Can someone explain Ratio Decompression?

[ucfdiver]

One thing that's worth mentioning is the PDF's that are referenced only apply for about 30 minutes of deco. Beyond that there are other things to account for, such as minimum stop times for deep and deco stops, as well the amount of time that you spend on deco gases below the 70' bottle. That is, until you reach max deco. Then it's a new ball game.

Minimum stop times only come into pay after 30min deco? hugh?

Beyond short & simple dives ratio deco reminds me of showing up to my AC circuit analysis lecture the day I forgot my calculator-- not an experience I care to have underwater.

 

[amascuba]

Well, not exactly. For the referenced PDF's, dives with about 30 minutes of deco, each deep stop is 1 minute long and each deco stop is a minimum of two minutes long. I've done up to about 45 minutes with out changing deep stop times or the 2 minute minimum deco stop time. Beyond that, I'd change the strategy. The PDF's showing set points are really just a short cut for the bigger ratio deco picture. To calculate your deco for dives that are about 90 minutes of deco or even into max deco, you would need to determine your o2 segment and then it's easy to figure out the rest of the deco from there.

I use ratio deco as a planning tool. My team and I just don't hop in the water and figure out deco when we want to come up. On the surface we plan our dive and run through ratio deco and rock bottom then.

Minimum stop times only come into pay after 30min deco? hugh?

Beyond short & simple dives ratio deco reminds me of showing up to my AC circuit analysis lecture the day I forgot my calculator-- not an experience I care to have underwater.

 

[limeyx]

Minimum stop times only come into pay after 30min deco? hugh?

Beyond short & simple dives ratio deco reminds me of showing up to my AC circuit analysis lecture the day I forgot my calculator-- not an experience I care to have underwater.

And you are welcome to that I use a form of ratio deco for every dive i do from recreational to cave to 250ft+ ocean dives with far more than 30 mins deco.

There are obviously cases where the basic rules of thumb break down, as we already talked about, the internet examples quoted are usually geared toward 30 mins BT (not deco time) but relatively deeper dives.

Its pretty clearly stated in the info that outside those ranges, you need to apply different rules, most noticeable as we already discussed, for longer bottom times in the 80-120 range, if one mistakenly applies the incorrect rules, you'd end up doing significantly more deco than was needed.

 

[limeyx]

Well, not exactly. For the referenced PDF's, dives with about 30 minutes of deco, each deep stop is 1 minute long and each deco stop is a minimum of two minutes long.

The "public" documents (which I would caution anyone against applying too literally as there are multiple updated versions) usually consider up to 30 min BT, which for the deeper dives most definitely isn't just 30 mins deco

 

[amascuba]

Thanks for the correction. I suppose it would have helped if I had read it sooner than two years ago. I just remembered that they were limited in scope of their application and was thinking that it was limited to about 30 minutes of deco. That said, if anybody is intersted in ratio deco, i recommend the workshop. It's much more in depth than what the PDF's show.

The "public" documents (which I would caution anyone against applying too literally as there are multiple updated versions) usually consider up to 30 min BT, which for the deeper dives most definitely isn't just 30 mins deco

 

[Kevrumbo]

Well, not exactly. For the referenced PDF's, dives with about 30 minutes of deco, each deep stop is 1 minute long and each deco stop is a minimum of two minutes long. I've done up to about 45 minutes with out changing deep stop times or the 2 minute minimum deco stop time. Beyond that, I'd change the strategy. The PDF's showing set points are really just a short cut for the bigger ratio deco picture. To calculate your deco for dives that are about 90 minutes of deco or even into max deco, you would need to determine your o2 segment and then it's easy to figure out the rest of the deco from there.

. . .

Are you referring to the other method of Cascading Ratio Deco for dives 150'/45m and beyond? It essentially generates similar profiles, but is easier to implement from a programming and manual arithmetic point of view (see attached).

Note: 70'-30' (N50)Time / 2 = 120'-80' (35/25) Time (5 stops) denotes:

"The 70' to 30' (Nitrox 50) Time quantity divided-by-two equals The 120' to 80' (35/25 trimix deco gas) Time (shaped over 5 stops)". . .

 

[limeyx]

Thanks for the correction. I suppose it would have helped if I had read it sooner than two years ago. I just remembered that they were limited in scope of their application and was thinking that it was limited to about 30 minutes of deco. That said, if anybody is intersted in ratio deco, i recommend the workshop. It's much more in depth than what the PDF's show.

Indeed.

 

[Peter Guy]

Every time I see this thread and its title "Can Someone Explain Ratio Decompression" I can't help but think the only possible post is:

No.

 

[Kevrumbo]

Thanks for the correction. I suppose it would have helped if I had read it sooner than two years ago. I just remembered that they were limited in scope of their application and was thinking that it was limited to about 30 minutes of deco. That said, if anybody is intersted in ratio deco, i recommend the workshop. It's much more in depth than what the PDF's show.

30 minutes of deco limit sounds to me like the max duration of a single AL40 bottle of nitrox 50 for RD1:1 dives of 150'/45m.

IMO that's a more logical & likely explanation for the misunderstanding . . .

 

[amascuba]

30 minutes of deco limit sounds to me like the max duration of a single AL40 bottle of nitrox 50 for RD1:1 dives of 150'/45m.

IMO that's a more logical & likely explanation for the misunderstanding . . .

An AL40 does last about 30 minutes of deco as a 70 bottle, but there is more to it. Minimum stop times are applied to the various depths from 0 - 30, 31 -60, 61 - 90, 91 - 120, and 121 - 150 minutes of deco at the O2 Segment. Using Cascade Deco, you determine how much deco you need on O2 at 20' and then figure out the rest of the deco based on that. From there, depending on how long the deco segment is, determines the minimum time for each stop and even how you curve the deco. Your Cascade Deco document, while mostly accurate, leaves out a lot of information. You can also use it for dives shallower than 150'.

For example. On at 400' dive for 150 minutes, you wouldn't do 2 minute stops at each deco stop and then finish off the deco segment with the remaining time. Nor would your deep stops be one minute long. Your total run time is going to be about 10 hours after you take into account for ascent to deep stops, the deep stops, the deco at the four different deco segments, and a six minute ascent from 20.