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TTS Calculation
Posted by: Titus ()
Date: May 19, 2019 12:25PM


I'm a OSTC user since several years now (OSTC 2N) and just bought the new OSTC2 with hwOS Tech.
Just playing with the computer to understand how the deco planning is calculated.
I have set the computer to basic setting for Deco:
- Deco model : ZH-L16 (no GF)
- Sat/Desat : 85/115
- Gas : Air only (no deco gas)
- Last stop @3m
- speed : 10m/min (no variable speed)

I have calculated the deco for the following dive :
54m / 4' -> No Deco ; TTS = 6' ; 0' left

Trying to understand the TTS first : I guess the TTS is rounded to the upper value for security reason (54 m @ 10m/' gives 5,4' rounded to 6'). No prob !

What means "0' left" ?
rem: I did another simulation for 60m/2', get No Deco and "1' left"

Second calculation :
54m / 8' -> deco 3m/5' ; TTS = 14'
I try to understand how the TTS is calculated.
If a no deco dive at 54m gives a TTS of 6', then adding 5' of deco stop should give me a TTS of 6' + 5' = 11'. I have here a TTS of 14' !!! 3' more !

From where come these 3' ?
As a former OSTC user, I guess that 1' additional can be added to reach the surface from the stop at 3m. Ok good behavior to slow down the ascent when we are near the surface.
Ok for this 1' ... but still 2' to explain.

Any idea about these additional minutes ?

My only idea is that the ascent speed between 3m and surface is a 1m = 1' ... which is very very slow. 10m/1' gives 30 sec (which is too fast I admit); rounding to 1 min looks a good practice .... rouding to 3 min looks a bit over-killing especially when you dive with buddies not having such "conservative" measure and managing your deco on TTS to have harmonized deco plan inside the group.

Thanks a lot for sharing your advise and experience.


Edited 1 time(s). Last edit at 05/19/2019 12:35PM by Titus.

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Re: TTS Calculation
Posted by: Arno ()
Date: May 19, 2019 03:26PM

As I recall correctly the OSTC now indeed uses a 1m / minute ascent rate from (I believe 6m) to the surface.
Hence the longer time. Personally I think it's good practice to have an ascent rate this slow for the last 6 meters especially on a deco dive. It makes me much less "tired" after a dive when doing a proper ascent including a very slow last few meters.

I believe a second reasoning in this is to be extra safe on gas usage and calculation, but imho at these depths a minute more or less doesn't really add up to the total gas consumption.

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Re: TTS Calculation
Posted by: Titus ()
Date: May 19, 2019 04:21PM

Hello Arno

thanks for your feedback.

For what I noted, the 1m/1' ascent rate is valid from 3m. There is no such ascent time increase when 6m stops are present.

I'm also in favor to slow down a bit the ascent speed when reaching the surface but 1m/1' is a bit slow at my idea. (I do prefer to increase my last stop with a few minutes)
my main concern is not the speed in itself nor the gas consumption, it is more the way we will plan the dive with our buddies.

When I take a group of divers using several computers (OSTC, SUUNTO, SCUBAPRO, MARES, ...) during recreational (deco) dive, you run risks to have for the same dive a lot of difference in deco simulation, even if the computers are based on the same deco algo (Buhlman w/o GF). All this will lead to a smaller bottom time due to a (a bit) exaggerated ascent time in the last 3m.

Ok this can explain the additional TTS time. Can H&W confirm this ?

But what about the "0' left" sometime present when calculating deco ?



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Re: TTS Calculation
Posted by: Ralph ()
Date: May 19, 2019 06:43PM

Arno is completely right!

The OSTC adds 1 minute per meter for the final ascent to the TTS, so with a last stop set to 3 meters it's 3 minutes, with last stop at 6 meters it will be 6 minutes (indeed!). When the last stop is finished, the TTS will disappear and "NDL 240" or alike will be shown, so these extra minutes won't acctually counted down on you any more when doing the final ascent. But for your own favour, you should still try to do it that slow - the "real" last stop is the surface, watch how the saturation percentage value goes up on the last meters!

The "xx left" in the deco calculator is an estimate of the remaining NDL time. The deco calculator is geared to calculate a deco schedule, if it doesn't find any stops that need to be done, it gives an estimate on how many more minutes would be possible. "0 left" hence means you're on the edge between NDL and deco.

Regarding deco plans from different computers: Suunto doesn't do Bühlmann, they use the secret RGBM algorithm. Other brands that claim to do Bühlmann generally do it, but often add lots of company-own "safety additions". There are only few computers that really do "straigth Bühlmann", the OSTC is one of them. When you are into serious deco diving, make a good decision which computer will be your companion...


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Re: TTS Calculation
Posted by: Arno ()
Date: May 19, 2019 07:10PM

Ralph Wrote:
There are only few
> computers that really do "straigth Bühlmann", the
> OSTC is one of them....

Wasn't is so that OSTC "secretly" adds 1 meter depth in deco calculations as an extra conservatism factor? I might not recall correctly, but I do recall having read something like this in the past.
It may still do straigt Buhlmann, but then with the added conservatism that is not visible to the diver himself.

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Re: TTS Calculation
Posted by: Ralph ()
Date: May 19, 2019 08:31PM

You are pretty good informed!

This one meter was added in the stops table calculation, but not in the calculation of the real tissue pressures and hence ceiling / ascent limit depth. In consequence, the OSTC constantly assumed that you are hanging one meter below stop depth, but then adjusted its pessimistic assumption every couple of seconds to what's done in real. If a stop was calculated like that to be 3 minutes, it actually cleared some time earlier. But as the seconds are not shown, no one notices... ;-)

Well, in the next firmware version, this 1 meter thing will be thrown out!

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