Pressure Height: The height above mean sea level as determined from pressure measurements used in this work with the N.A.C.A. Standard Atmosphere.
Section 4. Flight Techniques
The general techniques of preparing and launching controlled altitude balloons are patterned after those of the smaller radiosonde balloons. The treatment of large, manned balloons has been studied, however, and information of considerable value has been gleaned; as from the National Geographic Society reports of the flights of Explorer I and Explorer $ \mathrm{II}^{11,12} $ , and from the book by Upson and Chandler $ ^{15} $ . From these and other studies $ ^{13,14} $ , and from original experimentation with General Mills advice, a satisfactory technique of handling controlled-altitude balloons has been developed.
A. Inflation
The lifting gas used for these large balloons has been helium. The choice of gas was made on safety considerations. Hydrogen, however, has several advantages over helium. It will lift 9% more than helium and, due to its lower specific gravity, requires but 50% of the ballast release that helium requires to correct for disappearance of superheat at sunset. Helium, on the other hand, leaks and diffuses at a rate but 70% that of hydrogen. However, for long flights, hydrogen would probably have more over-all economy of ballast.
Inflation has been made through a low-pressure, diffusing manifold, feeding from a number of helium tanks simultaneously to the balloon. The smaller balloons have been inflated inside a hangar, permitting very exact weigh-off of the balloon's free lift, thus predetermining the rate of rise fairly well. The plastic balloons larger than 15 feet in diameter have generally been inflated out-of-doors, as no hangar large enough for interior