helium

[M&P+4]

OK - may be a dumb question but....I'm not smarter than my 5th grader.

I'm sitting around the campfire with my kids and one of them asks - "Does helium affect bouyancy differently than air"? I'm 99.9999% sure it doesn't bbut thought I would ask.

[brettbyers]

With all my classes I have taken it was never brought up in discussion. I am like you and not 100% sure but with all my trimix dives I have never noticed a difference when switching gasses.

[theskull]

The answer is a qualified yes.

Helium is enough lighter than air that a set of doubles with a high helium mix will be lighter and less negatively buoyant than the same doubles filled with air. So you might need to wear a little additional weight. For a low helium mix, such as 20/30, you are not likely to notice any difference.

theskull

[brettbyers]

Even with helium in my tanks they still weigh a ton.

[SLIM]

Then if they way a TON you dont have enough He in them.

TS is right. Fill your tanks with He the weight will change not only in the water but also on land. Take a AL80 and fill it with a high He content and it will float very easy with more PSI of gas in it then if it was Air.

SLIM

[M&P+4]

I would have never guessed it - I figured that the high pressure would negate any differences that there might be.

[brettbyers]

I found a thread on TDS that was discussing the same thing. Here is a page from a guy that weighed his tanks to show the difference.

Re: Helium : How much does it weigh?!!!
Okay, here are the numbers. For starters, the cylinders in question are Pressed Steel LP80s, all from the same batch (6-01), and Pressed Steel LP95s, two from one batch and two from another, but all from the same year (1996).

All four sets are set up identically: Dive-Rite 2" bands with bolts, Sherwood 300BAR isolator manifold with two rubber handwheels and one low-friction handwheel, two stainless steel DIN plugs, four stage clamps, four D-ring holders, and two 2-inch D-rings. All fills are to 3600psi, resulting in 218 cubic feet in the 80s and 260 cubic feet in the 95s.

All weighings were on a set of digital bathroom scales. This fact destroys most of the scientific credibility that otherwise might exist.

The 80s weigh 76.5 pounds empty, 85.5 pounds full of 20/40, and 91.5 pounds full of EAN34.

The 95s weigh 92 pounds empty, 107.5 full of 17/35, and 110 pounds full of EAN34.

This results in weights of 0.069 pounds/cubic foot for EAN34, 0.043 pounds/cubic foot for 20/40, and 0.059 pounds/cubic foot for 17/35.

Not purely scientific, but interesting (if you're a "numbers guy").

[scubado]

the issue isn't compressibility or high pressure, I'm guessing the issue is the weight of the gas that is being compressed and put into the cylinders. Normal air has a weight (to the point that an 80 cu ft cylinder weights 6.4 pounds more when filled with normal (80/20) air). I haven't read or even thought to research the issue of EANX blends and their variance in weight.

[Charlie19]

With all my classes I have taken it was never brought up in discussion. I am like you and not 100% sure but with all my trimix dives I have never noticed a difference when switching gasses.

You wouldn't notice any difference because you are not switching the gas that inflates your wing, just breathing mix.

There is a difference in buoyancy, but you would never know it because you only inject enough gas in the wing to obtain neutral buoyancy--and you never know how much that is. When breathing a mix which contains helium you still would inject enough gas to obtain the same neutral buoyancy, and even though it may be less gas, you would never know how much less. Further, given the partial pressure of helium in the mix, the difference would hardly be noticeable even if you could or did know how much gas it took to obtain neutral buoyancy at any given depth.

[theskull]

You wouldn't notice any difference because you are not switching the gas that inflates your wing, just breathing mix.

There is a difference in buoyancy, but you would never know it because you only inject enough gas in the wing to obtain neutral buoyancy--and you never know how much that is. When breathing a mix which contains helium you still would inject enough gas to obtain the same neutral buoyancy, and even though it may be less gas, you would never know how much less. Further, given the partial pressure of helium in the mix, the difference would hardly be noticeable even if you could or did know how much gas it took to obtain neutral buoyancy at any given depth.

That's why my initial answer was a qualified yes. You won't notice any change during the dive when you change breathing mixes. But you will notice a big difference between dives if on one dive you have doubles with a high helium mix . . . and then on another dive you have your doubles full of air. You may need to wear some weights on the helium dive if you are normally pretty close to balanced when diving air.

theskull

BTW--have you seen the latest prices on helium ?!!! I just re-planned my next deep dive to a depth I can handle on air after getting the quote for a double 104 fill of 17/50.

[brettbyers]

You wouldn't notice any difference because you are not switching the gas that inflates your wing, just breathing mix.

There is a difference in buoyancy, but you would never know it because you only inject enough gas in the wing to obtain neutral buoyancy--and you never know how much that is. When breathing a mix which contains helium you still would inject enough gas to obtain the same neutral buoyancy, and even though it may be less gas, you would never know how much less. Further, given the partial pressure of helium in the mix, the difference would hardly be noticeable even if you could or did know how much gas it took to obtain neutral buoyancy at any given depth.
So the answer is no? Here is my take on this. I am set up for weight so at the end of the dive with less than half a tank I am neutral buoyant so I don't pop up to the surface, so at the begining of the dive I am a little over weighted. I put a helium mix on my back that weighs five pounds less, I won't notice because I can still get down and by the end of the dive when my tanks are less than half full the weight of my tanks would only be half the difference that they were to start with. Would I really notice a two to three pound difference at that point? That is why I said I have never noticed a difference when going from one type of mix to the next.