Question calculating NDL is difficult

[Etmutt]

Where did you get shearwater values? Did match water density ? I recall shearwater uses European definition

 

[L13]

fortunely, the values are very similar to mine. Unfortunately, there is also a difference from shear water. Thank you for letting me know though.

You will have a very difficult time exactly matching Shearwater (or any other computer). They all have small differences in interpretation or implementation of the base algorithm, or slightly different values for physical constants (like water density). But will be close on the same algorithm and settings.

Often, differences also occur because of assumptions about the ascent rate and whether a ceiling is allowed if it clears before it is reached with ascent rate assumptions.

 

[BlueTrin]

I am curious, why not just use the NDL dive tables, like we have for the last 50 years?

SeaRat

I think @int wants to compute real time NDL as a toy project for his shiny new Apple Watch which can support apps and has a depth sensor

I have a few friends who have similar projects of making their own dive app just as a fun project.

 

[John C. Ratliff]

I’d like to caution people trying to do the below, as there are a lot of assumptions and testing that goes into a decompression meter. We are not talking about some fun project, but upon a project that would be used in a life support capacity. I mentioned using the no-decompression tables for a reason, in that they have been used for a very long time, and if used correctly, will protect against decompression sickness. A “real-time NDL“ ignores the fact that the tables assume that the diver starts bottom time counting from the surface dive to the time the diver leaves to begin his/her ascent, and uses the deepest depth achieved as the recorded depth. As mentioned above, divers depth fluxuates, and this provides a safety factor when using the NDL tables. But creating your own does not seem to me to be a good idea, as there may be no safety factor, and the algorythm used may not be the best for that application (app).

SeaRat

I think @int wants to compute real time NDL as a toy project for his shiny new Apple Watch which can support apps and has a depth sensor

I have a few friends who have similar projects of making their own dive app just as a fun project.

 

[JohnN]

fortunely, the values are very similar to mine. Unfortunately, there is also a difference from shear water. Thank you for letting me know though.

Given the imprecision of any current decompression algorithm to predict a divers risk for DCS, your exercise is like measuring with a micrometer, marking with chalk, and cutting with an axe

 

[Etmutt]

I’d like to caution people trying to do the below, as there are a lot of assumptions and testing that goes into a decompression meter. We are not talking about some fun project, but upon a project that would be used in a life support capacity. I mentioned using the no-decompression tables for a reason, in that they have been used for a very long time, and if used correctly, will protect against decompression sickness. A “real-time NDL“ ignores the fact that the tables assume that the diver starts bottom time counting from the surface dive to the time the diver leaves to begin his/her ascent, and uses the deepest depth achieved as the recorded depth. As mentioned above, divers depth fluxuates, and this provides a safety factor when using the NDL tables. But creating your own does not seem to me to be a good idea, as there may be no safety factor, and the algorythm used may not be the best for that application (app).

SeaRat

on the other side, the apple watch is opening up a range of possibilites.

Why not to imagine an open source app linked to one of the major open source dive software (for example like sub surface).
People with some level of understanding of apple watch developpment could create & sell an app like the oceanic, once properly tested.

once apple fix their watch and crank up waterproofing to a more suitable degree, i would be more than happy to buy one

 

[Ski_Lounge]

I can't speak much about how to implement deco algos, but it's always fascinating reading about them. As to why you may not get an exact match across implementations (like Subsurface vs Shearwater), there is a really insightful article by one of the Subsurface contributors on his blog below. He addresses some not-quite-deterministic aspects of model implementation (and in general has some in-depth explanations of Buhlmann as well as other deco algos).

Why is Bühlmann not like Bühlmann – The Theoretical Diver

thetheoreticaldiver.org

 

[BlueTrin]

I can't speak much about how to implement deco algos, but it's always fascinating reading about them. As to why you may not get an exact match across implementations (like Subsurface vs Shearwater), there is a really insightful article by one of the Subsurface contributors on his blog below. He addresses some not-quite-deterministic aspects of model implementation (and in general has some in-depth explanations of Buhlmann as well as other deco algos).

Why is Bühlmann not like Bühlmann – The Theoretical Diver

thetheoreticaldiver.org

If you implement Bühlmann as a toy project you run immediately into the issues highlighted in this link. But for moderately deep dives it does not make a very large difference.

Makes a much bigger difference if your first stop is deep as it will have a knock off effect, as the article stated.

 

[OTF]

Both OP and the Theoretical Diver mention water vapor as a factor in deco models but I've never run into that before. It's logical that water vapor makes up some percentage of the inspired gas (especially on CCR!) but how is it estimated and what are the models doing with it?

 

[BlueTrin]

Both OP and the Theoretical Diver mention water vapor as a factor in deco models but I've never run into that before. It's logical that water vapor makes up some percentage of the inspired gas (especially on CCR!) but how is it estimated and what are the models doing with it?

According to this thread, Buhlmann Calculation Help

It looks like you should use them to account for the composition of the inspired gas.

Maybe someone with more knowledge than myself can clarify better?