average depth and tables?

[svs]

I guess that problem may happened from a fact that safety stop will be incorporated into average on most computer logs but not accounted in tables.

 

[atedeschi]

Ianr33 that is what I was trying to state and I believe you stated it better.

To the op idk why you need a formula just take the average depth from the bottom portion of your dive.

Right before I make my ascent I look up what my average depth is and then time and pull out my tables and follow the deco schedule.

I also started slow with this method comparing it to my dive computer and it was pretty close or even more conservative sometimes. But you have to use your brain and think about what your doing. I would do a dive like ianr33 posted as the second example.

May I ask why you want to do average depth tables?

 

[RTee]

I would tend to think that experimenting with computer and tables should assist you with your query. If you are into the habits of doing repetitive dives, use the average depth you get from your computer with the dive time of your first dive, plug these into the tables or RDP and see what NDL you get for a subsequent dive at different depths and compare with what the dive planner function of your computer gives you. I did this exercise a couple times in the past and I ended up pretty darn close to what Suunto was limiting me on my second dive.

 

[Blackwood]

I think Walter is referring to the fact that using dive tables the way they were designed won't necessarily prevent decompression injuries, and the more liberties a diver takes with a table, the greater the likelihood of error.

Not to be argumentative, but: source?

I will agree with the notion that using a more aggressive methodology is riskier by table definition, but am genuinely curious if there have been any studies into DCS events resulting from using tables in ways other than suggested by their instruction sets.

 

[Thalassamania]

can you use the average depth from your computer to log dives with tables.

Lets not get into why I want to do this because it is not really relavant.

Would you consider this safe enough to use to make a subsequent table dive?

No.

Source?

The folks who cut the table perhaps?

I have been using average depth for almost two years now. I just take from the bottom timer and use those tables. Most of my profiles are cave type therefore same in and out depths or saw tooth ones on wrecks where I'm up in down going inside down to say enigne room then back up to a deck above then down in to a cargo hold.
My max depths so far doing this have only been to 210' and beening doing it on closed and open circuit.
Buddy of mine have been doing this for years with max depths over 300'.

I recommend the bt and/ or a tec 2g they are the only ones on the market that do average depth on the fly.

You're rather lucky.

So, just so I understand the question, we're trying to determine if it would be ok, for instance, to take a dive that was 20 minutes at 100' and 20 minutes at 50' and base our next dive as if it were a 40 minute dive to 75'?

Since the pressure effects of sea water are linear with depth (not sure if the resultant effects of nitrogen loading are correspondingly linear though) then shouldn't this averaging be valid?

I'm not that experienced so consider all of this pretty much an addition to the OP's question.

Pressure varies directly with a change in depth, but the nitrogen uptake is not a linear function, it is exponential and the controlling tissue jumps around depending on the depth and the time.

It would be more accurate to say that the tables were not validated using average depth, not that you are violating model assumptions. All the models assume ongassing is a function of inspired gas pressure which is explicitly a function of depth. Assuming the gas load is a function of maximum depth for a multilevel dive violates the models. It is a conservative approach, but wrong. If most of the dive is spent above the maximum depth the gas load will be less than assumed by the tables.

The lack of validation likely has more to do with the difficulty in designing the experiment and limited resources. Ratio deco advocates have been diving using depth averaging long enough to say that there has been a practical validation. And of course the bottom time provided by many computers on a multilevel dive looks a lot like a depth averaged dive. But a quantitative statement cannot be made about the relative safety of deco planning using depth averaging because the work has not been done. The number of dives performed by ratio deco advocates has likely exceeded the number of dives performed to validate the “official” methods. There does not appear to be a high bends rate on RD but then no one is keeping count so it is hard to know.

Of course you should know what you are doing before going out and trying this.

You are violating model assumptions which are always a bit more complex than just, "ongassing is a function of inspired gas pressure which is explicitly a function of depth." The purpose of a set of tables is to minimize the possibility of DCS so your statement: "Assuming the gas load is a function of maximum depth for a multilevel dive violates the models. It is a conservative approach, but wrong. If most of the dive is spent above the maximum depth the gas load will be less than assumed by the tables." is NOT correct. Rather, it is a good idea. The table model exists to draw a line that you should not cross, not to assure that you have the maximum bottom time available without getting bent. If you want to use a different model, knock yourself out ... there are plenty out there, but applying a table to a set of conditions that it was neither designed nor tested for is playing Russian Roulette with your spinal cord.

It seems like there should be a formula that would provide an accurate "average" dive depth. Obviously not really an average in the common sense of the word. But a depth that can be used to plan the next dive based on the times and depths of the previous dive.

When you make a repetitive dive calculation you have to throw away a bunch of information. You can really only use an evaluation of the saturation of the single compartment that, based on your activity, is most likely to give you trouble.

Not to be argumentative, but: source?

I'm genuinely curious if there have been any studies into DCS events resulting from using tables in ways other than suggested by their instruction sets.

Not to be flippant, but what you are asking for is a study of the efficacy of astrological decompression.

 

[atedeschi]

I dnt believe it's luck. If you take the depth and times from a dive computer every 15 or 30 seconds how ever it records and take the average depth of the bottom portion of your dive I bet they are pretty close.

 

[SailNaked]

to be honest I was not considering using it to plan a dive with, as in, plan to use the average as a means of determining deco for that dive.

I was more interested in using a series of dives downloaded from my computer and then logging (converting) them using tables and the average depth from the computer over the past say 24-36 hours to then make a table dive. I think from the replies I'm going to die, be fine.

Good to know that inverse profiles are bad for this kind of thing.

I have tried to convert computer dives to tables using max depth and this is meaningless as many are deco dives on the tables and it would take hours to do a reasonable job plotting them as multilevel.

 

[Thalassamania]

I dnt believe it's luck. If you take the depth and times from a dive computer every 15 or 30 seconds how ever it records and take the average depth of the bottom portion of your dive I bet they are pretty close.

The reality is that decompression models draw a line, and lines are not real, biology is fuzzy. Just because you did used a concept that is based on coming with a reasonably close approximation of a true multilevel calculation, and so far that has kept you on the "safe" side of the fuzz, doesn't mean that that will be the case next dive, or the dive after. You have, in fact, been lucky. My spine deserves more than "pretty close."

 

[Mr Carcharodon]

You have, in fact, been lucky. My spine deserves more than "pretty close."

So where exactly are all the crippled people that used depth averaging? If it is as dangerous as you suggest they must be legion.

I am sure all the sucessful dives have just been a coincidence.

 

[Thalassamania]

Depth averaging is what you'd call a fortuitous fit to multi-level calculations, it has not got a basis in decompression theory. It does seem to "work" most of the time, just when it's not going to work is somewhat unpredictable ... most of the folk I know who use averaging do so with lots of rather arcane "personal" adjustments that are based on what they perceive to be excursions into the fuzzy area that resulted in sub-clinical DCS. It's not a risk I'd choose to take, I've grown quite attached to bowel, bladder and sexual functions, not to mention ambulation.

But that was not the OP question anyway.