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Discussion Starter · #1 ·
Was diving with a couple of friends yesterday and, as always, I dived with my Tusa IQ-700 and Uwatec Smartcom dive computers. On the second dive we noticed our NDL getting quite low so we started to ascend in order to maximise our bottom time. We got to 10m and we still only had 8 minutes of bottom time so we swam around for a little at that depth before ascending a little more. As soon as we hit 9.5m the bottom time shot up to 144 minutes on my Tusa and 99 minutes (the maximum NDL that will be displayed on my Smartcom).

I know it's down in some manner to the algorithms used, but what causes that tiny decrease in depth to change the NDL by such a substantial amount? I've noticed it before but only started really thinking about it yesterday. Thought I'd pass it onto you folks to help me out.

Cheers for any info you can offer me about this:)
 

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Nigel Hewitt
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I know it's down in some manner to the algorithms used, but what causes that tiny decrease in depth to change the NDL by such a substantial amount? I've noticed it before but only started really thinking about it yesterday. Thought I'd pass it onto you folks to help me out.
Yes.
To the mathematics going shallower does work like that.
The shorter period comartments are now clear-to-surface where before they would soon gas-on enough to require a stop and only the longer ones (that can produce stops from shallow dives are now able to have an effect.
 

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A short fat well off crap cave diver. Likes wrecks
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Because they are crap dive computers.

I had the same problem with my Suunto Vytec. The deco depths were like an on off switch. 7m sit there for ever 5m clears no problem :angry:

I had to get out of the water with the Sinnto showing 40+ odd mins of deco on a couple of Red sea dives where the reef was at 8m but it wouldn't give me any deco credit for being at 8.


For no deco diving they are fine but the issues you describe can be frustrating.

ATB

Mark Chase
 

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His Holeyness
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It's considered that up to 10mtrs there is no problematic absorption of nitrogen, so you can dive up to 10mtrs for ever.

So if you rise to above 10mtrs. and you've still some no-deco time left, that time will last forever, or whatever the maximum time you computer will allow

I'm sure its explained in the PADI manual somewhere . . .


ColinM
 

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Discussion Starter · #7 ·
It's considered that up to 10mtrs there is no problematic absorption of nitrogen, so you can dive up to 10mtrs for ever.
Doesn't that go against the RDP though, which suggests that even a dive to 10m has a no-decompression limit?
 

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A short fat well off crap cave diver. Likes wrecks
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Is it the dive computers that are "crap", or the algorithms they are based on? Surely a computer can only be as good as the theoretical calculations it's supposed to work out.

No its the computer. My VR3s were fine (thats how i knew to get out when i did) and my Shearwater is fine too. I can deco out at 10m if I like. They all work on Bhulman ZHL16C The RGBM thing with Suunto is not true its just a name only thing.

ATB

Mark Chase
 

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A short fat well off crap cave diver. Likes wrecks
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Doesn't that go against the RDP though, which suggests that even a dive to 10m has a no-decompression limit?

Yeah but Air the NDL at 10m is over eight hours which is a bit of a push even on twin 15s :D

Do it on a CCR with 50% 02 and you will be there all day :D

ATB

Mark Chase
 

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No social integrator
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As Nigel said - the controlling compartment just changed.
 
G

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FWIW,
The original decompression model mafde up by Haldane at least 100 years ago found that:
the each tissue compartment could tolerate an over pressure gradient ( i.e. difference between dissolved gas pressure and ambient gas pressure) of 1:2,
i.e. the outside water pressure could be no more than 2 times the tissue dissloved gas pressure.
At 10m this is on the very limit of 1:2 bar gradient and any deeper it becomes a 'decompression' dive (i.e. requiring controlled ascent rates),
when not diving deeper than that presumably the computer stops caring about your deco obligations as you can handle the bubbles without symptoms.
(i.e. the coke bottle, you, is not fizzy enough to fizz when opened).
Hope this helps!


Spunkmire accepts no responsibility for any statements from him, anywhere, at any time, ever. Accuracy is guaranteed by the tiny elves living inside my head.:)

 

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The smell of freshly turned delrin is more powerfu
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as dive computers get better the math is better. I am guessing that it would be easy to build a safe computer to dive with. But one that gives you the longest time underwater at depth would be something a little harder.

Mark pointed out he had 40 mins deco showing and just got out :) he can do that because he knows the other computers well enough to trust them.

some people decide that a computer is pointless and work out their own deco or follow tables.

But I think following that computer might just keep you safer and alive longer.
but that saftey stop at 5 or 6 meters might look diffrent even now just by what you have seen for your self ?

David
 

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Rich D
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582 Posts
ok - so a question

i dive with a quantum...and havnt done many deco dives, when required i tend to stop hanging on my smb at around 5m - and noticed the deco does appear to clear extremely slowly - would I be better having the stop any deeper/shallower than 5m?
 

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Kit Junkie
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804 Posts
I've noticed the depth of the final stop (when in deco) has a huge effect on times with my Suunto. Just at or below 3m and it clears quickly, at or just above 6m and it can take ages. Although I'm above 50 and should be putting in conservatism factors I subscribe to Mr. Hewitt's concept of having your computer show the fastest way up but then adding conservatism ad hoc. A combined bubble and tissue tension model seems also to make sense and the Suunto RGBM appears to account for the bubble side of things.

In practice I try to ascend as slowly as possible but never more than 10m/min, use as rich a nitrox mix as I can to stay within 1.4ppo2 and make the ascent from 3m to the surface as slowly as possible, I aim for 3m/min.
 

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Kit Junkie
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If you read the specs they use a combination, Bubble and Buhlmann. I suppose they could be lying, but why?
 

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GUE Instructor
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If you read the specs they use a combination, Bubble and Buhlmann. I suppose they could be lying, but why?
yeah they use a combination. They use a traditional model in the maths, and a bubble model in the name. They have only licensed the rights to use the name "RGBM" not the bubble model itself, unless this has changed in the last year or so, just like Mark Chase stated earlier in the thread.

Why would a corporate entity lie? Is that a serious question?
 
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