Whats the best mix for air top offs

[Timbobaggins]

Standby for blast off.

Typically i dive in the 65-80M range.
Typically i use a 16/40 or a 14/50

This is on a CCR.

I have three sets of diluent cylinders.

I am going somewhere where trimix pumping is out of the question and we are taking a portable compressor. Hopefully weather permitting we should get 7-8 dives in the 65-80M range

I usually take a 15l to decant from.

However with a gas mix air top programme (which i have ) i do not really need to take the 15L to decant from. If i put the right mix in my dil cylinders and then top off with air.

So what would be the best mix to start with.
Suggestions greatfully recieved.

[Padowan]

Why not dive a Heliair mix, then you can just put Helium in your 15, then decant and air-top?

[nickb]

100 Bar of 10/90 air-topped to 232 bar = 16/39
120 Bar of 5/95 air-topped to 232 bar = 13/49

These are Ideal Gas Law calcs and the real values will probably put the He up a bit and the O2 down, but not by anything worth getting upset about.

PM me if you want a copy of the spreadsheet:

[Padowan]

OK, just re-read your post and you want to try and avoid taking the 15, and just taking 3 sets of Dil Cylinders. If you are planning on doing 5-8 dives, then you're looking at at least one top off per cylinder, and maybe 2 top ups.

If we make some asumptions:
You start each dive with the dil at 200 bar, and you end the dive on 100 bar, that should provide sufficient contingency.

You could start with a Heliox dil, then your first top up will be the first time you introduce any N2 into the Dil, this will give the the max possible extension of the Helium while keeping the N2 as low as possible.

If you start with a 15/85 Heliox, then a 100bar top off will take this to about 18/40 Trimix, which is a little hot for 80m, but fine for 60-65m.

[Timbobaggins]

To Nick and Padowan thanks.

Nick i have pm'd you for spread sheet.

will have a look at calc's and try to work out whats best to take.

Ideally i want to start first dives with 230 bar diluent

so i should only use 70-80 bar dil max (unless i have to do a dil flush!!)

so will be topping off from around 150-160 bar ish

[nigelH]

How much gas do you use?
I tend to find DIL usage runs on the dive count not duration.

OK my thoughts would be to put the wing and the suit on an air source. Even running the wing on DIL (suit on air) I've done a week of 40m stuff cross filling a single DIL bottle from a 7L of mix. No air top-offs at all. If I'm carrying serious bailout I don't worry about the DIL being full as the moment I come off the loop it's too small to consider.

[Padowan]

I would also try and avoid using really low O2 percentage dils, for the sole reason of maximising your top-off potential - that "surface convenience" reason, gives quite some "underwater disadvantages" that i don't think are worth the convenience.

10% is about as low as I'd want to go for the depths you're talking about and even then, your dil-PPO2 at maximum depth (80m) is a long way from setpoint, meaning that the unit's gonna be working harder on the descent to get the PPO2 to SP, and following a dil flush at depth, is gonna then use a lot of O2 to get things back up again. 15% is a great starting mix for the first dive as it's surface breathable and will reduce the O2 injection requirement on descent, and be more useful in an SCR situation, probably somewhere between 10 and 15% O2 will give you an optimum balance of being OK to breathe very shallow and maximising top off potential.

I think that 5/95 Heliox is too much, and another point to mention if you do decide to use Heliox (on the first dive from each tank), is whether you have dived Heliox before and explored your body's physiology with respect to Deco profiles when there is no N2. Helium can be unforgiving if you follow any sort of bend-and-mend type (Bulhman/VR3) profile.

[Mark Chase]

Originally Posted by Timbobaggins

Standby for blast off.

Typically i dive in the 65-80M range.
Typically i use a 16/40 or a 14/50

This is on a CCR.

I have three sets of diluent cylinders.

I am going somewhere where trimix pumping is out of the question and we are taking a portable compressor. Hopefully weather permitting we should get 7-8 dives in the 65-80M range

I usually take a 15l to decant from.

However with a gas mix air top programme (which i have ) i do not really need to take the 15L to decant from. If i put the right mix in my dil cylinders and then top off with air.

So what would be the best mix to start with.
Suggestions greatfully recieved.

Take a 3ltr pony with you and run wing and suit inflate off the pony full of air.

Then your three dilluient cylinders will easily last 8 dives. I generaly use less than 50 bar on a three hour dive 60-80m running it like this.

TBH I run a 1.5ltr cylinder for wing and suit and I have an emergancy whip on my 50% in case I run out.

ATB

Mark

[Timbobaggins]

I do run suit inflation off a 3l pony bolted to rebreather case (i use air for inflation)

i carry two 7's side slung

so my 3 inboard dils have just got to last for 6-8 dives. TBH if we do 6 dives i am happy i can comfortably get those out of my thre dils. And my 3 O2's.

Its only if i get freak weather and we squeeze a 7th and eighth dive in.

Not going till late June so hope weather improves!!!!

Will probably take a 12/55 or a 12/60.

Padowan, you are correct have never dived heliox, always trimix so i know what my body will take so to speak.

Assuming a 12/55 is pumped to 230 bar if i run it down to 100 bar and then top off with air to 180 bar i should end up with a 16/30 which should be good for the 65M range dives.