Question Dive time management - multi-level vs. square profile

[LFMarm]

Hi all,

Would love to hear feedback on the approach I am envisioning for managing real life multi-level long deco dives on CCR. Below is my approach and a couple of simulated examples to show how it would work in the water. Configuration is CCR with on-board oxygen; 2 AL80 tanks for diluent and deco gas.

Approach

1. Calculate the bailout volume based on a square profile dive to the bottom for the square profile dive
2. Calculate the Deco Time (DT) allowed on BO as DT = Total Dive Time - Bottom Time for the square profile dive
3. Set one computer (C1) to conservative GF (50/60) and monitor its TTS (TTS_1) — this will limit the dive duration if all goes well and you finish the dive on CCR. It is limited by scrubber time
4. Set another computer (C2) to aggressive GF (70/85) and monitor its TTS (TTS_2) — this will limit the dive duration if you need to bail out in OC and is limited by Deco time (based on available gas for bailout)
5. You need to start the ascent as soon as one of two conditions happens: TTS_1+Current Dive time=Scrubber time or TTS_2=Deco Time

Example 1 (dive limited by scrubber)

• Planned dive (square profile @ 30m) with BT of 100 min (95 min on CC and 5 on OC) with Air and EAN90 both in AL80; scrubber time 180 min; diluent time on CCR 180+ min, O2 time on CCR 210 min; using GF 70/85 and ppO2 1.1, the DT=total dive time-bottom time=78 min
  • C1: you need to go up when TTS_1=180 min (scrubber) - dive time until then
    • Diluent and O2 are not limiting factors as both are more than the scrubber (180 min)
  • C2: you need to go up as soon at TTS_2=78 min (BO)
    • Scrubber, diluent and O2 are more stringent on C1
• Actual dive (multi-level @ 25-30m). After 112 min of bottom time between 25 m and 30 m, you have TTS_1 68 min (total dive time = 112 + 68 = 180 min | limiting factor) and TTS_2 44 min (TTS_2 = 44 min < DT = 78 min | not the limiting factor) so you start your ascent as you have reached the limit on C1

Example 2 (dive limited by Bail Out gas)

• Planned dive (square profile @ 60m). 60 m with BT of 26 min (21 on CC and 5 on OC) with TX 14/50 and EAN60 both in AL80; scrubber time 180 min; diluent time on CCR 180+ min, O2 time on CCR 210 min; using GF 70/85 and ppO2 1.1, the DT=total dive time-bottom time=61 min
  • C1: you need to go up when TTS_1=180 min (scrubber) - dive time until then
    • Diluent and O2 are not limiting factors as both are more than the scrubber (180 min)
  • C2: you need to go up as soon at TTS_2=61 min (BO)
    • Scrubber, diluent and O2 are more stringent on C1
• Actual dive (multi-level @ 48-60m). After 30 min of bottom time between 48 m and 60 m, you have TTS_1 62 min (total dive time = 30 + 62 = 92 min < 180 min | not limiting factor) and TTS_2 61 min (TTS_2 = DT = 61 min | limiting factor) so you start your ascent as you have reached the limit on C2

Keen to hear your thoughts on this approach to translate planned profiles into real multi-level ones.

 

[inquis]

Using max-depth TTS as a proxy for deco gas is commonly done in my circle of buddies, and I've looked at enough plans to convince myself it's a valid strategy for extending multi-level dives within that gas constraint. [EDIT: more correctly, that is for extending multi-level OC dives. See below for how this may be insufficient for some choices of BO/deco gas when on CC.]

I don't get using different GFs, though. Do you think the off-gassed inert gas in the loop makes 60% on CC the equivalent risk to 85% on OC? Not to mention it seems really easy to reference the "wrong" TTS in the various comparisons.

 

[LFMarm]

I don't get using different GFs, though. Do you think the off-gassed inert gas in the loop makes 60% on CC the equivalent risk to 85% on OC? Not to mention it seems really easy to reference the "wrong" TTS in the various comparisons.

My rationale is that if all goes well in CC I want to be very conservative (50/60); if I am in a bailout to OC situation, I want to get out of the water faster and hence allow for lower conservatism (70/85). Using 50/60 in case of OC bailout would limit significantly the allowed bottom time.

 

[beldridg]

I think it is good to do this type of analysis, especially when you are first learning to dive on rebreathers and perform bailout needs.

As @inquisit mentions, there is a much more practical and pragmatic way to do the planning in an everyday diving situation.

Ideally, if you are carrying the "right" gasses for bailout, your bailout TTS would be the same or less than on a rebreather (especially if you are carrying Nitrox 80 or O2) since your PO2 will be 1.6-ish vs. 1.2 or 1.3.

However, in quite a few situations, for the dives and bailouts you are mentioning (60m/200' and two tanks), it is far more likely that you will have a deep bailout and then Nitrox 50 which is less efficient at the shallower and longer stops.

Therefore, personally, I plan on somewhere between 125%-150% of the on-loop TTS as my bailout TTS and make sure I have enough bailout gas to cover that plus a safety margin.

I usually mark my bailout tanks with the amount of cubic feet of gas I have in them and I can do the calculation "on the fly" whenever I'm doing dive planning or during the dive.

One other thing worth thinking about: the Shearwater @+5 is a very useful guide as you do a multi-level dive. If your @+5 is more than 0, you are still on-gassing and adding to TTS. If it is less than 0, you are off-gassing and lowering your bailout TTS.

I have asked Shearwater for a simple Bailout TTS to display similar to SurfGF, @+5, etc. but have yet to see it. They have recently included a bailout deco schedule in the Dive Planner during the dive which somebody recently told me about and which I played with a bit.

Regards,

- brett

 

[inquis]

If your @+5 is more than 0, you are still on-gassing and adding to TTS

Minor nit: Delta+5 > 0 or @+5 greater than TTS. (The point stands, however, that this info is very useful. Fingers crossed for a BO-TTS.)

 

[beldridg]

Minor nit: Delta+5 > 0 or @+5 greater than TTS. (The point stands, however, that this info is very useful. Fingers crossed for a BO-TTS.)

Yes, correct. Still fighting jet lag after returning from Palau.

- brett

 

[inquis]

I plan on somewhere between 125%-150% of the on-loop TTS as my bailout TTS and make sure I have enough bailout gas to cover that plus a safety margin.

I think this is critical and should be underscored. OP, the computer is expecting that you will continue on CC at the current setpoint, and the displayed TTS value reflects that. It cannot be used as a direct proxy for the OC ascent/deco gas.

It's only after you switch to BO on the computer that it would update TTS to reflect the use of the OC gasses & depth-dependent ppO2s. That's the TTS you can compare directly to the planned deco time, but by then it's too late. If that updated TTS is higher than the CC-mode (pre-bailout) TTS, then your OC deco plan is insufficient. (Thus the padding @beldridg employs.)

 

[inquis]

To elaborate on the above, if it's going to work at all, you need to make sure you're comparing the max-depth CCR-TTS value to the value shown during the multi-level dive (the CCR-TTS). I ran one profile on which it worked (the multi-level OC gas required was within the amount estimated by the max-depth plan), but that hardly proves it always works. The only thing it did was to show it's still a possibility. Obviously, the (max-depth) OC deco schedule forms a basis for the OC gas quantity to carry, but that time should NOT be used in a comparison to the TTS shown during the multi-level dive (which will be the CCR-TTS). Unless I'm mistaken, that's what the OP was suggesting. Run more plans to prove to yourself it works.

 

[LFMarm]

Super helpful. Thanks! Let's really hope for a BO-TTS from @Shearwater

 

[beldridg]

Super helpful. Thanks! Let's really hope for a BO-TTS from @Shearwater

They have it, but it isn't super easy to get at.

/* From memory */

During the dive when you have exceeded NDL, if you go into the dive planner, it will show you your deco schedule on CCR. Then if you right-click it will give you the bailout deco schedule including a total TTS. It also allows you to right click again and get the gas usage in cubic feet (assuming you program in your SAC or RMV rate).

I'm not sure when they added the bailout deco schedule but somebody on my last dive trip told me about it. I'm hoping to document it assuming I can get some good pictures of my Petrel on my dive on Thursday this week.

I do still think it would be helpful to have an option to display the BO TTS as an option to display where you can show @+5, SurfGF, etc.

Regards,

- brett