What is the standard?

[gcbryan]

What is the standard against which all computers/tables/short-cuts (ratio deco) are based upon? I understand that most are based on Haldane/Buhlman models with some fine tuning and some are RGBM influenced but regarding something like NDL's what is the starting point?

I also understand that there is no magic line but PADI tables show NDL's of 55 minutes and 20 minutes for 60 fsw and 100 fsw respectively. V-planner on a nominal setting shows 58 minutes and 16 minutes respectively. A Nitex Duo for instance shows 45 minutes and 16 minutes at those depths.

In decompression diving this issue is dealt with using gradient factors and other means of making a decompression schedule more or less conservative but that still has to be based on a nominal value (85% of what). Minimum deco addresses this by just including some deco in every dive. So my question has only to do with what are the nominal values rather than how they are actually dealt with.

Are the Navy tables the closest to a pure nominal printout of the times/depths for a pure Buhlman model?

Also, it's interesting that dive computer models that are the most conservative generally allow more settings to make that computer more conservative but not less. Liberal computers generally don't have settings to make them more conservative. Kind of goes against logic.

 

[Walter]

What is the standard against which all computers/tables/short-cuts (ratio deco) are based upon?

There isn't one standard.

 

[gcbryan]

There isn't one standard.

Ok, using Bühlmann ZH-L16 or whatever is the most commonly used algorithm what are the 60 fsw and 100 fsw NDL figures in the purest form of this algorithm?

 

[Charlie99]

There is no such thing as a "SAFE" table or dive profile or dive computer. It is simply a matter of higher or lower risk. The change in the risk of DCS is much more gradual than most divers would believe. In other words, 16 at 100' and 20 minutes at 100' have nearly the same risk of DCS. Bruce Wienke has some info on this in his book Technical Diving in Depth, For example to change the relative DCS risk by only a factor of 5, one would have to increase an 80' air dive exposure from 15 minutes to 60 minutes. This 15 to 60 minute range far exceeds the variation from the most liber to the most conservative computers, and far exceeds the full range of tables from conservative DCIEM to liberal USN was less than 2 to 1 risk factor).

http://www.scubaboard.com/forums/2783824-post7.html
has similar 5 to 1 risk factor limits for other depths --- for 100' air dive it is the outrageously large spread of 8 minutes to 50 minutes bottome time (USN table is 25 minutes). It sure makes the difference between 16 and 20 minute NDL look pretty small, doesn't it.

======================
On Buhlmann NDLs ......

At ftp://decompression.org/pub/Baker/
download the No-Stop Time pdf, as well as the Understanding M-Values pdf. That will give you the info needed to answer your question on 60' and 100' NDLs for ZH-L16a/b/c.

(That article doesn't have info on what the models assume for CO2 + H2O vapor pressure, but whether you use a low number of 1.6fsw or a high one of 2.0fsw doesn't affect the results much, particularly if you are interested in deeper than 40' NDLs).

You have to test each of the compartments to see which one reaches its limit in the shortest time at any given depth and breathing mix. That "controlling compartment" halftime will typically be around half or 2/3 of the NDL time --- i.e. it will be around 40 minute halftime for the 60' NDL, and will probably be the 10 minute compartment for the 100' NDL.

I've posted a spreadsheet that use that basic NDL equation to generate a table equivalent to the PADI RDP and wheel -- you could download that and modify it for your use. Or just crank out
t = (-1/k)*ln[(Pi - P)/(Pi - Po)] by hand for each compartment.

See ftp://decompression.org/pub/Baker/No-Stop Time.pdf for definitions of symbols. Remember that all pressure are absolute pressures, and don't forget factor in the inert fraction of your breathing gas to get inspired inert pressure.

Charlie Allen

 

[Dr Deco]

Hello gcbryan:

DCS Limits ?

It might seem surprising to divers, but there does not exist an actual bends-no bends limit. Unlike something like the boiling point of water, a physical DCS limit does not exist. Why might this be? Mainly, it is because DCS is the result of the growth of a gas phase within the body. The smaller the amount of separated nitrogen, the less will be the physiological effect. Limb pain, for example, does not go from none to painful with a small change in free gas load. In a similar fashion, this sensation will vary from diver to diver.

The no-decompression limits [NDLs] will vary from laboratory to laboratory as they reflect the responses of their test population. Where to set a “limit” is up to the table designers to determine. However, realize that there does not exist an actual bends/no bends break point.

Cushion?

We might see a similar effect were we to fix a room temperature for comfort. This would be different for a group from Maine compared to a group from the desert of New Mexico. There will be some flexibility.

All table designers wish to put some conservatism, some "cushion," for safety. How much this is will reflect what NDLs they use. There is also the more objective Doppler ultrasound bubble detection system that was used for the PADI tests. This is also influenced by the test population. It was not hundreds of thousands of divers.

The US Navy had one population when it worked out its table NDLs. Professor Buhlmann had a different group, as did I with the PADI dive test series. Military and commercial tables tend to be less conservative [longer bottom times] than recreation tables, and they reflect differences in treatment capabilities.

Dr Deco :doctor:

 

[gcbryan]

Hello gcbryan:

DCS Limits ?

It might seem surprising to divers, but there does not exist an actual bends-no bends limit. Unlike something like the boiling point of water, a physical DCS limit does not exist. Why might this be? Mainly, it is because DCS is the result of the growth of a gas phase within the body. The smaller the amount of separated nitrogen, the less will be the physiological effect. Limb pain, for example, does not go from none to painful with a small change in free gas load. In a similar fashion, this sensation will vary from diver to diver.

The no-decompression limits [NDLs] will vary from laboratory to laboratory as they reflect the responses of their test population. Where to set a “limit” is up to the table designers to determine. However, realize that there does not exist an actual bends/no bends break point.

Cushion?

We might see a similar effect were we to fix a room temperature for comfort. This would be different for a group from Maine compared to a group from the desert of New Mexico. There will be some flexibility.

All table designers wish to put some conservatism, some "cushion," for safety. How much this is will reflect what NDLs they use. There is also the more objective Doppler ultrasound bubble detection system that was used for the PADI tests. This is also influenced by the test population. It was not hundreds of thousands of divers.

The US Navy had one population when it worked out its table NDLs. Professor Buhlmann had a different group, as did I with the PADI dive test series. Military and commercial tables tend to be less conservative [longer bottom times] than recreation tables, and they reflect differences in treatment capabilities.

Dr Deco :doctor:

Thanks for the reply. Yes, I understand that there is no black and white line between no DCS and certain DCS. So it looks like the differences between tables or different computers is different degrees of padding as well as differences in the test populations. I understand that military and commercial dive operations have chambers onsite or nearby and are more liberal. I hadn't considered that the test population sizes were small enough that there would be large differences in raw date between different test groups.

The original reason for the question was just to attempt to figure out what the raw numbers were so that it would be easier to figure out what degree of padding was going on in each of the different computer models.

Thanks again.