Erm does X-Y=Z?

[Nick Kay]

From a personal (wussy, self-preservation, getting on a bit, dive with my son, more than my life is worth to go back alon) perspective:

1. Would switch to the back gas and complete deco as if I'd been on the back gas all the time

2. Generally:
- Have a nitrox computer with the back gas set
- Have an air computer as a backup
- Have a run-time slate as a (set of) further backups - unless its an ERD-type or Trimix dive in which case the rutime slates take precedence

3. What I meant, was that, depending on your safety / personal / gf settings (dependent on s/w used), you MIGHT be safe to get out the water at that point anyway, so whichever way you re-calc'd you MIGHT be safe

4. As a further example of my "wussiness", I go along with Jack Ingle's view that once you've finished all your mandatory deco, (dependent on gas remaining, cold, etc) why not add extra time on for a shallower stop (if safe to do so), i.e. finished your deco at 6m, add additional time on at 3m

AND...

I did say

So... Why not have a slate with a set of KYAG / GOOJ runtimes???

[Scuby]

Originally Posted by Nick Kay

- Have an air computer as a backup

Out of interest, how's that work? Surely you'd bend it on the first dive you did and then not be able to use it for the rest of the weekend? Or do you just use the nitrox for safety margin? If not I can imagine the constant "you're going to die" beeping would annoy me to the point of smashing the thing!

David

[Nick Kay]

Originally Posted by Scuby

Out of interest, how's that work? Surely you'd bend it on the first dive you did and then not be able to use it for the rest of the weekend? Or do you just use the nitrox for safety margin? If not I can imagine the constant "you're going to die" beeping would annoy me to the point of smashing the thing!

David

1. For Trimix, ERD I run ProPlanner at 20/20
2. When I finish the deco, I tend to find that the air computer has something like 5-10mins left, so do the "air computer" deco. But I can get out of the water if I want to according to the runtime slate
3. If I do get out of the water "early", then for the next dive:
- drop to gauge mode on the Nitrox computer & use runtimes & take a dive watch (Trimix, ERD or "std" dive)
- use the nitrox computer with watch/slate as backup

4. For "std" dives, I use the Nitrox computer set to whatever %age
5. As per (2) above

KYAG / GOOJ slates as backups

Forgot, also generally have a reduced size/laminated set of Nitrox tables (about A6 size folded in half) for 32 & 36 mixes with me

If all that lot has failed and for whatever reason, can't use buddy's deco profile, then someone somewhere is sending a message

[Finless]

Whilst pondering my navel (which appears to have headed south to the dark side of the moon) and the question I originally asked I have decided that my 50/50 assumption of deco times is probably OK?

If, as previously stated, the rate of off gasing nitrogen at the final stop is not linear then it must be faster initially and slower towards the end of the deco as the "pressure diff" between N2 in me and the breathing gas gets closer to parity. Therefore by the time I have completed 50% of my accelerated deco time I must have off gassed more than 50% of the excess nitrogen in my system?

This should mean that I would be OK, if need be, to change over to my back gas and complete 50% of the deco time required when using the back gas only for deco?

Logic (well, as I see it) would suggest there must a point at which it is safe to say that now I have completed X% of my accelerated deco time IF I have to switch to back gas gas I only need to complete Y% of my back gas deco time?

It all makes sense to me.

[Scubee]

Originally Posted by Nick Kay

KYAG / GOOJ

OK... I have to ask at the risk of accusations of girlieness or blondness...

At a guess, I would say these are worst case scenario/loss of deco gas back up plans but what does it stand for?

No doubt something bleedin obvious!

[Garf]

Kiss your ass goodbye / Get out of Jail

Personally, I would just double up all the stops on the way up, and then breathe the backgas down to a bare minimum on the last stop before doing a monumentally slow ascent. But then I'm in no rush to get to the surface.

[Finless]

Originally Posted by Garf

Kiss your ass goodbye / Get out of Jail

Personally, I would just double up all the stops on the way up, and then breathe the backgas down to a bare minimum on the last stop before doing a monumentally slow ascent. But then I'm in no rush to get to the surface.

Generally, I totally agree with you BUT there are circumstances where you, well .....I, want out asap. My dive on the Ash last year was hugely uncomfortable at the deco stops and, although I didn't lose any deco gas, I wanted out asap!

Also, please note that for my example, I have deco run times for both accelerated deco and for just my back gas - as we all do for every deco dive, don't we? Therefore, I know how much time the last stop is using accelerated deco AND I also know how long the last stop is using just back gas.

Therefore, given these particular circumstances, do you think that doing 50% of the accelerated stop time and then switching to back gas and doing 50% of the back gas stop time would get you out inside the safety line?

[Garf]

Originally Posted by Finless

Therefore, given these particular circumstances, do you think that doing 50% of the accelerated stop time and then switching to back gas and doing 50% of the back gas stop time would get you out inside the safety line?

there are far wiser on me than here, but my instinct would tell me no. My reason for this is that the high mix will not have had enough time to actually do you any real favours before you swap back off it onto your lean mix.

that being said, you'd be talking about doing 13.5 mins instead of 20 mins, so you "might" get away with it, but I wouldn't plan for it

[Finless]

Originally Posted by Garf

there are far wiser on me than here, but my instinct would tell me no. My reason for this is that the high mix will not have had enough time to actually do you any real favours before you swap back off it onto your lean mix.

that being said, you'd be talking about doing 13.5 mins instead of 20 mins, so you "might" get away with it, but I wouldn't plan for it

I don't know so much, surely the first part of the deco is the bit in which the greatest volume of nitrogen offgassing is done ........ or am I looking at too simple a model in my mind.

As already stated I would do the full backgas deco time, regardless of how much accelerated deco I had done (well, unless I was within a minute of completing it) or as much as I could do with the gas left.

Where's the smug clever git when you one!

[Scubee]

Originally Posted by Garf

Kiss your ass goodbye / Get out of Jail

Thank you