The conservatism of RD?

[grazie42]

Hi all,
A thread on another board about RatioDeco left me with some questions, because I´m impatient and that thread has lost some of its momentum I´ll ask the question here where we (hopefully) won´t get into arguments about the pre-requisits of RD...I was wondering if someone would be willing to take the time to add a collumn of GF-decoplanner (30/85), it would be nice to see it all in one table ...

Here´s a comparison I made between RD and V-planner +2 (all according to AG´s RD-paper):
I did this in metric so the figures in feet are rounded, BT=30 mins for all profiles:
T1-range(?) 21/35 with 50%______V-plan +2__________RD______Diff
35m/120ft_________________________18mins_________15min s_____+3
40m/135ft_________________________25mins_________25min s_____+/-
45m/150ft_________________________32mins_________30min s_____+2
50m/170ft_________________________39mins_________40min s_____-1

T2-range(?) 18/45 with 50% +O2____V-plan +2_________RD_______Diff
55m/180ft_________________________45mins__________40mi ns_____+5
60m/200ft_________________________56mins__________50mi ns_____+6
65m/220ft_________________________65mins__________ 60mins_____+5
70m/240ft_________________________74mins__________ 70mins_____+4
*I didn´t reset all my configs for this so the numbers on V-planner vary a bit depending on different descent/ascent-rates, last-stop depth etc but not significantly

[amascuba]

The profiles weren't generated with decoplanner, but with the same settings as decoplanner (buhlman 30/85), so they should be the same numbers that decoplanner would generate. (should being the key word) It's also worth noting that as you get to the deeper portions of both your profiles that your bottom mix and deco mixes aren't the most efficient combinations of gases.

Here you go:

profiles:
T1-range(?) 21/35 with 50%______V-plan +2________GF30/85__________RD______Diff
35m/120ft_________________________18mins______________ 22min_________15min s_____+3 (v-plan) +7(GF30/85)
40m/135ft_________________________25mins______________ 19min_________25min s_____+/- (v-plan) -6(GF30/85)
45m/150ft_________________________32mins______________ 27min_________30min s_____+2 (v-plan) -3(GF30/85)
50m/170ft_________________________39mins______________ 38min_________35min s_____+4(v-plan) +3(GF30/85)

T2-range(?) 18/45 with 50% +O2____V-plan +2________GF30/85_________RD_______Diff
55m/180ft_________________________45mins______________ 41min__________45mi ns_____+/-(v-plan) -4(GF30/85)
60m/200ft_________________________56mins______________ 55min__________50mi ns_____+6 (v-plan) +5(GF30/85)
65m/220ft_________________________65mins______________ 69min__________ 60mins_____+5 (v-plan) +9(GF30/85)
70m/235ft_________________________74mins______________ 83min__________ 65mins_____+9(v-plan) +18(GF30/85)

Also, one thing that I like to do is make my stops divisible by 5, which usually means me rounding up the times.

For example: For the 120ft profile, I would give myself 20min of deco because in my head it's much easier to say 20/2 = 10 and 10/5 = 2.

That would give me the profile of:

70fsw - 2min.
60fsw - 2min.
50fsw - 2min.
40fsw - 2min.
30fsw - 2min.
20-0fsw - 10min.

Of course that is a linear curve and I would want to take advantage of the oxygen window at 70 and 60fsw, so I would steal time from the 50 and 40fsw stops and put them at the 70 and 60fsw stops, so the new times would be:

70fsw - 3min.
60fsw - 3min.
50fsw - 1min.
40fsw - 1min.
30fsw - 2min.
20 - 0fsw - 10min.

For me, that would be a nice conservative profile. Straight buhlman actually gives you 18min of deco with 50% I believe. I would have to double check.

[grazie42]

Thanks!
The RD-text onlys says buhlman 30/85 so you posted what I was looking for...

Looking at your table, it just makes me wonder why they´d say that RD would generate profiles similar to V-planner +2 & Buhlman 30/85...especially for the deeper range the differences are more than trivial...I´m also surprised at the difference between v-planner & buhlman...

[amascuba]

Quote: (Originally Posted by grazie42)

Thanks!
The RD-text onlys says buhlman 30/85 so you posted what I was looking for...

Looking at your table, it just makes me wonder why they´d say that RD would generate profiles similar to V-planner +2 & Buhlman 30/85...especially for the deeper range the differences are more than trivial...I´m also surprised at the difference between v-planner & buhlman...

V-planner uses VPM-B as the deco algorithm I believe, which is a bubble model. Bubble models basically say that disolved gases will start to form bubbles much deeper in the water column, during ascent, and therefore call for deeper stops.

Straight Bulhman is a disolved gas model, which like the US Navy Tables, typically has longer, but shallower stops by default. Over time people started adding gradient factors into the model so that they don't push the M-Values so much. Therefore basically making it a gypsy-rigged bubble model in a sense. I don't know much scientific data the gradient factors have other than by trial and error over a period of time.

In all reality decompression theory == best guesstimate and doesn't account for individual physiology.

[Data]

Quote: (Originally Posted by grazie42)

50m/170ft_________________________39mins______________ 38min_________35min

looking at the imperial calculation, shouldn't 170fsw come to 40min using RD?

[amascuba]

Quote: (Originally Posted by Data)

looking at the imperial calculation, shouldn't 170fsw come to 40min using RD?

Yep, that's right.

[grazie42]

The "real" difference between 45m & 60 meters is 17ft...so 50m is really 167ft...
The "conservative" option would be to +10mins (I used +5mins), if you´r going to be really accurate you should add 5mins+5*0,67=8,35mins...I only counted on "full" 3m/10ft changes...

You are correct though...I propably should have done +10 on the 170ft level for RD...I´ll change it in my table above...

Thanks for helping me get the comparison fairer...

[Data]

Quote: (Originally Posted by grazie42)

The "real" difference between 45m & 60 meters is 17ft...so 50m is really 167ft...
The "conservative" option would be to +10mins (I used +5mins), if you´r going to be really accurate you should add 5mins+5*0,67=8,35mins...I only counted on "full" 3m/10ft changes...

You are correct though...I propably should have done +10 on the 170ft level for RD...I´ll change it in my table above...

Thanks for helping me get the comparison fairer...

while you're at it, in the T2 graph, 180fsw should be 40min and 235fsw round up to 240fsw comes to 70min of deco.

maybe that's part of the problem, metric and imperial conversions.

[grazie42]

Corrected that too...

I think you´re right...it seems like silly misstakes but I´m afraid there´ll always be rounding-errors...or maybe I was just stupid when I did the original last night....

[grazie42]

I just revised the compilation with my mistakes corrected to make for easier viewing...
Here you go:

profiles:
T1-range(?) 21/35 with 50%______V-plan +2________GF30/85__________RD_____________Diff
35m/120ft_________________________18mins__________22mi n_________15mins_____+3 (v-plan) +7(GF30/85)
40m/135ft_________________________25mins__________19mi n_________25mins_____+0 (v-plan) -6(GF30/85)
45m/150ft_________________________32mins__________27mi n_________30mins_____+2 (v-plan) -3(GF30/85)
50m/170ft_________________________39mins___________38m in________40mins_____-1(v-plan) -2(GF30/85)

T2-range(?) 18/45 with 50% +O2____V-plan +2________GF30/85_________RD_____________Diff
55m/180ft_________________________45mins___________41m in_________40mins_____+5(v-plan) +1(GF30/85)
60m/200ft_________________________56mins___________55m in_________50mins_____+6 (v-plan) +5(GF30/85)
65m/220ft_________________________65mins___________69m in_________ 60mins_____+5 (v-plan) +9(GF30/85)
70m/235ft_________________________74mins___________83m in_________ 70mins_____+4(v-plan) +13(GF30/85)