How did you feel after that Deco ?

[DevonDiver]

Just think of ambient as meaning 'surrounding'.

At 10m, the ambient pressure surrounding a bubble on the body isn't 2ata....its the inert gas pressure in the surrounding tissue.

 

[joshk]

That is not bottom gas, it is Deco gas, It was exploration in a quarry.

Rephrase: Why use 50% for deco after a short 42m dive?

 

[Remy B.]

Two of those dives a day, and the water is cold so you want to reduce your Deco time.

 

[Kronek]

Rephrase: Why use 50% for deco after a short 42m dive?

Two of those dives a day, and the water is cold so you want to reduce your Deco time.

At first I agreed with Joshk, thinking 100% would be the better gas for deco time. but after running them with 100 and 50, the deco time is the same within 30 seconds. 100% being 30 seconds faster.

Personally I would still use 100%, but for this dive, time wise either is the same.

 

[DevonDiver]

50% has additional benefits. 100% really cuts down your options.

It's absolutely what I'd want to use for extended range dives with a single deco.

You can switch to it at 21m. This gives a nice curve with a good differential from depth.

I like to get off-gassing kick-started as soon as possible. There's a lot we don't know about deco....but there are theories about compartments working in series (i.e. DCIEM). Smashing the fast tissues early in the ascent could pay dividends with slower tissues that you wouldn't get from delaying your accelerated deco until the 6m stop was reached. Whatever your views on deep stops, getting off bottom mix early helps reduce bubble scores.

50% gives you an alternative gas to access at mid-ascent depth, so it's got some bailout type uses. If you had a low on bottom gas scenario, the emergency ends at 21. You'd not have to skip any stops. If using 100% you'd miss stops all the way to 6m. Ouch.

Lastly, if anything unforseen happened later in the deco phase, then you've already been decompressing... so when you hit the surface the repercussions could be less severe.

The only reason I can think of to use 100% as a sole deco gas is if you're in a situation where blending isnt easy or possible.

I hear some people say that 100% is good because it covers surface O2 options. That's BS. You should aways have a separate and dedicated O2 source on the boat.

I'd use 50% for a bunch of dives, even if they're no-stop. It's a good ascent gas for cleaning out the tissues.

 

[PfcAJ]

100% makes sense on longer bottom times at depths 100ft and shallower.

Other than that it's 50%, then 50% plus 100%.

 

[DevonDiver]

100% makes sense on longer bottom times at depths 100ft and shallower.

Interested as to your reasoning why?

Because your deco would typically be shallow only?

50% is basically a deco and a bailout/pony/stage in that depth range. That's a big advantage.

But my thinking is probably skewed by the type of diving I do at those ~100ft depths. Normally 60+ mins bottom time on wreck penetration. 50% would give me a certain flexibility in some gas scenarios.

 

[Remy B.]

I find very important to start flushing deeper as did mention Andy, I will choose a 50% then a 80%, and Deco with two gases, I see them as more flexible gases with less compromise than the 100%, I understand AJ preference for 100%, it will be good for cave as you don't have the conflicts with waves and boats in the area, nor much currents at surface, for wreck diving I believe the 80% give you a little more flexibility and the difference between both are small.

 

[kensuf]

For shallower than 100' to 110', the 100% instead of 50% option is preferred in my mind.

Your first stop will rarely be deeper than 50' and even then it should only be for a minute or two max.

The 40' stop is rarely longer than 5 minutes and the deco advantage there by bringing 50% gas is negligible.

I'll agree that the 30' stop can get a bit longer, but 50% over bottom mix is only saving you 30% of your time and that stop is nowhere near as long as the 20' and 10' stops.

Using 100% at 20' and 10' instead of 50% will shave 1/3rd of your deco time off at those stops.

Finally, if something goes tits up you can use the remaining 100% on the surface as an emergency gas.

As for bailout gas, there's an old saying, plan your dive and dive your plan. If you're planning on spending 10 minutes at 30' on deco, plan adequate reserves.

Here's two quick examples, 110' for 90 minutes on 30%. Both with a 50% deco schedule and a 100% deco schedule.

MultiDeco 4.07 by Ross Hemingway,
ZHL code by Erik C. Baker.

Decompression model: ZHL16-B + GF (40/85)

DIVE PLAN - 110' for 90 minutes on 30% w/50% deco gas
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 40/85

Dec to 110ft (1) Nitrox 30 60ft/min descent.
Level 110ft 88:10 (90) Nitrox 30 1.30 ppO2, 94ft ead
Asc to 70ft (91) Nitrox 30 -30ft/min ascent.
Asc to 50ft (92) Nitrox 50 -30ft/min ascent.
Stop at 50ft 1:00 (93) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 4:00 (97) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 10:00 (107) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 15:00 (122) Nitrox 50 0.80 ppO2, 1ft ead
Stop at 10ft 28:00 (150) Nitrox 50 0.65 ppO2, 0ft ead
Surface (150) Nitrox 50 -30ft/min ascent.

Total run-time: 150 minutes

MultiDeco 4.07 by Ross Hemingway,
ZHL code by Erik C. Baker.

Decompression model: ZHL16-B + GF (40/85)

DIVE PLAN - 110' for 90 minutes on 30% w/100% deco gas
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 40/85

Dec to 110ft (1) Nitrox 30 60ft/min descent.
Level 110ft 88:10 (90) Nitrox 30 1.30 ppO2, 94ft ead
Asc to 50ft (92) Nitrox 30 -30ft/min ascent.
Stop at 50ft 1:00 (93) Nitrox 30 0.75 ppO2, 41ft ead
Stop at 40ft 6:00 (99) Nitrox 30 0.66 ppO2, 32ft ead
Stop at 30ft 14:00 (113) Nitrox 30 0.57 ppO2, 23ft ead
Stop at 20ft 10:00 (123) Oxygen 1.60 ppO2, 0ft ead
Stop at 10ft 17:00 (140) Oxygen 1.30 ppO2, 0ft ead
Surface (140) Oxygen -30ft/min ascent.

Total run-time: 140 minutes

 

[DevonDiver]

Finally, if something goes tits up you can use the remaining 100% on the surface as an emergency gas.

As for bailout gas, there's an old saying, plan your dive and dive your plan.

You shouldn't consider 50% as a bailout gas, but you can consider 100% for surface oxygen?

Part of the dive plan should be an adequate supply of surface O2....just as you'd plan an adequate supply of in-water gas.

Plan your dive....