Are dive computers making bad divers?

[The Chairman]

There is a simple method called Depth Averaging

This has to be one of the most horrid inventions ever created to deal with NDLs. It relies on the most problematic instrument that every diver owns: their brain. This eventually convolutes into ratio deco which is indeed the most horrid of inventions. Don't fall for the hubris that maintains that this is 'simple' and 'easy'. That only applies to getting bent.

 

[Storker]

It is difficult to combine tables and computers unless you are doing truly square profiles.

I don't agree.

I've used my tables for pre-dive planning for most of my dives, but when I dive I use my computer. Given my style of diving, where the dive leader requires you to give a max depth and a max run time before splashing, a decent plan is pretty useful. Yes, most dives are not square profiles. However, how to do multi-level planning with the good old PADI RDP has been published, and for rec dive planning purposes it's more than good enough for me. I've even compared a few profiles with a computer simulation tool to check on the sensibility in the table planning approach.

One of my most typical profiles is planned as a two-level dive: 25m for 25minutes, then 15m for 15minutes. Safety stop, ascend. That's 55 minutes run time. According to the RDP, it'll put me very close to the NDL. According to Suunto, it'll put me slightly outside the NDL. So , I plan my dive as MAX 25m for MAX 25 minutes, then MAX 25 minutes for MAX 15m. I know that I'll probably spend less than 25 minutes at 25m, and less than 25 minutes at 15m, so that's OK. In addition, I have calculated my min gas, i.e. my minimum tank pressue at 25m and my minimum tank pressure at 15m. Those are my limits, and as required I tell the dive leader my max depth (25m) and my max run time (60 minutes). Sometimes I run pretty close to my NDL, and sometimes I run pretty close to my min gas pressure. I still have my "plan", which I made using my good old, according to some people here obsolete, tables. It definitely does not limit my diving, since I'm free to adjust my profile as long as I don't violate my predetermined limits. And if I want to have more freedom, I might add a contingency depth - say 30m - to that plan which I discuss with my buddy before splashing. My dive leader is happy, since I don't violate the limits I inform him about before submerging, my buddy is happy since we've agreed on how long and how deep we're going to go, and I'm happy since I'm pretty certain that if SHTF I and my buddy will have enough gas to complete a sensible ascent with no fear of running OOG.

Underwater my dive is, of course, controlled by my PDC. If it tells me to get the heck out of Dodge, I'm going to get the heck out of Dodge. No matter whether that's due to my tank pressure or my residual bottom time. And my repeat dive will also be controlled by my PDC. But then I like to get a solid SI (about a couple of hours or more) before splashing again, so I rarely incur a hefty penalty due to residual nitrogen.

They do advocate that.

I guess I'm not the only one who sees a certain... discrepancy between accepting 30m END narcosis and on-the-fly depth averaging...

 

[CAPTAIN SINBAD]

So let me get this straight,

I can spend 11 minutes a the top of a wreck at 70', go to the sand at 130' for 11 minutes and surface?????. My average depth is 100' which has a limit of 25 minutes so my 22 minutes is well within that limit......

Tables are not built for average depths.

Notice how you are magically appearing from a depth of 70 feet to a depth of 130 feet while not spending any time in the middle? Even the descent and ascent times between those two depths have been ignored because it will bring depth averaging closer to the results you are calculating with your computer.

How about trying the following more true-to-life profile instead. You know your total bottom time at 130ft is 10 minutes so start with that deepest part first.

8 minutes at 130 feet
8 minutes at 100 feet
8 minutes at 70 feet

The above profile gives you 24 minutes which is closer to the total time of 25 minutes you have calculated. Nowhere in that progression have you pushed into deco.

 

[KenGordon]

I will not comment as I cannot do it politely. But let's look at how true "nowhere have you pushed deco" is.

I had to set multi deco to imperial for this too. That's how annoyed I am.


Depth Stop Run Mix pO2 EAD

Des130-221--
Lvl1307:5010211.04130
Asc100-1121--
Lvl1008:0019210.85100
Asc70-2021--
Lvl708:0028210.6570
Asc20-2921--
Stp204:2034210.3420 4:20 at 20ft
Sfc--3521--

Dive # 1, ZHL-C+GF 100/100
Elevation = 0 ft
CNS = 7%
OTU's = 20
Decozone start = 73 ft
Gas 21 = 2450 ltr.


You just skipped 4 minutes of stops on 100/100 gf. On a more reasonable 45/85 12 minutes.

Depth Stop Run Mix pO2 EAD

Des130-221--
Lvl1307:5010211.04130
Asc100-1121--
Lvl1008:0019210.85100
Asc70-2021--
Lvl708:0028210.6570
Asc30-2921--
Stp300:4030210.4030. 40 Sec at 30 ft
Stp201242210.3420. 12 minutes at 20ft
Sfc--4321--

Dive # 1, ZHL-C+GF 45/85
Elevation = 0 ft
CNS = 7%
OTU's = 20
Decozone start = 73 ft
Gas 21 = 2633 ltr.

And of course if I do the profile the other way round the deco goes from 4 to 8 minutes. Obviously. It is more complicated than adding up and dividing.

PS, for any 'basic' divers still here. The gf numbers like 45/85 are knobs on some maths. If you can't follow the maths how do you choose those numbers? Choose 100/100 (an aggressive choice) and a dive might be no stop. On 30/70 (a conservative choice) the same dive may have stops. Now it is a technical dive (by SB convention).

 

[Steve_C]

I will point out that IF averaging is appropriate, then what Sinbad said is STILL wrong unless you are spending the same time at each depth. Typically on a dive I spend different amounts of time at different depths. Then IF averaging was appropriate you would need to do weighted averaging.

 

[KenGordon]

That is fine, we can all do (7*130 + 8*100 + 9*70)/3 in our heads or we are not real divers surely?

 

[boulderjohn]

So let me get this straight,

I can spend 11 minutes a the top of a wreck at 70', go to the sand at 130' for 11 minutes and surface?????. My average depth is 100' which has a limit of 25 minutes so my 22 minutes is well within that limit......

Tables are not built for average depths.

Notice how you are magically appearing from a depth of 70 feet to a depth of 130 feet while not spending any time in the middle? Even the descent and ascent times between those two depths have been ignored because it will bring depth averaging closer to the results you are calculating with your computer.

How about trying the following more true-to-life profile instead. You know your total bottom time at 130ft is 10 minutes so start with that deepest part first.

8 minutes at 130 feet
8 minutes at 100 feet
8 minutes at 70 feet

The above profile gives you 24 minutes which is closer to the total time of 25 minutes you have calculated. Nowhere in that progression have you pushed into deco.

Like Ken Gordon, I ran Captain Sinbad's plan through multi-deco, and I also ran packarat12's plan. It might be hard to follow what Ken wrote becuse it did not printout in an orderly fashion, but at the least conservative setting possible (100/100), Captain Sinbad's plan called for 6 minutes of deco, and at a more typical setting, it called for 12. Packarat12's plan called for 9 minutes of deco (after only 22 minutes of bottom time). It would only call for 6 if he had done the deepest part of the dive first. Of course, that was his question--does it matter in averaging what order the numbers are reached.

So I would like to see some justification for the claim that Captain Sinbad's plan does not go into deco. Under what algorithm is that true? Are you saying that because you left the bottom within the limits you are OK? Leaving the bottom within limits only works if you head for the surface at the rate prescribed by the tables.

---------- Post added December 18th, 2015 at 05:16 PM ----------

That is fine, we can all do (7*130 + 8*100 + 9*70)/3 in our heads or we are not real divers surely?

For me, only when I'm narked.

 

[KenGordon]

Under the algorithm of adding it all up and dividing.

Sorry about multi deco output it seems Ross is too busy arguing with Simon M to make his text formatting work.

I now have to check all my ascent rates etc in multi deco having switched back and forth between imperial and metric. Normalisation? Who needs it?

 

[boulderjohn]

Sorry about multi deco output it seems Ross is too busy arguing with Simon M to make his text formatting work.

Don't blame Ross. The program on which ScubaBoard currently runs does not handle that sort of thing very well at all.

 

[CT-Rich]

Seems to me, going back to the OP in this thread, that not using a computer makes bad divers.... (at least the ones that think there are shortcuts in math at 130 fsw....)