Diving at Altitude, Compensate or not?

[NJMike]

Holy bubbles, Batman!

I am not experienced enough to figure out how and when to 'wing it' or 'pad' or 'add a little extra' and 'should be okay'.

I'm a 'by the book' kind of guy. I believe in this: that the people who spent countless hours researching, studying, and analyzing data know far more than I do. And when the result of that is a table or two, then that's the way to go (for me). Not following the tables is akin to saying "I haven't done any research, and hence have not analyzed any data, but my knowledge is sufficient to know when to override the results of your knowledge..."

Bottom line: I'm too scared to not follow the tables!

But hey, how about a follow up question? Suppose you live at sea level, but are going diving at 6000'. After diving (by the tables), it's time to start home. If the return trip home was always downhill.....no problem.

But suppose it went downhill from 6000' to 2000', but then back up (mountains?) to 4000', then all the way downhill to sea level.

Would that 2000' ascent in the middle of the trip be a factor?

PS: I have the NAUI Advance book open now and will be reading the section on altitude diving...

 

[Dr Deco]

Hello NJ Mike:

Ascent

The ascent before return to sea level would be a factor in DCS. How much depends on:

a. how large an altitude change was the ascent,
b. how long were you at the dive site before the ascent (an off gassing issue),
c. how fast was the ascent to the higher altitude (an off gassing issue).

All of these must be factored into the use of altitude/dive tables.

Tables, etc . . .

As has been mention many times on this FORUM over the years, tables are computed for the most sensitive individuals. A table with a 2% DCS incidence means that 98% of individuals could do that dive with no problem under the same conditions. you do not know if you are a “DCS-sensitive” diver until you have had a “hit,” however.

The range between “most susceptible” divers and “least sensitive” individuals is not a matter of a couple of feet, as many divers believe. It is quite considerable for altitude exposures, and I have not actually seen numbers for exposures to depth (i.e., determination of the NDLs in a laboratory).

Deco Models

All models are ultimately fitted to data derived from the field. The more cases of DCS that are available in the database, the more useful the data set for the determination of the model constants. One hundred “clean” dives are not as valuable as a few dives in which DCS presents in a portion of them.

As one can imagine, most recreational dives will not have any DCS hits. This means that the data is not as valuable as you might think simply because the modeler has collected, let us say, 1,000 dives. In fact, most dives are “bends free,” and thus are not as valuable as one might initially think. This makes deco modeling very difficult. Additionally, field dives are not to constant depths [generally] for a defined duration; the compartment gas loads must be individually calculated for each exposure.

If some degree of complexity is added, it will require considerable data to “fix” the constants. In some cases, one might argue that the model parameters are not really necessary and simply a pious fiction in the modeler’s mind. Simply because a model has a very complex mathematical structure does not mean that the underlying concept is correct. It might work because of the many fitting constants.

Models will often have computer programs analyse the data and then adjust the several constants to give the best fit. In some cases, the constants will assume values that do not appear to be realistic in the physical world. Most likely, this means that the computer is forcing the variable, and the interpretation [meaning] of that variable, in the modeler’s mind, is incorrect. It could also mean that something important has be omitted from the model. :huh:

Altitude

If the dive profiles are relatively simple, very often one can just dive a single table – to a first approximation – and have a favorable result. Most like, this results from the margin of safety in the table. This comment is only to give an explanation, and it is not to excuse sloppiness or suggest that the correct method [i.e., more complete use of several tables] is superfluous. :no

Dr Deco :doctor:

 

[NJMike]

I have decided to put off my Christmas week dive for a couple different reasons...one being that it is apparent that I need a little more prep/training.

Thanks for all the comments.