Deco-dive with travel gas, bottom gas and four deco gases

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mobu
Posts: 58
Joined: Tuesday 27. March 2018, 20:01

Deco-dive with travel gas, bottom gas and four deco gases

Post by mobu »

Hello,

in the OSTC plus I can set up until 5 gases. But if I need a travel gas, a bottom gas and four deco gas, how do I set it up?

Actually, I use five gases, but the travel gas and one of the deco gas I have to configure twice. Once as travel gas and once as deco gas, and both at the same depth. So I get 6 gases to configure.

Thank in advice
mobu

OSTC plus #16088
heinrichsweikamp
Posts: 4370
Joined: Sunday 13. May 2007, 18:07

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by heinrichsweikamp »

Hi,

6 different gases is a bit unsual but: Configure the deepest deco gas (likely not too different from your bottom mix) manually during the dive. That's the best I can recommend without knowing your gas setup and the dive you want to do.

Regards,
Matthias
mobu
Posts: 58
Joined: Tuesday 27. March 2018, 20:01

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by mobu »

Hi Matthias,

I know it's no usual but I like to use a gas that pulls me off the bottom in case of problems with the bibo (emergency gas).
I could not use it if it was not necessary, and then I would have 4 gases, one of the four would also be the travel gas.
But I also want to use the emergency gas that I have as a deco to minimize a problem with the ICD and keep the PpO2 above 0.85 during the ascent.

Gas setup would be:
Travel gas: Triox 30/30
Botton gas: TX 12/65
Emergency and deco gas: TX16/45
Deco gas: Triox 30/30
Deco gas: NX50
Deco gas O2

What you recommend would be a solution. Although I'd rather not have to manually set it while diving at those depths.
I have also tried not setting travel gas on the OSTC and using bottom gas from the start. But when doing the simulation I get the error.

Regards
heinrichsweikamp
Posts: 4370
Joined: Sunday 13. May 2007, 18:07

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by heinrichsweikamp »

Setup your Bottom gas as "First" (With MOD setup as Depth in the Gas Setup). Select your 30/30 Deco gas after starting your dive. No need to setup a manual gas then and you'll have correct TTS with all deco gases taken into account when selecting your Bottom gas again (At the Bottom).

Regards,
Matthias
mobu
Posts: 58
Joined: Tuesday 27. March 2018, 20:01

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by mobu »

Hi Matthias,

I wold like to make sure that I have understood you correctly.

Predive. Setup gas list:
Gas1, first, TX 12/65, depth MOD
Gas2, deco, TX 16/45, depth 51 m.
Gas3, deco, TX 30/30, depth 42 m.
Gas4, deco, NX50, depth 21 m.
Gas5, deco, O2, depth 6 m.

Just after starting the descent:
- select manually 30/30 (as if it were travel gas). If I have planned the change from travel gas to bottom gas at 30 m, I should change to gas3 (30/30) as soon as possible and once the descent has started, for example at 2 m. Is that so?
What I understand from your answer is that I switch to bottom gas on the bottom, but I can't breathe the 30/30 on the bottom (Ppo2 is 3,1)

In these types of dives the begining of the descent is usually one of the most stressful parts. That's why I plan the switch from travel gas to bottom gas at 30 m. and not much earlier. Manually switching to 30/30 in the first meters of the descent could increase that initial stress.

Does the decompression profile change a lot if I setup gas1 as first, with the MOD depth, and go on with that gas to the bottom? Basically, what I'm telling the OSTC is that I'm not going to use a travel gas, although in reality it does.
Trying this solution, when calculating the decoplan I get an error that says: "No usable gas for bottom depth found!" Then I go to the list of gases and setup gas1 depth to the MOD. So it works. But, if I change some data and recalculate the decoplan, the depth of gas1 has returned to 0m. So I have to re-setup gas1 depth to the MOD. Is this normal?



Regards
Ralph
Posts: 708
Joined: Saturday 24. June 2017, 11:31

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by Ralph »

Hi, not a direct answer to your questions, but maybe some background information: two calculations run in parallel

1) your actual decompression obligation - this is computed solely from the trajectory of the gasses shown in the lower left of the display. The trajectory will start with the gas tagged as 'first' in the gas list, then continues with the gases you switched to (manually, parallel to the actual gas regulator switching).

2) the ascent calculation to determine the stops, TTS, CNS @ end of dive, gas needs etc. This calculation is done every couple of seconds throughout the whole dive. It starts with the current deco status as of 1) and then calculates the ascent, --> swapping gasses by their MOD settings <--, i.e. always picking the gas with the shallowest MOD >= current depth along the calculated ascent. Thereby it picks among the gases tagged as first and work until reaching the first stop, then the selection is opened to all gasses, i.e. now also the deco gases are included into the selection.

Therefore:

a) to decrease the workload at the begin of the dive, you can tag the travel (deco) gas as 'first'. So it will become your active gas right on submerging. Once you pass its MOD, you will be prompted to change your gas. Therefore, ...

b) ... your bottom mix will have to be set up as 'work' with sufficient MOD. Remember: while not yet at 1st stop, gas slection picks among 'first' and 'work' gasses only.

c) On ascent, your travel / deco gas will be picked / prompted once passing its MOD, --> even if this MOD is > your actual 1st stop depth <--. This is because the gas is tagged as 'first' and not 'deco', so the gas selection algorithm can pick it any time. The ascent calculation will pick it and base the stops table, TTS, etc. on the assumption that you are going to switch to it in reality, too. If you don't switch, the calculation will be a little bit too optimistic as the bottom gas has lower ppO2 than the travel/deco gas the computer things you will be on or switch to immediatly. Anyhow, if you don't do it, all the time while you haven't is computed correctly and once you switch to the first "real" deco gas your real deco and the computed one are in full sync again.

d) If you want to suppress the travel gas in the ascent calculation, and thereby also stop it from being prompted to switch to for during ascent, you can set it to 'lost' state once you have started the descent, actually any time between leaving the surface and reaching the bottom depth. Ideally when switching from travel to bottom gas. --> A gas that is selected (shown in the lower left) will be used in the deco calculation, actually canceling its 'lost' state <--.

e) In case you want to use the travel/deco gas, e.g. because you have to abort the dive or are low on back gas during the ascent, you can switch to it via the menu at any time, even if it is set to 'lost'. Once you picked it and it is shown in the lower left, the ascent calculation does not treat it as lost any more but as available and all calculation will be correct and also the prompt for switching to the next deco gas will come as expectable.

Hope that helps,
BR
Ralph
mobu
Posts: 58
Joined: Tuesday 27. March 2018, 20:01

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by mobu »

Hi Ralph,


Thank you very much for the reply. It has cleared up a lot of things for me.

Just a few questions in paragraph C and E:

- On the ascent, do I have to manually (via the menu) select the gas1, configured as trip, to use it as deco? Or is it going to be the computer itself that suggests me the change, to the depth established in the gas setup (same depth at which I changed from travel gas to bottom gas)?

- You said: "If you don't switch, the calculation will be a little bit too optimistic as the bottom gas has lower ppO2 than the travel/deco gas" but I understand that the calculatión will be a little bit too conservative precisely because the bottom gas has lower ppO2 than the travel/deco gas. Is it so?

- In paragraph E you say: In case you want to use the travel/deco gas you can switch to it via the menu at any time. Would this have to be done in case that gas1 had been selected as lost, (paragraph d)?


Thanks in advice.

BR
Ralph
Posts: 708
Joined: Saturday 24. June 2017, 11:31

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by Ralph »

Hi,

you always have to select the gases maually, either via the menu or by acknowleding the gas-switch promt, the computer will never decide on its own that you have done a gas switch without you telling the computer. --> This accounts for the calculation of your deco obligation, leading tissue, super-saturation, ceiling, first stop depth, and also the CNS.

The other thing is the calculation of the --> complete <-- ascent, to get data on TTS, gas needs, CNS at end of dive, etc. Here the computer will make assumptions on you doing the gas switches as indicated by the gases' MOD and type. As said, the 'best gas' is the gas with the shallowest MOD <= the current depth in the ascent calculation and it is elected from the 1st & bottom gases until arrival at the 1st stop, and then from all available gases. So gases set to 'lost' will not be picked by the calculation. So:

- you have to select gas1 manually to tell the computer that you are breathing it now. For the mathematics of the deco calculation, there is nothing special with 'deco gases', as said this setting only controls in what phase of the dive the ascent calculation will assume you will be switching to this gas.
If it is not of type 'deco', the computer will assume (and also prompt you for this gas) as soon as you are at <= its MOD. Side node: the promting function also checks the ppO2 limits, the ascent calculation purely looks at the MODs only.

- If the computer assumes you will switch to a better gas, i.e. a gas that is better than the current one because it has a shallower MOD than the current one, and you do not switch but stay on the gas with the deeper MOD, you continue breathing a gas that is sub-optimal whereas the computer thinks you will breath a gas that is better. So the resulting TTS, gas consumption and so on will be too optimistic: shorter TTS than your sub-optimal gas switching will yield, less gas consumption than you will actually need due to the longer TTS. Perhaps only the CNS at end of dive will be over-estimated as the computer thinks you'll be breathing higher ppO2s.

- As said in the into, you always have to tell the computer what you are breathing. And you can pick any gas at any time. It is only the ascent calculation that will or will not pick different gases along the calculated ascent. If you set a gas to 'lost', you excude it from being picked by the ascent calculation. But you can select it, telling the computer you are breathing it now. When doing so, the ascent calculation will cancle the 'lost' state and will calculate with this gas until it finds a better one because of entering the MOD area of another gas whose MOD is shallower than your current gas.

Setting a gas to lost also stops it from being propmted for, so if you have an emergency gas that you do not want to touch if the dive goes uneventful: configure it, then set it to lost. Now this gas will neither be prompted for nor picked by the caclulation. But when you are urgend to use it, you can simple revoke its 'lost' state and it will be back in all prompting & calculation.

all clear now?
BR
Ralph
mobu
Posts: 58
Joined: Tuesday 27. March 2018, 20:01

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by mobu »

Hi Ralph,

I think so. I have the feeling that we do not understand some things because of my level of English, which is not very good. Sorry about it.

I know that I always have to select the gases maually, either via the menu or by acknowleding the gas-switch promt. My question refer to paragraph C was if the computer prompted me the gas-switch or I had to select via menu.

I think I understand it, although I want to read it several times and be clear that I have understood it.

Any case, I would want to comment on something that, I don't know if it's a bug, and that I'm realizing now.

I have setup the gases in the following way to do a simulation on the computer:
Gas1 12/65 first Depth 106m (MOD)
Gas2 16/45 deco Depth 51m
Gas3 30/30 deco Depth 42m
Gas4 nx50 deco Depth 21
Gas5 O2 deco Depth 6

If, once the gases have been setup in this way, I go to the home screen where the configured gases are seen, I notice that the depth of gas1 is 0m.
This doesn't allow me to do the simulation because it gives me an error that says: "No usable gas for bottom depth found!"
Even though I setup again that gas1 with the MOD depth, it doesn't change it and it's always 0m.
However, if I change the depth of that gas1 to a value of 100m. or less, then it does save it and I can run the simulation. Is that normal?

BR
Ralph
Posts: 708
Joined: Saturday 24. June 2017, 11:31

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by Ralph »

Hi,

I will have to check the 0 m MOD issue.

All gases will be prompted for, unless they are disabled or set as lost. If they are deco, they will be prompted for once the 1st deco stop is reached on tje ascent.
Anyhow, best is always to try out with the simulator mode.

BR
R.
mobu
Posts: 58
Joined: Tuesday 27. March 2018, 20:01

Re: Deco-dive with travel gas, bottom gas and four deco gases

Post by mobu »

Hi Ralph,

Colud you check the 0 m MOD issue?
Thanks.

BR
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