Passing through a gas change.

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edhowarth
Posts: 32
Joined: Saturday 30. April 2011, 17:56

Passing through a gas change.

Post by edhowarth »

As I'm hoping to do some diving later on this year, and it now being spring, I upgraded my Plus to V3.13 and had a happy few hours doing some simulations, just to get back to familiarity.

One thing I noticed on doing a 2 gas dive is, if I ignore the option to change to an enabled deco gas on ascent, the computer keeps on telling me to change even when I have gone way, way shallower. I think on an old version it would give up after a few minutes/metres and adjust times accordingly.

I'm happy with the way it now works, and I suppose the way to get rid of the nagging screen is to go to the gas list and select lost gas. (By the way, I think the way the gas list now works is greatly improved compared to before).

But I just want to be sure, that when the screen is nagging me to change, the deco times (and TTS) are assuming I change to the deco gas at that instant? And if I don't, then the minutes will keep on extending, so even though it might say 10 minutes TTS, it will actually take 17 to slowly count down?

I'm just thinking if something serious goes wrong at a gas change depth, and I'm totally task loaded and I can't change to a deco gas, will the computer get me up to the surface with the correct deco?
Ralph
Posts: 657
Joined: Saturday 24. June 2017, 11:31

Re: Passing through a gas change.

Post by Ralph »

Hi,

you have to distinguish between the displayed as-of-now data and the calculated decompression plan data:

The as-of-now-data such as current depth, next stop depth (lower right), ceiling & (super-)saturation (custom view) are computed on the basis of what you have done up to now. So for the deco-related data, you need to have told the computer which gas(es) you were breathing up to now.

For the planning data (duration of the current stop and the all the next stops, TTS, gas needs for ascent, ...) you need to breath what the computer is prompting you for, because its calculation assumes you will breath the best gases throughout the remainder of the dive. And what the computer prompts you to switch to is the gas it has figured out to be the best gas for the depth you are currently at.

If you do not switch, the computer continues to calculate the as-of-now data based on your currently selected gas. And it will continiously calculate an ascent plan based on the assumption that you will switch now. If you decide you will NOT switch to the gas the computer is prompting you for, or any other gas that will become the best gas later on in the dive, you need to tell the computer so that it will exclude that gas from the choice of gases it is electing the best gas from. You do this by setting the gas to the 'lost' state.

BR
Ralph
Last edited by Ralph on Saturday 27. March 2021, 18:50, edited 2 times in total.
edhowarth
Posts: 32
Joined: Saturday 30. April 2011, 17:56

Re: Passing through a gas change.

Post by edhowarth »

Perfect. Thanks Ralph.
ocramx
Posts: 3
Joined: Sunday 26. July 2015, 12:10

Re: Passing through a gas change.

Post by ocramx »

I would like to know if I have understood correctly how the gas change works. During the ascent, at the programmed altitude, the computer signals to change gas. At this point, if I DO NOT confirm change gas, the computer does NOT recalculate the decompression plan and the TTS remains the SAME as if I were using decompression gas, so the computer waits for me to do something or give some command. If I decide not to change the gas, I have to tell the computer that I have lost the gas. At this point the computer recalculates the entire decompression plan according to the gas I am re-breathing. Did I get it right?
Ralph
Posts: 657
Joined: Saturday 24. June 2017, 11:31

Re: Passing through a gas change.

Post by Ralph »

Yep, that's how it goes. Just to repeat in other words:

The OSTC calcuates TWO decompression plans in parallel:

Plan 1: your current (super-)saturations and the resulting ceiling / 1st stop depth and current CNS <- based on which gases were breathed during the dive, i.e. were selected & shown in the lower left corner. Thus the plan 1 tracks the history of the dive until the point in time as of now.

Plan 2: this plan picks the result of plan 1 and makes a projection calculation how an immediate ascent would look like regarding follow-up stops, CNS at the end of the dive, gas needs etc. This calculation assumes the diver is changing to the available gases as of their assigned change depths.

So when you are at a stop and get a gas change advice:
- the plan 1 continues to be calculated based on the currently active gas.
- the plan 2 assumes you are switching now to the highlighted gas and will continue to switch whenever prompted to.

So the net effect of not switching will be that the plan 2 yields longer and longer deco schedules with each calculation cycle as you are staying on a sub-optimal gas, so plan 1 accumulates more uptaking and plan 2 will calculate a longer deco to get rid again of the increasing uptaking.
If you want to stop the plan 2 from using a specific gas, you need to tell the OSTC you don't have it (anymore), hence set it to 'lost'. Once done, you'll get a sudden and sometimes even drastic immediate change in the plan 2 results, for example when ditching the availability of O2, as plan 2 will now figure out a deco schedule without that gas.

By the way: the calculation cycle time varies between 1 second (near to end of NDL), 2 seconds (begin of the dive) and 5-6 seconds when there are lots of long stops.

If fTTS (future Time-To-Surface) or CCR B/O calculation is enabled, the OSTC will calculate an additional 3rd plan, which will be a variant of the plan 2 with some tweaks: fTTS assumes the ascent will not be initiated immediatly but the diver will stay another set number of minutes at the current depth before starting the ascent, the B/O plan calculates an OC bailout ascent in plan 3 while plans 1 & 2 are calcuated in CCR or pSCR mode.

BR
Ralph
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