No decompression limit equation question

[Victor Kmita]

Hi everybody!

My goal is to be able to calculate NDLs. I'm currently using Buhlmann's M-values, and I can use the "Schreiner equation" to calculate the tissue pressures at any point in a dive. I cannot figure out how to calculate NDLs for the compartments at points in a dive.

I'm using Deco For Divers as a reference, and I know there's a "No decompression limit equation" but I'm having trouble with it. With it I can calculate the P(no_deco), partial pressure at which ascending has to be started. Should I use that value and solve, say, "Buhlmann's equation" for time? I want a minute, or second value for my NDLs not a partial pressure.

Is there some other equation I can use that's better?

Thanks for reading

 

[dmaziuk]

dmaziuk/diy-zhl and scroll down to "no-stop time"

 

[Victor Kmita]

dmaziuk/diy-zhl and scroll down to "no-stop time"

Thanks for the link! That's an awesome compilation of the implementations of all the interesting deco equations.

I just can't imagine the best way to calculate no-stop time is so inefficient...

 

[Victor Kmita]

dmaziuk/diy-zhl and scroll down to "no-stop time"

Actually I have a question. What does dmaziuk/diy-zhl return?

Thank you for your time!

 

[dmaziuk]

Actually I have a question. What does dmaziuk/diy-zhl return?

If you go up to the top of the project, there are a couple of other notebook files, "dive.ipynb" has an example implementation of a dive profile using that code. However I don't think I got around to adding no-stop time in it.

You can click on "launch binder" button in the top-level README and wait forever for it to come up, when it does you'll be able to open .ipynb files and play with them yourself. Or if you have a linux box handy, you can download the Dockerfile, build the container and run the notebooks locally.

 

[dmaziuk]

I just can't imagine the best way to calculate no-stop time is so inefficient...

No-stop diving was never their concern... But anyway, it tries to guesstimate something that hasn't happened, the only good way to do it is a well-calibrated crystal ball.

 

[EFX]

If you're already calculating a ceiling after calculating the TC pressures then starting at 1 minute calculate the pressures and the ceiling. For NDL dives the ceiling is negative. Keep adding 1 minute and recalculate until the ceiling is zero. The cumulative dive time is the NDL time. To see how to do this get my spreadsheet in the Dive Calculation Spreadsheet thread within this forum and open up VBA to see how I calculate it.

 

[dmaziuk]

Actually I have a question. What does dmaziuk/diy-zhl return?

Sorry, I wasn't paying attention.

This is straight from page 226 of Powell. It returns "P no_deco", whatever that's supposed to be. It's not very useful actually.

The useful version is in Erik Baker's "calculating no stop time" -- he re-arranges it for time t and has a working example with an excellent commentary.

I forgot -- I did start looking at it but never got around to cleaning up the notebook and adding Erik's stuff.

 

[wstorms]

just wanted to point out it might be a good idea to add some padding to your NDLs if you are actually going to dive with your own tables. Pure Buhlmann is considered quite aggressive by todays standards.
Then again, could be you are simply doing the calculations for fun now everybody is locked in

 

[dmaziuk]

just wanted to point out it might be a good idea to add some padding to your NDLs if you are actually going to dive with your own tables. Pure Buhlmann is considered quite aggressive by todays standards.

If you have empirical data showing that ZH-16C at GF70..85 is "too aggressive" (Freadrich), and/or that longer ascent times result in "better" DCS scores (Spisni), you should publish it in a peer-reviewed journal.

Otherwise, I wouldn't.