Adjusting nitrox mix in twinset?

[seeker242]

So my AL80 doubles set is currently filled with 32%. The next dive I want to do necessitates 28%. According to a blending calculator, you can bleed off 1,100 psi and fill with air to get 28%. However, will this work with doubles given that it has to mix thru the manifold? Or, would it be better to just drain them to 500 and refill? Does it matter?

 

[packrat12]

bleed off what you need, fill the rest with air. The gas in the tank will mix naturally, just have the manifold open.

 

[joe10540]

It would seem like the pressure would stay equalized between the two tanks as long as the manifold was open and the fill should be the same in both tanks. I'm not a blender but when I have had my doubles filled they always fill from one post and I analyze from both and always get the same readings. I'd be curious to hear from some of the more experienced people too.

 

[tbone1004]

it works just fine with doubles so long as the crossbar is open. If you are concerned, you can bleed off 1100, then fill 550 per side, but it's not really going to change anything.

What depth are you going to that you need to blow down that much? With AL80's, unless you're truly going to 130ft for the whole dive, I'd just dive EAN32. You'll be a bit hot if you're at 130ft, but you'll be ok

 

[seeker242]

What depth are you going to that you need to blow down that much? With AL80's, unless you're truly going to 130ft for the whole dive

"Deep specialty" training dives and I'm insisting that we go to max depths. I don't think they will allow 32 as that would be 1.6 ppo2, instead of 1.4

 

[tbone1004]

"Deep specialty" training dives and I'm insisting that we go to max depths. I don't think they will allow 32 as that would be 1.6 ppo2, instead of 1.4

you likely won't be going to max depth on the first dive though, so if your dive plan says you have a "shakedown" dive of some sort, I would just dive on that mix for the first dive and keep topping off with air until it goes down

 

[Jack Hammer]

What depth are you going to that you need to blow down that much? With AL80's, unless you're truly going to 130ft for the whole dive, I'd just dive EAN32. You'll be a bit hot if you're at 130ft, but you'll be ok

Really? Your gonna blankly recommend someone uses a PPO2 of 1.6 for a dive? Aren't you some sort of instructor too? Really, dude?

OP, bleed off your tanks and top with air. Its not that difficult and will mix fine in doubles. It can be done through one valve.

 

[tbone1004]

Really? Your gonna blankly recommend someone uses a PPO2 of 1.6 for a dive? Aren't you some sort of instructor too? Really, dude?

OP, bleed off your tanks and top with air. Its not that difficult and will mix fine in doubles. It can be done through one valve.

I didn't blankly recommend it, i asked what depth. If you're planning on staying at 130ft for the whole time, then of course not, don't dive EAN32 at that depth. Bad idea. if you're planned depth is 100-120ft and you may hit 130 for a minute or so, then no, I don't have a problem with it. His instructor may, but I don't. I personally don't like going beyond 100ft without helium, but that's just me. I get narc'd easily.

For a "deep specialty" there is also going to be no decompression. That means at 130ft, on EAN32, NDL is going to be something like 10-15 minutes depending on algorithm/tables, etc. 15 minutes. If you subscribe to the NOAA CNS table, that's 33% of the max for a single dive. 15 minutes on AL80s at 130ft, estimated consumption of 45cf is about 900psi assuming they are full. Top off with air after that dive and you're back to pretty close to EAN28. Top off with EAN28 for the rest of the course if it's banked.
Odds are, the first dive is going to be longer than 15 minutes and a lot shallower than 130ft. No instructor would just take the first dive of a course to the max of the limits, that's insane. So yeah, I'd keep EAN32 in there for the first dive and top off with air as the dives get progressively deeper. If the first dive of that course is to 130ft for the duration of NDL's, then I'd find a new instructor

 

[2airishuman]

So, maybe I'm being either pedantic or overly hung up on terminology here, but doubles do not "mix" through the manifold. If the two cylinders in a twinset have somehow ended up with different mixes, they will stay that way and will not "mix" through the manifold even if the isolator is left open and even if a substantial amount of time has gone by.

Instead, what is going on here, is that bleeding from either post with the isolator open draws gas in equal amounts from each cylinder in the twinset. Filling using either post with the isolator open fills gas in equal amounts to each cylinder in the twinset. Proper blending depends on this, which is why it is vital to leave the isolator open during these operations. Done this way, each cylinder ends up with the same mix, because 1) each started with the same mix, 2) each had the same amount of gas bled, and 3) each had the same amount of gas filled.

When someone blends gases by adding gas to one cylinder in a twinset with the isolator closed, out of ignorance or carelessness, the cylinders will end up with different blends. The only three practical ways to clean up the mess are 1) to drain the twinset and start over, 2) to close the isolator, analyze each side, and bleed/fill the two cylinders individually as required to reach the target mix, or 3) to use a booster to force blending to take place.

 

[tbone1004]

@2airishuman that is correct. people have died from assuming that gases mix through the manifold. They will mix as pressures are changing *i.e. bleeding off or filling*, but they don't really mix by sitting around.