Predator/Petrel lost gas procedure ?

[Dr. Lecter]

This is turning out to be an interesting thread at least to me. Is the following correct then? Using the example above; If you don't switch off the 50% the unit will calculate you deco assuming you made the 50 switch, therefore resulting in a shorter deco vs what you actually require?

No - it's only running deco calculations off a gas you have defined, turned on, and made active by selecting it from the menu. If you don't make the switch off backgas and onto a deco mix, it will continue to calculate your deco obligation as if you're still breathing backgas--because as far as you've told it, that's what you're breathing. If you're at 70' and have 50% programed in and turned ON, the SW will tell you that you have a better gas available and suggest you switch to it (by displaying your active gas in contrasting yellow alert text), but it won't do anything different about your deco calculations until and unless you change the active gas, thereby indicating you've made a gas switch. Great thing about Shearwaters...they almost never do your thinking for you Controller models have some limited and mildly annoying exceptions to that rule, but I digress.

However, until you've turned OFF any defined deco gasses you have ON, you've also told it that you're carrying and can use those deco gases. For TTS calculations, which unlike your current deco status are predictive for the entire rest of the dive, it will assume you'll use those deco gasses. If you don't switch gasses, though, the deco calcs will be spot on...your TTS will only be accurate once you switch (or once you reprogram the gasses it's treating as available for the dive).

 

[ajduplessis]

I think I am missing something here .... The unit will let you know you have a better gas to use by highlighting it for you in yellow. This I understand and have seen before. However the TTS will be calculated based on what is selected (active) and not what is available (on)? Is this correct?

 

[Dr. Lecter]

TTS will be calculated based on what is selected (active) and not what is available (on)? Is this correct?

No. Deco stops/time will be calculated based on what is selected/active. TTS will be calculated based on what is available/defined and on. If you want an accurate TTS, only have on gasses you actually have available.

For divers like us, this is basically a non-issue: TTS is not something we're relying on, and we've already planned our dives and contingencies...turning off the 50% if you've lost it is merely window-dressing your displayed TTS. Where this becomes more important is unthinking divers who program in and leave on a bunch of gasses, only take a single tank of air, and then think they're OK because TTS is only a few minutes...not realizing that the SW is calculating TTS based off all the various nitrox blends they programmed in and left on, and that on air alone they've got 30 minutes of deco.

I truly have a hard time believing that anyone is that dumb, but the SW manual specifically addresses that scenario, and I trust they have a reason for discussing it.

 

[ajduplessis]

So TTS could be different vs actual in this case??

 

[Dr. Lecter]

So TTS could be different vs actual in this case??

Assuming that "this case" means:

1) you've got say 21/00, 50/00, and 99/00 "ON" in the SW;

2) have "active" on the SW and are breathing the 21/00;

3) have run up a deco obligation at depth and are now at 70'; and

4) only have access to the 21/00 and 99/00, with your 50/00 bottle having magically gone missing...

yes, TTS will be shown with the SW assuming that in the very next moment you will switch to 50/00 and proceed with the deco schedule from there until you switch to the 99/00 and finish the dive. TTS will ​actually be somewhat longer, because you will not switch off the 21/00 until 20' when the 99/00 stops begin. If you were to enter the define gasses submenu, though, and turn the 50/00 from "ON" to "OFF", TTS would recalculate to include only 21/00 and 99/00 and then be accurate.

At no time, however, is the SW doing deco calculations off anything except the gas you have selected/active...those are the only ones it "knows" you have breathed. It assumes you will breathe all the "ON" gasses later on during deco, which is where the TTS calc can diverge from the deco calculations.

It may be helpful to think of a distinction between what the SW "knows" you have done from the start of the dive up until a given moment in time--all gasses made active and given depth/time values--and what the SW "assumes" you will do from that given moment until the end of the dive--which involves taking the former information, comparing it to the "ON" gasses, and arriving at a deco schedule and TTS. Whether you follow that schedule or whether you do the full deco obligation on 21/00 is entirely up to you (and your gas supply)...if you don't switch off the 21/00, the SW will happily calculate your off gassing with that breathing mix until you make a gas switch or clear deco.

 

[ajduplessis]

I am slowly starting to recall why I back then decided to dive with all gasses off. Need to go back and check my notes........... For me its trying to eliminate mistakes, even dumb ones.

 

[lv2dive]

For me, Petrel TTS is a double check against plan... If TTS is showing a time that is different than what is expected then that is a clue something is going on... Either a gas switch wasn't made (on the computer), a gas was not programmed as active in the computer before the dive, etc. (Assuming no changes in dive plan)

I am not sure I can think of the benefit of diving with all planned deco gases inactive and having TTS reflect an inaccurate value. I can't think of how one would switch to a different gas accidentally even if it is active.

What am I missing (honestly)?

 

[steady1570]

I am not sure I can think of the benefit of diving with all planned deco gases inactive and having TTS reflect an inaccurate value. I can't think of how one would switch to a different gas accidentally even if it is active.

What am I missing (honestly)?

I think one issue would be as Dr Lecter mentioned - suppose yesterday you were diving a mulit-gas profile and activated a load of different mixes. Today to relax you jump in for a rec dive with just air and wander around at depth until TTS shows zz mins. "Thats ok, my SAC is xx and I have yy Bars, no sweat". Except the TTS is assuming you change to 50% then O2.... uh oh. Or if that seems unlikely, people play around with their computers in the surface intervals (I know I do), so you have to check that you switch all the inactive gasses off again.

I guess then, as a default safety position, you could just make sure that you only have 1 active gas each time, then you can't leave anything on by mistake. It also gives you a worst case scenario of TTS. It's not an approach I think I would take, but I can see the logic behind it, especially as when I was diving OC I would mix in properly planned tec diving with less-planned (ahem) recreational ones.

-Mark

 

[rhwestfall]

why wouldn't you want the worst case TTS, hence gasses off? If you are using a pre-defined schedule, this seems to be an excellent emergency plan tool.. or, have we transgressed to the computer being the plan?

I'm in AN/DP right now, so this is perfect timing for this discussion. Instructor is slowly adopting PDC's in his diving and we are having good discussions.

Thanks for any insight.

 

[victorzamora]

why wouldn't you want the worst case TTS, hence gasses off? If you are using a pre-defined schedule, this seems to be an excellent emergency plan tool.. or, have we transgressed to the computer being the plan?

I'm in AN/DP right now, so this is perfect timing for this discussion. Instructor is slowly adopting PDC's in his diving and we are having good discussions.

Thanks for any insight.

This was actually a huge point of discussion during my Deco training. Now that we have bulletproof computers that are running full-blown algorithms (as opposed to questionable computers running simplified algorithms)....does it make sense to have the distrust we do? Nothing replaces proper planning, agreed, but at what point is it "kosher" to run two Petrels instead of cutting tables? The Petrels will do the same work as your deco program on your computer, but in real time.

My deco training was fantastic. It was more of a give-and-take with my instructor (I read a lot, he has a lot of experience) and we covered FAR beyond the minimums and started talking about different books, perspectives, current trends like: VPM seemingly starting to fall out of favor, GFs have started swinging from 30/85 towards 30/70 or 40/70, advanced deco strategies like leaving a heated-vest canister with your first deco bottle for additional off-gassing. It was super interesting, and I got a very honest look at what his thoughts and opinions were. The one sticking point we had was we wanted to trust two computers, but at what point can we do so properly? It'd be much easier to plan a dive for gas, and then just ride two computers back out....but when does that become accepted?

---------- Post added May 13th, 2015 at 09:47 AM ----------

I think one issue would be as Dr Lecter mentioned - suppose yesterday you were diving a mulit-gas profile and activated a load of different mixes. Today to relax you jump in for a rec dive with just air and wander around at depth until TTS shows zz mins. "Thats ok, my SAC is xx and I have yy Bars, no sweat". Except the TTS is assuming you change to 50% then O2.... uh oh. Or if that seems unlikely, people play around with their computers in the surface intervals (I know I do), so you have to check that you switch all the inactive gasses off again.

I guess then, as a default safety position, you could just make sure that you only have 1 active gas each time, then you can't leave anything on by mistake. It also gives you a worst case scenario of TTS. It's not an approach I think I would take, but I can see the logic behind it, especially as when I was diving OC I would mix in properly planned tec diving with less-planned (ahem) recreational ones.

-Mark

I don't know anybody that would do a single-tank rec dive with enough depth and enough bottom time to rack up enough deco on enough mixes for it to matter.