Ongoing discussion of Ratio Deco

[TSandM]

This is probably a stupid, naive question, but don't decompression programs often (if not always) tell you at what depth offgassing begins? And if you delay below that depth, it's got to be additional bottom time, right?

 

[rjack321]

This is probably a stupid, naive question, but don't decompression programs often (if not always) tell you at what depth offgassing begins? And if you delay below that depth, it's got to be additional bottom time, right?

Sort of. If you set decoplanner to a GF of 01/XX then you get the depth at which you get one compartment just starting to off gas. But its probably done in 10-30seconds. What's the remainder of that time? Who knows.

Richard Pyle was diving per tables years ago and figured out that something was happening even deeper and slowing down was good. Even if that was below an hypothetical/mathematical offgasing threshold.

So you can add that mid-level time into you BT if you want, or use to to shallow up your average depth a bit. But in either case its not a strict 1:1 relationship (medium depth time does not = BT nor DT).

RD requires scubamath. Highly precise estimates and averages don't fit very well to the overall approach. plus-minus 10ft is more like it.

 

[limeyx]

RD uses "80% ATA (75% depth)" as an approximation of the commonly calcuated start of decompression zone where the leading compartment has a gradient to off-gas. Just the same as running a VPM-B profile (deco zone starts ~2 ATA less than bottom) or running a (GF Low=5 to 10) Buhlmann profile. /QUOTE]

Why not simply adjust the low GF value in Decoplanner to incorporate the deep stops and the high GF to get the total deco time in line? This allows for extension to profiles not quite at the "RD sweet spot" and that use an accepted GUE approach (the science behind Decoplanner)...GF 5/90 or thereabouts yields very GUE-RD profiles.

I think the RD approach is great because it can be used "on the fly" - but why do that when most dives are planned? Decoplanner is a very good tool. Why limit the power of a tool that has a GF adjustment. After all, the low GF applies only to the deepest stop, and the high GF to the shallowest - all the other stops are linear interpolations of GF setting by depth (deepest to shallowest).

Stow the Napalm, just use the flamethrower - Good thread.

Because

1) Why struggle wrestling with an application trying to get it to meet a profile that you already know you want?

2) I dont think any combo of gradient factors will give the RD shape to a profile
3) Ratio deco has the benefit that you can change the plan without having to memorize all kinds of tables. I have cut dives short, or extended them, or turned up at a shallower or deeper site enough to find this really useful.

 

[limeyx]

Well think of it this way...
You do a 25 min dive at 150ft. First deep stop is 100ft

You start ascending and, gas permitting (deco and backgas), you see something really cool at 120ft. You spend 5 minutes looking at it. Total time now is 30 mins (you had an instant descent) You haven't started deepstops or deco. You're still "on the bottom phase" for this 5 mins.


...
Clear as mud?

I would do something similar and to an extent it is down to experience.
This is why it helps to use a bottom timer and the MDL version of ratio deco right from the start -- you get used to multi-level up and down profiles at the recreational levels first where I think you have a bit more leeway either side.

Then when you start going deeper, you have more of a basis.

I track avg depth in my head then when I download the dive, I can use the computed average (over just the time we spent on the bottom) to compare. I am usually close enough for my liking.

Long cave dives are definitely more tricky though if they vary in depth a lot. This is something that I definitely need to work on.

 

[grazie42]

I think I have a better understanding of how you use RD now...
My main concerns with RD would be:
1. Keeping track of avg. depth
2. Deco obligation
3. Gas management

If you only use RD to account for changes to a planned profile then I really don´t see a problem...
I had visions of people just jumping in, "doing their thing", keeping track of avg. depth, calculating their deco obligation and figuring out if they had enough gas "on the fly"...that´s obviously not the way you do it..

One of the most surprising things in my cave-course was the amount of mental work you are expected to do during a dive. Keeping track of "waypoints" (ie at what time and depth did we hit the "florida room", cornflakes, the lips etc and how much gas did you and your closest buddy have left?). I now try to find waypoints in all my dives and remember them, just to get into the habit of "paying attention"...

As long as you have a "plan" before you hit the water, I´d expect most people to be able to adapt it somewhat to circumstances...If nothing else, I always carry a table for 5+5 (depth & time)...After diving different profiles, over time, you start remembring the different decoplans and how they vary with depth and time (I try to memorize my decoplan before every dive so it tends to stay with me)...

I think this thread and the other one have made me realize that RD is both a lot more and a lot less than I thought..if that makes sense...

Thank you rjack321 and limeyx for taking the time to explain this stuff...

 

[ScubaMilo]

There was something about the "tone" of your post that REALLY pissed me off last night, I´ll just put it down to the limitations of the medium and move on...sorry...

Sorry if my post came off as having a "tone" I was simply trying to convey my point as clear as possible no "tone" intended.

I respect the the fact that you have taken the time to compare the different profiles and deco aproaches.
This is basicly the point that I was trying to make about the different approaches.
Some times they match and other times they don't.
It's a good Idea if you are aware of the times when certain program stray from the norm so that you can plan accordingly.

No hard feelings I hope,(none here anyway)
Milo

 

[limeyx]

I think I have a better understanding of how you use RD now...
My main concerns with RD would be:
1. Keeping track of avg. depth
2. Deco obligation
3. Gas management

If you only use RD to account for changes to a planned profile then I really don´t see a problem...
I had visions of people just jumping in, "doing their thing", keeping track of avg. depth, calculating their deco obligation and figuring out if they had enough gas "on the fly"...that´s obviously not the way you do it..
...
Thank you rjack321 and limeyx for taking the time to explain this stuff...

No problem. here's how I (and every DIR diver I personally dive with does it)

-- this stuff is covered in the sections of "GUE EDGE" but we (bad us) dont always use that acronym.

1) Plan the exposure for the dive (one of the 'E's) so we might say let's average 150 feet for 25 mins. This could mean go out at 160 and back at 140, or just cruise at 150.
Then we might say "Well, if the vis sucks at 150, we will come up to 130" (if it is a reef or a tall wreck)

2) Based on the exposure, we calculate the likely deco amounts.
This is nice because say we planned 25 mins at 150, but decide instead to dive at 130. We can either do 25 mins and cut the deco to 15 *or* we can do 10 mins more and keep the deco at 25 -- we discuss this before hitting the water.

3) deco profile -- for T1 it's easy. We need the depth for 1st deep stop (usually 100 or 90 feet), and then the profile for 70-20 feet -- we usually only need 15,20,25 or 30 min profiles for T1 type diving. We also have a "min" deco (usually 1's from 70 feet and a few mins at 20) if we cant find the wreck or the vis sucks

4) calculate Minimum gas/rock bottom and turn pressures.

5) Dive. Generally I will not modify a turn pressure or a Rock bottom if we change the dive, so the planned exposure is generally the max we intend to do.

6) on ascent, the deco captain takes over (either at 1st deep stop or at 70 feet -- doesnt really matter, just agree beforehand)

7) team follows deco captains signals. If team has a violent disagreement with deco plan, they can voice their concern to the captain. RJack has a nice technique where he writes the plan in big numbers in his wetnotes and passes it around the team.

8) in T1 depths, generally you spend more time arguing over a minute or 2 here and there than it's worth! BUT what you are really doing is building experience and TEAM skills for more aggressive diving also -- eventually you should manage to all be in sync enough to not need to negotiate deco.

GUE is very very good at making people do dive plans before they hit the water, but I can definitely see how just by looking at ratio deco, the planning aspect might not be so obvious.

GUE tries to make sure all the "hard" stuff happens on the surface, and simplify everything that needs to happen underwater (as probably other agencies do too -- I just am only familiar with GUE) -- this is one reason for putting the MOD on deco bottles rather than the mix -- no need to calculate anything underwater.

These techniques have served our teams well so far.

 

[rjack321]

I think I have a better understanding of how you use RD now...
My main concerns with RD would be:
1. Keeping track of avg. depth
2. Deco obligation
3. Gas management

If you only use RD to account for changes to a planned profile then I really don´t see a problem...
I had visions of people just jumping in, "doing their thing", keeping track of avg. depth, calculating their deco obligation and figuring out if they had enough gas "on the fly"...that´s obviously not the way you do it..
..

Jump right in? Heck no!

The plan, roughly consists of...
(I'm only diving T1 profiles so this is limited to 21/35 or (better for me) 18/45 +50% deco gas)

A minimum gas as PSI
Max deco typically as minutes (100 psi in an AL40 is 1 min of deco on average amazingly)
Max depth
Max time

Those general constraints end up being:
60-75cf of backgas as rockbottom depending on max depth
30 mins deco
170ft max
70 min max runtime

Those are really the physical constraints dictated by backgas and deco gas capacities.

The point is that you can "trade" depth for time or vice versa in RD quite easily.

Of course because of your gas limits you can't go doing 50 min, 150ft dives. You don't have the backgas for rockbottom, the deco gas for the deco, nor the backgas in case you lose your deco gas and have to deco out on that (2x time or 1.5x time by sharing the 50%).

So you do have to know your "trading limits" and those happen to be ~170ft max depth, 30 mins deco. And those are the T1 limits.

Beyond T1 you've got additional resources to trade amongst time/depth/deco. So its a more complicated playing field.

 

[grazie42]

Those general constraints end up being:
60-75cf of backgas as rockbottom depending on max depth
30 mins deco
170ft max
70 min max runtime

So you do have to know your "trading limits" and those happen to be ~170ft max depth 30 mins deco. And those are the T1 limits.

Beyond T1 you've got additional resources to trade amongst time/depth/deco. So its a more complicated playing field.

I understand what you´re both saying but take a look at the part I quoted above...

Does that really look very different from:
-"let´s jump in, no deeper than 170ft max AVG. depth and no more than 70min RT or 30mins of deco, RB is 65-75cf, agreed?
-"agreed!"
-"let´s dive already!"
-"dive, dive, dive!"

 

[limeyx]

I understand what you´re both saying but take a look at the part I quoted above...

Does that really look very different from:
-"let´s jump in, no deeper than 170ft max AVG. depth and no more than 70min RT or 30mins of deco, RB is 65-75cf, agreed?
-"agreed!"
-"let´s dive already!"
-"dive, dive, dive!"

It may not *look* so different, but it really is.

We dont just say "oh 65cft" each of those items has detail behind it. Each person calculates their Minimum gas PSI, and their turn pressure. We go over the deep stops and shape of the S-curve etc. We discuss who is #1, #2, #3, who is going to run deco, who shoots the bag if needed etc.

It *is* true that the dive leader feels the dive is better at 140 than 160, then they can turn to the team, indicate we level off of 140 and then adjust the dive -- which is also easy. The team leader just turns to #2, indicates "Hey, 5 more mins in that direction?" everyone consults their turn pressure, decides if they have gas for 5 more mins out (and depending on the dive plan) 5 more mins back and OK's or turns the dive.

Dive planning goes something like

G(oal) -- the goal of the dive (explore this wreck, try to go a bit further into the cave than last time, video this fish etc.)
U(nified team) -- who is #1, #2, #3, who is captain, who does deco etc. etc. + any hand signals that we might have a difference on
E(quipment) -- primary reel, video camera etc -- this is anything beyond the standard basics.
E(exposure) MAX depth, time, Expected depth, time
D(eco) deep stops and shallow stops for max and expected deco. Including depths and times and +/-5 min, +/-10 feet. Also "min deco" -- missed the wreck. Also, can we convert a deeper dive to shallower profile if necessary or should we just abort if conditions are crappy
G(as) minimum gas, dive plan (1/3, 1/2 or all usable), turn pressures, what bottom stages if neccessary we have
E(nvironment) -- if we expect silt, rocks, where the dive site is etc. etc.

This is in addition to

- Modified valve drill (are valves for all bottles -- backgas, argon, deco, stages) fully open
- Modified S -- can long hose be fully deployed
- Equipment check (top to toe, including all pockets)
- Bubble check.

So yes, things can be flexible underwater (and it only takes 1 person to say let's not mess with it and end the dive), but just jump in the water? Not really.

EDIT: And this really all goes right back to the most important GUE concept -- the unified team (which I honestly at first thought was something of a "hogwash" idea)

If you trust your team, all of this becomes "easy" If you are diving with a brand new team, ideally you
1) dive recreationally first
2) start off with simple, fixed plans until you get comfortable.

DIR/GUE makes (1) and (2) very much easier, but nothing substitutes for actually going diving with someone.