>>13249209The simplest way to get close is to install a calibrated vent in the balloon, so that it will vent extra gas instead of popping once it reaches the altitude where it expands to its maximum volume. After this point it will continue to rise a bit further until the air around it is too thin to float it any higher. Here it should reach equilibrium. Unfortunately if you use a common rubber balloon, this equilibrium won't be perfectly stable, so if winds or temperature variations cause the balloon to descend, increasing air pressure will cause the balloon to shrink and continue gradually sinking. This may be acceptable depending how long of a flight you want. But if not, to overcome this you must use a high-tensile superpressure balloon made from inelastic materials. Such advanced gas balloons have proven capable of remaining airborne for up to a year.
