Discrete Liquid/Vapor Detectors for use in Liquid Helium
M. J. DiPirro and A. T. Serlemitsos
Code 713
NASA/Goddard Space Flight Center
Greenbelt, MD 20771
Published in Advances in Cryogenic Engineering 35, 1617 (1990)
Simple devices have been constructed and tested which can discriminate between liquid helium and its vapor. The devices are 0.25 mm doped silicon cubes suspended from 0.05 mm diameter stainless steel and manganin wires. Its resistance is strongly dependent on temperature and voltage. A small current is passed through the device heating it and lowering its resistance. The degree of self heating is dependent on whether the device is immersed in liquid or is surrounded by vapor. The voltage across the device then indicates the presence of liquid or vapor. The doping level is selected such that the liquid reading is a few volts and the vapor signal is < 0.5 volt over a wide range of current. The device is operated at a power sufficient to boil away any liquid helium film present, but not so large a power as to cause boiling in the bulk liquid. Typically the operating characteristics of a device are nearly independent of its operating temperature over a fairly wide range, for example, from 1.0 to 2.1 K. The devices are meant to operate in the milligravity environment of space. Tests simulating thick superfluid films which would be present in this case indicate less than 0.3 milliwatt per detector is sufficient to boil away these thick films. The detector response time under these conditions is less than 50 milliseconds. 
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