Question Ratio : Where does the 30 msw / 20 min NDL come me from ?

[Tom Agneessens]

Hello,

Can you help me in finding the origin of the simplified rule that says that one can dive NDL (no-deco) for 20min @ 30msw on air / 45à50min @ 20msw ?

This seems to be a generally accepted rule of thumb in ratio calculations (Gue,UTD,ISE,...)
Except for the IANTD OW & NAUI tables, that come close to it, I cannot find the rationale behind it?

Thank you in advance

Tom

 

[rjack321]

Hello,

Can you help me in finding the origin of the simplified rule that says that one can dive NDL (no-deco) for 20min @ 30msw on air / 45à50min @ 20msw ?

This seems to be a generally accepted rule of thumb in ratio calculations (Gue,UTD,ISE,...)
Except for the IANTD OW & NAUI tables, that come close to it, I cannot find the rationale behind it?

Thank you in advance

Tom

DSAT air tables

 

[wnissen]

Hello,

Can you help me in finding the origin of the simplified rule that says that one can dive NDL (no-deco) for 20min @ 30msw on air / 45à50min @ 20msw ?

This seems to be a generally accepted rule of thumb in ratio calculations (Gue,UTD,ISE,...)
Except for the IANTD OW & NAUI tables, that come close to it, I cannot find the rationale behind it?

Thank you in advance

Tom

I can tell you where it doesn't come from, if that is helpful. At least in English, Haldane was the first to formulate a coherent theory of decompression. However, his original published tables show a no stop time of just 15 minutes for 20 m / 66 ft. Interestingly, that is the deepest depth for which he shows any no-stop time, so any time at 30 m / 100 ft. is regarded as a decompression dive. I believe that Haldane's tables have been revised based on Navy experiments.

Haldane's decompression model - Wikipedia

en.wikipedia.org

 

[Tom Agneessens]

Thank you.

 

[Stefan Liden]

Here is a recent article explaining the rationale:

Rules of Thumb: The Mysteries of Ratio Deco Revealed

Is it a secret algorithm developed by the WKPP to get you out of the water faster sans DCI, or an unsubstantiated decompression speculation promoted by Kool-Aid swilling quacks and charlatans? British tech instructor/instructor evaluator Rich Walker divulges the arcane mysteries behind GUE’s...

gue.com

 

[Subcooled]

Hello,

Can you help me in finding the origin of the simplified rule that says that one can dive NDL (no-deco) for 20min @ 30msw on air / 45à50min @ 20msw ?

This seems to be a generally accepted rule of thumb in ratio calculations (Gue,UTD,ISE,...)
Except for the IANTD OW & NAUI tables, that come close to it, I cannot find the rationale behind it?

Thank you in advance

Tom

It is an experimental fact.

Stay longer at those depths and the probability that you'll get bent grows too big.
Many beautiful models can be built, but they will always containt parameters to set - experimentally!

 

[Subcooled]

I can tell you where it doesn't come from, if that is helpful. At least in English, Haldane was the first to formulate a coherent theory of decompression. However, his original published tables show a no stop time of just 15 minutes for 20 m / 66 ft. Interestingly, that is the deepest depth for which he shows any no-stop time, so any time at 30 m / 100 ft. is regarded as a decompression dive. I believe that Haldane's tables have been revised based on Navy experiments.

Haldane's decompression model - Wikipedia

en.wikipedia.org

With no limits for ascent rate and with no safety stop?

Please bear in mind that the commonly used ascent rate used to be twice the current one.

His model is perfectly valid for goats

Please correct me if I am wrong!

 

[dmaziuk]

His model is perfectly valid for goats

Please correct me if I am wrong!

You aren't, it's perfectly valid for goats. Also men:

Haldane ran his experiments on some animals, illustrating the difference between different kinds of animals such as goats, guinea-pigs, mice, rats, hens and rabbits, but his main work and results were done on goats and men.

Wikipedia says so, so it must be true.

 

[lowwall]

It was mainly goats. But also mice, men, dogs, and other people's experiments and observations. Take a look, it's surprisingly easy to follow.

 

[Subcooled]

You aren't, it's perfectly valid for goats. Also men:


Wikipedia says so, so it must be true.

True, as the previous post by @lowwall shows.
Dive tables can be found at the end.

I was able to find a cleaner version (with OCR) here.