Min Deco for 30/30

[PfcAJ]

But a 4 hour dive on 30% is not the same deco as a 4 hour dive on 32%.

If you run it in decoplanner is barely different. Like 5mins difference.

 

[rjack321]

If you run it in decoplanner is barely different. Like 5mins difference.

not in multideco, I don't remember the difference but it was perhaps 12-15mins of deco I think? I forget the settings I was using it was 3+ years ago for me (and I felt crappy too).

I just think people should to their own research on what is "close enough" for O2 purposes and that it depends on the dive in mind.

 

[PfcAJ]

not in multideco, I don't remember the difference but it was perhaps 12-15mins of deco I think? I forget the settings I was using it was 3+ years ago for me (and I felt crappy too).

I just think people should to their own research on what is "close enough" for O2 purposes and that it depends on the dive in mind.

VPM or ZHL?

 

[rjack321]

Regarding EAD, the only formula I've seen is for nitrox only and only examines the nitrogen component. One of the many things debated in this thread is whether Helium should be considered just as bad as Nitrogen in terms of EAD, and if not, by how much? If you completely ignore Helium, then 25/25 at 100' has an EAD of 51'. If you consider Helium just as bad as Nitrogen, 25/25 at 100' has an EAD of 93'. For comparison, EANx 32 at 100' has an EAD of 81'.

Inerts are not part of the EAD formula:
(1-O2%) x (D+33) - 33 / 0.79

In Table form:
https://c1.staticflickr.com/2/1379/702248872_0249563570_b.jpg

The fact that you think the EAD has anything to do with N2 is concerning. EAD is addressing O2 (only). END addresses N2 (for sure) and O2 depending on the instructor and agency.

 

[rjack321]

VPM or ZHL?

ZHL I thought. I will recheck when I get home. At the time I felt like poo and since I was assuming -20% I though maybe 30% is more different than 32% than I expected.

 

[PfcAJ]

ZHL I thought. I will recheck when I get home. At the time I felt like poo and since I was assuming -20% I though maybe 30% is more different than 32% than I expected.

Please check. I dont own multideco, would be interesting to see if the two programs produce different results.

I did 80ft for 240min, 40/85GF with 32% and 30% (balance nitrogen) with oxygen deco.

 

[Kevrumbo]

on a min-deco dive i treat 3030 like 32% and i dive them exactly the same
always have

It's "faster" in that a tissue reaches saturation faster due to helium being less soluble.

I wouldn't object to your ideas if your suggestion was to treat helium the same as nitrogen for decompression purposes, but that's not what you're saying.

Pretending that 25/25 can be dives the same way as 32% is just reckless. But I guess I have come to expect that from anything UTD puts out regarding deco.

It's faster to move in and out of the tissue but doesn't dissolve nearly as much based on the solubility indexes mentioned in the Meyer-Overton rule.

How exactly is it wreckless to treat 25/25 the same as 32/00 for NDL?

Attacking UTD and not giving a good explanation doesn't really help anybody. For the record I've train with both GUE and UTD, if agencies matter.

The reason I was bringing up Meyer-Overton is because it makes use of gas solubilities. Helium is indeed less soluble, we can agree on that. What we are at odds with is how Helium reacts in dissolved gas theory. When descending, or more appropriately, when there is a pressure gradient setup between a higher ambient pressure under water and a lower internal pressure of the tissue, the tissue would reach full saturation sooner with Helium that it would with Nitrogen. However, by the same property, it also exits the tissue at the same rate when the opposite gradient is setup, as long as the gas is in free-phase (not dissolved). This is all based on Henry's law. However, since Helium does not dissolve as fast as Nitrogen (about 4.47 times less soluble), gas exits the tissue at a much higher rate than Nitrogen (in theory based on Graham's law.).

The canonical example is the "sugar and sand" scenario. If you pour 1 teaspoon of sugar, simulating Nitrogen, into water at a constant rate until the glass is "saturated", the sugar eventually dissolves because of the solubility of sugar. If you pour 2.65 teaspoons of sand into a glass, simulating Helium (2.65 because that's how Buhlmann would model Helium: moving 2.65 times faster into the tissue) until that glass is saturated, it would saturate much sooner no doubt. However, the sand (Helium) would be able to be removed at the same rate it came into the glass because of it's low solubility, where the sugar (Nitrogen) would have to come out of solution first.

There's also the work done by B.R. Wienke that suggests higher soluble gases causes larger bubbles, and larger bubbles are in theory related to DCS (Buhlmann). (Deep Helium • ADVANCED DIVER MAGAZINE • By B.R. Wienke and T.R. O’Leary)

Keep in mind this is all mad science of decompression theory, i.e. the wild west of the medical research world. We are way off topic from the OP. His original question was if it was "valid" to have tables that use the same NDL (I'm assuming "min-deco limits" means NDL here) for Nitrox 32% as 30/30. In my opinion, it certainly is and is very conservative to do so, based on the theories above.

From Bruce Wienke in Technical Diving in Depth:

Helium NDLs are actually shorter than nitrogen for shallow exposures . . . Reasons for this stem from kinetic versus solubility properties of helium and nitrogen, and go away as exposures extend beyond 150 fsw, and times extend beyond 40 min or so.

Helium ingasses and outgasses 2.7 times faster than nitrogen, but nitrogen is 1.5 to 3.3 times more soluble in body aqueous and lipid tissue than helium. For short exposures (bounce and shallow), the faster diffusion rate of helium is more important in gas buildup than solubility, and shorter NDLs than nitrogen result. For long bottom times (deco and extended range), the lesser solubility of helium is a dominant factor in gas buildup, and helium outperforms nitrogen for staging. Thus, deep implies helium bottom and stage gas. Said another way, transient diving favors nitrogen while steady state diving favors helium as a breathing gas.

Lynne (RIP) had a great explanation and concept of the above:

Really, it's fairly simple. Solubility determines the total number of gas molecules that can occupy a given volume, for example, a cc of blood. Diffusivity determines how QUICKLY that number of molecules can get in there. Helium diffuses very quickly, so the total number of permitted molecules will accumulate fast. But there won't be as many of them in a given volume as there are nitrogen molecules.

Imagine an elevator. There's a sign above it that says, "Men - 12; women - 3." The door opens. 12 men can get in, but they walk slowly, so it takes a full minute to fill the car. Only three women can get in, but they're moving fast, so it takes 10 seconds to get all of them in.

The point is because of Helium's high diffusivity/low solubility in transient NDL diving, it's better to clean-up on Oxygen rather than doing a min deco profile on 30/30 or 25/25 backgas alone. IOW -You're risking the "chokes" especially after repetitive min deco dives on Triox backgas only, because of venous blood supersaturation returning to the lungs alveolar bed with potential numerous diffuse Helium microbubble formation. You still need to breathe O2 to effectively wash out the He and N2 inerts at least on the last repetitive Triox dive of the day. . .

 

[sea_otter]

Please check. I dont own multideco, would be interesting to see if the two programs produce different results.

I did 80ft for 240min, 40/85GF with 32% and 30% (balance nitrogen) with oxygen deco.

From MultiDeco:

Decompression model: ZHL16-C + GF

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 40/85

Dec to 80ft (1) Nitrox 30 60ft/min descent.
Level 80ft 238:40 (240) Nitrox 30 1.03 ppO2, 67ft ead
Asc to 40ft (241) Nitrox 30 -30ft/min ascent.
Stop at 40ft 0:40 (242) Nitrox 30 0.66 ppO2, 32ft ead
Stop at 30ft 27:00 (269) Nitrox 30 0.57 ppO2, 23ft ead
Stop at 20ft 58:00 (327) Oxygen 1.60 ppO2, 0ft ead
Surface (333) Oxygen -3ft/min ascent.


Decompression model: ZHL16-C + GF

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 40/85

Dec to 80ft (1) Nitrox 32 60ft/min descent.
Level 80ft 238:40 (240) Nitrox 32 1.09 ppO2, 64ft ead
Asc to 40ft (241) Nitrox 32 -30ft/min ascent.
Stop at 40ft 0:40 (242) Nitrox 32 0.71 ppO2, 30ft ead
Stop at 30ft 19:00 (261) Nitrox 32 0.61 ppO2, 21ft ead
Stop at 20ft 53:00 (314) Oxygen 1.60 ppO2, 0ft ead
Surface (320) Oxygen -3ft/min ascent.

 

[rjack321]

From MultiDeco:

I couldn't check what I did last night, power was out. But I did realize it wasn't multideco as I didn't own it at that time. So must have been VPM. Pretty sure I didn't use 80ft either. Most likely (retrospective) plan was 3.5hrs at 90ft using VPM+4. I will check that assuming my house has power now.

 

[rjack321]

I couldn't check what I did last night, power was out. But I did realize it wasn't multideco as I didn't own it at that time. So must have been VPM. Pretty sure I didn't use 80ft either. Most likely (retrospective) plan was 3.5hrs at 90ft using VPM+4. I will check that assuming my house has power now.

Nevermind I was wrong, the most I could figure out was 8 minutes difference of deco between 30 and 32. Perhaps I was changing conservatisms at the same time. I don't know it was 4 years ago and I haven't been stuck thinking/planning with EAN30 since that time.

I always used 30/30 with -20% EAD just as I did with EAN32. But around here 30/30 isn't very useful and diving 25/25 with -20% EAD is crazy aggressive but whatever.