Tec Dive computers and dive plans

[MaxTorque]

I have no experience of proper tecnical diving (yet ;-) ) and i was wondering what is the method that current dive computers work with a pre-planned dive?

Ie, lets say you plan a dive on some suitable software (MultiDeco or similar) with times and depths. Is that plan (and deco model!) then able to be loaded into a dive computer to give you electronic profile guidance, or is it a matter of manually ensuring your depth/vs time profile matches your plan.

And what happens if your actual dive for any reasons doesn't match (it should as plan the dive, dive the plan is king). You dive computer is going to be calculating actual near-real-time tissue tensions ( how often do tec computers do this? the math is trivial for a modern processor) but that is going to potentially depart from your planned stops in a worst case?


I guess this is more a question on "good housekeeping" and clearly must be well covered in Tec diving training, but i was wondering on what the general approach was, and if everyone follows a similar method?

 

[happy-diver]

Yeah, I do the multi deco thing which I have set up a particular way, and I print it out and memorise it

(1 hr dives) and write it on a slate with a few funny comments and the perdix which is set up similarly)

switches on automatically when I jump in the water (which they say you should turn on before jumping in the water) then when diving I follow the slate if the helium is not working, and always beat my computer
to the surface having followed the more conservative plan on my slate.
That's about as technically coordinated as I am
as I choose to do it so

 

[Centrals]

Write the dive profile on a slate with couple of contingency such as 5 mins too long, 5 mins short, 3m too deep.
Carry two bottom timers.
Tec computer as back up(optional).

IANTD has dedicated soft deco table to keep in the pocket.

Tables - Open Circuit

 

[Capt Jim Wyatt]

I guess this is more a question on "good housekeeping" and clearly must be well covered in Tec diving training, but i was wondering on what the general approach was, and if everyone follows a similar method?

Maybe I am just missing the question....? I don't follow what you are trying to ask.

 

[Marie13]

No, the info from MultiDeco can’t be loaded into Perdix. You have to make sure MD has the same settings as your computer.

If you have two computers the same (such as two Perdix), you don’t need to write dive plan from software on a slate, as you have a spare identical computer who will handle stuff if your primary computer fails.

 

[J-Vo]

Most people I dive with use two Shearwaters, so planning contingency profiles is not requisite.

I plan my dives using Subsurface and it tends to pretty well match the Shearwater profiles. I have noticed me friends that use Multideco don't get as good of a match. There is room for interpretation of implementation in Buhlman's work.

I will plan my maximum bottom time for about max depth square profile based on my available gases. Then all I write down is the TTS(time to surface), first stop depth, and minimum back gas for emergency assent to the first deco gas(min GAS). As long as I stay within these constraints I'm good.

What this allows is flexibility during the dive. At our usual training sink hole, a circuit of the bottom has a max depth of ~280'. So I plan 280' for 20 minutes. That's about all I can get out of AL80s of 100%,50%, and 21/35. Bottom gas will not be my limiter but I will still memorize and write down min gas. Now the actual dive profile starts at around 200' works it's way downslope to ~275', then back up to 200'. I can make the circuit and slowly move up the wall checking things out for a bottom time of around 35 minutes before I hit one of my 3 constraints (TTS, 1st Stop, Min Gas). No way I could accurately plan that exact profile, and it takes more then 20 minutes to complete the circuit.

To my knowledge the agencies all teach 3rds rather than min GAS, and max bottom time rather than TTS. Both are acceptable and more conservative than diving TTS.

 

[mc42]

I plan the dive first using Bühlmann ZHL-16C and GF around 45/75 or 55/70 on software such as MultiDeco or Subsurface, ratio deco, and previous similar profiles. Using ratio deco, for example, helps as a sanity cross-check for any data entry or configuration setting errors when using software.

Then the team discusses and reaches agreement on the overall plan. The plan will include rock bottom (gas to ascend to first switch depth with 2 divers), +/- depth and time, and lost deco gas calculations.

At this level (Tec 50) plans are usually simple enough to memorize the stops based upon standard gas switch depths (50% at 70’, 100% at 20’); e.g. 2 minutes each 10’ stop from 70 to 30, then 15 minutes at 20. We treat the bottom segment separate from the deco segment, so we don’t have strict runtimes written on a slate which could add a lot more mental load to try to realign or offset. We just tweak the deco segment if needed, based on when we get to the start of deco, and run the deco plan from time zero. I’ll stress that this works for us at this level with the conservatism we run; deeper dives may require more discipline as far as sticking to exact stops and runtimes.

We use the dive computers (All Shearwaters, as it happens) set to the same GF as we used during planning to act as our realtime tracker. We normally clear our planned schedule and the computer schedule within 1-2 minutes of each other. For conservatism, we always go with the longest, whether it’s the plan or the computer but I’d say the computer usually clears first which is a good indication everything ran to plan.

We all use two computers. I set my other, non-Shearwater to bottom timer mode.

 

[MaxTorque]

Lots of different methods!

The reason i ask is i'm just writing for interests sake a deco planner software app, and although the implementation of the Deco model is straight forward (in my current case the Buhlmann 16C with GF's) and from that it's easy to calculate for each tissue compartment the critical gradient as an ascent is made, it seems that there is the potential for a significant change in the tensions if a none continuous, ie descrete step deco stops profile is used!

For just for example, the compartment continuous model suggests that the first M value x GF limiting tension is hit at 10.5 meters depth on the ascent. For a continuous ascent, that's easy, you just stop, and at that stop that "ceiling" slowly winds itself upwards towards the surface, and if you follow it, that is a deco at the model tension limits


But for descrete steps,lets say 3m increments, do you want to stop prior to that point (more conservative) at 12m or after that point at 9m. And are you going to wait at that point for the amount of time it take for the ceiling to rise up above the next 3m point ie 6m, but you may find that as you start your ascent you can't get to 6m because the critical super saturation M value has been breached again by your chosen ascent rate up from 9 to 6m, so perhaps you want to wait until the M values are above the second stop point (3m)?

It's these subtlies that seem to drive alot of models into producing quite different profiles from the same basic half times and M values!

And then, the bit that led to my original question, how accurately do you need to follow, or not follow, this plan before you have actually introduced more "error" for want of a better word, than you originally had in the modelled profile in the first place!

It then struck me that being able to load the calculated plan into a dive computer and show that plan vs the actual profile has significant merit. That computer can then easily show targets and errors to the plan, and tissue loading delta's, and you then have a good idea in real time what the delta between your planned profile and your actual profile in either direction (higher or lower risk)

 

[atdotde]

Lots of different methods!

But for descrete steps,lets say 3m increments, do you want to stop prior to that point (more conservative) at 12m or after that point at 9m. And are you going to wait at that point for the amount of time it take for the ceiling to rise up above the next 3m point ie 6m, but you may find that as you start your ascent you can't get to 6m because the critical super saturation M value has been breached again by your chosen ascent rate up from 9 to 6m, so perhaps you want to wait until the M values are above the second stop point (3m)?

This never happens for realistic dives with realistic gradient factors. If a tissue was off-gassing so that you start your ascent to the next stop, it will keep off-gassing unless you go (significantly) deeper again. The worst thing that can happen (with stupid gradient factors) is that deco takes forever since even the saturation pressure at one stop does not allow you to proceed to the next stop.

BTW, in Subsurface, for Bühlmann-based deco, the condition that allows you to start the further ascent is that with the proper ascent rate you never violate the ceiling before reaching the next stop. This is different from "you can ascent once the next stop depth cleared" since it allows for the possibility that the next stop depth clears during the ascent to it.

 

[KenGordon]

Lots of different methods!

The reason i ask is i'm just writing for interests sake a deco planner software app, and although the implementation of the Deco model is straight forward (in my current case the Buhlmann 16C with GF's) and from that it's easy to calculate for each tissue compartment the critical gradient as an ascent is made, it seems that there is the potential for a significant change in the tensions if a none continuous, ie descrete step deco stops profile is used!

For just for example, the compartment continuous model suggests that the first M value x GF limiting tension is hit at 10.5 meters depth on the ascent. For a continuous ascent, that's easy, you just stop, and at that stop that "ceiling" slowly winds itself upwards towards the surface, and if you follow it, that is a deco at the model tension limits


But for descrete steps,lets say 3m increments, do you want to stop prior to that point (more conservative) at 12m or after that point at 9m. And are you going to wait at that point for the amount of time it take for the ceiling to rise up above the next 3m point ie 6m, but you may find that as you start your ascent you can't get to 6m because the critical super saturation M value has been breached again by your chosen ascent rate up from 9 to 6m, so perhaps you want to wait until the M values are above the second stop point (3m)?

It's these subtlies that seem to drive alot of models into producing quite different profiles from the same basic half times and M values!

And then, the bit that led to my original question, how accurately do you need to follow, or not follow, this plan before you have actually introduced more "error" for want of a better word, than you originally had in the modelled profile in the first place!

It then struck me that being able to load the calculated plan into a dive computer and show that plan vs the actual profile has significant merit. That computer can then easily show targets and errors to the plan, and tissue loading delta's, and you then have a good idea in real time what the delta between your planned profile and your actual profile in either direction (higher or lower risk)

It turns out that you are never so slow ascending so as to make the ceiling come to you in a three m increment. This is because there is more than inert gas - water vapour and o2 - so you are off gassing even at GF of zero. So calculate the ceiling, round to the nearest 3m below and plan an ascent to there at 3 or 10m/min, it will be safe.

These days with two computers what you do is work out the longest deco you can do (Ie max TTS for your deco gases - don’t try this for back gas deco), jump in and swim about until you hit that (or min gas if silly enough to be on OC) and then go up.

Obviously there are other constraints for lost gas etc, but the point is that you don’t have to have a slate with all the stops written out and slavishly follow them. Since most dives are not entirely at the target depth such plans are always much more pessimistic than necessary and you can end up getting out thinking you could have done another 10 minutes bottom time.

For backgas deco or only one deco gas the constraints are a bit more awkward because you may end up with no gas in case of a failure, so the length of dive, Ie bottom gas used, influences the max TTS.