25 replies to this topic

[Diverbrian]

Actually, I can think of a couple of reasons to have to dump gas to make my mix. They involve the fact that the shop that I work for will not allow the booster to be used with Helium (fear of burning out the booster pump because it so light) and oxygen (for obvious reasons with the heat involved). I can only boost the air or premixed EAN36 in the nitrox banks. That means that if I need a mix that requires me to add oxygen above the oxygen bank pressure I need to bleed a tank down first. Those are the conditions that I operate under for the ability to not have to maintain my own fill whips and fill station.

As to which deco mix that I prefer if only carrying one mix, I am in Jim's corner. It depends on the dive. I agree that oxygen cleans up more easily, but sometimes I would rather have the flexibility of going to a deco gas earlier in the dive. That is the choice of the dive team and I don't consider most informed decisions wrong. They are a matter of personal preference or preference of the dive team.

[6Gill]

I don't think there is any hard and fast "right" answer to this - everything depends on the type and parameters of the dive.


For OW diving, I tend to prefer 50/50 - these dives tend to be deeper max depths but shorter run times, so I think 50/50 makes more sense from a decompression standpoint (gets you off the higher PN2 gas sooner). Another advantage is that if you and your buddy are surfacing in an air-sharing situation, then you can "break" sooner, at 70 feet vs 20 feet. I think the deco part of the ascent is a little easier to manage of the divers are all breathing their own gas (shooting bags, etc).

That's my take on it anyway.

-JimG

I'm in the same choir...

V-Planner 3.76 by Ross Hemingway,
VPM code by Erik C. Baker.

Decompression model: VPM - B

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 130ft (2) Air 50ft/min descent.
Level 130ft 27:24 (30) Air 1.01 ppO2, 130ft ead
Asc to 70ft (32) Air -30ft/min ascent.
Stop at 60ft 1:00 (33) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 2:00 (35) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 3:00 (38) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 4:00 (42) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 18:00 (60) Nitrox 50 0.79 ppO2, 0ft ead
Surface (60) Nitrox 50 -30ft/min ascent.

Off gassing starts at 95.1ft

OTU's this dive: 54
CNS Total: 18.4%

118.3 cu ft Air
36.1 cu ft Nitrox 50
154.4 cu ft TOTAL


DIVE PLAN COMPLETE

130ft for half an hour on air doesn't seem extreme to me... and gets me out of the water in the 1 hour that most dive boats would want. I would be at the absolute limit of a 40cuft bottle on this plan.

Perrone,
I go away for a bit come back and herre you are planning air dives . Pm being sent

We have a Membrane system where we bank 40% and then add air to make whatever blend you want. Makes it easy.

So how do I get a 50/50 blend?

[Diverbrian]

6Gill,

I know how I would get 50/50 with prebanked . Prebanked simply means that I need less O2 pressure before I start topping off.

Nice to see you back!

[6Gill]

6Gill,

I know how I would get 50/50 with prebanked . Prebanked simply means that I need less O2 pressure before I start topping off.

Nice to see you back!

But the banks only have 40%
It's taking a bit to read through the back log,work got intresting and then diving didn't leave much free time.

[Diverbrian]

Let's see:

Start with 500 psi of O2. Top off 2500 psi of blend. It's all good and I don't need quite
the O2 bank pressure as I do starting with air. I know of very few people who boost oxygen up here. I know of more who will boost helium to make backgas. No one likes to send anymore helium back to the local gas supplier than they have to.

[pir8]

I don't think there is any hard and fast "right" answer to this - everything depends on the type and parameters of the dive.


For OW diving, I tend to prefer 50/50 - these dives tend to be deeper max depths but shorter run times, so I think 50/50 makes more sense from a decompression standpoint (gets you off the higher PN2 gas sooner). Another advantage is that if you and your buddy are surfacing in an air-sharing situation, then you can "break" sooner, at 70 feet vs 20 feet. I think the deco part of the ascent is a little easier to manage of the divers are all breathing their own gas (shooting bags, etc).

That's my take on it anyway.

-JimG

I'm in the same choir...

V-Planner 3.76 by Ross Hemingway,
VPM code by Erik C. Baker.

Decompression model: VPM - B

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 130ft (2) Air 50ft/min descent.
Level 130ft 27:24 (30) Air 1.01 ppO2, 130ft ead
Asc to 70ft (32) Air -30ft/min ascent.
Stop at 60ft 1:00 (33) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 2:00 (35) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 3:00 (38) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 4:00 (42) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 18:00 (60) Nitrox 50 0.79 ppO2, 0ft ead
Surface (60) Nitrox 50 -30ft/min ascent.

Off gassing starts at 95.1ft

OTU's this dive: 54
CNS Total: 18.4%

118.3 cu ft Air
36.1 cu ft Nitrox 50
154.4 cu ft TOTAL


DIVE PLAN COMPLETE

130ft for half an hour on air doesn't seem extreme to me... and gets me out of the water in the 1 hour that most dive boats would want. I would be at the absolute limit of a 40cuft bottle on this plan.

Perrone,
I go away for a bit come back and herre you are planning air dives . Pm being sent

We have a Membrane system where we bank 40% and then add air to make whatever blend you want. Makes it easy.

So how do I get a 50/50 blend?

We also have 100% O2 on hand to get higher blends as well as He for Trimix. But we were talking about Nitrox blends for back gas so thats what I mentioned.

[6Gill]

So how do I get a 50/50 blend?

We also have 100% O2 on hand to get higher blends as well as He for Trimix. But we were talking about Nitrox blends for back gas so thats what I mentioned.

No worries Bob,that what I thought but I was just amusing my tired brain

[DIVEAK]

As long as we follow the computer program there is no need for us to dump a tank.

[DIVEAK]

As long as we follow the computer program there is no need for us to dump a tank.

Stick with the training. 50% at 70ft with a 40 cf bottle is a good way to go and a standard approach to deco on dives within the Tech 1 limits (170) It will work within most dive plans. Nothing wrong with going "rich" so long as you don't exceed the PO2 limits... If you are more aggressive, your training is or should be beyond Tech 1 or something equivalent.

[PerroneFord]

Stick with the training. 50% at 70ft with a 40 cf bottle is a good way to go and a standard approach to deco on dives within the Tech 1 limits (170) It will work within most dive plans. Nothing wrong with going "rich" so long as you don't exceed the PO2 limits... If you are more aggressive, your training is or should be beyond Tech 1 or something equivalent.

Not sure if you've done caves, but this presents a problem in some shallow caves (the kind of stuff we are doing now). In OW it would fly pretty easily. But in the caves, especially since I'm not scootering, there may be 1000ft to go (swimming) to the exit if I deco on 70%. In fact, I don't remember there being a portion of the Peanut tunnel before the crossover that was 70ft. And that was nearly 1200ft from the entrance. So I lose the optimal oxygen window.

[DIVEAK]

Stick with the training. 50% at 70ft with a 40 cf bottle is a good way to go and a standard approach to deco on dives within the Tech 1 limits (170) It will work within most dive plans. Nothing wrong with going "rich" so long as you don't exceed the PO2 limits... If you are more aggressive, your training is or should be beyond Tech 1 or something equivalent.

Not sure if you've done caves, but this presents a problem in some shallow caves (the kind of stuff we are doing now). In OW it would fly pretty easily. But in the caves, especially since I'm not scootering, there may be 1000ft to go (swimming) to the exit if I deco on 70%. In fact, I don't remember there being a portion of the Peanut tunnel before the crossover that was 70ft. And that was nearly 1200ft from the entrance. So I lose the optimal oxygen window.

Gotcha. Cool stuff you are working out.
My cave experience is next to nil. I hope to add it to the list someday. Thanks for the post.