What is Ratio Deco?

[LiteHedded]

Well, that kind of speaks to my condition. I use a bottom timer and PDC. The PDC is either a VR3 or a Hockey Puck. I plan my dives and find myself glancing at my PDC along the way just to see if I'm on course, almost never look at the BT. No calcs involved, I have much more fun things to concern myself with. (And I love math, BTW)

In fact, from my solo view of life, even the timer becomes secondary. I'm reasonably sure that I could get by with a depth gauge and SPG. Time is just gas usage. If all DIR divers had the same SAC, I'm guessing they would only use a depth gauge and SPG and denounce those that need a timer. But what would be the point?

I find this whole subject most interesting. At first blush, all the top-end GUE/UTD divers appear to be Luddites. But after any small amount of work it becomes obvious that they can't be written off that way. The "rule makers" do indeed follow the science and simplify it for their masses using standard gasses and so forth.

I still can't pin down if this "dive computer thing" is all about dive standardization to where a PDC really isn't worth the price, an exercise in mental math that becomes easy and second nature, or just "branding".

As intriguing as the whole concept of DIR is to me, I'm pretty sure that I'm just not wired correctly to be one of them. I do love the fact that they are out there and pitching, though.

Take a class. I think you'll be surprised

 

[Doppler]

I wear a Petrel as a backup. When diving OC in caves or in openwater, I rarely us it for primary or principle information about my decompression status when on OC. I get that from other sources and one might say that the Shearwater simply confirms what I already suspect.

So why carry it?

Well, it's a good unit and bright enough to use as a backup light to exit a cave... which is actually a skill in the CCR Cave standards... but mostly wearing it is a function of covering my arse when teaching... and having it on my wrist has become a habit when dressing.

Most of my regular dive buddies wear one as well. Do we do what it tells us too? Not much. We often repeat the same dives or type of dive over and over... we are a boring bunch... we know what works for us and the computer is simply reassurance. Can we all create a decompression curve / ascent based on average depth, time, and gas; and modify that curve according to the dozens of variables that play a role in deco stress? Yep, I think so. In fact I know so. We regard that skill as necessary. But then, I still double de-clutch when dropping gears in a car with a manual shift, and because of this and other idiosyncrasies when diving, am probably a poor role model.

Bottom line for someone just getting into this deco stuff is this... take a class with an instructor who is going to teach you about decompression stress and run away from one who does not include a module about what to do to get your arse back to the surface without relying 100 percent on a PDC.

The rest of this is all academic...

 

[TSandM]

I believe the original proscription of computers for DIR divers had to do with avoiding the phenomenon we see with some recreational divers, where they get in the water with little or no plan, and swim around until the gauge says to come up. Having to run your tables, or design RD profiles, makes you take ownership of the dive. Having the tools to adjust your decompression according to what you actually DID gives you almost the flexibility of running a computer, without the passivity. In addition, when those proscription were first promulgated, we didn't have units like the X1, that will run the entire desktop version of V-planner (making it moot whether you generate your profiles on a laptop or on your wrist).

This has been an interesting conversation, but I believe little of it has been relevant to the OP, who I believe was really asking about the use of depth averaging and min deco for the recreational diver.

 

[DaleC]

From a rec perspective (I only know basic air decompression values at best) I sort of understand depth averaging and non reliance on PDC's, though it took a while to get there. I began by cutting square profiles with tables, even though I own a PDC because I wanted to understand what the computer was telling me. So I wouldn't fall into the "strap it on and forget" camp. But, over time, I also have gained quite a bit of understanding of the dives I do and how depth/time/pressure affects my next move. I don't think I do what GUE does but it works and I could definitely forgo my PDC on most dives. Actually, I think understanding "rock bottom"
values played a big part in this, as does the 120 rule (air).

At certain depths I know I can stay this long, at other depths less. I have to turn at this pressure, ascend to a shallower depth at this pressure, seek safety stop range at this pressure. I would say I dive my SPG far more than my PDC. Like lowvis, if I have an SPG I don't really need a timer because most decisions are based on pressure. I generally use an SPG, though I could do basic dives based on time/depth only (aka J valve vintage diving) but that requires thinking in square profile terms. I really usually don't even need a depth gauge most of the time as I can judge relative depth by fauna/light conditions.

However, having said that, there are a few things that could throw me off my game. Many of my decisions are based on knowing my behavior with a St72 tank. Different tank sizes, or unfamiliar terrain/conditions or stressed breathing rates ie. swimming in current could mess up my non reliance on gauges. With twins I could easily go into deco if I don't monitor time/depth and know the NDL's. In unknown terrain I could easily misjudge my actual (vs perceived) depth. With a prolonged stressed breathing rate I could easily burn through more gas than I have allotted for specific depths.

The other thing that can really throw me is task loading or myopia due to shooting video. If I become too fixated on a subject I can easily lose track of what I'm doing divewise.

I know I'm not describing depth averaging or RD. I'm looking forward to learning more about that in the near future but I sort of get the brain vs computer debate. I totally get why new divers should learn what is going on behind the algorithm so they can gain a better understanding of why, what they do now, affects what they do next. PADI used to try to do that by emphasizing exercises using the tables but I believe now they teach that as optional info. Too bad IMO.

 

[BRT]

From a rec perspective (I only know basic air decompression values at best) I sort of understand depth averaging and non reliance on PDC's, though it took a while to get there. I began by cutting square profiles with tables, even though I own a PDC because I wanted to understand what the computer was telling me. So I wouldn't fall into the "strap it on and forget" camp. But, over time, I also have gained quite a bit of understanding of the dives I do and how depth/time/pressure affects my next move. I don't think I do what GUE does but it works and I could definitely forgo my PDC on most dives. Actually, I think understanding "rock bottom"
values played a big part in this, as does the 120 rule (air).

At certain depths I know I can stay this long, at other depths less. I have to turn at this pressure, ascend to a shallower depth at this pressure, seek safety stop range at this pressure. I would say I dive my SPG far more than my PDC. Like lowvis, if I have an SPG I don't really need a timer because most decisions are based on pressure. I generally use an SPG, though I could do basic dives based on time/depth only (aka J valve vintage diving) but that requires thinking in square profile terms. I really usually don't even need a depth gauge most of the time as I can judge relative depth by fauna/light conditions.

However, having said that, there are a few things that could throw me off my game. Many of my decisions are based on knowing my behavior with a St72 tank. Different tank sizes, or unfamiliar terrain/conditions or stressed breathing rates ie. swimming in current could mess up my non reliance on gauges. With twins I could easily go into deco if I don't monitor time/depth and know the NDL's. In unknown terrain I could easily misjudge my actual (vs perceived) depth. With a prolonged stressed breathing rate I could easily burn through more gas than I have allotted for specific depths.

The other thing that can really throw me is task loading or myopia due to shooting video. If I become too fixated on a subject I can easily lose track of what I'm doing divewise.

I know I'm not describing depth averaging or RD. I'm looking forward to learning more about that in the near future but I sort of get the brain vs computer debate. I totally get why new divers should learn what is going on behind the algorithm so they can gain a better understanding of why, what they do now, affects what they do next. PADI used to try to do that by emphasizing exercises using the tables but I believe now they teach that as optional info. Too bad IMO.

This reminds me of my fisherman friend in Mexico. He has a deep fear of overfilled tanks. His deco strategy starts with leaving the bottom at 600 psi, and too much air in the tank screws up his system. After watching me for several dives he said, with wonder in his voice, "so you just watch your computer and don't have problems?" I, of course am sticking with him till I get close to deco and riding the computer up above him until I can't see him anymore. I know, I know. Not a good dive plan, but it beats staying on the bottom till you get low on air.

 

[Slamfire]

I believe the original proscription of computers for DIR divers had to do with avoiding the phenomenon we see with some recreational divers, where they get in the water with little or no plan...

I can see that. But I think your second reason is more relevant for the proscription at that time:

In addition, when those proscription were first promulgated, we didn't have units like the X1, that will run the entire desktop version of V-planner (making it moot whether you generate your profiles on a laptop or on your wrist).

Planing a dive is a basic tenet that should be universally encouraged at all levels regardless of technological advancements. The depth of the implications of the second reason is not as basic nor as readily apparent. The proscription came out at a time when manufacturers liked to "improve" algorithms into the realm of proprietary information that divers usually had no access to. It was very difficult, if not impossible, for a diver to formulate a plan that would match the PDC. Therefore a choice had to be made. Either you plan your dive ignoring whatever the PDC might do or follow the PDC blindly.

If you plan before hand there would be no point in having a PDC because, a)your plan will most likely not match the PDC and b)the PDC would lock you out once it detected that its plan was violated. In the best of cases the lockout would turn the computer into a bottom timer, which would have been a much better option to begin with.

If you decided to follow the computer as a priority over your previously manually formulated plan, you would be de facto jumping in with little or no plan even if you thought you had one. Why? Because manufacturers would twist the algorithms with unclear extra conservatism mechanisms. For example, I have a computer that will tell you that you have to do x mins of deco at 10'. Decoing at 10 ft was not practical on this dive. Fifteen ft was doable. Even though the computer was still proclaiming that you only had to do x mins deco, the x mins went by and 10 more went by and the computer would still not clear me out. Nowhere in the computer's documentation did they ever described that this sort of penalizing would happen under these conditions. There is no warning in the PDCs screen to alert you that time slows down at fifteen. There was no way for me to predictably know how much more time I would need to be down there. If I wanted to follow the computer I had to follow it blindly because it left me no other choice.

In short following PDCs from that time prevented you from properly planning a dive -- and this is not necessarily because the diver was negligent or lazy. As you pointed out, there are now some computers today that don't suffer from those vices.

 

[PfcAJ]

I had a great dive yesterday, but it was a GREAT example of how ratio deco breaks down.

At 150' using 21/35, the ratio is 1:1 using 50% within a bottom time range of about 30mins.

We did 80mins at 140' using 18/45. Guess what the ratio was? STILL 1:1, but with two deco gases (100% and 50%). The above ratio would have bent us like a pretzel because it just doesn't apply to the increased bottom time.

In either case, you can still use 'ratio deco' to derive an ascent schedule, but its imperative to check it against deco software to make sure the setpoint of choice holds up, and if it doesn't, you've got to adjust accordingly.

 

[LiteHedded]

there are plenty of ratios to be found and they're derived from the algorithms.
to pick a ratio and just say it's the one and only way to go regardless of the software seems silly to me

 

[Kevrumbo]

I had a great dive yesterday, but it was a GREAT example of how ratio deco breaks down.

At 150' using 21/35, the ratio is 1:1 using 50% within a bottom time range of about 30mins.

We did 80mins at 140' using 18/45. Guess what the ratio was? STILL 1:1, but with two deco gases (100% and 50%). The above ratio would have bent us like a pretzel because it just doesn't apply to the increased bottom time.

In either case, you can still use 'ratio deco' to derive an ascent schedule, but its imperative to check it against deco software to make sure the setpoint of choice holds up, and if it doesn't, you've got to adjust accordingly.

Use the 1:2 RD schedule since you're actually breathing the standard gas 18/45 with 50% and 100% O2; the reference set point is 60m (200'), so you will have to subtract 6 time intervals (your working depth is 140' which is 60' less than the 200' set point reference, or 6 time intervals of 5 minutes per interval for a total of 30 minutes) from your adjusted 1:2 ratio deco time. You should come out with 130 minutes of deco: split in half, that's 65 minutes deco time each for 50% and 100% O2 deco stops; finally shape the profiles as needed along with your initial deep stops. . .

 

[PfcAJ]

65min is 15mins less than 20/85 suggests, and I don't care to get much more sporty than 20/85 on a dive like that.

---------- Post added November 17th, 2014 at 03:53 PM ----------

Wait wait wait, are you suggesting that dive called for 130mins of deco on 50% and o2?

Thats a lot...