Multideco... tips and tricks?

[Wibble]

On this particular dive I made some configuration changes that probably had some affect on my consumption, mainly I switched from ADV\MAV to MAV only to have better control over minimum loop volume (admittedly I did like the ADV better, especially for having busy hands, but I must give it several more dives before deciding). On top of that I was leading which we all know can have quite the effect on your consumption and air management. Also, I am still having a very difficult time managing my CCR trim and buoyancy at shallow depths. All excuses and also facts. I use Garmin for my air integration. For reference a previous dive at the same location with relatively consistent displayed RMV values, 1h50m average depth 21m I went from 179 BAR to 89 and a supposed average RMV of 8.7L. Another dive, same location, 1h54m average depth 21m I went from 184 to 89 and a supposed average RMV of 6.4L.

That will be the cause of the high oxygen consumption.

Buoyancy and trim on a rebreather is a hard skill to master and just takes time for it to click into place. Ironically being at deco is probably the best practice you will get, although it is always harder when there’s people around watching.

For MOD3 you absolutely must have stability and be able to hold that depth without wagging fins as your instructor throws yet more tasks at you.

Regarding loop control, given a constant depth, not a lot happens as your oxygen is metabolised. An ADV is good for the busy descent as your hands are busy with the shot and faffing with kit. However, once you’re on the bottom you don’t really need the ADV and in fact it can be a real problem if your loop is slightly low on volume and the ADV fires, diluting the loop… which the controller notices and fires in some oxygen then the loop is above min loop…. so you breathe out some from your nose…. then the ADV fires…. Running the diluent manually is far better as you, the human in control, can predict when you need more diluent.

For the ascent and deco phase, the ADV is a darn nuisance as the PPO2 of the diluent will be very low and even a tiny injection will ruin your setpoint possibly requiring a loop flush — wasting diluent and oxygen.

Personally, once I’m at 9m/30ft I’ll change the setpoint to low (0.7) and run the oxygen manually at 1.3-ish. When the loop volume drops, add some oxygen. Every 15 mins or so I’ll breathe some loop gas out to dump some off-gassed inert gas, not too much though.


Bottom line, it takes time to get all this sorted out. It really is a good idea to pause sometimes to get a skill mastered before making things harder!

 

[mr_v]

I'm trying to learn how to use multideco properly on CCR; so thank you for pointing out the setpoint issue. It is supposed to be switched to 1.1 at 20 meters on descent but I'm sure I left some things messy while screwing around with settings in multideco.

Ok, let's focus on the quote above - using MultiDeco. MultiDeco is useful but not that user friendly by today's standards. I am not surprised if you messed up your setpoint. You can begin your MultiDeco journey with simple dive plans that include constant depth and time. For each dive, you'll have to create 3 schedules. This planning is crude but if you don't nail it, creating more complex plans will be hard.

Let's say that you start without any dives defined in MultiDeco. Add you bailout gasses as "deco mixes" but keep them disabled. Mod 3 will require at least 3 gas switches, so think about what you want to use.

The first schedule is your expected dive, e.g., "80m for 20 min with 10/70 DIL & 1.3 setpoint." Click Calculate. Write the schedule down in wet notes. Disable the dive in MultiDeco.

The second schedule is your "deeper, longer dive", e.g., "85m for 25 min with 10/70 DIL and 1.3 setpoint." Click Calculate. Write the schedule down in wet notes. Disable the dive in MultiDeco.

The third schedule is your BO schedule that determines your gas needs. To do create that schedule, you create a two-level dive. The first level is the descent. The second level is the ascent. Presume that each level takes 50% of your time. E.g.:

"80m for 10 min with 10/70 DIL & 1.3 setpoint"
"80m for 10 min with 15/55 OC"

Enable bailout gasses. Click Calculate. Write the schedule down in wet notes. Click Details. Write down gas needs for all but the first gas (15/55 in the example).

Go back to dives and change the OC leg time from 10 min to 15 min, then go to Calculate->Details, and note the number for the first OC gas (15/55 in the example). Use that number for 15/55 needs. You will use this number to account for increased heart rate during BO. Essentially, you're presuming that you'll breathe as much gas in 1 min as you would in 5 in case of a bailout.

I am sure there are other ways of accomplishing the same thing, but breaking down MultiDeco use into simple steps is a good start so that you understand what you're doing. It is also getting pretty late here and I am very tired, so please do not use my post as the only guide. I am sure I missed something.

In my experience, most people are okay with the first two plans, but bailout calculation/decisions are the hardest due to dive profile influence, e.g. you'd plan an open water bailout differently from a cave bailout.

 

[rjack321]

Ok, let's focus on the quote above - using MultiDeco. MultiDeco is useful but not that user friendly by today's standards. I am not surprised if you messed up your setpoint. You can begin your MultiDeco journey with simple dive plans that include constant depth and time. For each dive, you'll have to create 3 schedules. This planning is crude but if you don't nail it, creating more complex plans will be hard.

Let's say that you start without any dives defined in MultiDeco. Add you bailout gasses as "deco mixes" but keep them disabled. Mod 3 will require at least 3 gas switches, so think about what you want to use.

The first schedule is your expected dive, e.g., "80m for 20 min with 10/70 DIL & 1.3 setpoint." Click Calculate. Write the schedule down in wet notes. Disable the dive in MultiDeco.

The second schedule is your "deeper, longer dive", e.g., "85m for 25 min with 10/70 DIL and 1.3 setpoint." Click Calculate. Write the schedule down in wet notes. Disable the dive in MultiDeco.

The third schedule is your BO schedule that determins your gas needs. To do create that schedule, you create a two-level dive. The first level is the descent. The second level is the ascent. Presume that each level takes 50% of your time. E.g.:

"80m for 10 min with 10/70 DIL & 1.3 setpoint"
"80m for 10 min with 15/55 OC"

Enable bailout gasses. Click Calculate. Write the schedule down in wet notes. Click Details. Write down gas needs for all but the first gas (15/55 in the example).

Go back to dives and change the OC leg time from 10 min to 15 min, then go to Calculate->Details, and note the number for the first OC gas (15/55 in the example). Use that number for 15/55 needs. You will use this number to account for increased heart rate during BO. Essentially, you're presuming that you'll breathe as much gas in 1 min as you would in 5 in case of a bailout.

I am sure there are other ways of accomplishing the same thing, but breaking down MultiDeco use into simple steps is a good start so that you understand what you're doing. It is also getting pretty late here and I am very tired, so please do not use my post as the only guide. I am sure I missed something.

In my experience, most people are okay with the first two plans, but bailout calculation/decisions are the hardest due to dive profile influence, e.g. you'd plan an open water bailout differently from a cave bailout.

You realize that multideco has an OC bailout plan mode? You don't have to force feed it an OC ascent.

And you can also set your OC consumption rate, although the app doesnt allow you to set that differently for different stops. But any OC tech diver should be able to tell you how much 15/15, 21/35, 50%, and O2 is required for this BO plan using their personal stressed consumption choices across the whole ascent. I'd bring 3x 80s and a 40 myself, otherwise its a realllly long way from 80m to 21m. Or you crash and burn on volume if you bring a deco gas you can breathe deeper than 21m. Or you crash and burn even worse if you overstay at 80m even 5mins. In theory with only 24 mins on 50% you could use a 40 of 50% but that's cutting it close on both deco gases. But I'd rather have leftovers to cover for each other a bit.

So 3x 80s + 40 it is for me. Plus a safety diver meeting us around 30m with spare gas if we were drifting and couldn't hang any on a mooring.

 

[mr_v]

You realize that multideco has an OC bailout plan mode? You don't have to force feed it an OC ascent.

Yes, I know that. And as stated above there are many ways to create the same dive plan. I choose to force feed OC ascents because in my mind it is more straightforward. I'd do the same for more complex dives, like multi-level cave dives. When I played with BO modes, the app crashed on me several times, so now I break down the dives as stated above - I know that functionality works w/o issues and frustration ;-)

And you can also set your OC consumption rate, although the app doesnt allow you to set that differently for different stops. But any OC tech diver should be able to tell you how much 15/15, 21/35, 50%, and O2 is required for this BO plan using their personal stressed consumption choices across the whole ascent.

Yes, and I still use MultiDeco to double verify my BO estimates, check gas density etc.

 

[inquis]

Presume that each level takes 50% of your time. E.g.:

"80m for 10 min with 10/70 DIL & 1.3 setpoint"
"80m for 10 min with 15/55 OC"

Enable bailout gasses. Click Calculate. Write the schedule down in wet notes. Click Details. Write down gas needs for all but the first gas (15/55 in the example).

Are you not shortchanging your deco gas by doing that? (The effective mix in the loop is 14/67, so lower PO2, higher PHe.)

It's not a huge difference in this case, but why not do the full 20 min on CCR for the more realistic deco obligation, with two SAC values to capture the variable demand? (You can still force an OC leg of 1 min for an explicit switch and avoid the built-in BO mode if you want.)

 

[mr_v]

Are you not shortchanging your deco gas by doing that? (The effective mix in the loop is 14/67, so lower PO2, higher PHe.)

It's not a huge difference in this case, but why not do the full 20 min on CCR for the more realistic deco obligation, with two SAC values to capture the variable demand? (You can still force an OC leg of 1 min for an explicit switch and avoid the built-in BO mode if you want.)

I create 3 plans for each dive.

The full 20 min on the CCR is the first plan.

What you quote above is the bailout scenario that calls for the bailout half way through the dive. Hence, two levels (but you can also use the BO mode in MultiDeco, if that's your thing). The bailout plan assumes a compromised loop, so all deco is done on OC.

I tested these plans multiple times with my consumption rates and tanks - gas volumes were on the conservative sides, which is what I wanted. I also used tables to do the dive and the deco. My dive computer was on and the differences between the dive computer and the actual plan as I dove it, were minimal.

 

[rjack321]

What you quote above is the bailout scenario that calls for the bailout half way through the dive.

Curious why?
If you have the full time at depth covered with BO what is this 1/2 BT plan accomplishing for you mentally?

 

[inquis]

I create 3 plans for each dive.
The full 20 min on the CCR is the first plan.

Right, so that's the nominal plan where you stay on CC. The second plan you mentioned is the deeper/longer plan, also completely on CC. These would presumably be used if your computer went out or maybe your computer is your backup. Either way, no BO involved.

What you quote above is the bailout scenario that calls for the bailout half way through the dive.

You've greatly inflated the initial BO gas vol (as you're allowing for 15 mins at depth and the ascent). This will easily handle the initial BO after the full bottom time on CCR, so I see no issue with that. The issue is with the later deco gases, as they were calculated based on nitrogen loading from 10 mins on CCR + 10 min OC which for this choice of OC gas has a shorter deco obligation than 20 mins on CCR.

I suspect that you're trying to cover a BO at the far end of a cave or wreck where you have to get back to the entrance/upline. Again, what you've outlined is great for characterizing the initial BO gas, but it does shortchange the subsequent gas volumes.

 

[mr_v]

I suspect that you're trying to cover a BO at the far end of a cave or wreck where you have to get back to the entrance/upline.

That's correct. The shortchange is minimal, and it all works out at the end with plenty of gas to spare. In my planning, I have more than enough gas to cover deep and shallow cave portions. You can play with numbers and add extra minutes to each gas change to mitigate the mid-range gas shortcharge.

@rjack321 - it is my cave bailout profile that assumes a BO scenario at the deepest penetration point. MultiDeco can do the same for you in the Bail section - you select "return to exit time." As stated in my initial reply, the plan is a gross oversimplification of how one _might_ create a dive plan and check gas volumes. This type of planning gives me mental clarity. YMMV.

The reality is simple - the OP should not learn how to do MOD3 dives on SB. The instructor should tell the OP what's expected for a successful MOD3 completion.

 

[rjack321]

@rjack321 - it is my cave bailout profile that assumes a BO scenario at the deepest penetration point. MultiDeco can do the same for you in the Bail section - you select "return to exit time." As stated in my initial reply, the plan is a gross oversimplification of how one _might_ create a dive plan and check gas volumes. This type of planning gives me mental clarity. YMMV.

That's fine I gotcha. I was thinking these were 80m MOD3 training wreck dives not an overhead.