average depth and tables?

[ucfdiver]

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.

Not really...tissue loading to depth isn't a linear relationship, but within certain parameters, we can simplify things and maintain a reasonable level of risk. 10min @ 20ft then 10min @ 100ft isn't anywhere near the same as 20min @ 60ft, look at the tables and it will sort of make sense. Neither are a deco dive, but you get the point...

 

[Herk_Man]

Ya, I knew it didn't feel right. When you take it to the extremes, something doesn't add up.

 

[atedeschi]

I guess I should add you should use sone common sense doing it this way. Most of your averages on normal dives are 10 to 20ft different then the max depth. If it is more then that you have to use judgement and see wat ur profile looks like.

 

[ucfdiver]

Ya, I knew it didn't feel right. When you take it to the extremes, something doesn't add up.

The UTD online classroom is excellent for expanding what you were taught in almost any level of diving course on how your body handles gas loading. It's well worth the money. I think the ratio deco for basic open water diving explained in that course is the best way to teach new divers that I've heard.

 

[Mr Carcharodon]

So, if you plan your dives, and especially repetitive dives, using average depth instead of maximum depth, you are violating one of the assumptions of the model, .

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.

 

[Trace Malinowski]

Source?

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.

Most tables warn against using them in creative ways. I would consider the manufacturer's warnings to be a valid source.

 

[ianr33]

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

While I would be fine with averaging depth during the bottom section of a dive,using computer average depth for an entire dive seems pretty dubious.

e.g. Spend 10 minutes at 100 feet.Ascend to 80 feet for another 10 minutes and average the depth as 90 feet for 20 minutes. No problem.

20 minutes at 100 feet followed by 20 at 10 feet gives an average of 55 feet for 40 minutes . Using that as an average is a Bad Idea. Diving air, 55 for 40 is well with NDL but in reality the dive was right up to NDL

 

[Web Monkey]

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

Let us know how it works out for you, then we'll have an answer the next time someone asks.

Terry

 

[Herk_Man]

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.

 

[Rainer]

20 minutes at 100 feet followed by 20 at 10 feet gives an average of 55 feet for 40 minutes . Using that as an average is a Bad Idea. Diving air, 55 for 40 is well with NDL but in reality the dive was right up to NDL

Not the greatest example. You just did TWENTY minutes of back gas deco at 10' (for a bottom portion that itself didn't violate NDLs). You're going to be just fine.