Would you really know what was going on if your computer went into Deco...?

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Spectre

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H2Andy:
if you go to the limit on air and start your ascent, you CAN NOT violate your Nitrox NDL unless the extra deco takes you there, right?

Well, that's what I'm stating. You can summize that to be the case, but you never really can be sure. With the variations of depth on a dive, your computer is going to sample the actual dive, not the square profile that a table is designed around. For example; I'm staring at a dive profile from my computer download. The max depth was 124 feet. The bottom time was 24 minutes before the computer went into deco. At what point would that dive have gone into deco if it was set to air? If I dove it on tables, beginning my ascent at 16 minutes, it's likely the computer set to air wouldn't have kicked into deco. So if I ran the dive to the point that it kicked into deco, how can I be sure how much time I have between the time the computer kicked into deco and the dive actually becomes a dive requiring deco stops? The next question is how far into deco can I run it and still not have required stops?

ok... all you have to do now is not exceed your MAX BOTTOM time on the nitrox mix, and you'll be safe. if the deco takes you past that (at least six to ten extra minutes, btw) then you call the dive.

The point I'm trying to make is that the suggestion of running the computer in air leaves a lot of unknowns in place. Yes, if you don't exceed your max bottom time on the mix, you'll be safe; but given the nature of tables vs. computers, it's likely that to get your computer into deco on air, you will need to exceed that nitrox bottom time; which now takes the whole dive profile into a crap shoot as to if it _is_ safe or not.

Granted, you could go with two computers, one watching your actual dive, one watching your theoretical dive. You could also go to your depth, and sit there for the whole dive, to ensure that you don't need to go over your nitrox planned bottom time; but given the simulators you can play around with "what if's" to your hearts content without putting yourself at risk.

The safest answer to the question at hand is to play around with the simulator in your computer. There are other related topics that could be useful information to those interested, and possibly another thread might be good on this. Those questions revolve around situations where divers actually did end up going into unplanned deco [which is what this request is meant to prepare them for]. A thread analysing those incidents could be very useful in preparing yourself for how to handle it. What caused it, how did they handle it, and what lessons did they learn to prevent it or deal with it better should it happen again.
 

H2Andy

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yeah i see what you're saying. last post from me, not to beat a dead horse.

my point is that i will retain ultimate control of the dive. i won't let the computer
take me into Nitrox NDL. if in doubt at any point, i'll call the dive.

i am sort of paranoid about this. i carry tables with me on every dive (air, n32 and n36) AND i have a chronomoter running in addition to the computer keeping time.
that's just me.
 

Genesis

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H2Andy:
yeah i see what you're saying. last post from me, not to beat a dead horse.

my point is that i will retain ultimate control of the dive. i won't let the computer
take me into Nitrox NDL. if in doubt at any point, i'll call the dive.

i am sort of paranoid about this. i carry tables with me on every dive (air, n32 and n36) AND i have a chronomoter running in addition to the computer keeping time.
that's just me.

Why?

You know what the time was at the last time you looked, right?

Let me posit something Andy.

You're on a dive with your computer. You know you're within the NDLs when you look at it, and at 20 minutes dive time.

You look again, and its blank - its puked. You're not sure exactly WHEN it puked, but you know it has.

Ok, now what?

Well, the dive's over, right?

If you have a buddy, you have a depth gauge. That's all you need. Make your ascent, doing a stop profile for a deco dive (even though you know its not). Assuming you were at 80', do 1 minute @ 50, 1 @ 40, 2 @ 30, 2 @ 20, 5 @ 15, then do a 1'/10 second ascent from there to the surface.

If you go back and run this profile through some decompression software, you'll find that if you were "over time" by a few minutes you're still WELL within the "ok" boundary doing the ascent this way.

Since you look at the computer every few minutes, you couldn't have had more of an obligation than this.

BTW, if I'm going to carry one redundant device, I want a depth gauge rather than a second timing device. I can estimate time (by counting) and get reasonably close. If I don't have external references depth is much harder to estimate accurately.
 

H2Andy

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Genesis:
If you have a buddy, you have a depth gauge. That's all you need.

"IF" is the key word

and if i did any deco diving, i'd agree with you that
a depth gauge would be the thing to have redundant.

since i limit all my dives to NDL, i'd rather have a second
watch.
 

DandyDon

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I can certainly be guilty of making things too complicated, but at others times - I see others doing that, I try to summarize and/or simplify as needed...

Since I finally studied parts of the owners manuaul that I thought I'd covered - but hadn't, and found the Sim Mode, we practiced with that at a kitchen table. That took some focus, fumbling, recovery, and was a good experience.

When my home-bud gets certified for Nitrox, we can then practice underwater, for the real, hands-on feelings OW students get with their pool and open water sessions. We'll take Andy's suggestions - using the only controlled enviroment we have within hours here, the Santa Rosa NM (Turn) Blue Hole...

We'll arrive after a 1500 feet increase in elevation to 4800, for an 84 foot FW dive adjusted to 110, we'll round up to a Padi Group D, wait 1:19 hours to come down to Group A, and accept that we have 4 minutes of Residual N2 to carry forward the rest of the day for 32% Nitrox.

Then, we'll plan the adjusted 110 foot dive for less than 25 minutes on EANx 32%, to keep us under NDLimits. We'll set our Oceanic computers on Air or 21% Nitrox (same things, different settings) and make the dive. The computers may automatically read the altitude at start-up as 3, 4, or 5,000 feet, as they have a 1,000 foot variance, but if the computers work with adjusted 100, 110, or 120 foot depths on Air, they will go into Deco well before we hit the 25 minute NDL for 32% EANx. Since the puters could be as much as 2,000 feet apart on their start-ups, we may have to wait for the last one, but - even though Oceanic puters are set a couple of minutes more liberal than Padi tables, we'll hit Deco on them no later than 22 minutes into the dive, still 3 minutes short of EANX 32% Padi limits.

We'll ascend at 30 feet per minutes, do a one minute stop at 42 feet, then work off our Deco obligations at the altitude adjusted, suggested depth of 12 feet, then add 3 minutes to that Safety Stop. Of course, we'll also carry a depth guage and a timer each, and we'll een carry an extra Oceanic puter.

Then, we will have carefully worked thru a hands-on, in-water computer deco dive that actually didn't not violate Padi NDLs for air, so if we ever see it again in water, we'll know - "we've done this drill before." We might do it again now and then to stay practiced.

Sounds good to me...!

thanks, don
 

Spectre

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Don,
If you are going to do this, I would suggest that you also add any personal conservatism and altitude into the computer as well. I don't know what you're using, but I know with the Suunto I can change it to have a personal conservatism of 0, 1, or 2, and an altitude setting of 0 [sea level] 1 [5000 feet] and 2 [10000 feet] I believe. By spiking them both to 2 it will add more conservatism into the dive, and thus will kick into deco sooner.
 

DandyDon

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Spectre:
Don,
If you are going to do this, I would suggest that you also add any personal conservatism and altitude into the computer as well. I don't know what you're using, but I know with the Suunto I can change it to have a personal conservatism of 0, 1, or 2, and an altitude setting of 0 [sea level] 1 [5000 feet] and 2 [10000 feet] I believe. By spiking them both to 2 it will add more conservatism into the dive, and thus will kick into deco sooner.

Good idea. Two of the three Oceanics are Verso Pro, which can be set more conservative. We'llk do that with those two, and leave the older Oceanic on Air to double check our real dive.

The altitude settings are automactic in Oceanic, albeit rough.

don
 
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