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aka Chimp 1 or Mavis...
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Discussion Starter · #1 ·
Seeing as the other thread has gone the way normal YD threads do, I thought I would start another one... :)

I was driving back this evening and I was thinking what happens with BSAC tables when you have a low or high pressure day.

It is not uncommon to get low pressures of 970-980mb (change of sea level to plus 1300-1000ft respectively) and high pressures of 1030mb (change to minus 510ft).

Unless you have a barometer with you, how do you know what the pressure is and therefore the additional deco or measures you have to take?

Serious question as I do not see the difference between diving on a low pressure day at the coast and diving a lake at 1000ft above sea level on a 1013mb day (1013.2mb is taken as the pressure for the international standard atmosphere (ISA)). Hence why I don't see why you need to take altitude into account in the UK.

Over to the BSAC altitude specialists...
 

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Nigel Hewitt
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7,142 Posts
Not BSAC but I put the Bühlmann into a spread sheet and played with some numbers for air/nitrox.

As a test I considered all compartments saturated to the point where you can just surface. Of course you can never get there but it gave me a selection of worst cases to choose from. NB: this is not saturated to some depth but gassed off to the point where that compartment says 1 bar? I can cope with that.

Then I ran 10 minutes of surface deco ie: breathing 0.79bar N2.
Then rework the max compartment pressures, then convert it to altitude in meters.

The five minute compartment, unsurprisingly, gases off like mad and gives me 6450m, the 635 minute compartment just 20.6m. More predictive is the 38minute compartment at 560m.

For a final column I wondered how long it took to gas on enough for a compartment to become relevant at, say 30 minutes at 40meters (Mondays dive) and it came out as everything up to 27 minutes.

I don't seem to be finding much to worry about....
Does anybody want to check my maths?
altitude.xls
 

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aka Chimp 1 or Mavis...
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10,108 Posts
Discussion Starter · #3 ·

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Registered
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Seeing as the other thread has gone the way normal YD threads do, I thought I would start another one... :)

I was driving back this evening and I was thinking what happens with BSAC tables when you have a low or high pressure day.

It is not uncommon to get low pressures of 970-980mb (change of sea level to plus 1300-1000ft respectively) and high pressures of 1030mb (change to minus 510ft).

Unless you have a barometer with you, how do you know what the pressure is and therefore the additional deco or measures you have to take?

Serious question as I do not see the difference between diving on a low pressure day at the coast and diving a lake at 1000ft above sea level on a 1013mb day (1013.2mb is taken as the pressure for the international standard atmosphere (ISA)). Hence why I don't see why you need to take altitude into account in the UK.

Over to the BSAC altitude specialists...
Gareth,

There a 4 sets of air tables. Each has it's own elevation/pressure range. So yes, your example is fair, there is no difference - if they both end up with the same ambient pressure at the dive location. If you are borderline, you go to the next level - more conservative.

You'll have to ask Tom Hennessy why it matters...

Adrian
 

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aka Chimp 1 or Mavis...
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10,108 Posts
Discussion Starter · #5 · (Edited)
Gareth,

There a 4 sets of air tables. Each has it's own elevation/pressure range. So yes, your example is fair, there is no difference - if they both end up with the same ambient pressure at the dive location. If you are borderline, you go to the next level - more conservative.

Adrian
Adrian, What are the other altitude/pressure ranges as I haven't seen 88's before? Would you be able to give a couple of examples to see what the penalties are please? Do the tables also take into account how long it from surfacing to crossing the 'high point'?

Just done a google on Tom :) I know now why you said speak to him :) However, I have a feeling it will go into more detail than I can probably cope with but I will drop him a line nonetheless.

Thanks
 

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The King Of The Divan
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1,848 Posts
From my dim and distant Beardy past, I seem to think that I spent my time calculating altitude impact before the 88 tables came out. It was also for deep dives (on air) in the early-mid 80's so we would have been using the RNPL tables below 50ms.

Shame, I have binned all my old BSAC manuals which I think had these details in. These were the big white manuals with drawings in (a la Joy of Sex :teeth:. hmm thinking about it; both books had a preponderence of beards...:embarassed:) as opposed to the glossy photo manauls which followed them.

Snash
 

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I came, I saw, I dived
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552 Posts
I haven't got my tables handy, but I seem to remember that they recommend that when diving in low pressure you should adjust to use the relavent tables that correspond to the drop in pressure.

I'll try to find them later to check
 
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