by GLOC

Don't dive BSAC 88's

Yes, I reached that conclusion but they do have a lot of stuff regarding altitude and diving and I can't say I've seen much in the way of 'how to calculate' elsewhere. Not that I've set out to find it before

Originally Posted by Garf

I've never understood this. As GLOC says, the pressure differential in air is so much smaller than it is under the water you are at most talking about the equivelent of a couple of metres. Even then you have had a long surface interval before "ascending" further, and then an incredibly slow "ascent" that probably works out at about a metre per hour!

I just don't get this post dive altitude stuff.

As I understand it the issue isn't just POST dive.
For example, while large commercial jets don't seem to be an issue, pressurised cabin, time after landing etc. anyway, it does say this :
"Note that diving at sea level (Level 1) after flights of less than 90 minutes in a pressurised aircraft (Level 4) requires table B (level 1) to be used, unless a SURFACE INTERVAL of at least 10 hours at level 1 has elapsed after landing"
For those unfamiliar with BSAC tables they start with Table A for a clear start so the B tables are the next along, generally after a dive with a suitable SI

Hence why I particularly raise the question of level changes BEFORE a dive

[NotDeadYet]

There is nothing to worry about in the UK.

The question came up on another forum a few months ago and I decided to modify my deco software to take into account an ascent to the highest road altitude in the UK after the dive. I couldn't calculate a single dive that would be problematic.

When I go cave diving in the Doubs it isn't unusual to 60m dive in the morning at 900m alt, ascend to 12-1400m going over the mountains into Switzerland, drop to 450m for a 30m dive, back up to 900m through another pass, drop to 300m for a dive late afternoon then back to camp at 900m. I've never had any bother. I've also done 100m+ dives in Loch Ness and driven back over Rannoch Moor with no issues either.

Cheers,

Stuart

[David Pye]

Originally Posted by GLOC

Don't dive BSAC 88's

I can't see anywhere in the SE of the UK where this would case an issue.

1mb equals 30ft or 10m of air pressure. So if the highest point is a 1000ft/330m (which I doubt), then you are talking about a pressure difference of .33of a bar or 3m difference in water pressure. TBH I can't see that making a difference by the time you have dekitted, loaded the stuff in the car and driven off.

Sorry to chip in on this, but aren't you a decimal point out?

1mbar = 10m of air pressure
so:
330 metres of air = 33mbar pressure difference = 0.03 of a bar, instead of .33?

David

[tootricky]

Originally Posted by David Pye

Sorry to chip in on this, but aren't you a decimal point out?

1mbar = 10m of air pressure
so:
330 metres of air = 33mbar pressure difference = 0.03 of a bar, instead of .33?

David

You might have thought that GLOC's maths was wrong but I don't hink it it is.

1mbar = 1/1000 = 0.001bar

330 * 0.001 = 0.33bar

Unless I'm missing something obvious.

EDIT: Yes, I missed something obvious: 1mbar = 10m air pressure
( 330 / 10 ) * 0.001 = 0.033bar

Cheers/Nic