Feature Request : Bailout TTS

[beldridg]

I saw the other feature request thread and thought that it was a good idea to post just one of the 4 or 5 I have.

This only applies to rebreather divers.

In rebreather diving, you always bring “bailout” tanks in case your rebreather completely fails. In fact, most dives on a rebreather are limited by how much bailout gas you bring and not the rebreather scrubber or gas on-board.

It would be nice to know “if I bail out right now, what is my revised Time To Surface” given the bailout gasses programmed into the computer and "active."

On any given dive, I know how many cubic feet and what types of gasses I have. What I don't know is how my TTS would change if I had to bailout.

I know that I could "bailout" temporarily to see the data and then switch back to CC, but it would be so much easier just to have it as an additional field.

This could be implemented in a similar fashion as @+5, SurfGF, etc.

- brett

 

[rsingler]

It's an excellent idea! Yes, you can briefly switch to bailout mode and get that answer, but it's cumbersome.
Alternatively, the method I have used is to (laboriously) sit down with Subsurface and run dive profiles.
Typically, I've found that accelerated deco with whatever mix I'm diving as my dil and EAN 50 at 70 feet for bigger dives, is close to double my running TTS on CCR. In other words, if I have 12 min TTS on CCR, it's roughly 25 min TTS if I bail out.

On top of that, my bailout ascent gas requirement for my RMV ends up being typically less than 2.5cf per minute of CCR TTS (or 1.25 cf per minute of bailout TTS) across a variety of profiles (+/- 20%). So I just keep an eye on CCR TTS, and know that with an AL80 of B/O gas (plus a 40 of EAN50 for bigger dives) I'm good up to 15 min of CCR TTS (which also adds some padding for increased RMV in an emergency). Inside a wreck, of course, I also need to add horizontal b/o requirement to my ascent numbers.

Run the numbers. You'll be surprised at the consistency of B/O gas requirements over a range of depths and profiles when divided by TTS.

Examples:
80' x 120min; Air dil @ 1.2; GF 50/70
CCR TTS 28min; B/O TTS 81min
B/O Ascent gas = 70cf Air @ 0.6 cfm RMV
70cf/28 min CCR TTS = 2.5cf per min of CCR TTS

132' x 40min; 21/25 dil @ 1.2; GF50/70
CCR TTS 34min; B/O TTS 69 min
B/O Ascent gas = 69cf 21/25 @ 0.6cfm RMV
69cf/34 min CCR TTS = 2.0cf per min of CCR TTS

 

[_Ralph]

Here’s a neat trick for running at a 1.2


OC deco averages out to ~1.2 as well! (Aside from O2 deco)

_R

 

[Wibble]

I use a spare Perdix on the other wrist which I set to 90:90 to know the min time to surface and a backup should the other computers (Petrel and Nerd) go tilt. I have this set to CC mode with "assumed" PPO2 of 1.2.

(know this is not the same as you're asking for!)

 

[rjack321]

It's an excellent idea! Yes, you can briefly switch to bailout mode and get that answer, but it's cumbersome.
Alternatively, the method I have used is to (laboriously) sit down with Subsurface and run dive profiles.
Typically, I've found that accelerated deco with whatever mix I'm diving as my dil and EAN 50 at 70 feet for bigger dives, is close to double my running TTS on CCR. In other words, if I have 12 min TTS on CCR, it's roughly 25 min TTS if I bail out.

On top of that, my bailout ascent gas requirement for my RMV ends up being typically less than 2.5cf per minute of CCR TTS (or 1.25 cf per minute of bailout TTS) across a variety of profiles (+/- 20%). So I just keep an eye on CCR TTS, and know that with an AL80 of B/O gas (plus a 40 of EAN50 for bigger dives) I'm good up to 15 min of CCR TTS (which also adds some padding for increased RMV in an emergency). Inside a wreck, of course, I also need to add horizontal b/o requirement to my ascent numbers.

Run the numbers. You'll be surprised at the consistency of B/O gas requirements over a range of depths and profiles when divided by TTS.

Examples:
80' x 120min; Air dil @ 1.2; GF 50/70
CCR TTS 28min; B/O TTS 81min
B/O Ascent gas = 70cf Air @ 0.6 cfm RMV
70cf/28 min CCR TTS = 2.5cf per min of CCR TTS

132' x 40min; 21/25 dil @ 1.2; GF50/70
CCR TTS 34min; B/O TTS 69 min
B/O Ascent gas = 69cf 21/25 @ 0.6cfm RMV
69cf/34 min CCR TTS = 2.0cf per min of CCR TTS

Why are you bailing to (just) air on a deco dive?

 

[rsingler]

Just for the computations.
B/O to air above 120' plus O2 at20'.
He in deeper bailout mixes, again w/ shallow O2.
I left the shallow O2 off the summary, but mentioned it in the text. That's why the air requirements aren't huge. That's why the 2.5cf per min CCR TTS works.

Or am I misunderstanding your question?

 

[-Sven-]

I also would like to have Bailout TTS.

 

[oceance]

@Shearwater, are you still considering adding B/O TTS in one of the next firmware updates? Thanks

 

[Alekseolsen]

Just adding here.

Did my RB course last week, and had a hard time searching for this function which I was amazed to find was not present!

 

[Wibble]

When you switch from CC to Bailout, it'll be running the calculations with the appropriate gas you've selected. Think it'll recalculate, simply cannot imagine that once set to bailout mode it would continue following the PPO2 from the unit.