Q Our club is considering an APRS (Automatic Position Reporting System) balloon project. The payload—a GPS receiver, packet TNC and 2-meter transmitter will total about 4 pounds. How much helium is the flight going to require?
A Your ordinary weather balloon, when filled with helium, is lighter than air and floats upward as descending air pushes it out of the way. Like a bubble in water, the helium goes up to make room for the air going down. The buoyant force that acts on the helium is equal to the weight of air that the helium displaces.
A cubic foot of air weighs about 0.078 pounds, so the upward buoyant force on a cubic foot of helium is about 0.078 pounds. A cubic foot of helium weighs only about 0.011 pounds. The difference between the upward buoyant force on the cubic foot of helium and the weight of the helium is the amount of extra weight that the helium can lift: about 0.067 pounds. Since your payload is going to weigh in at about 4 pounds, you’d need about 60 cubic feet (4 / 0.067) of helium. I’d recommend 100 cubic feet to provide a good rate of climb and to take into account other weight factors.
From QST June 2000
