Operation of Liquid Helium Level Sensors at Elevated Pressures

Abstract

Operation of standard liquid helium level sensors depends on the difference in heat transfer between the gas and liquid phases. At increased pressure inadequate cooling by the liquid causes the resistive zone to penetrate below the liquid surface when the liquid level rises. Thus the level detector records the minimum level and does not follow any subsequent rise in the level. Measurements of indicated levels as a function of heater current at pressures up to 0.2 MPa (29 psia) were made using an American Magnetics sensor to determine the proper heater currents. Reduced heater currents were found to permit operation at pressures up to nearly the critical point.