Average Depth Diving?

[mempilot]

Understood, but the post was about multilevel diving where some significant portion of the dive is spent at shallower levels, potentially less than 50'. And, you agree that you need to know the residual N2 for repetive diving.

Time spent shallow will primarily impact slow compartments which have long NDLs. As long as you're not doing a lot of repetitive multi-day diving (e.g. liveaboards) those will not be controlling compartments. The faster compartments have higher m-values and are already probably loaded up to the partial pressure of that depth. If you just came up from a 100 fsw dive for 30 mins on EAN32, what is happening to your slower tissues at 50 fsw isn't going to start affecting your NDL for awhile, and unless you're taking a nap there, the "bottom time" and "ongassing" time of the dive can be assumed to be over -- you can hang out at 50 fsw and when you start to ascend the real issue is still decompressing those faster compartments which were loaded up at 100 fsw. The stop at 50 fsw made some slower compartments more loaded, and it shifted your controlling compartment slightly slower, but unless you've stayed there so long that you're running up against your NDL at 50 fsw the problem is still those faster compartments. Clearly you're still ongassing into slow compartments, but they aren't going to be important until you start planning your next dive and figuring residual nitrogen in slower tissues left after your SI...

 

[lamont]

one thing about depth averging is that mathematically you're more-or-less approximating an exponential with a straight line. that works *only* if you're doing small deltas around a large value.

so where depth averaging works is when you're looking at a segment of profile with a max depth and min depth which are actually fairly close, generally with outliers that are only brief and where the max depth does not come at the end of the profile segment. so, a dive where someone bounced down to 110 fsw and then made their way up to 90 fsw for an an average depth of 100 fsw works. the reverse profile going from 90 to 110 is not going to be the same. depth averaging an entire dive profile from 110 to 50 fsw doesn't work at all because the differences in how you're ongassing into different compartments there isn't close to linear.

gotta remember, though, that the people who depth averaging on-the-fly generally are okay if they make a 'mistake' and go into a few mins of deco, and they always do stops which are more conservative than what agencies recommend, and often spend 10 mins or more going from 30 fsw to the surface. they keep larger reserves of gas than normal recreational divers, practice good rock bottom rules and dive assuming that a straight ascent to the surface isn't an option. try to do depth averaging without having the rest of the stuff together, and you probably will be asking to get bent when you push it and both of you run OOA with a deco obligation...

 

[lamont]

Understood, but the post was about multilevel diving where some significant portion of the dive is spent at shallower levels, potentially less than 50'. And, you agree that you need to know the residual N2 for repetive diving.

most of the time you can just eat lunch, let the faster compartments offgas and do it again though. if you're doing dives where slower compartments are getting hammered, then you're going to need to worry about it more, depends on your profiles.

i haven't been on any liveaboards so, honestly, i just don't track residual nitrogen. on a liveaboard i might need to, but i'd probably pad my safety by using EAN32 and doing something like 1@30, 3@20, 5@10 stops...

 

[To_Narced]

Ok I've spent 2 days reading this thread, weighing all sides of the average depth diving argument and listening to some members call others "idiots". What I havent been able to figure out is what is the usefullness of average depth diving? I mean why would anyone want to do it? My closest dive buddy is a GUE tech diver who had learned to calculate deco on the fly and yet he would only use this skill in the event of a unforseen incident. An example would be if he had to decend past his planned profile to assist a diver and in doing so violated his dive plan. Mabye I'm missing something here but as far as I can see there is no argument which can make average depth diving safer, or necessary in any diving situation besides a emergency. I hope by posting this I wont be labled a "idiot".

 

[miketsp]

Ok I've spent 2 days reading this thread, weighing all sides of the average depth diving argument and listening to some members call others "idiots". What I havent been able to figure out is what is the usefullness of average depth diving? I mean why would anyone want to do it? My closest dive buddy is a GUE tech diver who had learned to calculate deco on the fly and yet he would only use this skill in the event of a unforseen incident. An example would be if he had to decend past his planned profile to assist a diver and in doing so violated his dive plan. Mabye I'm missing something here but as far as I can see there is no argument which can make average depth diving safer, or necessary in any diving situation besides a emergency. I hope by posting this I wont be labled a "idiot".

I think that's quite a nice summary.
Why do it? What advantage does it bring?
As far as I can see, if people are doing it and getting away with it, then it's because they are diving a combination of SIT & profile that is already so conservative it leaves a lot of room for error.
Because there is no way I can accept that exponential functions can be approximated "on the fly".

After thinking about this for a while I really think that this thread should be pulled on the grounds of encouraging an irresponsible technique which does nothing to promote safe diving, one of the objectives of SB.

 

[mempilot]

All a bunch of experimentation if you ask me. There are no controls in place to ensure conservatism. A cx can do it, but there is no way a diver can tell me their exact average on a dive with any accuracy. The broader the depth range, the more inherrent error in trying to calculate this manually during a dive.

I'm surprised this thread hasn't been closed. Seems talking about overfilling a tank a bit is crime here, but promoting unsafe dive profiles is not. In theory, this sounds good. In practice, I doubt anyone would come close to a computer in tracking their N2 and O2 on a multilevel dive using average depth. I like to see someone's example pre-dive and then compare it to their computer post-dive.

 

[RTodd]

All a bunch of experimentation if you ask me. There are no controls in place to ensure conservatism. A cx can do it, but there is no way a diver can tell me their exact average on a dive with any accuracy. The broader the depth range, the more inherrent error in trying to calculate this manually during a dive.

I'm surprised this thread hasn't been closed. Seems talking about overfilling a tank a bit is crime here, but promoting unsafe dive profiles is not. In theory, this sounds good. In practice, I doubt anyone would come close to a computer in tracking their N2 and O2 on a multilevel dive using average depth. I like to see someone's example pre-dive and then compare it to their computer post-dive.

I can accurately tell you my average depth at each level of a dive within less than 5 feet. Your individual hydration during the dive matters more than a 5 foot difference in a in a theoretical deco model. Lamont has given a pretty good basic explanation of this. Until what he says seems completely obvious, no, you should not try using depth averaging because it is an advanced skill and requires a better "feel" and awareness of the dive profile than most divers have. Hence, the reason your OW instructor taught you the most conservative, fool-proof, method of using the maximum depth for tables. (Not a bad practice, just not necessary for more advanced divers. And, completely unpractical for dives at extreme depths.)

If you do actually want to start understanding this stuff, try planning, using 32% nitrox, multi-level dives with at least 20 minutes at 100' on the first dive and say 10-20 minutes deeper than 90 on all subsequent dives. (Shallower would work too, but this will show you that it works for pretty much all recreational dives.) Use any deco software you want. (Tables would probably work too, but I have never tried it.) Assume a one hour surface interval between dives and plan up to 5 dives. Try to find any material effect whatsoever of being shallower than 50'. (I am talking about nitrogen loading effects here, not the necessary stops in this range that are outside the scope of this topic.)

In doing this, you do have to keep the bottom times reasonable. So, in your gas planning, assume an aluminum 80 tank. My working SAC rate for recreational dives is between .3 and .4 if you want to use that. Although, an unrealistic .1 SAC would probably work or come close to working too - never had the need to find out. Amount of gas sets your max dive times more than the no-deco limits. You can even ignore any reserve planning for these purposes since the reserve air would start to be used to pad safety once you are above 50' anyway.

As for those blowing a fuse because I was poking fun at all of UPs dislcaimers, you may want to consider following the thread more carefully. While I will admit I think the fall out is kind of funny, it does detract from the thread and that was not my intent. So, I won't use any more disclaimers. I highly recommend you start putting into practice everything you read on the internet regarding decompression.

 

[boomx5]

Ok I've spent 2 days reading this thread, weighing all sides of the average depth diving argument and listening to some members call others "idiots". What I havent been able to figure out is what is the usefullness of average depth diving? I mean why would anyone want to do it? My closest dive buddy is a GUE tech diver who had learned to calculate deco on the fly and yet he would only use this skill in the event of a unforseen incident. An example would be if he had to decend past his planned profile to assist a diver and in doing so violated his dive plan. Mabye I'm missing something here but as far as I can see there is no argument which can make average depth diving safer, or necessary in any diving situation besides a emergency. I hope by posting this I wont be labled a "idiot".

Depth averaging, just like Ratio Deco is a tool. If you know how to safely use it, then it works just fine. It's not something we always do, but it is a tool at our disposal none the less. On the other hand, we plan all our dives using Ratio Deco and adjust to the "deco on the fly" if we need to.

I think some people are making a bigger deal out of this than there needs to be. Some people use it, and some people don't and arguing over it really is sensless.

 

[Walter]

Some people use it, and some people don't and arguing over it really is sensless.

True, there's no point arguing. Anyone wanting to use unsafe methods should feel free to do so. The only problem is when other people read posts and start thinking it's safe.

 

[Wijbrandus]

Where did you guys learn all this stuff? I have yet to be trained on anything beyond tables - though my training is strictly rec.

I'm assuming this is all advanced tech training stuff, but I'm wondering what classes specifically teach these alternate methods.

Very interesting reading, but I think I'll stick with my tables for now.