This was the fourth flight made with a single, 7-foot, .001" polyethylene balloon (Figure 37), carrying a light load to relatively low altitudes.
Figure 37: 7-Foot polyethylene balloon
On Flight 74, the automatic ballast meter showed that a ballast flow of 200 grams per hour was required by an automatic ballast valve on such a balloon. Flight 84 was launched in August, 1948 with a low-altitude barograph and no altitude controls to ascertain how long such a balloon would stay up. Using radar and helicopter that balloon was tracked for nearly 2 hours at an altitude of 12,500 feet with a load of 3 kilograms. It was still floating when lost.
On Flight 86, a fixed ballast leak was used, set at 170 grams per hour. After an early failure of the radiosonde transmitter, this balloon was followed with a plane; a floating level of about 14,500 feet was maintained for 4 hours, with a rise of "ceiling" of about 1200 feet per hour.
This balloon was observed during descent and was still distended, indicating that the lifting gas had been replaced by air both before and during descent.