how to calculate where your first decompression stop is

[rhwestfall]

Anyone have it in, say, FORTRAN for geezers?

The FORTRAN code, IIRC, is in Deco for Divers... I think that is where I saw it last (maybe elsewhere). Now, if you had a compiler that still worked,,,, (my discs are somewhere, but it was for a DOS based 286/386 machine.....)

 

[TrimixToo]

Anything is possible but many things don't seem like they're worth the effort. My way is to put the planned dive into MultiDeco and read it off the screen, along with gas consumptions and contingency plans. Then, I follow my primary Shearwater on the actual dive.

 

[BlueTrin]

+1 for subsurface on desktop to play with something free

The FORTRAN code, IIRC, is in Deco for Divers... I think that is where I saw it last (maybe elsewhere). Now, if you had a compiler that still worked,,,, (my discs are somewhere, but it was for a DOS based 286/386 machine.....)

You could try one of these interpreters if the code isn’t too complex:

LFortran

This is LFortran Compiler

dev.lfortran.org

Online Fortran Compiler - fortran

Online Fortran Compiler - The best online Fortran programming compiler and editor provides an easy to use and simple Integrated Development Environment (IDE) for the students and working professionals to Edit, Save, Compile, Execute and Share Fortran source code with in your browser itself.

www.tutorialspoint.com

 

[inquis]

how do you calculate where your first decompression stop is based on depth and time? I think you have to factor in estimated tissue loading

Yep. If you're talking about Buhlmann + GF, you then take that tissue tension across until it intersects the GFlow line. Say you drop to 100 ft. Here's the situation at GF 40/85 if you get involved taking pictures and overstay NDL by 7 minutes when running GF 40/85 (20 minute bottom time):

There are 16 tissues/compartments/constants, but #2 is controlling, so I've drawn it. Start at the star on the right (100 ft depth and tissue tension of about 90 fsw). The first stop is where your tissue tension crosses the GFlow line (here 40% of the way from ambient to Buhlmann's M-Value line). The goal is to approximate the red path until you exit at GFhigh (80%). Realistically, though, you ascend (move left in the graph), then stop & wait (move down in the graph) until your next horizontal movement can make it the next multiple of 10 ft before running into the red line. So a stair-step path instead of straight diagonal. The first stop will be at 30 ft in this instance, as the GF80 line is crossed at 26 ft depth.

That's the "behind the scenes" info. I second the recommendation to play with the planner in Subsurface. That gives you the information you'd use on [Edit: before] an actual dive. [ETA, your computer is repeatedly doing these calculations on the dive].

 

[OMyMyOHellYes]

I just look at the Shearwater and do what it tells me.

 

[Wibble]

so for academic purposes, how do you calculate where your first decompression stop is based on depth and time?

It's a good question.

Obviously there's the "run the Buhlmann algorithm. The algorithm will be available somewhere on the internets even if it is in Fortran!

The deco curve does have patterns. Your first few stops will be shorter than the later stops. The curve flattens out and typically half your total decompression time will be at 6m/20ft. Typically a dive to 45m/150ft will have 1:1 bottom time to deco time (e.g. 1h on the bottom is 1h of deco). At 60m/200ft this is 1:2 (e.g. 40mins on the bottom and 80mins of deco). 75m/225ft will be 1:3...

Example: 70m dive for 35mins with total 2h20 runtime
(i.e. 35mins on the bottom, 105mins deco = 1:3 ratio)​

All of those patterns vary to a certain extent depending on the depth, time and gases used.

Prior to the advent of reliable and easy to use computers -- hat tip to Shearwater and others -- people used a manual mental arithmetic approximation which is generally called "Ratio Deco". This requires you to average your depth in 5 minute (ish) intervals and remember these numbers. When it comes to the ascent (you have a pre-planned max bottom time!), then you will do some arithmetic which, with practice, gets easier (apparently). GUE (Global Underwater Explorers) use this (or maybe not nowadays?) and definitely UTD (Unified Team Diving) still use and promote their method of doing this (they blather on about this on their podcasts).

Apparently it's really easy: add the numbers, divide by something, halve the depth in ATA, blah blah blah. Probably why they need a team!

Sorry, it's 2023. I look at my computer to tell me what the first stop is and for how long. My second computer will confirm this. And even my third one if diving deeper. If I loose interest then my computer(s) will recalculate the stops.

Computers are good at running algorithms. Humans aren't. Divers definitely aren't as they started off as humans and their IQ halves as soon as they drop in the water. Mine does anyway.


However, for NDL diving, you'll memorise your max bottom time (30mins on 32% at 30m/100ft). You then ascend quickly to the first 3m/10ft increment past half way up (e.g. 15m) and stop for 30 seconds, then slowly ascend by 3m/10ft, stop, ascend... until you reach the surface. This isn't true decompression as such, more a way of slowing the ascent within No Deco Limits. On the other hand, most others just do a 3min stop at 5m/15ft.

 

[RobPNW]

My days of being a double major in physics and math are over. My plan is to look at the computer and enjoy the dive

 

[TrimixToo]

Anything is possible but many things don't seem like they're worth the effort. My way is to put the planned dive into MultiDeco and read it off the screen, along with gas consumptions and contingency plans. Then, I follow my primary Shearwater on the actual dive.

I forgot how interested you are in deco theory. Sorry.

Time, gas tension, gas density (He vs. N2, mostly), current tissue loading, and probably other things I'm forgetting come into play to answer that question. I think I'd want to study them one at a time, and then integrate them afterward. The current best approximation seems to be Buhlman with gradient factors but as usual Rule #1 applies, which is to say if Simon says something different he's right! ;-)

It's been quite some time since I've read up on this and I have forgotten a lot. However, the one practical thing I remember is that the first 50' of ascent are "free." That is, unencumbered by deco stops. You might be able to ascend farther than that, but you can always ascend at least that far with reasonable risk.

From experience, we all develop a sense of about how deep the first stop recommended by our computers will be. For example, depending on the usual BTs in the 20-30 minute range the first stop for most of my 200'-ish dives has been around 120'-130'.

 

[Member002]

The answer really depends and changes based on how familiar the audience is with the given concepts. The other big factor is that it's all pretty theoretical (And largely validated by just whether people are getting bent or not). You pick the algorithm/technique, work through it, and you get that strategies best guess for your deco obligations.

For Buhlmann, this talk by Simon Mitchell is excellent for walking through some of the core concepts. Deco for Divers is also excellent reading.

The tl;dr is:

1. Calculate tissue loading (for each of the 16 theoretical compartments), with something like the Schreiner Equation, for each planned level of the dive.
2. Based on the tissue loading you now have, calculate the first stop based on the maximum tolerable pressures (factoring in GF Low), for each compartment. Pick the deepest result. That's your ceiling. That's your first stop.
3. Then you loop through and redo steps 1 and 2, but you "add" the simulated stop length, increasing the stop length until the next ceiling reaches the next stop depth.
4. Do step 3 for each stop (using a GF that "slides" from GF Low to GF High as described in the video) until you get to the surface.

It's more nuanced than that, and there are other ways you can tackle it, but that's a quick example of how Buhlmannis often calculated by computers. The reason you'll get minor differences in results is because it's up to the developer how to actually implement some of these steps, beyond the well understood math, which causes (likely unimportant) minor differences.

 

[kaylee_ann]

I forgot how interested you are in deco theory. Sorry.

Time, gas tension, gas density (He vs. N2, mostly), current tissue loading, and probably other things I'm forgetting come into play to answer that question. I think I'd want to study them one at a time, and then integrate them afterward. The current best approximation seems to be Buhlman with gradient factors but as usual Rule #1 applies, which is to say if Simon says something different he's right! ;-)

It's been quite some time since I've read up on this and I have forgotten a lot. However, the one practical thing I remember is that the first 50' of ascent are "free." That is, unencumbered by deco stops. You might be able to ascend farther than that, but you can always ascend at least that far with reasonable risk.

From experience, we all develop a sense of about how deep the first stop recommended by our computers will be. For example, depending on the usual BTs in the 20-30 minute range the first stop for most of my 200'-ish dives has been around 120'-130'.

so I've talked to a couple of people. what I'm understanding now is the general idea that your first decompression stop is based on how far you want to get between the depth at which the pressure gradient is enough to begin off gassing efficiently, and the M-value that shows the maximum "safe" tissue saturation. So if you use gradient factors of 65/85, your first stop would create a tissue pressure gradient at 65% of that M-value; conversely, you will surface, at most, at 85% of the M-value. if I misremembered, or mis-typed, sorry!
(correct me if I'm wrong)
I've been reading through the calculations in Deco for Divers, and for some reason, I'm not figuring out where to get certain values. There's a table, but for some reason, when calculating the b-variable, I keep getting a negative number. Very annoying, but a work in progress.