calculating the "No Stop Time"

[marc.collin]

hi

in this document, we can see the calculation for the NDL.
http://www.frogkick.dk/library/deco/Erik Baker/ndl.pdf

for a constant depth

t = (-1/k)*ln[(Pi - P)/(Pi - Po)]

We can read

The inspired pressure is the ambient pressure minus the water vapor pressure(Buhlmann value) times the fraction of inert gas:

Pi = (Pamb - PH2O)*FN2 = (40 msw - 0.627 msw)*0.79 = 31.1 msw

if we use nitrox 32, 0.79 should be 0.68?

The initial compartment pressure will be due to breathing atmospheric air at thesurface for a long time (first dive of the day):

Po = (Pamb - PH20)*FN2 = (10 msw - 0.627 msw)*0.79 = 7.4 msw

if it's not the first dive, what thing should be modified in this calculation?

is there any other way to calculate ndl?

is there any diving computer who use this calculation? surely we are never at the same depth... a little delta is possible

 

[SanDiegoSidemount]

There are a number of different ways to calculate this. I use Min Deco because it's so simple to figure out on the fly.

Min Deco has a "set point" of 100 feet / 20 min for air. (100 feet / 30 min for Nitrox 32). For every 10 feet shallower, add 5 minutes. For every 10 feet deeper, subtract 5 minutes.

Min Deco also presumes multiple stops on the way up. So stop for 1 minute at 50% of your average depth, and 1 minute every 10 feet above that to the surface. 30 FPM ascent rate.

This "set point" model works well for dives from 60-100+ feet. The model ends up being pretty conservative for shallow dives, so for anything 60 feet or less, you can also use depth+time = 110. In other words, 60 feet / 50 mins, 50 feet / 60 mins and so on. Same stop strategy: 1 min at 50% average depth and every 10 feet shallower.

Been diving this way a while now, and works well. A lot simpler than all the crazy math above.

 

[EFX]

Answering your questions in order:

If we use nitrox 32, 0.79 should be 0.68?

Yes. When diving nitrox substitute the decimal fraction of nitrogen for FN2.

If it's not the first dive, what thing should be modified in this calculation?

The NDL of any repetitive dive is going to depend on the residual nitrogen loading from previous dives. Using the constant depth equation P = Po + (Pi - Po)(1 - 2^(-t/half-time)), find P for the first dive. Po will equal surface pressure. We then need to find P for the surface dive: Po = P, Pi = surface air pressure, t becomes time at the surface, and half time is the half time of the tissue compartment from Baker's table. Inspired air pressure is derived from the ambient water (or air) pressure from the computer's pressure sensor. Once you have P for the surface interval we can find the P for the second dive: Po = P, Pi = air pressure at depth, and t = time at depth. Solve for P. Using the the value of Po = P and P = Mo solve for t to get NDL. Follow this procedure for any additional dives.

The solution for P is only for one tissue compartment (TC). Each TC will have its own N2 loading (P) and NDL.

If you enjoy doing these calculations you might want to use the ascent/descent equations from Stuart Morrison's paper on DIY Decompression.

Is there any other way to calculate ndl?

I don't know.

Is there any diving computer who use this calculation? surely we are never at the same depth... a little delta is possible

All computers solve these equations (or modifications to the equations) at every time base interval. My Mares Puck uses 20 seconds. Other computers use shorter time intervals (this gives more accuracy). Referring to the constant depth equation, at the end of every time interval the value of Po becomes P (Po = P) and the equation is solved for a new value of P, with t equal to the time interval. When the TC becomes saturated (say at the surface) Po = Pi and the equation reduces to P = Po where the initial TCP equals the final TCP. The computer does these calculations for all TC's. The shortest NDL from all the TC's is displayed on the computer.

Calculations are performed that take into account personal conservative settings, altitude, gradient factors, number of repetitive dives, minimum NDL, etc. These variables are used to modify NDL's or deco stop times.

 

[igor p]

hi

in this document, we can see the calculation for the NDL.
http://www.frogkick.dk/library/deco/Erik Baker/ndl.pdf

for a constant depth

t = (-1/k)*ln[(Pi - P)/(Pi - Po)]

We can read


Pi = (Pamb - PH2O)*FN2 = (40 msw - 0.627 msw)*0.79 = 31.1 msw

if we use nitrox 32, 0.79 should be 0.68?


Po = (Pamb - PH20)*FN2 = (10 msw - 0.627 msw)*0.79 = 7.4 msw

if it's not the first dive, what thing should be modified in this calculation?

is there any other way to calculate ndl?

is there any diving computer who use this calculation? surely we are never at the same depth... a little delta is possible

Hi Marc,

if it is not the first dive you account for actual tissue status before the dive.

But before going too deep into theory a question:
Are you interested in math used to calculate it or need just a tool to calculate NDL between dives etc?
Mix, nitox, air?

Igor P

Sent from my PAP4500DUO using Tapatalk 2

 

[marc.collin]

Hi Marc,

if it is not the first dive you account for actual tissue status before the dive.

But before going too deep into theory a question:
Are you interested in math used to calculate it or need just a tool to calculate NDL between dives etc?
Mix, nitox, air?

Igor P

Sent from my PAP4500DUO using Tapatalk 2

more interested in math

---------- Post added April 30th, 2015 at 10:27 AM ----------

If you enjoy doing these calculations you might want to use the ascent/descent equations from Stuart Morrison's paper on DIY Decompression.

When i read DIY DECOMPRESSION, By Stuart Morrison and i think dive computer implementation, i have a couple of question.


With a diving computer, we can do calculation every second without problem.

In a dive it's almost impossible to descent at the same constant speed, a little gap (+,-) will occur. So we will need to recalculate every time Schreiner.

Same thing for the dive, depth can vary from a couple of cm.

 

[dberry]

IMO, it can be "fun" to think about the equations and how dive computers use the models, or even play with the models in one's own software. But it's REALLY important to remember that these are empirical models based on remarkably approximate data for different compartments, and that variability the physiologies of different divers - or the same diver on different days - can be HUGE and likely are more important than the depth variations you're asking about.

Sorry if you already know this, but one can't predict NDL times with great accuracy with a semi-empirical model while ignoring other variables (which are unknowable, for practical purposes.) That's why the models build in conservative assumptions and fudge factors, and then most dive computers add further padding in the interest of safety for the majority of diver physiologies.

You can, of course, create your own simulations of how an individual model will respond to a hypothetical dive, but that's not at all the same thing as knowing what's actually happening in YOUR body compartments during that dive, or ultimately, whether you'll be bent when you surface.

If Pete were here, he's mumble something about measuring with a micrometer and cutting with an axe

That said, I'm a data geek and get (minor) kicks from simulating and plotting stuff like this... I just remember it's not reality.

 

[igor p]

more interested in math

---------- Post added April 30th, 2015 at 10:27 AM ----------




When i read DIY DECOMPRESSION, By Stuart Morrison and i think dive computer implementation, i have a couple of question.


With a diving computer, we can do calculation every second without problem.

In a dive it's almost impossible to descent at the same constant speed, a little gap (+,-) will occur. So we will need to recalculate every time Schreiner.

Same thing for the dive, depth can vary from a couple of cm.

Correct, in UW computer software you need to recalculate depth, tissue status and all rest on the fly every X secconds. In general Schreiner equation is used for tissue status calculation.



Sent from my PAP4500DUO using Tapatalk 2

 

[EFX]

When i read DIY DECOMPRESSION, By Stuart Morrison and i think dive computer implementation, i have a couple of question.

With a diving computer, we can do calculation every second without problem.

In a dive it's almost impossible to descent at the same constant speed, a little gap (+,-) will occur. So we will need to recalculate every time Schreiner.

Same thing for the dive, depth can vary from a couple of cm.

You didn't actually ask a question but I'm assuming it could be: If the computer only calculates every x seconds how can it be accurate considering there could be constant depth changes during those time periods?

Well, how much accuracy do we need? The pressure differences and the resulting calculated P values from cm depth changes are negligible especially in light of the fact that it's a model approximating the loading in the body. Considering that most dives are spent at constant depth or slowly changing depth it won't matter.

 

[igor p]

IMO, it can be "fun" to think about the equations and how dive computers use the models, or even play with the models in one's own software. But it's REALLY important to remember that these are empirical models based on remarkably approximate data for different compartments, and that variability the physiologies of different divers - or the same diver on different days - can be HUGE and likely are more important than the depth variations you're asking about.

Sorry if you already know this, but one can't predict NDL times with great accuracy with a semi-empirical model while ignoring other variables (which are unknowable, for practical purposes.) That's why the models build in conservative assumptions and fudge factors, and then most dive computers add further padding in the interest of safety for the majority of diver physiologies.

You can, of course, create your own simulations of how an individual model will respond to a hypothetical dive, but that's not at all the same thing as knowing what's actually happening in YOUR body compartments during that dive, or ultimately, whether you'll be bent when you surface.

If Pete were here, he's mumble something about measuring with a micrometer and cutting with an axe

That said, I'm a data geek and get (minor) kicks from simulating and plotting stuff like this... I just remember it's not reality.

Sorry I can not agree. I have built my own decoplanner and UW computer. It implements known decompression algorithm with clasical safety settings. It's results are in line with most known Shearwater tech computers. I believe they are quite good in calculating safe deco profiles for different (a lot of different) divers. The conservatism or safety settings are normal algorithm settings and no added further paddings even for CCR dives used there.

But on the other side, it is true, decompression models are only mathematical models that try to describe/simulate what happens in the divers body.

More or less, I believe, they are quite succesful as the injury incidence is quite low.

Igor P

Sent from my PAP4500DUO using Tapatalk 2

 

[marc.collin]

Sorry I can not agree. I have built my own decoplanner and UW computer. It implements known decompression algorithm with clasical safety settings. It's results are in line with most known Shearwater tech computers. I believe they are quite good in calculating safe deco profiles for different (a lot of different) divers. The conservatism or safety settings are normal algorithm settings and no added further paddings even for CCR dives used there.

But on the other side, it is true, decompression models are only mathematical models that try to describe/simulate what happens in the divers body.

More or less, I believe, they are quite succesful as the injury incidence is quite low.

Igor P

Sent from my PAP4500DUO using Tapatalk 2

do you have any web page about you uw computer?