DIR Answer Required

[Snowbear]

I got the following PM from someone who wants to try this from reading it on the internet. This person seems to be taking a part of what I said and wants to apply it to his or her dives without understanding how it works.

This is something that I think should be addressed in the thread, not just via PM...
My answer to this person follows (slightly edited).

Hi Snowbear.

I have a few questions about depth averaging. I've been reading the following thread, http://www.scubaboard.com/showthread.php?t=140939&page=3

I have a concern when the math is no longer simple, ie., 10 min at 100 fsw and 10 min at 80 fsw to give 20 min at 90 fsw. The examples given on scubaboard usually simplify the problem, but the example you gave in the above thread hit the nail on the head and will help to illustrate my concern.

Here's a totally hypothetical example.... say at the first 5 minute mark, I'm at 100'. So my average is 100' for 5min.
Say at 10 min, I'm still at 100', so my average is 100' for 10 min.
I'm going to start heading up, so say at 15 min I'm at 80', making the average 90' for 15 min.
Another 5 at 80' makes the average 85' for 20 min. Being on EAN32, I'm still well within my NDL
At 25, I'm at 70'. At this point, I want to keep it simple in my head, so instead of saying 77.5 at 25, I'll call it 80 at 25.
Etc, etc....

10 minutes at 100 fsw and 5 minutes at 80 feet gives 15 minutes at 93.3 fsw, not 90 fsw.

10 minutes at 100 fsw, 10 min at 80 fsw, and 5 min at 70 fsw gives 86 fsw, not 77.5 or 80 fsw.

Here's how I calculate the average depth: 100x10/25 + 80x10/25 + 70x5/25 = 86 fsw.

What you seem to be doing is averaging your current depth with your previous average and ignoring the time spent at each. You did state that some error doesn't matter, but I still look for as much accuracy as I can get.

What do you do if you spend 25 min at 80 fsw and then 5 min at 40? 30 min at 60 is far from the ndl but 30 min at 73.3 fsw is close.

What about 8 min at 80 fsw, 1 min at 70 fsw, and then 2 min at 50 fsw? Do you plan your dive so that it is in 5 or 10 minute increments at each depth to make depth averaging easier?

Thanks in advance. I'm hoping your answers will help clear it up for me as I would like to start trying depth averaging without bringing a laptop with me.

Cheers,

Don't forget the rest of it. Depth averaging is only a piece of the puzzle and it's not precisely accurate. For me, it is close enough. A few feet either way will only get you bent if you blindly follow someone else's deco without knowing why they are doing what they're doing. This includes allowing a computer to run it for you.

As I said in the thread - it's a rough average and it's a running average. The snapshots are every 5 minutes, not 10. And not 8, then 2, then 4, etc... The "profile" you have above is not a dive I would do if I could help it. Part of the whole concept is paying attention to your dive. If I were to spend 25 minutes at 80', then for some reason wanted to get to 40', it would take me most of the next 5 minutes getting there and no, I would not call it 60' for 30 minutes. I would have paid attention to what I was doing and called the average more like 70 or 75'.

What I would do is note that I got shallower than 3 ata at about 30 minutes. Once I got to 2 ata, I would make sure to spend that 30 minutes ascending ever slower and slower and slower to the surface.... shaping the curve

It would be impossible for me to teach you to do this via the internet. This is only a brief outline of what I do and does not include all the pieces. It would also be unwise for you to try it without understanding it just because you read about it here.

 

[TSandM]

Part of the issue here is that DECOMPRESSION IS NOT A PRECISE THING. I can't remember which recent thread it was, but we spent some time talking about the differences among various decompression algorithms, different tables, and different computers. There was up to an 18 minute difference in NDL among models. This is the problem with dividing dives into "no decompression" and "decompression". It gives you the idea that you are "safe" to proceed directly to the surface if you are at 120 feet for nine minutes, but damned if you do so after eleven, and that simply is not true. Under the right circumstances, and even given a correct standard ascent, you may get bent at 9; under other circumstances, you may be fine at 11.

It makes much more sense to me to think about dives in terms of loading, as I think Charlie99 wrote. The dive I did yesterday with a max depth of about 25 feet resulted in pretty minimal loading, and I could have gone directly to the surface at any point -- probably even at an excessive rate of speed -- with little risk. When you are pushing your NDLs at 80 feet, it's a different story, and you should be far more cautious with your ascent rate, and introduce stops. Exactly what stops and for how long can be argued, and are.

Depth averaging techniques, if done carefully, probably don't introduce much more variability than exists among the models . . . but doing depth averaging and on the fly deco requires diligence, attention, education, and diving skills. GUE, the "no-computer" people, don't just tell you to throw the tables out the window . . . they teach procedures appropriate to recreational dives to recreational divers, and don't introduce the more complex ideas until they are teaching technical classes, where people are assumed to have a different level of education and skill.

But, as Snowbear said above, all dives ARE decompression dives, and if you turn your thinking around that way, you can begin to build a knowledge base and skill set that will work for any diving.

 

[onfloat]

As a recreational dive, I would plan 25/25 for the gas, and plan to keep my average depth a bit shallower to increase bottom time without exceeding MDL. This isn't a GUE standard gas, but it's within their standards for gas mixes.

As a Tech dive, I'd do 25/25 for 35 minutes of BT at an average depth of 120', and do 20 minutes of deco on a 50% bottle.

I'm not sure why you thought onfloat made the statements I did, and for the record, I dive with the heretics (i.e. AG with NAUI) now. I did DIR-F and RecTriox with GUE, and the numbers in my original post are from those classes.

~ Jason

Jason,
I just took the tech diver class with Joe here on Oahu. Big discusion about 25/25 being used in that class after my class report.

Proud to dive with the heretics,
Onfloat

 

[onfloat]

So what GUE standard gas would you use for the Sea Tiger if you did intend to go over to the floor at 127'?

Isn't it a bit deep for 30/30? Does recreational triox cover deco planning for other standard GUE gases suitable for the Sea Tiger at 127', or are you supposed to get additional training beyond Rec Triox before doing such a dive?

Yep, I'd be using 25/25 as taught by 5thd-x for a standard gas.

 

[darkpup]

Jason,
I just took the tech diver class with Joe here on Oahu. Big discusion about 25/25 being used in that class after my class report.

Proud to dive with the heretics,
Onfloat

Yep, we had the same lecture in Part 2 of our class this past April. I've seen the material in my RecTriox and 2nd DIR-F class, but the way it was explained in the NAUI course really brought it home. Either that, or I was finally ready to hear it. Sometimes this stuff takes time to sink in.

~ Jason

 

[MXGratefulDiver]

**** MOD POST ****

OK all ~ most of the non-DIR 'answers' to the original post, as well as the ensuing discussion, have been split off and moved over here so that discussion could continue.

A few posts were irrelevent and out of place after the split, so were deleted

And of course ~ the requisite reminder of the DIR forum 'special rules'

**** END MOD POST ****​

Oh?

I didn't realize that doing a 127 foot dive on an AL80 was a "DIR answer".

... Bob (Grateful Diver)

 

[*Floater*]

It would be impossible for me to teach you to do this via the internet. This is only a brief outline of what I do and does not include all the pieces. It would also be unwise for you to try it without understanding it just because you read about it here.

I think you could teach it via the internet if you wanted to.

 

[Snowbear]

Oh?

I didn't realize that doing a 127 foot dive on an AL80 was a "DIR answer".

... Bob (Grateful Diver)

It's not and I don't think anyone in this thread said it is. The posts advocating that answer were split and moved. If I missed a post actually advocating such a dive, please point them out so they can join the other thread

The original poster asked for DIR answers. It's up to her to accept or reject the ones she got. It seems she is more inclined to the latter than the former.... Again - it's her choice.

Hopefully someone else, somewhere else is going to stumble across this thread some other day and glean some useful information from it, though

 

[jagfish]

Nice cleanup Snow...

I think it's hard for folks to appreciate how much effort it takes to unravel and reorganize a long thread like this one...

 

[MXGratefulDiver]

Nice cleanup Snow...

I think it's hard for folks to appreciate how much effort it takes to unravel and reorganize a long thread like this one...

Actually, I do appreciate it.

I just don't see why someone would come here and post a thread titled "DIR Answer Required", and then so obviously hijack her own thread with non-DIR answers.

Well, actually I do ... I just thought it wasn't allowed here.

The post I was referencing ...

http://www.scubaboard.com/showpost.php?p=1815420&postcount=89