SeaJay
Contributor
True.
False.
Firstly, let's get some of those variables out of the way:
1. I don't know of any computer that does not factor in repetitive dives... So let's assume that this one does. Let's also assume that this is the first dive in a series. That keeps things simple and gets rid of that variable.
2. Work load is somewhat taken into account with AI computers... They measure breathing rate, and thus have a rough estimate of your workload. If the load is higher, you'll use more gas. Since this diver is near his NDL, but still has plenty of gas, this diver either has a bunch of gas available to him or his work load is low. Again, to keep things simple, let's assume that his work load is low and his consumption is respectable.
3. I haven't looked into which computers actually take temperature into account... Perhaps the popular brands do and perhaps they don't. Temperature effects quite a bit when it comes to DCS (think about how much "fizzier" cold Coke is when it's cold or poured over ice cubes), but typically that would also manifest itself in poor air consumption. Since we've assumed his air consumption is good, let's assume a moderate temperature with the correct exposure protection - say, 78* in a 3 mil.
...So we've assumed a very nice dive with little effect from the "variables" you were talking about.
Still think there's no problem?
While we're at it, let's get another variable out of the way... Let's say that this diver is diving open ocean, and he's diving clear water and can see the anchor line (the "upline") that's around 50 feet away.
Since he's a good buddy, he signals to his fellow diver that it's time to ascend, right? There's a slight current at the top, so they'd want to do the ascent on the upline, right?
Still see no problem?
The two of them get together and head for the anchor line. It takes them about 40 seconds to get there. Now the computer's reading 1 minute. They begin their ascent. Since most computers will "beep" at you if you exceed an ascent rate of 30 fpm, they head up at just under that rate.
Our diver now has 1 minute of NDL left.
One minute later... Where are they on the upline?
See the problem now?
They're at 90 feet with zero bottom time left. They've gone into deco. Not a problem if that's what they were planning... But they weren't.
I say this out of personal experience.
Another minute goes by. They're now at 60 feet, and their computer is now telling them that they have a mandatory deco stop at 15'... Which is a little unnerving because they've never done a deco stop before. They weren't trained to do deco stops. Not yet, anyway.
Yet another minute goes by, and they're at 30 feet... With the computer beeping something unintelligible. Our buddy did great by reading the owner's manual for his computer many times, but still... This is the first time he's dived it past his NDL - which, by the way, his owner's manual tells him to never do.
At 15', our hero can't really make out what his computer's telling him (it's all new to him), but he's been diving for a couple of years, and he remembers the plan for stops if he blows his deco. Eight minutes if under 5, right? Or is that 12 under? Five minutes at 12 feet? Shoot. He wishes he could remember...
Okay, let's stay safe. Let's do... 15 minutes at 15'. Sounds good.
Good thing he had all that extra gas, isn't it? What if he hadn't? What if his rate was worse than it is? By the way, with all of this kicking around and anxiety of doing deco, guess what's happened to his breathing rate?
What would you do if you had a mandatory deco of 15 minutes at 15 feet (he thinks) but only 5 minutes worth of gas?
Now, hang at 15 and think about it for a quarter of an hour and consider how that little thing on your wrist just about put you in a world of trouble...
