Test data for conservative computer?

[gcbryan]

What/where is the test data that conservative computers are based on? The Navy tables are the basis for the tables used in most OW classes. Most test data was collected to complile those tables was it not? The liberal computer more or less reflect the OW tables. Conservative computers are based on what?

Other than just reducing a NDL number to make a conservative computer was there any actual data involved to base these numbers on?

I recently looked a two computers in plan mode where both computers were cleared of any nitrogen from any previous dive. At 60 fsw one computer allowed :57 of NDL time and the other :45. As the depths increases the differences become smaller, by 160 fsw they were both the same :07. At 30 fsw one allowed 200 minutes and one 260 minutes.

I understand that this difference is an argument against dive computers and for bottom timers, for using your brain and for planning your dive before you dive. But what is Vplanner based on?

I also understand that most liberal computers are only liberal for the first dive and would be the most conservative computer in many cases for the second dive. For those just doing one dive a day however different computers give wildly different numbers. I'm just wondering if there is any test data involved in the more conservative numbers or is it just pulling numbers out of one's *****?

I've know about these differences for a long time but only recently really compared two computers directly. This question doesn't really affect me that much either since most of my diving is deep followed by substantial time shallow so both computers allow me to do my usual dives. This is mainly about intellectual curiousity concerning the basis for both types of dive computers.

 

[in_cavediver]

What/where is the test data that conservative computers are based on? The Navy tables are the basis for the tables used in most OW classes. Most test data was collected to complile those tables was it not? The liberal computer more or less reflect the OW tables. Conservative computers are based on what?

Other than just reducing a NDL number to make a conservative computer was there any actual data involved to base these numbers on?

I recently looked a two computers in plan mode where both computers were cleared of any nitrogen from any previous dive. At 60 fsw one computer allowed :57 of NDL time and the other :45. As the depths increases the differences become smaller, by 160 fsw they were both the same :07. At 30 fsw one allowed 200 minutes and one 260 minutes.

I understand that this difference is an argument against dive computers and for bottom timers, for using your brain and for planning your dive before you dive. But what is Vplanner based on?

I also understand that most liberal computers are only liberal for the first dive and would be the most conservative computer in many cases for the second dive. For those just doing one dive a day however different computers give wildly different numbers. I'm just wondering if there is any test data involved in the more conservative numbers or is it just pulling numbers out of one's *****?

I've know about these differences for a long time but only recently really compared two computers directly. This question doesn't really affect me that much either since most of my diving is deep followed by substantial time shallow so both computers allow me to do my usual dives. This is mainly about intellectual curiousity concerning the basis for both types of dive computers.

What you are seeing is different decopression models at work. All the dive tables you see are created using one of the mathematical models available. Different models have different attributes and will generate different profiles and hecnce different tables. Some such as the PADI DSAT tables are strictly no deco diving while others such as Buelman's ZL-16C allow it. Instead of rehashing the different models, search the archives/google with these for more information: RGBM, Haldane, Buelmann, Pyle, Wienke, Baker, Bubble Model

 

[gcbryan]

What you are seeing is different decopression models at work. All the dive tables you see are created using one of the mathematical models available. Different models have different attributes and will generate different profiles and hecnce different tables. Some such as the PADI DSAT tables are strictly no deco diving while others such as Buelman's ZL-16C allow it. Instead of rehashing the different models, search the archives/google with these for more information: RGBM, Haldane, Buelmann, Pyle, Wienke, Baker, Bubble Model

I understand (within reason) RGBM (bubble model) and have read some of Bruce Wienke's work and I understand Pyle stops. I don't understand why there is this large a difference in the no deco times with different rec computers.

 

[AndyNZ]

I understand (within reason) RGBM (bubble model) and have read some of Bruce Wienke's work and I understand Pyle stops. I don't understand why there is this large a difference in the no deco times with different rec computers.

Off the top of my head, likely reasons are going to be:

1. using tissue compartments with different half times
2. using more/less numbers of tissue compartments
3. different washout rates for tissue compartments
4. different m-values for tissue compartments

I don't think it's particularly surprising to see such variation in dive computers once you realise the various parameters.

 

[scubamickey]

I understand (within reason) RGBM (bubble model) and have read some of Bruce Wienke's work and I understand Pyle stops. I don't understand why there is this large a difference in the no deco times with different rec computers.

Then you don't understand Wienke's work or what bubble mechanics in general is. Try reading some basic books on diving physics before you tackle decompression algorithyms. It's the only way to really understand what you are seeing.

 

[gcbryan]

Then you don't understand Wienke's work or what bubble mechanics in general is. Try reading some basic books on diving physics before you tackle decompression algorithyms. It's the only way to really understand what you are seeing.

Since you do understand and have read the books I thought that maybe you or others could answer the questions.

I understand that you can setup and measure many or fewer compartments but if the useful numbers within recreational diving vary that much then the numbers and the ways that they are measured border on being useless within the recreational realm.

 

[scubamickey]

Since you do understand and have read the books I thought that maybe you or others could answer the questions.

I understand that you can setup and measure many or fewer compartments but if the useful numbers within recreational diving vary that much then the numbers and the ways that they are measured border on being useless within the recreational realm.

The problem is that you are dealing with very advanced physics and mathematics. It's not black and white and in the end it's all theories--albeit theories from geniuses in the field like Weinke. It's impossible to answer the questions without going into a long discussion. There are a couple of folks on the forum that are good at simplifying the subject (and I don't mean that in a bad way, we are talking about heavy stuff here). Maybe one of them will respond here if we keep this thread alive.

In the meantime, I am trying to find some resources for you that you can look at yourself and maybe help with your questions. For me it was Weinke's work. But I can't find any online summaries of his work right now. I'll keep trying.

 

[gcbryan]

The problem is that you are dealing with very advanced physics and mathematics. It's not black and white and in the end it's all theories--albeit theories from geniuses in the field like Weinke. It's impossible to answer the questions without going into a long discussion. There are a couple of folks on the forum that are good at simplifying the subject (and I don't mean that in a bad way, we are talking about heavy stuff here). Maybe one of them will respond here if we keep this thread alive.

In the meantime, I am trying to find some resources for you that you can look at yourself and maybe help with your questions. For me it was Weinke's work. But I can't find any online summaries of his work right now. I'll keep trying.

In my experience those who really understand anything can explain it to other's without the jargon of that field. I had to learn a lot of concepts when learning to fly a helicopter but I can explain those same concept to others without having to use complicated concept how that I understand them.

I'm not picking on you by the way. I just find it hard to believe that so many on this board claim to understand and yet no one really seems to be able to explain.

I'm not asking about going into a detailed explaination of dissolved vs free phrase, pressure gradients, how gas moves within the body, how one deco model can call for shorter times but deeper stops and another calls for longer times but shallower stops.

I just saying that for computers that are mainly recreational in nature for one to indicate that one is has a deco obligation while another says you have 20 minutes before you have a deco obligation is to make a recreational dive computer almost meaningless.

 

[bradshsi]

I just saying that for computers that are mainly recreational in nature for one to indicate that one is has a deco obligation while another says you have 20 minutes before you have a deco obligation is to make a recreational dive computer almost meaningless.

I don't see how you can make that generalization. Computers and tables are all approximations of reality. Because we don't know all the factors that can trigger DCS (nor can any computer take into account things such as fitness, hydration, obesity etc), different computer makers have added different fudge factors to avoid the possibility of bending too many people.

If they truely were meaningless, than why should anyone buy one ? The answer is that a flawed approximation is better than no answer at all.

An example of this is back in the 80's when we were designing PWR nuclear reactors, we didn't have good computer models for 2 phase flow. You were lucky if you could get within 50% of the actual measured value. But we needed to know for safety reasons, so you used the best computer model you had and added a big enough factor to avoid frying your core. It's the same principle here.

 

[gcbryan]

I don't see how you can make that generalization. Computers and tables are all approximations of reality. Because we don't know all the factors that can trigger DCS (nor can any computer take into account things such as fitness, hydration, obesity etc), different computer makers have added different fudge factors to avoid the possibility of bending too many people.

If they truely were meaningless, than why should anyone buy one ? The answer is that a flawed approximation is better than no answer at all.

An example of this is back in the 80's when we were designing PWR nuclear reactors, we didn't have good computer models for 2 phase flow. You were lucky if you could get within 50% of the actual measured value. But we needed to know for safety reasons, so you used the best computer model you had and added a big enough factor to avoid frying your core. It's the same principle here.

The interesting thing here I guess is that I'm not aware of any more incidents of DCS among any one of the rec models compared to the others so the gray area may be more of a line than a large gray area or there would be more hits among the more liberal dive computers.