compensating for helium

[lermontov]

has anyone got a link for a spreadsheet or table to compensate for high helium % in Al80's - allowing extra lead for lighter tanks or even better a formula - and i wonder what if any variation to allow for BO scenarios -meaning if I allow extra weight for helium content will that balance out for a lighter empty deep BO tank

 

[kierentec]

Weight yourself with the tanks empty/near empty. That’s the lightest they’ll ever be.

 

[grantctobin]

has anyone got a link for a spreadsheet or table to compensate for high helium % in Al80's - allowing extra lead for lighter tanks or even better a formula - and i wonder what if any variation to allow for BO scenarios -meaning if I allow extra weight for helium content will that balance out for a lighter empty deep BO tank

An interesting thought, but to what end? Once you hit the depth in open water where an AL80 isn’t enough deep bailout to survive the 5/10/X minutes you’d plan to need to get your **** together and hit your first gas switch, switch to a pair of 7L in a rack or an LP85/13L of 10/70 or similar and go on. If you’re in a cave, a bailout pair of 80s of 32 or 30/30, a pair of LP85s of 18/45, a pair of LP104s of 15/55, a pair of PST LP120s of 10/70 or 8/80 are all fungible in terms of weighting.

As Kieren said, they’ll be lightest empty.

 

[lermontov]

Weight yourself with the tanks empty/near empty. That’s the lightest they’ll ever be.

sometimes the best answer is the most obvious

 

[bubblemonkey2]

I think the math is not too hard if you ignore the "non-ideal" gas properties at higher pressures... (but maybe you are asking for the Real Gas version?)

"Ideal gas" trimix density:
d (g/L) = ATM*(1.429*O2_fraction+0.178*He_fraction+1.25*N2_fraction)

"ATM" is like ~200(bar) for a full cylinder, and multiply the "d" by 11 liters internal cylinder volume:
air: 2833g
10/70: 1139g
So roughly add 1700g or (3 to 4 pounds) to the 10/70 cylinder to balance it out, which can be done with a cam strap and a single lead weight brick (for a BO stage cylinder)

Bit of a sidetrack, but:

The "nearly empty cylinders" weight check makes sense for a twinset, because you can't really ditch it.

For multiple stage cylinders though, it is optional, or even dangerous to do an "everything empty" weighting. This could potentially result in serious overweighting that exceeds the buoyancy capacity of your typical "tech" wing.

If your wing is already half full or more because you've weighted the rig for the rare situation of having several empty cylinders attached to you, then how much wing capacity is left for a flooded CCR loop or drysuit flooding scenario? Not much!

Rather than being overweighted combined with several negative cylinders that will normally remain full, it may also be possible in an emergency to jettison or exchange empty bailout cylinders one by one--they are no longer of any use to you!

 

[Raacerx]

I agree with the above post; it's good to do your weight check with near empty back gas incase you need to bail out for deco, but stage/deco tanks are made to be easily ditched or swapped.

It's not safe to calculate your weight needs based on empty back gas AND fully empty stage tanks, because it's not a realistic situation and will cause you to be overweight at depth.

The actual math for calculating the weight of helium in normal 11-12L tanks shows me that we are talking a couple KG at most on a seriously hypoxic mix.

I'm not going hardcore and only doing 60m trimix dives with generally 50% or less HE, but I use the same weights for trimix dives as I do for deco dives on a backgas of air, and never noticed any difference even during bailout deco drills.