Liquid Helium Level Indicator

摘要

A device to detect the level of liquid helium in stainless steel Dewar is described. Various alternate methods of liquid helium detection are described, as well as the method selected to measure the varying resistance of carbon at low temperatures. A meter-expanding circuit, utilizing a Zener diode, is shown to be an aid in metering the change of resistance. The problems involved in Zener diode metering circuits, including linearity and current versus voltage characteristics, are examined experimentally. The results indicated that the basic circuit, with some modification, could be used in other applications of meter expansion. Operation of the final instrument is described showing the advantages of its simple design. The result of trade-off of optimum values is shown. The electrical characteristics of the Zener diode make it suitable for discriminating between two slightly different input voltage levels. (Author)