Launching a helium balloon requires inflation, tethering, release and recovery. A balloon inflated to a diameter of 4 feet (34 cubic feet) is capable of lifting a payload of around 2 ½ pounds.
Helium: Helium is lighter than air and hence will rise when it displaces air. A small tank of compressed helium can typically hold 40 cubic ft. of the gas when allowed to go to normal atmospheric pressure. Larger tanks can hold over 200 cu. ft., compressed with a pressure of 2500 p.s.i. but these are heavy and require secure dollies to move.
Inflation: For balloons of a few feet diameter it is easy to simply put the neck of the balloon over whatever outlet port is on the helium tank. The balloon neck is often a larger diameter than the port, in this case the excess diameter of the balloon can be manually wrapped around the port and held tight by hand as the helium is let into the balloon. There is only a slight amount of pressure inside a typical weather balloon, even when it is filled, so it is not difficult to prevent the gas from escaping out the neck when filling. It is easier to fill the balloon if there is a flexible hose coming from the helium tank onto which the balloon neck can be wrapped and taped (see photo).
If it is necessary to go directly from the tanks metal nozzle, make sure the neck of the balloon is straight with the helium exit nozzle so the gas can move freely into the main part of the balloon. When the balloon becomes large (3 or more) the lifting force may require a second person to gently help hold the balloon down so it is not pulling up against the nozzle.
When helium is released from the tank quickly, the drop in pressure as it leaves the storage tank causes it to become quite cold (expanding gas is the principle behind refrigeration). If you are using your hand to hold the balloon in place on the tank, it could become painfully cold. You can temporarily close the valve and stop the process, use a glove, or screw a PVC pipe fitting onto the tank valve with a section of PVC pipe. When inflation is complete, close the tank valve, spin the balloon to twist the neck closed and tie or tape the neck (with fiber strapping tape) tightly to keep it twisted. It is a good idea to place the metal tether attachment ring around the neck then double the neck back up (open end up) and tie or tape the resulting loop of the neck securely with the ring (key ring) at the bottom of the loop (see photo). It is usually necessary to have two people working with the balloon when one has to tie the neck close. One person holds the balloon from floating away and keeps the neck twisted, the other wraps and ties the string or tape (make multiple wraps and knots).
Inflation diameter: The amount of helium required depends on two factors, the total weight to be lifted by the balloon (including the weight of the balloon itself and the total length of tether line) and the amount of wind present. Wind speeds in excess of 10 mph may cause difficulty for allowing the balloon to gain altitude and in its clearing of hazards. In this case more helium must be added to allow for wind. Below is a table giving the total lift force for different volumes of helium. Balloon diameter is most easily measured by measuring the diameter of its shadow if it is sunny. Volume = 4/3 p radius^3
Balloon Tie-off: When inflation is complete, Twist the balloon neck closed a few turns. Spiral wrap fiber strapping tape around the neck several times around to maintain the twist. Without cutting the tape, place the 3" metal ring around the neck and double back the balloon neck (the opening is now pointed up). Continue wrapping the tape several times around the outside of the doubled neck, the ring will be hanging at the bottom of the loop in the neck. The ring will provide the attachment point to tether the balloon.
Tethering: We have generally used nylon 3 strand string with a 100-pound rating. It often comes on rolls of 500 or 1000. We have tied the end to a quick connect ring which can be closed over the metal ring secured to the balloon neck. The quick connect should be some type of lightweight shackle such as a D ring or other shackle or clip from a hardware store. A moderate pet leash type clip should suffice. We have had bad luck using fishing gear swivel clips and have even broken one rated for 100#.
Release: As the balloon ascends, tether line is payed out, it helps to wear gloves for this. Every 50, a bright flag should be tied on the line per FAA regulations. We used fluorescent plastic non-adhesive tape and a strip about 9" long for these flags. For speed of estimation of string length we would measure the string in arm span lengths. Each arm span of a person is approximately as long as their height. Nine or ten arm spans is equal to 50 more or less. This can also serve as a general estimate of the total length of string released.
Retrieval: If the tethered balloon has been out on a 1000 tether line, it is worth the effort to make a large winding board (see photo below) to wrap the string around when pulling it back in. One person (wearing gloves) can pull the line down, while another wraps it over the ends of the winding board. If it is necessary to bring the balloon down quickly, two people can "run" the line down, the first person holds one spot on the line in place while the other runs along the ground pulling the string as it slides though their (gloved) hands. The line brought to the ground is left laying out in a straight line. They can then return to where the first person is, bring the line in hand over hand and leaving it on the ground on the way back. Since this process leaves all the line layed out on the ground, care must be used to avoid tangling. It can be wound up later. At this point we have found no easy way to retrieve helium from used balloons. It may be possible to leave the balloon filled although it will generally pop very easily after one use in sunlight.