Predator/Petrel lost gas procedure ?

[a878bob]

I am thinking of the best practice for Predator/Petrel lost gas. I haven't had it happen. So thinking out loud.
If diving 50%/100%, and loose my 50%, can you continue on back gas, don't touch the Petrel, switch at 20', it will yell at you to switch, but will it update accurately doing it this way? I would think the TTS would be wrong but other wise ok. Or is it better to go into the menu, define gas, and turn off the 50%?

 

[Dr. Lecter]

Until and unless you switch gasses, the SW computer will continue to calculate your deco information using the selected gas. Yes, TTS will be wrong--though you can always go into the Dive Setup menu during the dive and turn the 50% gas to OFF--TTS will then recalculate and the SW would stop suggesting there's a better gas available until you're at 100% O2 depth.

 

[ajduplessis]

To my knowledge (have not checked and I might be wrong) the SW units will keep re-calculating TTS throughout the dive even if you are not using a defined gas.

 

[Dr. Lecter]

To my knowledge (have not checked and I might be wrong) the SW units will keep re-calculating TTS throughout the dive even if you are not using a defined gas.

They will keep recalculating TTS, but the calculation will always assume (among other things) that you're able to use all gasses defined and turned on, and that you'll use them at their deepest available depth on up to the next gas switch. If you have backgas as the active gas on the SW at 70' and you haven't switched the 50% off, you'll see the yellow contrasting text indicating the SW thinks you should have switched already and its calculating TTS based on you performing the switch as it's suggesting. Since you're not performing the switch as time passes, however, it keeps recalculating that switch and the TTS value on into the future, all while tracking your off gassing with the active gas.

So if you're not switching from backgas to 50% at 70' and you want an accurate TTS, you need to turn the 50% off. At that point it will calculate based on the two "available" gasses, backgas and O2, and you'll see TTS increase.

 

[ajduplessis]

I will take your word on that. I dive with my deco gasses switched off.

 

[rhwestfall]

I dive with my deco gasses switched off.

out of curiosity, why? Are you following a defined (cut tables) plan, and then using the computer as the "emergency" of lost deco gas?

 

[victorzamora]

I am thinking of the best practice for Predator/Petrel lost gas. I haven't had it happen. So thinking out loud.
If diving 50%/100%, and loose my 50%, can you continue on back gas, don't touch the Petrel, switch at 20', it will yell at you to switch, but will it update accurately doing it this way? I would think the TTS would be wrong but other wise ok. Or is it better to go into the menu, define gas, and turn off the 50%?

It will only assume you're breathing what you're telling it you're breathing. All deco calculations will be "accurate" (marked with chalk, I know) but TTS will be wrong and your Petrel will think you're an idiot. Turning off lost gasses will make your TTS accurate. I highly recommend turning off your lost gas anyway, because accidentally switching gases (especially with the "new" gas switch mode that I LOVE) could be rough.

 

[ajduplessis]

out of curiosity, why? Are you following a defined (cut tables) plan, and then using the computer as the "emergency" of lost deco gas?

Yes, partly because my Petrel is backup to a predefined run and I like to see worst case scenario wrt TTS during the dive. It also eliminates accidental gas switches, not that I believe they happen easily with SW computers. We typically plan for thirds or TTS, whichever happens first is the signal to turn the dive.

 

[Dr. Lecter]

I will take your word on that. I dive with my deco gasses switched off.

Interesting choice - at least you'll never find yourself surprised with how much backgas deco you have should you ever need to complete it.

 

[ajduplessis]

They will keep recalculating TTS, but the calculation will always assume (among other things) that you're able to use all gasses defined and turned on, and that you'll use them at their deepest available depth on up to the next gas switch. If you have backgas as the active gas on the SW at 70' and you haven't switched the 50% off, you'll see the yellow contrasting text indicating the SW thinks you should have switched already and its calculating TTS based on you performing the switch as it's suggesting. Since you're not performing the switch as time passes, however, it keeps recalculating that switch and the TTS value on into the future, all while tracking your off gassing with the active gas.

So if you're not switching from backgas to 50% at 70' and you want an accurate TTS, you need to turn the 50% off. At that point it will calculate based on the two "available" gasses, backgas and O2, and you'll see TTS increase.

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?