WATER VAPOUR MEASUREMENTS

The primary objective of the flight was to evaluate and compare different water vapor detection instruments at high altitudes, with a specific focus on correlating data from multiple hygrometers, gravimetric water vapor traps, and microwave refractometers. The flight aimed to assess the performance of these instruments under operational conditions, verify their accuracy against existing atmospheric models, and determine the reliability of their measurements in a real-world high-altitude environment.

In the image at left we can see an scheme of the distribution of sensors in the payload (click to enlarge). The first set of instruments were hygrometers designed to determine the frost point of atmospheric water vapor. Two of these were alpha radiation hygrometers developed by Honeywell. These instruments used a Peltier-cooled surface to induce condensation or frost formation, with a radioactive Polonium-210 source emitting alpha particles. A detector measured the reduction in energy of these particles as they passed through the condensed water layer, allowing for precise frost point determination. The control system adjusted the cooling power to maintain the equilibrium frost point, and the readings were telemetered to the ground for analysis. These hygrometers operated continuously during the flight and relied on ram ventilation for air intake.

The second type of hygrometer was an optical frost-point instrument developed by the Bendix Corporation. Like the Honeywell units, it also used a Peltier cooler to regulate its sensing surface's temperature, but instead of alpha radiation, it employed an optical detection method. A light beam was reflected off a mirror surface that was subject to frost accumulation. When frost formed, the beam was interrupted, triggering the heater to remove the frost before the cycle repeated. This method provided an alternative means of determining frost point based on optical signal changes rather than particle attenuation.

The third type of hygrometer was developed by the Ballistic Research Laboratories. This unit employed a constant cold source of dry ice-alcohol (or in this case, frozen ethyl alcohol) instead of a cycling Peltier element. A mirror with an embedded thermistor was attached to a rod immersed in the cold bath. The frost point was measured by monitoring the temperature of the mirror while an optical system detected frost accumulation. When frost formed, the heater was activated to clear the mirror, and the cycle repeated. Unlike the Honeywell and Bendix hygrometers, this unit used forced ventilation via a fan rather than relying on ram air intake.

In addition to the hygrometers, two gravimetric water vapor collectors were included in the flight, both developed and operated by General Mills, Inc. The first was the Dual Molecular Sieve unit, which functioned by adsorbing water vapor onto a specially prepared molecular sieve material. The adsorbent was stored in sealed canisters, which were exposed to airflow at the desired altitude. A high-volume blower flushed the sampling system, drawing air through the adsorbent beds where water vapor and carbon dioxide were selectively removed. The collected water vapor was quantified post-flight by analyzing the adsorbent and correlating the results with air mass flow measurements.

The second gravimetric instrument, the Goldsmith Vapor Trap, was built by General Mills based on designs from previous work conducted in the United Kingdom by Goldsmith and colleagues. This unit functioned by drawing air through a coiled stainless-steel tube immersed in liquid nitrogen. The extreme cold caused water vapor and carbon dioxide to condense and freeze within the coil. The amount of water collected was determined post-flight by weighing the frozen sample and comparing it to the total sampled air volume.

Finally, two microwave refractometers were included to measure changes in the atmospheric index of refraction, both designed and manufactured by Bendix Corporation. These instruments operated by drawing air through a sampling capacitor integrated into a microwave circuit at approximately 20 MHz. As the air's refractive index changed with altitude and humidity, the capacitance shifted, altering the circuit's frequency. This signal was compared against a stable reference frequency, allowing for real-time calculation of refractive index variations. These refractometers were flown to determine their operational viability at high altitudes and to compare their data with the hygrometers in the lower atmosphere.

Balloon launched on: 10/17/1962 at 9:17 cst
Launch site: University of Minnesota Airport, New Brighton, US  
Balloon launched by: General Mills Inc.
Balloon manufacturer/size/composition: Zero Pressure Balloon 681.000 cuft (2 mil)
End of flight (L for landing time, W for last contact, otherwise termination time): 10/17/1962 at 14:25 cst
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 5 h
Landing site: 185 miles E of Minneapolis, Minnesota, US
Payload weight: 791 PDS

The main balloon was launched on October 17, 1962, at 9:17 AM CST from the General Mills flight facility near New Brighton, Minnesota. For the mission was used a dual balloon launch system to manage an extremely long 500 ft flight train, allowing operation in higher ground winds. In this system, a main balloon with attached parachute was positioned upwind, while a smaller launch balloon was placed directly over the package, both having equal free lift. The launch sequence began with the main balloon, followed by the release of the load and launch balloon, with the main balloon taking up slack as it rose faster.

The launch faced complications from increased ground winds of 15-18 knots causing high strain on the balloon, and a wind shift that caused the load line to catch on a ground anchor. This resulted in the main balloon pulling the launch balloon and load to the ground before the system freed itself and ascended. During this "launch impact," the AMQ-9 transmitter was knocked from the package and the Bendix hygrometer was disabled.

The balloon reached a float altitude of 80,700 ft at an average rate of 624 fpm but remained at this height for only 30 minutes before beginning a slow descent. The flight ended at 2:25 PM CST with an impact 185 miles east of Minneapolis in 20-25 knot winds. Due to the strong winds at impact, the tension switch meant to release the balloon failed to actuate. The main load struck the ground multiple times before coming to rest in a grove of trees, scattering equipment and sustaining considerable damage.

• A STUDY OF HIGH ALTITUDE WATER-VAPOR DETECTORS General Mills (1963)
• The Flight Record Scientific Ballooning Nº 7, November 1962