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the kind of human wreckage that you love
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Discussion Starter · #1 ·
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 :eek:mg: and, personally, my dives were shallow and short
 

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New Member ? Its the same one i've always had
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Can't really answer the questions but I remember similar issues being raised during my SD course in Plymouth regarding driving home across Dartmoor.
 

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aka Chimp 1 or Mavis...
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Don't dive BSAC 88's :D

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!
 

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The King Of The Divan
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GUE Instructor
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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.

:(
 

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Don't dive BSAC 88's :D

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
 

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the kind of human wreckage that you love
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Discussion Starter · #7 ·
Don't dive BSAC 88's :D
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

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
 

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Jesus don't want me for a sunbeam
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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
 

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Apprentice houseplant
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Don't dive BSAC 88's :D

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
 

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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 :D 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
 

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the kind of human wreckage that you love
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Discussion Starter · #11 ·
Sorry to chip in on this,
chip away :teeth: I'm a wee bit lost and will need to do some reading so accurate postings will help.

So while I'm coming from a 'not very clued in' corner It was recently mentioned that even Stoney could be considered an altitude dive given the required atmosperic pressures!

I remember when I mentioned wanting to dive Vivian that someone jokingly mentioned I could tick off and altitude dive aswell

While I admit I don't have a firm grasp on the causes of change in AP I'm pretty sure that heatwave last July must have caused some affect, somewhere. It's not just about the height is it
 

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aka Chimp 1 or Mavis...
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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
I'm going home :( And I thought I was supposed to be good at maths...posts amended accordingly.
 

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aka Chimp 1 or Mavis...
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You might have thought that GLOC's maths was wrong :D but I don't think it it is.

1mbar = 1/1000 = 0.001bar

330 * 0.001 = 0.33bar

Unless I'm missing something obvious.

Cheers/Nic
Yes, David is right, you too are out by a decimal point. 330m equals 33mbar therefore 33 * 0.001 = .033
 

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Jesus don't want me for a sunbeam
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I dug out the figures I used in my calcs and based it on 300m on air:

For a 60min dive at 30m you would have to spend 37min at sea level after the dive before it was safe to drive to 300m altitude. For a 30min dive at 30m then it is only 13min. It was based on a 50mb reduction from sea level (based on an algorithm I have which calculates it which I got from Erik Baker).

As I remember it, when I'd factored in time it took to get to altitude and dekitting then even at the highest road pass it was totally irrelevant.

Cheers,

Stuart
 
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aka Chimp 1 or Mavis...
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For a 60min dive at 30m you would have to spend 37min at sea level after the dive before it was safe to drive to 300m altitude. For a 30min dive at 30m then it is only 13min. It was based on a 50mb reduction from sea level (based on an algorithm I have which calculates it which I got from Erik Baker).
Thanks Stuart. The only thing that I can think of is that might cause an issue is the max ascent rates expected in an '88 table. Not that people do 18m/min unless they want a bend :)
 

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Jesus don't want me for a sunbeam
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Thanks Stuart. The only thing that I can think of is that might cause an issue is the max ascent rates expected in an '88 table. Not that people do 18m/min unless they want a bend :)

I don't know how '88 works, the calcs were based on M11F6 deco model which is quite conservative.

Cheers,

Stuart
 

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I'm going home :( And I thought I was supposed to be good at maths...posts amended accordingly.
ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh hahahahahahahahahahahahahahahahahahahahahaha

Back to Speak 'n' Spell and "my little pony" for you.

you plank.
 

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aka Chimp 1 or Mavis...
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ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh hahahahahahahahahahahahahahahahahahahahahaha

Back to Speak 'n' Spell and "my little pony" for you.

you plank.
Well, you missed it too, Miss Smarty Pants :)

Eye'll ave em bak wen u are finished wit dem then.
 
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