Helium drysuit bouyancy

[50BarBill]

This one is a bit of a hypothetical, I’m not sure you want to use your helium trimix just to fill your drysuit
But if you were to fill with a helium mix instead of regular air would you experience more buoyancy due to helium being the lighter gas ?

 

[rongoodman]

Only by a few grams. Buoyancy has much more to do with the amount of water displaced than the weight of the gas displacing it.

 

[50BarBill]

and I guess it would be extremely cold

 

[AJ]

This one is a bit of a hypothetical, I’m not sure you want to use your helium trimix just to fill your drysuit
But if you were to fill with a helium mix instead of regular air would you experience more buoyancy due to helium being the lighter gas ?

Probably. My partly helium filled stages float more than comparable air filled stages.

 

[rduquesnoy]

This one is a bit of a hypothetical, I’m not sure you want to use your helium trimix just to fill your drysuit
But if you were to fill with a helium mix instead of regular air would you experience more buoyancy due to helium being the lighter gas ?

Due to circumstances I once did a dive where I had to use a helium mix for my drysuit.

The difference in buoyancy was negligible. At least I can't remembe it being noticeably different from a 'normal' dive. The cold wasn't too bad, but in all fairness it wasn't a very long dive.

 

[OTF]

No noticeable difference in buoyancy. It's a little bit colder and costs a few extra bucks. I do it on mild trimix dives when I don't want to carry a separate suit inflation bottle.

 

[David Carron]

The total buoyancy of a drysuit is given by its displacement minus its weight minus the weight of the gas it contains. The only thing that changes in your thought experiment is the difference in the weight of gas, so we'll work that out.

If you have a reasonable 10 litres of gas in your drysuit at 90m that's (to a reasonable approximation) equivalent to 100 litres at standard temperature and pressure.
Air at standard temperature and pressure has a mass of around 1.29 grammes per litre, helium around 0.178 grammes per litre.

So unless I messed up somewhere, the mass of air required to fill your drysuit is around 129 grammes, the mass of pure helium around 18 grammes for a total difference of around 111 grammes.

This is all really an upper bound on the buoyancy difference since 10 litres in your drysuit is probably a bit too much and you're presumably not injecting pure helium.

 

[broncobowsher]

Due to circumstances I once did a dive where I had to use a helium mix for my drysuit.

The difference in buoyancy was negligible. At least I can't remembe it being noticeably different from a 'normal' dive. The cold wasn't too bad, but in all fairness it wasn't a very long dive.

Same here. Somehow left the drysuit bottle at home and only had mix for dil/bailout. It was a mild dive, water was between cool and cold, not extreme. Probably a little extra undergarment than needed. Thought I was going to freeze. Didn't notice anything different. But was far from pushing the limits as well.

 

[Wibble]

Probably. My partly helium filled stages float more than comparable air filled stages.

An ali80 has circa 2100 litres of gas under great pressure (210 Bar); your drysuit contains a couple or three litres at ambient pressure.

Hence a few grams difference.

Although, if using helium, you’d need much thicker underclothes to cope with cold from the greater heat conduction

 

[inquis]

Since we've moved into tangents... One thing not yet mentioned is the increased likelihood of DCS (primarily in the form of skin bends), since helium is small enough to diffuse through the skin.