My wife threw a surprise 40th Birthday party for me, a month early. Cake and balloons were all over the place so I was having fun. As the Geeky dads stood around, we looked at the helium tank and wondered how many helium-filled balloons it would take to lift a toddler. We were purely concerned about their safety, of course.
Helium (He) is the second element on the periodic table, an inert noble gas and a safer choice for balloons than highly flammable hydrogen (H). Helium is lighter than air and able to lift about one gram per liter of gas. It has a number of industrial uses beyond being a standard at parties.
First, we need to know how much the kid weighs. Our 2 year old is roughly 30 pounds (13,440g), clothed. Second, we need to know how much gas each balloon holds. Although the balloons are not perfect spheres, we’ll consider them as equivalent to spheres with a one foot diameter. Subtract 3 grams from the lifting force of each balloon to allow for the weight of the string and balloon.
The volume of each balloon needs to be calculated:
The volume of a sphere is 4/3 * pi * r3, where r is the radius of the balloon. So first determine the radius of the sphere (the radius is half the diameter). Cube the radius (multiply it by itself twice: r*r*r), multiply by 4/3 and then multiply by Pi. If you are measuring your balloon in feet, that gives you the volume of the balloon in cubic feet.
One cubic foot of helium will lift about 28.2 grams, so multiply the volume of the balloon by 28.2.
Divide by 448 — the number of grams in a pound — to determine the number of pounds it can lift. http://www.heliumuk.co.uk/helium.htm
Using the method above, each balloon containing .524 cuft of helium will lift 14.77 grams, or 11.77 grams of payload. Phew, we do not need to be concerned about a child lifting away from the party until we’ve attached 1,143 balloons to their wrist.
We did have to keep the toddler away from the older kids who were setting a bad example by breathing the helium from the balloons to raise the pitch of their voices. Maybe next time, we’ll fill Xenon balloons with gas to lower the pitch.
(Do not breathe helium for more than a few seconds, if at all. You don’t want the contaminants from the tank in your lungs and you certainly don’t want to let your brain seize up for lack of oxygen. Never, ever use the nozzle on the tank for anything but filling balloons.)