Blindly trust computers?
- Thread starter PfcAJ
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Sounds like good advice for that crowd...
I think I am seeing a disconnect with average depth use in practice vs running these schedules and seeing what is NDL and what isn't.
Extreme case, all numbers made up for ease of pre-coffee maths:
130' for 20 min then 30' for 20 min. Average is 80' so NDL around 60min so all good except you went into deco after 10 min at 130'. Average depth is bad.
However, in actual practice (assuming a 5 min to NDL conservatism applied by diver):
Descend to 130.
After 5 min, average depth is 130. NDL is 10, got 5 to go. Time to ascend.
Go up to 70'. After 5 min Average depth is 100'. NDL at 100' is 30 min, still got 20 min to go.
After 15 min, average depth is now around 80' or so. Ascend to 30'.
Average depth will move to somewhere around 40-50' where NDL is around 100 min. From here on, gas becomes limiting for most people.
So, in practice, monitoring your average depth every few minutes and then ascending before NDL for that depth will keep you safely in limits.
The discussion as to whether 2 computers are an adequate solution for the case of a dodgy computer is a different one, but I fully agree with AJ that gas planning and deco (avoidance) theory are woefully under-taught at the moment.
Extreme case, all numbers made up for ease of pre-coffee maths:
130' for 20 min then 30' for 20 min. Average is 80' so NDL around 60min so all good except you went into deco after 10 min at 130'. Average depth is bad.
However, in actual practice (assuming a 5 min to NDL conservatism applied by diver):
Descend to 130.
After 5 min, average depth is 130. NDL is 10, got 5 to go. Time to ascend.
Go up to 70'. After 5 min Average depth is 100'. NDL at 100' is 30 min, still got 20 min to go.
After 15 min, average depth is now around 80' or so. Ascend to 30'.
Average depth will move to somewhere around 40-50' where NDL is around 100 min. From here on, gas becomes limiting for most people.
So, in practice, monitoring your average depth every few minutes and then ascending before NDL for that depth will keep you safely in limits.
The discussion as to whether 2 computers are an adequate solution for the case of a dodgy computer is a different one, but I fully agree with AJ that gas planning and deco (avoidance) theory are woefully under-taught at the moment.
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OMG. That is the point that I was trying to make.
You just can't average those two. They are worlds apart.
And that is what I am also saying. One side seems to be saying: Take two weird depths and times, average them, and look there's deco.
On the other hand, those who have actually used avg depth are saying: Look at your average throughout the dive and ensure that you remain in NDL for that average continuously as you go up.
Doing it that way means you don't end up with weird examples, like 1000' for 1 second then up to 10' for an hour and getting an NDL dive. In practice, ignoring descent rates because I still haven't had a coffee thanks to you lot, at around 150' or so you will hit 0 NDL and have to immediately ascend, as you do, because NDL isn't linear, you will gain more NDL than you are expending in bringing the average up and so a drop to 150' and then coming up to a shallower depth will be doable on avg depth.
SAFETY WARNING
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Yeah, exactly.
Your 'challenge' made me stop and think about what it is that I'm actually doing. My best guess is that I'm binning times in no greater than 20' depth intervals in the midranges while adding a bit of 'English' for conservatism.
Deeper dives put me out of any sort of guessing. I just do a square profile max time on air and start to get creeped out if my PDC gives me a lot more than that.
Your 'challenge' made me stop and think about what it is that I'm actually doing. My best guess is that I'm binning times in no greater than 20' depth intervals in the midranges while adding a bit of 'English' for conservatism.
Deeper dives put me out of any sort of guessing. I just do a square profile max time on air and start to get creeped out if my PDC gives me a lot more than that.
AJ
Contributor
Panic?? What'd your computer say the last time you looked at it and it was working? Let's make it extreme and say you're at 100', 3 min to NDL with 800psi - do you panic? You do exactly what you were taught with an equipment failure - you ascend at a normal rate, complete your safety stop and surface.
If you even think you'll panic due to a computer failure at any point, you better find a second computer or another way to dive.
This whole trying to make an exact science using tables for rec multi level dives is yahoo. You're taught in OW to use your max depth or plan multi levels - it's a PITA but they also teach the benefit of computers.
The guys using average depth guesses I have to assume have done this a time or two and are making educated decisions based on experience - not something I'd be teaching most folks with 25 dives or so.
Heck, you can't even compare some computers - it depends on the math they run and the factors the user has set. I have dove a G2 set at MB0 next to a Shearwater altering the GF on the Shearwater from 45/85 to 50/70 and you really don't see much difference if you stay above 60', get down to 100 and you can see differences, pretty big ones. I wish the G2 would record data points on the ndl time.....
A new diver ought to look at tables and see where they'll be ndl roughly before a dive to gain some sense of awareness but I have to say yes, they should trust their computer. They're asked to trust their depth and SPG gauges correct? With all that though, situational awareness of when something ain't right has to take over and the dive should end at that point.
If computer shuts down I go to the other computer. Or if, as happened back in the days when I dove a Suunto as one of my two computers and at 100ft on shark tooth ledge Isaw I had very little NDL and then saw the Suunto was on air I switched to the other computer which was on Nitrox.
The Suunto had the trait I did not like that it defaulted to air at the start of the day. Even though I change batteries every year, the battery was weak. So after being set on the way out, it went to sleep after an hour and reverted to air for the first dive. Both my computers now start with whatever I used last which is essentially always 32%,
The Suunto had the trait I did not like that it defaulted to air at the start of the day. Even though I change batteries every year, the battery was weak. So after being set on the way out, it went to sleep after an hour and reverted to air for the first dive. Both my computers now start with whatever I used last which is essentially always 32%,
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