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ScubaCollie
23-08-2013, 07:29 PM
I qualified last year and did my pressure groups with Dive Tables.

I now have a computer 'Sunto Vyper' and it displays current depth, max depth, dive time, water temp, and no deco time.

I think I 'may' have fell into something tonight (not literally), I thought the benefits of a computer were to tell me my depth and safety stops and for when I finish a days diving I have all the details on it to complete my log book, however this evening ive been briefly discussing pressure groups with somebody and theyve said im on a computer. .......so ive been thinking that my computer doesn't tell me my pressure group does it?

But then I back tracked and thought about pressure groups and what they mean and about max bottom times etc., and realised that the dive tables tell you the max no deco bottom time you can dive......so therefore that no deco figure that keeps counting down on my computer is working out my pressure groups for me?

Therefore if I was to make 4 x dives in a day I dont need to work out my pressure groups with my tables, pen, and paper? Is it converting each dive of the multiple dives I do into presure groups and reducing my no deco dive time in each succeeding dive?

I.e if I do 1 x dive the no deco time may be xxxx but if I do the exact same dive profile half hour later the no deco time will be less due the new pressure group and the residual nitrogen left in my body?

Hickdive
23-08-2013, 07:59 PM
The computer doesn't work on tables or pressure groups, instead it uses an algorithm to model in real time your tissues' on gassing and off gassing.

Your tables are also based on an algorithm but, since they cannot know if you descended or ascended directly from your maximum depth they simply assume you spent your entire dive at maximum depth.

That's why a computer can extend your no-stop time as you go shallower in a way your table can't (well, with some tables you can but its a clumsy process). The problem with that is that it takes you well away from the parameters of your table and you can no longer tally information from your computer with tables and this worsens with the more dives you do in a sequence.

Pressure groups are just a handy way of guiding you round the table and, consequently the underlying algorithm. Since computers can do the maths in real-time they don't need to use pressure groups and work directly from the algorithm.

There's some more info here;

Dive computer - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Dive_computer)

holly
23-08-2013, 08:01 PM
I qualified last year and did my pressure groups with Dive Tables.

I now have a computer 'Sunto Vyper' and it displays current depth, max depth, dive time, water temp, and no deco time.

I think I 'may' have fell into something tonight (not literally), I thought the benefits of a computer were to tell me my depth and safety stops and for when I finish a days diving I have all the details on it to complete my log book, however this evening ive been briefly discussing pressure groups with somebody and theyve said im on a computer. .......so ive been thinking that my computer doesn't tell me my pressure group does it?

But then I back tracked and thought about pressure groups and what they mean and about max bottom times etc., and realised that the dive tables tell you the max no deco bottom time you can dive......so therefore that no deco figure that keeps counting down on my computer is working out my pressure groups for me?

Therefore if I was to make 4 x dives in a day I dont need to work out my pressure groups with my tables, pen, and paper? Is it converting each dive of the multiple dives I do into presure groups and reducing my no deco dive time in each succeeding dive?

I.e if I do 1 x dive the no deco time may be xxxx but if I do the exact same dive profile half hour later the no deco time will be less due the new pressure group and the residual nitrogen left in my body?

any repetative dives on the same day will result in a higer PG and shorter no deco time, not sure exactly how long it takes to off gas slow tissue compartments

anvill72
23-08-2013, 08:02 PM
Treat dive computers as electronic versions of the NDL tables. They follow similar models to estimate Nitrogen loading.

However, dive tables assume a perfectly square profile whereas computers take account of your actual depth, therefore giving you NDL credit when you go shallower.

Pressure groups are in the tables to allow you to navigate the table. I don't believe computers calculate pressure groups as such. They just keep running model calculations to track Nitrogen loading on a more granular time slice.

Chris_M
23-08-2013, 08:06 PM
I qualified last year and did my pressure groups with Dive Tables.

I now have a computer 'Sunto Vyper' and it displays current depth, max depth, dive time, water temp, and no deco time.

I think I 'may' have fell into something tonight (not literally), I thought the benefits of a computer were to tell me my depth and safety stops and for when I finish a days diving I have all the details on it to complete my log book, however this evening ive been briefly discussing pressure groups with somebody and theyve said im on a computer. .......so ive been thinking that my computer doesn't tell me my pressure group does it?

But then I back tracked and thought about pressure groups and what they mean and about max bottom times etc., and realised that the dive tables tell you the max no deco bottom time you can dive......so therefore that no deco figure that keeps counting down on my computer is working out my pressure groups for me?

Therefore if I was to make 4 x dives in a day I dont need to work out my pressure groups with my tables, pen, and paper? Is it converting each dive of the multiple dives I do into presure groups and reducing my no deco dive time in each succeeding dive?

I.e if I do 1 x dive the no deco time may be xxxx but if I do the exact same dive profile half hour later the no deco time will be less due the new pressure group and the residual nitrogen left in my body?

A dive computer will real time track your nitrogen exposure and calculate your limits. In planning mode you can still get it to tell you your No Deco Limits before you get in. They do not show pressure groups, partly as they usually run off a different algorithm to the RDP you would be use to from your PADI courses, and would therefore not be transferable.
You are correct in thinking that you would have different limits if you are to do the same dive twice with only half an hour, just like you would if you did it on tables.
Because computers work out your exposure in real time, thus turning every dive in a multilevel dive, on a 4 dive day you would get lots of no stop time on a computer, vs doing every dive on tables. I have done this test in Egypt doing a 3 dive day. 2 on the Thistlegorm and then Shark to Yolanda reef. Tables, I was pretty much bent having missed masses of stops. Computer I was fine.

ScubaCollie
23-08-2013, 08:06 PM
Thanks all, so it 'will' reduce the no deco time of the next dives in a sequence based on the immediate previous dives similar to what I would have performed if I used the pressure group tables?

Therefore I dont need to perform 'any' calculations myself as long as I remain in credit with my no deco time I can dive for as long and as quckly after each dive as I want?

chas49
23-08-2013, 08:10 PM
If you dive on tables (I'm assuming PADI, but that doesn't really matter), the NDL (and the pressure group for calculating residual nitrogen after a surface interval) is based on the assumption that ALL the bottom time is spent at the max depth. I don't know about you but I don't think I've ever done a dive where that's true.

So, what's your computer doing? It knows how deep you are and for how long all the way through the dive. By definition your depth for some (perhaps most) of the dive will be less than the max. Therefore your exposure will be less than the tables give.

And it knows how long the surface interval was, so it knows what you go in with....

(Caveat: the computer doesn't know - it works on a model and assumptions but it's certainly more accurate than tables.)

Tables are ultra conservative, computers are arguably more realistic.

My2p

Sent from my GT-P5110 using Tapatalk 4

Chris_M
23-08-2013, 08:10 PM
Thanks all, so it 'will' reduce the no deco time of the next dives in a sequence based on the immediate previous dives similar to what I would have performed if I used the pressure group tables?

Therefore I dont need to perform 'any' calculations myself as long as I remain in credit with my no deco time I can dive for as long and as quckly after each dive as I want?

Yes.

Your vyper has a nice little symbol on it when it thinks it is too soon to get in the water. I would always recommend having an idea what the ndl is before every dive, and a secondary bottom timer, watch. That way if your computer dies, you can still make sure you are in the No Deco Limit, and make a safe ascent, performing any required stops.

If you see me at a lake, feel free to collar me if you want a face to face explanation. Wont be for a couple of weeks though as my weekends are full.

Iain Smith
23-08-2013, 08:24 PM
Therefore I dont need to perform 'any' calculations myself as long as I remain in credit with my no deco time I can dive for as long and as quckly after each dive as I want?


Your vyper has a nice little symbol on it when it thinks it is too soon to get in the water. I would always recommend having an idea what the ndl is before every dive

Absolutely. Getting into the habit of always planning your dive is A Good Thing.

You'll also find that if you do lots of back-to-back dives (ie 4-5 a day) a) you'll feel absolutely shattered very quickly (repetitive air diving = significant decompression stress/subclinical DCI) and b) you will "load up" your computer to the point that you start getting very short no-deco limits.

Four dives per day for a week is doable...but you will feel like you've been hit by a truck. :-)

Iain

BenL
23-08-2013, 08:47 PM
I qualified last year and did my pressure groups with Dive Tables.

I now have a computer 'Sunto Vyper' and it displays current depth, max depth, dive time, water temp, and no deco time.

I think I 'may' have fell into something tonight (not literally), I thought the benefits of a computer were to tell me my depth and safety stops and for when I finish a days diving I have all the details on it to complete my log book, however this evening ive been briefly discussing pressure groups with somebody and theyve said im on a computer. .......so ive been thinking that my computer doesn't tell me my pressure group does it?

But then I back tracked and thought about pressure groups and what they mean and about max bottom times etc., and realised that the dive tables tell you the max no deco bottom time you can dive......so therefore that no deco figure that keeps counting down on my computer is working out my pressure groups for me?

Therefore if I was to make 4 x dives in a day I dont need to work out my pressure groups with my tables, pen, and paper? Is it converting each dive of the multiple dives I do into presure groups and reducing my no deco dive time in each succeeding dive?

I.e if I do 1 x dive the no deco time may be xxxx but if I do the exact same dive profile half hour later the no deco time will be less due the new pressure group and the residual nitrogen left in my body?

You won't get "pressure groups" like you find on a dive table but the Vyper does enable you at least to plan successive dives, the outcomes of which are based on previous history (residual nitrogen). You can enter a given surface interval, and then by adjusting the planned depth for the next dive it will recalculate the no deco time for the next dive.

ScubaCollie
23-08-2013, 08:51 PM
Many Thanks to all, didnt realise these computers were so cleaver, only been using it for about 6 months! ! :)

Chris_M, will take you up on that mate, although the only way I reckon I could get to see you is to dial 999!! LOL ;)

Out of interest, what does that symbol look like that you are referring to?

SilentDiver
24-08-2013, 08:29 AM
Try getting your hands on a copy of this book it is extremely good and will tell you everything you need to know about tissue loadings etc

Deco for Divers by Mark Powell (http://www.dive-tech.co.uk/deco%20for%20divers.htm)

Ian_6301
24-08-2013, 08:53 AM
You won't get "pressure groups" like you find on a dive table but the Vyper does enable you at least to plan successive dives, the outcomes of which are based on previous history (residual nitrogen). You can enter a given surface interval, and then by adjusting the planned depth for the next dive it will recalculate the no deco time for the next dive.

You do on my OSTC, but I'll grant you, that's not the norm.

The vyper is pretty good for the kind of diving that you're doing, but if you feel really tired after a day's diving, remember to bump the personal setting up a notch or two. This is likely the sub clinical dci mentioned above.

rongoodman
24-08-2013, 11:49 AM
You'll also find that if you do lots of back-to-back dives (ie 4-5 a day) a) you'll feel absolutely shattered very quickly (repetitive air diving = significant decompression stress/subclinical DCI) and b) you will "load up" your computer to the point that you start getting very short no-deco limits.

Four dives per day for a week is doable...but you will feel like you've been hit by a truck. :-)

Iain

These are trips where Nitrox really shines. That being said, I've done multiple liveaboard trips with 5 dives daily, and no tire tracks were to be seen on my person. (And I was in my 60s for all of them.)

Tim Digger
24-08-2013, 12:46 PM
Out of interest, what does that symbol look like that you are referring to?[/QUOTE]

On Suunto Vyper etc a continuous or flashing triangle top left means you should not dive again yet. Usually it stops after 1 hour ish. If you have done something silly that it calculates has left you with a greater than usual gas or bubble load to shed it will extend this time. If you ignore the triangle then it will penalise you with a shorter No stop dive time or deco.

RTFM But then it was about 6/12 after I got my first Suunto that I found out what the triangle ment. Hangs head in shame.
Tim Digger