Last Deco @6m, GF 30/80 and Air only

[Anonymous User]

Hi,

I'd like to have some limits for the GF settings when the last deco to 6m or a more visible warning for such useless configurations.
I used it for a 30min at 45m dive and got an endless 6m stop (About 2hours!). I went up to 3m meters ignoring the ceiling warning after a while and the deco finished within about 45mins...

Jostein

[heinrichsweikamp]

Hi,

Question is what is a "useless configuration"? We have some cave divers using the OSTC and GF>120% and you like to stay at 6m overnight.
Both is not recommended but can be useful for a particular person.

Problem in your example is the low GF_high of 80%. While this adds some safety to the model, in combination with the 6m stop and no deco gas you'll get very, very long 6m stops.

Due to the length of the stop (And the very slow decrease of N2 in your tissues) it actually reduces overall safety here (Gas loss, Hypothermia).

regards,
Matthias

[tiefunten]

Hmm, when you are using GF-settings, you should know what you are doing before the dive... Using air, a GF of 30/80 doesn't make much sense at all! If you like deeper stops, you can use 30/90, which is also very safe. BTW: When you set last stop to 6m, deco is always longer. If you really need a deeper last stop, you should think about using Oxygen at 6m.

[Remco]

and now you are supprised ?

I think most divers plan the dive and than dive the plan.
Now you wat went wrong...and i would not plan this dive on air only.

TX21/35 EAN50% runtime ...........65 minutes

[Hallgeir]

What I think would be very useful, is to have an automatical change of last stop depth from 6m to 3m when no deco gas is chosen, or made available...

[Regthing]

Hallgeir Wrote:
-------------------------------------------------------
> What I think would be very useful, is to have an
> automatical change of last stop depth from 6m to
> 3m when no deco gas is chosen, or made
> available...

Plan the dive! It has been said many a time on here that the OSTC will not make choices for you.

[fishwithlungs]

Halooo
Maybe it's about time to ban uselessly configured DIVERS from buying OSTC.
Any computer will just apply algorithm to the data it recieves(depth and time) based on settings,more mainstream ones will go to error mode if reasonable limits are exceeded.OSTC will simply update the info on screen,settings are responsibility of the USER.So is choice of appropriate gases for the dive.No computer can replace training,nor brains.

No offence meant,
Richard

[Hallgeir]

fishwithlungs Wrote:
-------------------------------------------------------
> Halooo
> Maybe it's about time to ban uselessly configured
> DIVERS from buying OSTC.
> Any computer will just apply algorithm to the data
> it recieves(depth and time) based on settings,more
> mainstream ones will go to error mode if
> reasonable limits are exceeded.OSTC will simply
> update the info on screen,settings are
> responsibility of the USER.So is choice of
> appropriate gases for the dive.No computer can
> replace training,nor brains.
>
> No offence meant,
> Richard

Absolutely no offence taken. My point is that there is no use in presetting the last deco stop depth if one should apply your logic. As all perfectly configured divers would be aware of, there is no use having the option of choosing 6m as the last deco stop if you are not decompressing on pure O2. So why have this function implemented as automatic preset default if the O2 is not on your active gas list? This would be a useful change in the computer for uselessly configured divers like me.

I have not used a computer for my diving for the last 8 years, and always plan my dives and dive my plan as it changes along the dive.I have calculated my deco on the fly. No problems. So why use a computer in the first place?

Get a grip! No offence meant,
Hallgeir

[PeterK]

"As all perfectly configured divers would be aware of, there is no use having the option of choosing 6m as the last deco stop if you are not decompressing on pure O2."

I would disagree with this position. There are several circumstances where last stop @6 m is useful, e.g. when diving in rough water. It's much more easy to maintain a deeper depth in rough water.
Personally I prefer to complete my decompression at 6m according to the selected GF high. The ascent to surface is done at 1 m/min.

However, this is absolutely up to you.

I think one of the main advantages of the OSTC is the flexibility to select the options that suits your needs. And I believe it should be kept that way.

[Hallgeir]

[quote]There are several circumstances where last stop @6 m is useful, e.g. when diving in rough water.[/quote]

Yes Peter, I agree. Isolated. This is also the only circumstance were I can see any reason for having last stop deeper than 3meter unless using oxygen. But if so, the OSTC will calculate your deco just as fine anyway. You just stop at 6m, and finish your deco with the best gas available.Thats all and a no brainer.

So what other reasons than using O2 is there to have a fixed last decostop of 6m?

What if we encounter a lost last gas scenario and find ourselves stuck with say ean50 as last gas? Is it then advicable to complete last stop on 6m?

I still believe that an automated setting of last stop 6meter with oxygene would be a positive advantage.

There must be something I've been missing here :p

[JeanDo]

This is a feature of the Bühlmann model. The trouble is that air have a 0.79 N2 ratio, so at 6m, ppN2 is 1,42 bar. If you have a look at you dive:
[img]http://www.heinrichsweikamp.com/file.ph ... in_45m.png[/img]

You will see that the C8 tissue (the last green curve segment on the right) is mostly flat. Because its ppN2 is near the 1.42 limit. The trouble is that the M0 value for C8 is 1.52 bar, meaning you can't surface until this tissue goes lower than 1.52 bar...
In short, your are doomed by the exponential diffusion with a very low gradient (1.42 - 1.52 = -0.1 bar).

The good news is that with a nitrox 50, you have 0.80 bar of ppN2 (at 6m). So the gradiant is a bit better (0.80 - 1.52 = -0.72 bar), and you expulse nitrogen even at 6m:
[img]http://www.heinrichsweikamp.com/file.ph ... in_45m.png[/img]

Anyway, the OSTC will compute what you actually do if your lost your gas, as long as let it know.

Hope this helps,

[fishwithlungs]

@ Hallgeir:

my comment was about the starting post.

But I agree with you,without o2 makes no sense to have last stop set @6m as default,save for one reason:

it is a bit of a trap to make sure one knows what one is doing.

and with the possibility to change so many custom functions user should know what are the consequences of any given setting.

if someone looks at last deco @6m and leaves it unchanged planning air only dive,then complaining about getting too much deco,that is raising both hair and questions about one's qualification for decompression diving.

personally,i set last deco @3m even with O2,simply because when ceiling has already moved up,i can do the o2 stop @5.9m - 6m.
else you can choose which looks better in logbook..ceiling violation or po2 too high



1.For the logbook,tissue loading info,ascent rate,stopwatch and avg depth,gradient factors,cns info and po2.OSTC is exceptional educational tool as well as great backup.
I compare my calculations on the fly against OSTC's as I did before with Suunto Vytec,Shearwater Pursuit and VR3.
I do not dive looking into the screen all the time,though i like to see it all the time and again OSTC wins in readability,forgetting the utter lack of info on Suunto,and complicated ways of displaying it on the other two.

2.Last year i didn't dive as much as usual,so between may 12th and dec 20th my OSTC logged 346 dives of all kinds.Do you think deco on the fly is fully applicable to dives to 20-30m 3x a day then week of 70-100m dives then back again?
Indeed i do deco on the fly or minimum deco but like to be able to compare to other models,as well as to see the details stated above.

@Peter K


Same here.Cheers

Happy 2012 to all

Richard

[wrobell]

JeanDo Wrote:
-------------------------------------------------------
> This is a feature of the Bühlmann model. The
> trouble is that air have a 0.79 N2 ratio, so at
> 6m, ppN2 is 1,42 bar.

shouldn't that be 0.79 * 1.6 = 1.26?

regards,

w

[JeanDo]

Arg... Absolutely right ! Sorry.
Even with the standard CF16=1m, you can count on 1.7bar at 6m... Not 1.8.

Nevertheless, the curve is nearly flat and your doomed for a long time...