Ratio Deco

[lamont]

I'm curious to know the answer to your question. What has AG done to prove/develop his version of ratio deco? I know he did some stuff with some divers and scientists in Italy and I think he concluded that UTD's ratio deco was better than Buhlmann GF (I think the GF they tested against was 30/85, but I can't be certain) but that was pretty recent (after UTD's version of ratio deco had already been taught to many people). I have never seen the peer reviewed and published white paper on this research though.

In that case the RD profile produced longer decompression than whatever the "control" decompression profile was. It was pretty unsurprising that the RD profile was cleaner.

It would be more interesting to study profiles where RD was equivalent in run-time or faster than the controls. That is what would theoretically make it "better" (less time and cleaner), but I suspect I know what the outcome of that kind of study would be.

 

[Slamfire]

When I dive with rd friends, we'll always crossreference their schedule with either vpm, buhlmann gf or both. Unless the dive is somewhat short, it is quite common for me to ask them for extra time in the shallow stops to make up for their extra-deep deep stops. They always oblige because it just adds conservatism and gas amounts are not an issue. But when I don't dive with them, I don't expect them to run their rd profile on vplanner or mvplanner to see how it compares to a more mathematically rigorous model. They were not trained to do that.

Lately they have acquired Shearwater Petrels, and one of them has asked me a lot of questions about how to use mvplanner. This important skill of validating your deco strategy against other models has been acquired independently of any UTD formal training. Why would you not do that? These days access to deco software is so easy. You can run it off your phone, you can run the petrel planner... And it only takes a few seconds.

 

[lamont]

GUE's ratio is just a trick that mimics's an algorithm's output and they spend a decent bit of time discussing the limits of applying a ratio method. You're still diving the algorithm, you're just using a handy ratio to get there. This is in contrast with UTDs version which has no algorithm. Critical difference. If GUE suddenly advocated something that was not inline with an algorithm I wouldn't be behind it at ALL.

The story about GUEs ratio deco as I heard it from a senior GUE Tech instructor is that they were doing project diving in a cave which had a 150 foot profile (Does Madison Blue make sense? Its the name I remember, but I've never dove it). It was noticed that for a 20 foot bottom time there was 20 mins of deco, for a 30 foot bottom time there was 30 mins of deco. For a deeper or shallower profile they noticed the +/- 5 mins for +/- 10 feet relationship. That is pretty much the extent of it, it was just noticing that around a given profile there was an easy way to predict roughly what the algorithm would produce. In class it was also taught and mentioned that for deeper, longer dives or shorter shallower dives that the relationship breaks down and that you have to validate the mental math against the algorithm.

This also matches what I was taught by my Physics/Math/Astronomy profs as ways to quickly roughly solve harder math problems by making a linear estimate in your head. Very useful tactic on tests to validate that your exact answer you crunch out on the calculator was correct and you didn't get a minus sign wrong someplace. If there was some major discrepancy between the simple solution using approximation and symmetry arguments and the answer the calculator produced, you know you needed to track down what side the mistake was on. Some of the profs would really try to beat that tactic into students since a lot of the students would just grind numbers on the paper and the calculator and not think about the result (equivalent to just blindly following the bend-o-matic and not understanding deco at all).

The "rule of 120" for NDLs is also a linear approximation that breaks down as you start going shallower or deeper as well.

I use "depth (feet) * 10" for rockbottom in LP104-sized tanks and "depth (feet) * 10 + 300" for rockbottom in LP80-sized tanks as well all the time and that works over a recreational depth range.

 

[LiteHedded]

The story about GUEs ratio deco as I heard it from a senior GUE Tech instructor is that they were doing project diving in a cave which had a 150 foot profile (Does Madison Blue make sense? Its the name I remember, but I've never dove it). It was noticed that for a 20 foot bottom time there was 20 mins of deco, for a 30 foot bottom time there was 30 mins of deco. For a deeper or shallower profile they noticed the +/- 5 mins for +/- 10 feet relationship. That is pretty much the extent of it, it was just noticing that around a given profile there was an easy way to predict roughly what the algorithm would produce. In class it was also taught and mentioned that for deeper, longer dives or shorter shallower dives that the relationship breaks down and that you have to validate the mental math against the algorithm.

This also matches what I was taught by my Physics/Math/Astronomy profs as ways to quickly roughly solve harder math problems by making a linear estimate in your head. Very useful tactic on tests to validate that your exact answer you crunch out on the calculator was correct and you didn't get a minus sign wrong someplace. If there was some major discrepancy between the simple solution using approximation and symmetry arguments and the answer the calculator produced, you know you needed to track down what side the mistake was on. Some of the profs would really try to beat that tactic into students since a lot of the students would just grind numbers on the paper and the calculator and not think about the result (equivalent to just blindly following the bend-o-matic and not understanding deco at all).

The "rule of 120" for NDLs is also a linear approximation that breaks down as you start going shallower or deeper as well.

I use "depth (feet) * 10" for rockbottom in LP104-sized tanks and "depth (feet) * 10 + 300" for rockbottom in LP80-sized tanks as well all the time and that works over a recreational depth range.

that's about right. and it works in the constraints of what you're certified to do. so outside of about 25-30 minutes of bottom time(shallower and deeper are ok with the ratio rules but long bt will break it), the ratio deco breaks down. it works for your average tech 1 or tech 2 dive but for any real exposures ratio deco is a bad idea

and in tech 1 and 2 they will show you how it breaks down on decoplanner and the safe ranges where it works pretty well

it approximates buhlmann 20/85 and vpm (which are very similar on these short bottom times) up until around 30 minutes

we certainly dont use it for any of our diving here in north florida. i would welcome a UTD diver to come to emerald sink and do 120 minutes at 200+ feet using ratio deco. they'll be out of the water way faster than me and that will give them plenty of time to call the ambulance

 

[MXGratefulDiver]

that's about right. and it works in the constraints of what you're certified to do. so outside of about 25-30 minutes of bottom time(shallower and deeper are ok with the ratio rules but long bt will break it), the ratio deco breaks down. it works for your average tech 1 or tech 2 dive but for any real exposures ratio deco is a bad idea

and in tech 1 and 2 they will show you how it breaks down on decoplanner and the safe ranges where it works pretty well

it approximates buhlmann 20/85 and vpm (which are very similar on these short bottom times) up until around 30 minutes

... that's pretty much how it was explained to me in the NAUI Trimix 1 and Trimix 2 classes where I learned it. We used it for pretty much all of our dives, but most of our tech dives around here are less than 250 feet and relatively short bottom times (30 minutes or less). One of the nice things about diving cold water is that thermal units often become the limiting factor ...

... Bob (Grateful Diver)

 

[LiteHedded]

... that's pretty much how it was explained to me in the NAUI Trimix 1 and Trimix 2 classes where I learned it. We used it for pretty much all of our dives, but most of our tech dives around here are less than 250 feet and relatively short bottom times (30 minutes or less). One of the nice things about diving cold water is that thermal units often become the limiting factor ...

... Bob (Grateful Diver)

it's a great tool for your average ocean tech dive. but the way UTD has taken the ball and run with it is where they lose me. all you have to do is run the actual schedule in software and see in some cases it's HALF the prescribed deco.

then when you ask them what science it's based on you get crickets. oxygen window and deep stops sound good on paper but there just isn't anything behind it. and leaving the deco shape of it out of it, there's nothing that backs up their ratios on longer bottom times. the ratios aren't linear as bottom time increases and we know risk goes up when bottom time goes up. so reducing the deco the longer the dive gets seems like insanity to me

now i've got to get Consuela in here to dust my blood cells from all this 20/85 diving i've been doing....

 

[nimoh]

when using UTD RD, at what point during the ascent does offdusting begin?

 

[decompression]

Sent from my iPhone using Tapatalk

 

[LiteHedded]

is that an official UTD hoodie?

 

[lamont]

The story about GUEs ratio deco as I heard it from a senior GUE Tech instructor is that they were doing project diving in a cave which had a 150 foot profile (Does Madison Blue make sense? Its the name I remember, but I've never dove it).

*Indian Springs apparently.

My memory is crap.