average depth and tables?

[Thalassamania]

Why you want to do it could be quite relevant. If the goal is just to transition from a computer dive to a table dive, one of the better methods I have seen is to check what NDL the computer is giving for various depths after your first dive, and compare that with your tables. You should be able to work out what pressure group you are in, and use that for subsequent table dives. The problem with using an average depth is that Nitrogen loading isn't linear with depth. If it was, tables and computers would both be a lot simpler. The order you do the depths matters too. If you don't believe that, you could do your deco stops at the beginning of the dive instead of the end... I really wouldn't recommend that as an experiment, unless you have some goats or Navy ratings to try it out on.

For details, see the papers in the AAUS Dive Computer Workshop Proceedings by Sharkey and by Emmerman.

 

[TSandM]

Thal, I think you're being a bit too stiff-necked about the RD thing. Nobody I know who is using RD is adjusting "by the phase of the moon". The algorithm generates times and you follow it. Some people I know who have had problems with DCS pad the shallow deco -- I believe I have read posts from people cutting tables who do the same thing.

The fact is that nobody really knows what is going on in the body with decompression, and ALL tables used for staged decompression diving are based on mathematical models of the behavior of gases in fluid and bubble complex systems. Some thing which seem extremely attractive and intuitive are turning out not to work as well as was thought. I was surprised, in reading the proceedings of the DAN Symposium on technical diving, what some of the research, both with Doppler and symptoms, is saying about the deeper stops called for by bubble model algorithms.

Just because you very precisely generate your tables on your laptop, doesn't mean the profile is any more "accurate" (in terms of what is happening to gas in your body) than someone who is using a different decompression program, or DOTF. All of them work, most of the time; all of them get people hurt, some of the time. Until one starts to look consistently risky, it's hard to say which is best and which is worst.

 

[Thalassamania]

Thal, I think you're being a bit too stiff-necked about the RD thing. Nobody I know who is using RD is adjusting "by the phase of the moon". The algorithm generates times and you follow it. Some people I know who have had problems with DCS pad the shallow deco -- I believe I have read posts from people cutting tables who do the same thing.

The fact is that nobody really knows what is going on in the body with decompression, and ALL tables used for staged decompression diving are based on mathematical models of the behavior of gases in fluid and bubble complex systems. Some thing which seem extremely attractive and intuitive are turning out not to work as well as was thought. I was surprised, in reading the proceedings of the DAN Symposium on technical diving, what some of the research, both with Doppler and symptoms, is saying about the deeper stops called for by bubble model algorithms.

Just because you very precisely generate your tables on your laptop, doesn't mean the profile is any more "accurate" (in terms of what is happening to gas in your body) than someone who is using a different decompression program, or DOTF. All of them work, most of the time; all of them get people hurt, some of the time. Until one starts to look consistently risky, it's hard to say which is best and which is worst.

I agree with you completely, and yes "phase of the moon" is hyperbole. My point is rather simple, RD fits a curve to a model that is already a fit, so you have a fit of a fit and then you make it "safe" by dailing in safety factor(s) based on a subjective interpretation of how subclincal DCS is making you "feel." Frankly, I have little problem with using this approach for a single dive. However, using this for repetitive dives or pretending that it is based on science ... those are real issues, and pushing it off on folks who don't have the background to tell an m-value from a glass of milk gives me informed consent heartburn.

 

[Blackwood]

I agree with you completely, and yes "phase of the moon" is hyperbole. My point is rather simple, RD fits a curve to a model that is already a fit, so you have a fit of a fit and then you make it "safe" by dailing in safety factor(s) based on a subjective interpretation of how subclincal DCS is making you "feel." Frankly, I have little problem with using this approach for a single dive. However, using this for repetitive dives or pretending that it is based on science ... those are real issues, and pushing it off on folks who don't have the background to tell an m-value from a glass of milk gives me informed consent heartburn.

I have to assume that there is a lot of wiggle room in deco models. I'd suggest as evidence the fact they spit out stops in 10' or 3m intervals at whole minutes. If the precise magnitude of pressure gradients, tissue tensions, etc. mattered, stops would be all over the board. "Ascend 26.385m and level off for 7 minutes 22 and-one-half-seconds." But convenience overrides precision.

Close enough seems to work, and within the prescribed confines*, it greatly simplifies my own diving. Maybe it's riskier than diving a traditional table, but I'm willing to take that risk.

*I wouldn't, for example, use it to plan a 2 hour cave dive.

 

[Thalassamania]

I have to assume that there is a lot of wiggle room in deco models. I'd suggest as evidence the fact they spit out stops in 10' or 3m intervals at whole minutes. If the precise magnitude of pressure gradients, tissue tensions, etc. mattered, stops would be all over the board. "Ascend 26.385m and level off for 7 minutes 22 and-one-half-seconds." But convenience overrides precision.

Close enough seems to work, and within the prescribed confines*, it greatly simplifies my own diving. Maybe it's riskier than diving a traditional table, but I'm willing to take that risk.

*I wouldn't, for example, use it to plan a 2 hour cave dive.

On one hand you are quite correct, there's wiggle (that's what I called fuzzy at the start of this thread). Ten foot stops are an artifact of staged decompression, not the model. The model could be used to compute 5 ft stops, 1 foot stops, or smooth profile deco, but as you observed, "convenience overrides precision." Actually, within the normal constraints of bottom mix choice, single deco gas, fairly short deco, and "personal" adjustments, RD normally yields a deco schedule that is slightly more conservative ... but for repetitive diving (or anything outside of the aforementioned limitations), I feel that RD may be a tar-baby.

My only real interest in it, is as a way to deal with a blown schedule.

BTW: for those who are interested, there a pretty good RD write up on wiki as well as this PDF from 5th D-x.

 

[Trace Malinowski]

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?

Here are a couple of problems for the board to help determine the safety of this practice. Use dive tables for the first two problems and ratio deco for the third. Given these problems, do you think taking the average depth from the first dive would be a safe way to plan a repetitive table dive? Why or why not?

Problem #1

A diver descends to 140 feet for 25 minutes using air. According to the US Navy Standard Air Decompression Table, what decompression stops must he make? What is his ascent time? What is his total run time? What is his letter group upon surfacing? What is the average depth of the dive? Comparing that depth average and dive time to the US Navy Tables, in what letter group would he be considered upon surfacing? Is this more conservative or less conservative than the letter group originally given?

Problem #2

A diver descends to 100 feet for a bottom time of 50 minutes using air. What decompression stops must she make? What is her ascent time and total run time? What letter group is she upon surfacing? What is her average depth? What letter group would she be if her dive was calculated based upon the average depth of her dive computer?

Problem #3

A Tech 1 diver using 21/35 with Nitrox 50 for decompression descends to 150 feet for 10 minutes then ascends to 130 feet for 20 minutes, then ascends to 120 feet for 10 minutes. Using depth averaging, construct her decompression and ascent profile. What will her UWATEC digital depth gauge/bottom timer read as her average depth upon surfacing?

 

[atedeschi]

U do not want to use average depth upon surfacing but you want average depth before you ascend from the bottom.

 

[Trace Malinowski]

U do not want to use average depth upon surfacing but you want average depth before you ascend from the bottom.

Is that what the OP is asking? I'm reading his posts to mean he wants to take the average depth displayed after the dives.

 

[atedeschi]

If that is true then it will not work.

Op can u explain what u want exactly.

 

[Blackwood]

I'm reading his posts to mean he wants to take the average depth displayed after the dives.

Hmm, that didn't even occur to me. You could be right, though.