Ratio deco #1

[ucfdiver]

Ratio deco is shoving a linear equation into what is conceptualized as a logarithmic process. Not the other way around.

Ha, you got me there.

 

[limeyx]

Ratio deco is shoving a linear equation into what is conceptualized as a logarithmic process. Not the other way around.

I would submit that ratio deco is recognizing and extrapolating patterns in decompression, yet also realizing within what limits each ratio/rule works/starts to break down.

It was made quite clear to us during cave2 that our standard RD profiles for "long shallow deco dives" (say 2 hours at 60-90 feet) was way overkill

Yet some of the VPM/Buhlman profiles we came up with seemed to provide shockingly short deco times.

 

[Blackwood]

I would submit that ratio deco is... extrapolating patterns in decompression

Not sure I agree. Rather, it's a linear fit to a non-linear curve. There's no real extrapolation going on (it's not like I say "okay, I have 3 minutes deco after 10 minutes bottom time and 4 minutes deco after 20 minutes bottom time, therefore I'll have 5 minutes deco after 30 minutes bottom time", or some such thing).

Ratio deco is a gross simplification - allowed by eliminating all but a few gas and depth combinations - that works pretty darn well within the first couple sectors of the total curve, but breaks down very rapidly once that sweet spot ("realizing limits") is exceeded.

 

[ucfdiver]

Yet some of the VPM/Buhlman profiles we came up with seemed to provide shockingly short deco times.

I've done dives up to 3 hours at around 80-100ft and followed Buhlman fairly close (slowed my ascent a few minutes starting at 1/2 depth) and never felt anything sub-clinical. Sometimes surprisingly short. They were doing Doppler testing at Ginnie Springs and found almost zero differences in bubbling from ratio, vpm, or buhlman. Small sample, but still interesting.

Ratio deco is great for river caves where our BT's are short. However, I still don't understand what it offers over tables.

 

[limeyx]

Not sure I agree. Rather, it's a linear fit to a non-linear curve. There's no real extrapolation going on (it's not like I say "okay, I have 3 minutes deco after 10 minutes bottom time and 4 minutes deco after 20 minutes bottom time, therefore I'll have 5 minutes deco after 30 minutes bottom time", or some such thing).

Ratio deco is a gross simplification (linear) that works pretty darn well within the first couple sectors of the total curve, but breaks down very rapidly once that sweet spot ("realizing limits") is exceeded.

I guess you could look at it that way, but ultimately the curve may not be well-formed, may be different for everyone and may not even resemble a curve especially as you get toward (or even believe in terms like "max deco")

I guess I look at it as a bunch of guidelines that start from some "fixed" point, about which I can make some reasonable predictions within a certain set of parameters (sufficiently vague ?

One point i would bring up against the "curve fit" is that I am not convinced we really even know what curve we are trying to fit in many of these cases, especially when we tinker with profiles

so I see it more as a set of rules of thumb, and "rough predictions"/guidelines

couple that with ways to "get things back on track" (such as adding a 100% bottle to profiles in the 150 range to "make" the ratio linear for longer bottom times) and I am not sure a curve fit is necessarily the way to look at it

(especially when you consider weird people like "Mr Bendy 321" with all his Helium allergies)

 

[limeyx]

I've done dives up to 3 hours at around 80-100ft and followed Buhlman fairly close (slowed my ascent a few minutes starting at 1/2 depth) and never felt anything sub-clinical. Sometimes surprisingly short. They were doing Doppler testing at Ginnie Springs and found almost zero differences in bubbling from ratio, vpm, or buhlman. Small sample, but still interesting.

Ratio deco is great for river caves where our BT's are short. However, I still don't understand what it offers over tables.

I think "ratio deco works best for short BT's" is something of almost a tautology. i.e. for ocean diving a huge percentage of dives fall in that "short BT" range, so probably more experience has been had there.

RD is a really nice tool because it allows you to tailor the deco to you, and to your dive if it changes radically from "the plan" but I for sure have far less understanding of how ratio works on longer bottom times (not surprising given my diving pattern)

And it doesn't have to be one set of ratios either. I know a (really weird guy who has been known to do a bit of cave diving in mexico whos "ratio" is
3 hours at 50 feet = min deco
4 hrs = 5 mins deco (on O2)
5 hrs = 10 mins deco

or something similar. Thats something that over time has proven to work for him, but wont be found probably in any formal "ratio deco" document

Now, if you used tables, you may have something of a challenging time deciding what contingencies to plan for on these longer bottom time dives (depending on how long the deco is of course)

For ocean diving, I have better examples than cave I guess.
- Rjack and I went to "dive a wreck in 180 feet" (that he had sounded). We get there and the max depth was 205 feet....it took about 2 mins to adjust our plan with no issues (obviously if somehow the wreck had moved to 300 feet this would not be practical)
- if I descend on a dive in 150 and dont find the wreck in 5-10 mins, I "know" it's basically a "min deco" dive --- I dont need to cut contingency tables for that.

I guess it's all what you are used to.

We did some deep diving a few months ago, and watching all the CCR guys writign down multiple contingencies for +5 mins, +10 mins, +5 mins + 10 feet, +5 mins + 20 feet etc until they ran out of wrist-slate space really seemed like a royal pain (especially given the very small & predictable differences in each actual profile)

 

[ucfdiver]

Are you calculating rock bottom on the fly, too? The deco on the fly thing seems great until you realize that you're still limited to the gas you've reserved. In all reality, using VPM+2, how much can you change your original dive plan and not violate your gas plan?

That's not to say that I don't find it convenient to know that if I stay 10ft off the reef, I add 10min of NDL...I'm just curious to see how much you really gain. I'm speaking from inexperience here admittedly, I'm not trying to claim to be an expert...no one around here uses ratio deco for anything but exceptionally short (back gas only) cave dives.

 

[AndyNZ]

Yup I said it... (I imagine that may raise an eyebrow or two).
Now is there going to be any computer companies out there to support it?
It is difficult to find an ideal timer with all the needs in a simple design.
Totally waiting for someone to come out with a timer that has:
Average Depth (resettable), Current Depth, Run Time (minutes & seconds) and a resettable stop watch function that shows not only minutes but also seconds.
And all this on the same screen would be fantastic....
Any one know of a timer or computer that does all this... even better... anyone know of a computer mfg. that is planning on supporting or writing programs for ratio deco style computers with all the standard mixes?

The OSTC Mk II.

I have one, it's awesome.

HW | OSTC Mk.2

 

[Rainer]

I know the RB requirements for any dives I'm willing to do (guess what, there are patterns there, too. ). If the bottom isn't at the planned depth, we'll take a moment to see if with a bit of tweaking, we can still come up with a viable plan. It's easy to do without wetnotes, but they're always there. I just don't see the need for tables for the usual dives I'm doing. For stuff outside my experience, I'll consult software and cut some tables (I'll also be looking for new ratios/patterns).

Edit: It *is* extremely easy (if needed) to calculate (conservative) RB on the fly. It's covered in GUE-F (and the basic idea translates to deeper dives).

Are you calculating rock bottom on the fly, too? The deco on the fly thing seems great until you realize that you're still limited to the gas you've reserved. In all reality, using VPM+2, how much can you change your original dive plan and not violate your gas plan?

That's not to say that I don't find it convenient to know that if I stay 10ft off the reef, I add 10min of NDL...I'm just curious to see how much you really gain. I'm speaking from inexperience here admittedly, I'm not trying to claim to be an expert...no one around here uses ratio deco for anything but exceptionally short (back gas only) cave dives.

 

[limeyx]

Are you calculating rock bottom on the fly, too? The deco on the fly thing seems great until you realize that you're still limited to the gas you've reserved. In all reality, using VPM+2, how much can you change your original dive plan and not violate your gas plan?

That's not to say that I don't find it convenient to know that if I stay 10ft off the reef, I add 10min of NDL...I'm just curious to see how much you really gain. I'm speaking from inexperience here admittedly, I'm not trying to claim to be an expert...no one around here uses ratio deco for anything but exceptionally short (back gas only) cave dives.

I think chris explained it decently below. Obviously one is limited by what they brought to the game so realistically it is usually used if you want to shorten the plan somewhat (didn't find the wreck, flooded suit, vis sucked etc)

Rock bottom is relatively easy for short/deep dives

If I assume stressed SAC of 0.75cft for dives deeper than 160, and (because I am lazy and didn't go back and rework the numbers with the lower stressed SAC) a 1.0 for 150 dives

then the range of RB is approx 64cft to around 100cft for BTs in the 20-30 min range (since RB depends on depth primarily but will also increase with BT due to longer deep stops)

so if I show up with 18/45 in decent doubles, a 40 of 50% and a 40 of O2

I have a variety of dives that are "easy" With 120s or so in the doubles.

The most I would look at is 25 mins at 200 (maybe pushing to 30 but thats really stretching the 50% gas)

I can also do 40 mins at 150, maybe even a little more and still have a reasonable rock bottom and runtime

This is pretty nice if like (say) this sunday, we are planning to hit a wreck in 190. It's possible conditions may not be great there, and we might have to divert to a wreck in 150, 120 or even 95 feet which might look better ... ratio deco makes this pretty trivial