Ratio deco #1

[limeyx]

EDIT: Chris's post is of course above

 

[rjack321]

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 have never used VPM or Buhlmann in the ocean.

With Al40s of deco gas in the ocean I am almost never breaking rock bottom or maximum deco limits. Because we essentially trade depth for BT back and forth the deeper dives just get shorter while if we're shallower we can extend them (on the fly). If its going to be square we roughly figure out a new plan on the boat. If we're multileveling up a wall we just dive - knowing what our max deco we can do with the gas we've got and rock bottom for max depth and shallower too, e.g. I know how much gas I need to get from 50ft to 20ft in 15mins so I can adjust if I had to (has never happened). We dive wherever we find the cool critters and then figure out the deco during the deep stops.

None of this requires futzing very much with rock bottom gas because: 1) its generally the 40cf 50% bottle which is the limiting factor (30mins on that) not backgas 2) the rock bottom for e.g. 180ft or 205ft isn't really different enough to worry about since we aren't bumping into either and just use 200ft for both anyway.

So the basic parameters are rock bottom psi for the chosen gas (e.g. 200ft for 18/45)
30mins max per deco gas in an AL40
Rock bottom for the BG will conveniently cover 1 lost deco gas.

These are pretty easy boundaries and we don't need to pre-plan, use software, or make tables for every possible depth/time combination. I can switch from a 190ft dive "plan" to a 130ft dive in about 30secs or less on the boat or in the water if the site turns out to be not there.

 

[Blackwood]

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

Sure we do: the one that DecoPlanner assumes. That's what RatioDeco intends to envelope, no?

Whether or not that has anything to do with reality is a different discussion.

 

[rongoodman]

Sure we do: the one that DecoPlanner assumes. That's what RatioDeco intends to envelope, no?

That was how I thought it worked, although I think I put it badly. The OP was asking about implementing RD on a computer, but since there are now dive computers capable of running the exact same software that RD is trying to emulate, why bother?

 

[Rainer]

RD != DP

You'd still have to tweak the output (add deep stops, extra time at gas switches, shaping, backgas breaks, etc).

That said, it's pretty trivial. You don't need a computer to tell you how.

That was how I thought it worked, although I think I put it badly. The OP was asking about implementing RD on a computer, but since there are now dive computers capable of running the exact same software that RD is trying to emulate, why bother?

 

[ucfdiver]

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)

That's all I have to do to avoid grabbing wetnotes and taking a quick glance? SWEET!

 

[rjack321]

Sure we do: the one that DecoPlanner assumes. That's what RatioDeco intends to envelope, no?

Whether or not that has anything to do with reality is a different discussion.

Step one is you have to realize that Buhlmann is wrong. In fact all models (not just deco models, hydrologic models, engineering models, etc) are wrong some just still have various levels of utility.

If they weren't wrong people wouldn't be tweaking them. Deep stops for Buhlmann, safety factors for bridge designs, etc.

RD has no aspirations of approximating Buhlmann and gets further away the more "dissolved" gas Buhlmann thinks you have. I think AndrewG would still be decompressing from the Pit last year if he was trying to fit to Buhlmann.

 

[PfcAJ]

Step one is you have to realize that Buhlmann is wrong. In fact all models (not just deco models, hydrologic models, engineering models, etc) are wrong some just still have various levels of utility.

If they weren't wrong people wouldn't be tweaking them. Deep stops for Buhlmann, safety factors for bridge designs, etc.

RD has no aspirations of approximating Buhlmann and gets further away the more "dissolved" gas Buhlmann thinks you have. I think AndrewG would still be decompressing from the Pit last year if he was trying to fit to Buhlmann.

I've got to disagree here.

RD can approximate whatever you want. If you identify a ratio (1:1, 2:1, 1.5:1, whateva) that lines up with your algorithm of choice (VPM+2, RGBM, Buhlmann 30/85, whateva) then you are using a form of "ratio deco".

There isn't one true "ratio deco".

 

[rjack321]

I've got to disagree here.

RD can approximate whatever you want. If you identify a ratio (1:1, 2:1, 1.5:1, whateva) that lines up with your algorithm of choice (VPM+2, RGBM, Buhlmann 30/85, whateva) then you are using a form of "ratio deco".

There isn't one true "ratio deco".

Apparently you weren't taught RD (if at all?) in the era when it was inside the secret decoder ring. What you are missing is that RD isn't actually trying to approximate some other model. Its often illustrated that way for noobs "see its basically VPM+2 time" but its a derived method unto itself and people are encouraged listen to their body and adjust. The guidelines expressed in the OP's class are just a starting point that work for most. In fact at the 2:1 ratio its already been tweaked compared to what it would take to fit decoplanner output. The setpoint was revised upwards from 220ft to 200ft based on experience (mostly AndrewG's since I for instance wasn't doing old 220' setpoint dives at that time otherwise I'd be bent like a pretzel).

 

[limeyx]

That's all I have to do to avoid grabbing wetnotes and taking a quick glance? SWEET!

I know you are somewhat tongue in cheek
I put a long-winded post I guess, because it's kind of hard to describe easily.

For those people really using it, it's really a very easy technique and works rather simply and well for a large number of dives (all of mine for instance)