And, being something of a math freak, I’ll take a stab at it anyways.
Take a board. Say, it’ll displace ( float when it’s completely under water ) some 230 lbs - call it a bit over 100 kilograms. This is more than a little high, but it’ll serve and it’ll make the math easy.
So, it has a volume of about 0.1 cubic meters. And lets say ( pulling a figure out of my hat, but it’s not too far off) that something like 80% of the volume of said board is gas. So, that’s 0.08 cubic meters of gas, call it 80 liters.
Note that I said ‘gas’ and not ‘air’ - the process of expanding the foam from a solid plastic to a foam is not necessarily done by adding air, rather it may be a chemical process which causes bubbles of gas to be trapped inside the plastic. We are not talking about whisking up egg whites for mousse au chocolat, y’know?
Typically, that gas will be carbon dioxide ( CO2 ) or nitrogen and it may not be possible to substitute helium. But, lets keep going. Like I said, I’m a math freak. It’s a personal problem, like…
Looking around for some good values for gas densities, I bumped into this
Density of Gases at STP in grams per cubic centimeter: Air 0.001293 Carbon dioxide .001977 Carbon monoxide 0.00125 Hydrogen 0.00009 Helium 0.000178 Nitrogen 0.001251
Not done yet, we need to change that to grams per liter so we multiply by 1000
Air - 1.293 grams per liter
CO2 - 1.977
Helium - 0.178
Nitrogen - 1.251
So, how much less weight are we talking about for a board that has around 80 liters of gas in it if we were somehow able to substitute helium for the CO2, air or Nitrogen in the foam?
Well, doing the math ( my favorite part, by the way )
1.293 ( air) grams/liter * 80 liters = 103.44 grams
1.977 ( CO2) grams/liter * 80 liters = 158.16 grams
1.251 ( nitrogen ) grams/liter * 80 liters = 100.08 grams
now, figure in helium -
0.178 grams/liter*80 liters = 14.24 grams
That is a considerable difference… but only if it’s a balloon with one hell of a lot of helium in it. The difference between a board with CO2 as the gas ( why not, it’s the heaviest ) and the same board using helium is about 144 grams. Around 4 1/2 ounces, if you’re going with US units, around a quarter pound. Say, less than the weight of a bar of wax. And it gives you an idea of how much volume there is in a helium balloon, for it to pick up a useful load.
And there’s more. See, helium is kinda like one of my ex-girlfriends used to describe me: it’s insidious. Though she used to say that almost everything else was insidious too - anyhow…
Put it in an even faintly permeable container under pressure and it makes its way out. Say, a permeable container like a fiberglass and resin skin that won’t let relatively large water molecules in…but a wee, bitty atom ( ‘cos Helium doesn’t form compounds, it’s a fairly noble gas like neon, argon and such ) like Helium, it can sneak out between the holes in those big ol’ resin molecules and plastic molecules that make up the foam like the breeze going through a screen door. Air is relatively big stuff, O2 molecules, N2, a few trace constituents like CO2 and such, it won’ go back in nearly as easily, or at all.
But the board isn’t under pressure… or is it? Put it on your rack, or on the beach, on a hot day, and there’s pressure in there, cos it heats up. Out goes the helium… and when the board hits the water and the gas inside cools off, well, lets just say it sucks, y’know? Boyles Law of Gases bites you. If, for instance, you go with a 20 degree F difference, air temp to water temp, the volume is gonna decrease by 1/20th - remember, it’s a 100 liter board, so about 5 liters ( think of it as 5 quarts) ain’t there any more, and suddenly the board looks like Starvin’ Marvin.
In case you were thinking about balloons - well, ever notice those kinda flat things on the shore, baloons and ribbons all together? I do. Those are rubber ( neoprene) balloons that were full of helium under a little pressure and the helium made its way out through the balloon itself, nobody put any holes in it. The Great Big helium baloons that they use to try to float around the world with have almost no pressure differerential between the helium inside and the air outside - that’s why they look kinda baggy when they take off.
And a 20 degree F difference isn’t a big deal - here, we get 40 degree F differences, 100 F air and 60 F water isn’t that odd in, say, August.
So - maybe helium isn’t such a good move, not for a few ounces… instead, skip that before-surf Quarter Pounder and you’ll be ahead of the game for much less effort.
hope that’s of use
doc…