An automated system for recording data during aerostat lift-loss test includes relative humidity measurements in the helium and outside air. These data show that initially dry helium increases in water vapor content, approaching that of the outside air, following closely theoretical prediction and permitting computation of the overall water vapor transmissivity of the material. The effect on buoyant lift is derived, showing that water vapor in the helium increases lift while that in the outside air decreases lift. The increase in water vapor in the helium gives an artificially low lift loss rate during initial phases of the lift-loss test. A correction can be made to eliminate this error.
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