Can someone explain Ratio Decompression?

[Wookie]

I cut tables with V-Planner. I usually cut my bottom time short by a couple of minutes because I am an easy bend. I usually deco for significantly longer than my tables call for, because I like being in the water, and I stay at 15 feet till the O2 is gone. How can Ratio Deco help me?

P.S. I'm not trolling, I really don't understand the theory, or I understand just enough to make assumptions, which we all know the results of....

 

[Slamfire]

Some of articles about it:

Team foxturd: Tech1 Ratio Deco - Explained

http://gps-tsc.upc.es/comm/jriba/personal_data_archivos/ratiodeco_riba_en.pdf

http://www.txfreak.de/ratiodeco.pdf

UTD offers a live webinar course. I bet they also have course recordings available.
Ratio Deco Classroom - Unified Team Diving

 

[TSandM]

Ratio Deco is a curve-fitting exercise, born of the observation that the outputs of a variety of decompression software programs (given standard gases and within certain depth/time product ranges) tend to have very similar shapes, and that those shapes could be fairly closely approximated by some simple arithmetic operations which can be done in the diver's head, even underwater.

The biggest advantage of RD is just that -- schedules can be adjusted or completely rewritten on the fly. If you are blown off your planned site and have to fall back on one with a very different profile, you don't have to have your laptop with you to rewrite your plan. If you have to abort your dive halfway through, it's easy to recalculate the required deco at the bottom of the upline.

I would imagine that, given your post, you would probably take the schedule RD gave you and massage it for your own needs, which seems to be pretty common among active technical divers.

If you were asking, "Why would I need to learn RD?", the answer really is that it's a very flexible tool to have in your toolbox. But again, it's based on standards gases and may not work well outside a certain range of times.

 

[Blackwood]

P.S. I'm not trolling, I really don't understand the theory, or I understand just enough to make assumptions, which we all know the results of....

It's not so much theory itself as it is application of theory (in partiular GF Buhlmann obligation with bubble-model and oxygen-window ascent strategies).

By limiting the number of gases to one or two combinations per depth range, one is able to isolate a major variable within decompression algorithms and is thus patterns which may exist are more apparent. The DIR standard gases facilitate that end. If you run 100 slightly different profiles using 21/35 bottom gas and EAN50 for deco in the 130-160 foot range, you'll see that [-]bottom[/-] deco time is generally about equal to bottom time +/- a little bit for small depth variations. [The system breaks down somewhat with extremely long bottom times, so let's limit the scope of this discussion to ~30 minutes]

The 'ratios' are only part of the system. In specific, along with depth (avg) and bottom time, the ratio allows you to estimate what the decompression obligation from DecoPlanner would be (at about GF 30/85, I believe, but it's been a while since I've fussed with it).

For "Tech 1" ratio deco, my total obligation less deep stops (which I'll define for brevity as stops before I reach my first bottle) is 1:1 on bottom time for dives at the setpoint (150 feet), five minutes less for each ten feet shallower, and five minute more for each ten feet deeper.

I then need to figure out where to spend that obligation. That's where the curve shaping Lynne mentions comes into play.

Ratio Deco as taught by Andrew (for example) involves S-curving to take advantage of the 'Oxygen Window', while others teach a linear or more exponentially shaped profile.

 

[Cave Diver]

It's not so much theory itself as it is application of theory (in partiular GF Buhlmann obligation with bubble-model and oxygen-window ascent strategies).

By limiting the number of gases to one or two combinations per depth range, one is able to isolate a major variable within decompression algorithms and is thus patterns which may exist are more apparent. The DIR standard gases facilitate that end. If you run 100 slightly different profiles using 21/35 bottom gas and EAN50 for deco in the 130-160 foot range, you'll see that bottom time is generally about equal to bottom time +/- a little bit for small depth variations. [The system breaks down somewhat with extremely long bottom times, so let's limit the scope of this discussion to ~30 minutes]

Is this a typo, or am I being dense. Shouldn't bottom time always = bottom time? I'm guessing that second should read "deco time"?

The 'ratios' are only part of the system. In specific, along with depth (avg) and bottom time, the ratio allows you to estimate what the decompression obligation from DecoPlanner would be (at about GF 30/85, I believe, but it's been a while since I've fussed with it).

For "Tech 1" ratio deco, my total obligation less deep stops (which I'll define for brevity as stops before I reach my first bottle) is 1:1 on bottom time for dives at the setpoint (150 feet), five minutes less for each ten feet shallower, and five minute more for each ten feet deeper.

I then need to figure out where to spend that obligation. That's where the curve shaping Lynne mentions comes into play.

Ratio Deco as taught by Andrew (for example) involves S-curving to take advantage of the 'Oxygen Window', while others teach a linear or more exponentially shaped profile.

What is the criteria used for determining when a 1:1, 1:2, 1:3 time vs. deco needed as referenced in the articles above? Depth, number of gasses, time?

 

[Blackwood]

Is this a typo, or am I being dense. Shouldn't bottom time always = bottom time? I'm guessing that second should read "deco time"?

lol. Good guess.

What is the criteria used for determining when a 1:1, 1:2, 1:3 time vs. deco needed as referenced in the articles above? Depth, number of gasses, time?

Depth is the primary variable. One could make an argument that bottom mix plays into it, but that itself is a function of depth. Divers may elect to add more deco gases than the system calls for to facilitate gas reserves, but they don't play into the ascent schedule.



Also note that I wasn't attempting a complete summary. I left out how to determine first deep stop depth, deep stop duration, and multi-deco gas diving (which is outside my personal experience).

 

[rjack321]

What is the criteria used for determining when a 1:1, 1:2, 1:3 time vs. deco needed as referenced in the articles above? Depth, number of gasses, time?

The "setpoints" are based on depth.

There are really only 3 setpoints now
at
150ft BT on 21/35 = DT on 50% (plus some deep stop time) (factor =1)
200ft BT on 18/45 = DT on 50% and DT on O2 (2x BT total plus some deep stops) (factor =2)
250ft BT on 15/55 is 1/3rd deco time on 50% and O2 (factor = 3)

multiple setpoint depth by factor then add 5min for every 10ft deeper or subtract 5mins for every 10ft shallower.

eg
25min at 180ft in 18/45 with 50% and O2 for deco
25 x2 = 50mins
20ft feet shallower than 200ft setpoint tho.
50-10mins = 40mins

40mins deco on deco gas.
20mins on 50% and 20mins on O2
add deep stops based on BT exposure
between 30-45min deco on 50%& O2 thus deep stops are 1min starting at 75% of depth (120ish) and 2 mins starting at 50% of depth (90ft) until the 70ft bottle is intersected.

last step is to shape deco gas time to create a schedule.

Change the BT to 30mins and the schedule would have the same deep stops but there would be 25mins deco on 50% and 25min on O2
30 x 2 = 60mins minus 10mins for being 20ft shallower

Change depth to 210 for 25mins?
25 x 2 = 50 plus 5 for being 10ft deeper = 55mins deco on 50% and O2 split evenly between those 2 gases. (some slight changes to deep stops for this too)

The basic premise is:
"Assuming standard gases, for a particular depth there is a relationship between BT and deco time which can be expressed as a ratio. For small changes in depth the product of this ratio can be adjusted linearly with an addition or subtraction of 5 mins per 10ft."

The ratios themselves were derived from Buhlmann with GF 30/85 with a bit of an eye towards Buhlmanns helium penalty. The ultimate shape of the deco is a bit of magic voodoo.

 

[rjack321]

Divers may elect to add more deco gases than the system calls for to facilitate gas reserves, but they don't play into the ascent schedule.

They may or may not count, depends on the depth/exposure.

Adding O2 to a 1:1 dive does not shorten the shallow stops by 50% due to having a second O2 window like you might assume (e.g. 10mins at 20-10ft does not become 5mins)

Adding 35/25 to a 250ft dive would probably be counted for 20% of the deco gas portion (by most peopl even though its on the line for being "required" as a 3 deco gas dive). Vs. adding 21/35 would not be used as a deco gas for most 250ft dives but it would reduce rock bottom.

It starts getting depth/time/exposure dependent way down here.

 

[Cave Diver]

The "setpoints" are based on depth.

There are really only 3 setpoints now
at
150ft BT on 21/35 = DT on 50% (plus some deep stop time) (factor =1)
200ft BT on 18/45 = DT on 50% and DT on O2 (2x BT total plus some deep stops) (factor =2)
250ft BT on 15/55 is 1/3rd deco time on 50% and O2 (factor = 3)

Thanks. Let me absorb this a bit and I'll come back with more questions.

 

[Blackwood]

They may or may not count, depends on the depth/exposure.

[snip]

Adding 35/25 to a 250ft dive would probably be counted for 20% of the deco gas portion

Interesting. Is it taught that way or is that simply a personal decision (250 is far beyond my knowledge base) ?