Fluctuations of the Weight Indicated by a Microbalance in the Pressure Range Between 1 and 10³ torr with the Sample at a Lower Temperature than the Beam

Abstract

Large fluctuations in the indicated weight of up to 0.1 Hz frequency and 50 µg amplitude, were observed when using a vacuum microbalance with suspended pans, of which one was cooled to 77 K and the balance beam kept at room temperature. The amplitudes increased with increasing pressure and temperature difference between sample and beam. In a nitrogen atmosphere, the fluctuations began at 200 torr, and in an argon atmosphere at 140 torr, while in helium and hydrogen no fluctuations were observed. When the balance pan was empty, the fluctuations were particularly large, and when loaded, they generally decreased. With horizontally suspended disks and vertical tubes, practically no fluctuations were observed.

A closer investigation of the phenomena showed that the reason for these fluctuations was convection at the pan due to thermal radiation from that part of the balance which was at room temperature. It turned out that the bottom of the pan was acting as a radiation receiver.

Several models are discussed for the interpretation of these observations. The fluctuations were greatly reduced when the pan was shielded with a metal disk attached to the suspension wire immediately above the pan.