Mass artifacts using a Cahn-RG microbalance in water vapor

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Abstract
Corrections for a ’’mass gain’’ artifact by the weighing mechanism have been found necessary when exposing a Cahn-RG-UHV microbalance in water vapor up to 1.3×103-Pa water vapor. The effect varies linearly with load from 0.2 to 0.9 g and amounts to an apparent mass gain of 27 μg per 100 mg of load at 1.3×103 Pa and 22° C. About two hours were required to reach equilibrium after exposure to water vapor at pressures between 1.3 and 2.4×103 Pa. Similar equilibration times are required in comparable pressure decrements, except that times ranging to as high as 33 h are necessary during final evacuation. Thus, the artifact depends on time, pressure, and load. The mass defect is tentatively ascribed to sorption by the polymer used for insulating the torque motor. Recoating the windings in the torque motor, which was done by the Cahn Division of Ventron, reduced the effect to 12.2 μg at a 100-mg load and 1.3×103 Pa, but the time, pressure, and load dependence of the artifact remained.

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