Blindly trust computers?

[JackD342]

The vast majority of OW rec divers dive infrequently. Most, taught tables or not, would need a refresher class before any vacation where they are going to dive, if they wanted to remember how to use tables anyway. They're going to go to Cozumel/Bahamas/etc etc and dive with a boat. That boat is going to take them to a spot that the boat knows, with depths the boat knows, and tell them to be back in an hour, and the divers will suck down their air in 45 minutes and surface with 30+ minutes left on their NDL.

That diver is NOT going to go take a dive theory course annually so they can have a reasonable chance of determining if the computer their diving, and their dive buddies computer, are full of crap. And for most of their guided, very conservative diving experiences, that's just fine. The tiny percentage of rec divers that DO dive regularly likely have a good idea if their computer is way off base just from experience. They don't need a course to tell them that the computer saying they have 45 minutes til they hit their NDL at 130 ft on air is not functioning properly.

YES,yes,yes!! Newsflash: very little in this sport is one-size-fits-all. There is a wide spectrum of divers with a huge variety of needs, ambitions, experience, training, and even personal preference. Those once a year tropical resort boat divers that are content to do just that and little more, if ever, are a significant driver of the agenda in most OW courses - "What does it take to train divers as young as 10 years, to dive safely and enjoyably, in today's world...." Mandatory use of tables really no longer fits that need. I would suggest that anything that increases use of dive computers, probably results in fewer of these divers ignoring NDL entirely as they just dive what the boat crew tells them to.

Using average depth can get you into trouble. It would only work effectively if NDL is a linear function of depth... which it's not. To see why, consider that the NDL for 70 feet is 40 minutes, while the NDL for 110 feet is only 16 minutes (I'm looking at the PADI RDP table.) I could spend 36 minutes at 70 feet be well within my NDL, but spending 18 minutes at 110 feet and another 18 minutes at 30 feet puts me into deco. Both scenarios have the same time-averaged depth.

Admittedly, understanding the definition of a function, the average value of a function, and the practical limitations of these definitions is not basic math. But it's important math for situations like this.

Thanks for the definitive example. I will also add that a lot of these discussions seem to forget that even what seems to be the simplest methods can break down once applied in the real world.

What was my maximum depth on this dive? Hmmm... did I actually look at my gauge at the deepest part, or am I just guessing? If I have a maximum depth indicator on my analog depth gauge, did I remember to reset to zero before this dive or not?
And let's for a moment accept that average depth might be valid. What supplies you with your average depth to use in your RDP calculation? I think at least 9 times out of 10 it is a dive computer, so why would we pretend the DC doesn't matter? (I do have a Casio watch that will supply me with dive time, maximum depth, and average depth, but how common are those, really?)

I think I am trying to say: "dive computer - good"
Also, RDP use is less and less integral to diving safely in the real world. Not quite as decremented as learning cursive in school, but we might just get there in time.

 

[yle]

Wut?! Run those numbers and tell me what the deco is? ... *pssst it’s none.


un th

I guess it depends what table you're using. I stated which table I was using in my post. Of tables that are taught to OW divers, it's a pretty popular one. At least in the US.

I'm okay with other tables providing slightly different numbers, and I'm all for anyone that wants to discuss this to cite their sources. But regardless of what your tables or other source suggests, my original point is sound: NDL is not a linear function of depth, and so using average depth for dive planning can cause safety problems.

 

[PfcAJ]

My point was to create an example of two dives with the same average depth and the same time which do not give the same NDL result. That is, an example to demonstrate that NDL is not a linear function of depth.

Even in your extreme example though it’s REALLY FRICKIN CLOSE. You don’t even end up with a mandatory stop, and the only reason you “violate” NDL is because of those extra 2mins. If you didn’t do that, avg depth would work perfectly.

Now compare “really frickin close” to the multitudes of Divers who can’t find their butts with two hands because they aren’t taught any method to dive except “follow the box” or “follow the DM” (who’s likely following their box).

 

[lexvil]

I'm going to suggest that was just coincidence. AL was only a distributor for Suunto (i.e. AL has no ownership in Suunto), and they gave that up a couple years ago (maybe three?) Huish, which owns Atomic, Bare, and collection of other scuba companies, has been the US distributor for Suunto for the past 2 or 3 years.

The suit was likely filed in California because the plaintiff lives in California... as do a lot of other divers.

That too.

 

[TMHeimer]

Is this really going to be beaten to death... again?

Of course.

On dives below 30' I use computer and analog. Once my SPG refused to go under 1000 PSI. Common sense told me something was amiss. It was toast. Good to have a backup, be it what I do or 2 computers. Unless you're at 20-30' and know you can CESA.

 

[yle]

Wut?! Run those numbers and tell me what the deco is? ... *pssst it’s none.


un th

If you need another example, try this one:

60 ft depth 50 minutes (my table has 55 min as NDL)

or

110 ft depth 25 minutes & 10 ft depth 25 minutes

Both dives have an average depth of 60 feet for 50 minutes. At least on my table, the first one is a no-deco dive and the second one is not.

Or hey, try this one:

80 ft depth 30 minutes

or

150 ft depth 15 minutes & 10 ft depth 15 minutes

Again, both dives have the same time and average depth. But I think the second dive incurs a deco obligation.

Please... feel free to use average depth to do your own dive planning, but it's irresponsible of you to suggest on a "basic scuba" forum that it's a viable option.

 

[dmaziuk]

Because basic math and basic science are both hard.

So when someone drives into a pond because Siri says there's a road there, that's because they failed basic math?

 

[Ryan Mcshane]

I guess it depends what table you're using. I stated which table I was using in my post. Of tables that are taught to OW divers, it's a pretty popular one. At least in the US.

I'm okay with other tables providing slightly different numbers, and I'm all for anyone that wants to discuss this to cite their sources. But regardless of what your tables or other source suggests, my original point is sound: NDL is not a linear function of depth, and so using average depth for dive planning can cause safety problems.

I used deco planner set to 20/85 (which is considered pretty safe) and @PfcAJ just showed you another from a mobile software.

We both use average depth to plan our dives and neither of us have died, horribly, twice. Where are the bodies??

 

[MaxBottomtime]

If you need another example, try this one:

60 ft depth 50 minutes (my table has 55 min as NDL)

or

110 ft depth 25 minutes & 10 ft depth 25 minutes

Both dives have an average depth of 60 feet for 50 minutes. At least on my table, the first one is a no-deco dive and the second one is not.

Or hey, try this one:

80 ft depth 30 minutes

or

150 ft depth 15 minutes & 10 ft depth 15 minutes

Again, both dives have the same time and average depth. But I think the second dive incurs a deco obligation.

Please... feel free to use average depth to do your own dive planning, but it's irresponsible of you to suggest on a "basic scuba" forum that it's a viable option.

You're looking at it all wrong. On the second dive you are in a deco dive after 16 minutes. After spending 15 minutes at 10 feet, you are no longer in deco. It works out the same as the first dive.

 

[letterboy]

So when someone drives into a pond because Siri says there's a road there, that's because they failed basic math?

No, you fail common sense.

Basic math is all it takes to calculate with tables etc.