Air or Nitrox as a diluent?

[Capt Jim Wyatt]

what do you see as the advantage to using air at 100ft instead of nitrox?

Ability to flush down below 1.3 @ 100 feet.

I firmly believe in diving the same dil gas as you'd use if doing that dive on open circuit.

I can dig it....

 

[tbone1004]

Just to expand this a little...

Dil should have a lower PPO2 so that you can flush down the loop. Thus a dil should be around 1.0 PPO2 at the bottom.

A bailout would, if using OC principles, be around 1.3 at the bottom.

So for deeper dives, say 60m/200', you'd be looking at a dil of 15% but a bailout of 18%. Or would you simply match your bailout to the dil - say 15/55 instead of 18/45?

My configuration uses "dilout" or "bailuent", so the gases are one and the same.

Ability to flush down below 1.3 @ 100 feet.

I can dig it....

What is the benefit of the ability to flush down below that? Do those outweigh the benefits of being able to do sensor validations at 1.2/1.3? I usually dive a ppO2 of 1.1, so yes when I dil flush, the ppO2 usually goes up a bit in Ginnie/JB/etc. but I would rather not suffer the deco penalties if I had to bailout or go to SCR mode from 4,000ft.

Part of this comes from a dilout/bailuent concept where on the sidemount rebreathers you don't have dedicated dil bottles, nor do you have them in the DIR style racks. Those are acceptable for open water with a DSV, but a 3l dil bottle has much better use as an inflation bottle in a cave than it does a dil bottle, and in any case it has no business being connected to a BOV.

 

[Capt Jim Wyatt]

I would rather not suffer the deco penalties if I had to bailout or go to SCR mode from 4,000ft

Right on...I'm a convert now especially since my dil gas is offboard.

 

[Wibble]

My configuration uses "dilout" or "bailuent"

Nice one

 

[rjack321]

So for deeper dives, say 60m/200', you'd be looking at a dil of 15% but a bailout of 18%. Or would you simply match your bailout to the dil - say 15/55 instead of 18/45?

This is me

I only really go over 1.0 on dil when diving a system that I know has a hard floor not exceeding about 105ft and using 32%. These dives often have large portions that are very much in the 32% range (70-95ft)

Otherwise dil is max 1.0 and I often end up with the next deeper gas due to uncertainty about the max depth.

If I had a cave with a max depth of 200ft (and I knew that) and the average depth was 130ft, I would probably bring 18/45. If the system had extensive sections at 200ft I would probably bring the deeper gas. Basically I'm not going to plan a 3 to 5 hour dive around a 5 min dip to 1.2 in the dil. If I was down there at 1.2 in dil for 30mins then yeah probably the deeper gas.

Every cave dive I have done for the last 4 or 5 years uses the OC BO as dil and the dive could in theory be done and "make sense" to me as an OC dive on the chosen BO/dil.

 

[Wibble]

The main challenge is the deep bailouts are really expensive to fill and difficult to top off without diluting the helium. Dil, on the other hand, is easy and cheap. I'm happy to be diving a stronger helium dil mix, but have to take the bailout as being a little more helium lean.

 

[rjack321]

The main challenge is the deep bailouts are really expensive to fill and difficult to top off without diluting the helium. Dil, on the other hand, is easy and cheap. I'm happy to be diving a stronger helium dil mix, but have to take the bailout as being a little more lean.

This is the reason I don't dil/bo in open water.
It's a huge pain for me to refill the larger BOs when I've only used a little. And the "optimal" BO gas is usually a shallower gas than the dil I would use.

e.g. why not dive 15/55 at 200ft for dil? But for BO I would take 18/45 or even 21/35 in a real pinch.

 

[JahJahwarrior]

That's what I love about diving backmount unit with inflation gas onboard and two offboard gases. One with lower o2 which is preferred for dil flush cell validations and second with higher o2 which is preferred for bailout and SCR situations.

I have my onboard inflation gas able to plumb in as dil too, primarily for recreational ocean diving and while entering before having offboard bottles on, and I think you guys just talked me into filling that with 32% instead of air. Negligible extra cost if any as it's usually a minimum fill situation, and that is a free way to reduce deco penalty while operating SCR.

 

[tbone1004]

This is the reason I don't dil/bo in open water.
It's a huge pain for me to refill the larger BOs when I've only used a little. And the "optimal" BO gas is usually a shallower gas than the dil I would use.

e.g. why not dive 15/55 at 200ft for dil? But for BO I would take 18/45 or even 21/35 in a real pinch.

what's the deco penalty of doing it in OW though? obviously I'm coming from a cave perspective where it really matters.

Todays dive was 3 hours bottom time in Ginnie. If you plug that into decoplanner on EAN32 60/80, it's 69mins of deco. If you had to bailout at 75min bottom time to air, you have a 117min deco. That's quite a bit and you also get to validate O2 cells at 1.3ppO2 instead of 0.8ppO2 if you normally run your setpoint up there. If you are using the "normal" configuration with 3l diluent and using a DSV, then the only advantage you have is a bit less O2 consumption, though you shouldn't do that in a cave IMO. Use the 3l for suit/wing inflation and plug offboard in.

Since we shifted the conversation away from the original question *shocker*, and went to trimix, then I still argue that the ideal option is to use whatever you would be diving in OC for that dive, but with practical exceptions. I'd probably keep a set at 10/70, 18/45, EAN32. No real need to have anything in between each of those gases and since you're in OW, the deco penalty is minimal.

@JahJahwarrior what is good about having a lower ppO2 to validate during dil flushes? You want to validate the cells at the setpoint that you're running the rebreather. If the gas is at 0.8 and you don't intend to run down there, the only thing you can do is see how fast the cells are responding. It's important, but nowhere near as important as linearity or current limiting.

 

[Superlyte27]

I run air in my left tank and 30-32% in my right tank for a dive to 100’

I want to be able to move a PPO2 quickly. I can do it a lot faster with air than 32%.

I hadn’t bailed out for almost a decade, but earlier this year I sheared off the OPV on the sidewinder in Wormhole. My simple hour long dive on CCR changed to an added 40+ minutes of added deco. because both of my tanks were air. I was also complacent in not having a bottle of oxygen at 20’

I don’t do that anymore. Dil 21 in left bottle, 32 in right bottle and certainly oxygen near me he surface.