Level detector for a rotating liquid helium bath

Generators with superconducting field winding may have a space partially filled with liquid helium around the center of the rotor. For that a level detector was constructed which operates under rotation in a pressure range between 3.10⁴Pa and 1. 3.10⁵Pa, which is not affected by magnetic fields and which allows a simple signal transfer from the rotor via sliprings. It consists of five separate carbon resistors placed on the rotor at different radii operating as liquid sensors, and a rotating electronics device which forms a stepped voltage signal corresponding to the number of sensors immersed in liquid helium. Principle and optimization of the liquid sensors as well as the design of the electronics device are described. The device was separately balanced. Final electronic adjustmend took place while the liquid sensors were immersed in a non-rotating helium bath. Two level detectors were successfully tested at speeds up to 3,000 rpm.