Commercial quartz oscillators of the tuning-fork type with a resonant frequency of ∼ 32 kHz have been investigated in helium liquids. The oscillators are found to have at best Q values in the range 10 5 -10 6 , when measured in vacuum below 1.5 K. However, the variability is large and for very low temperature operation the sensor has to be preselected. We explore their properties in the regime of linear viscous hydrodynamic response in normal and superfluid 3 He and 4 He, by comparing measurements to the hydrodynamic model of the sensor.

A magnetic sphere (radius 100 p, m), levitating inside a superconducting niobium capacitor, is immersed into superAuid helium.Vertical oscillations of the sphere can be excited and detected.At resonance we measure the velocity amplitude as a function of the driving force between 0.35 and 2.2 K. In the linear regime (laminar liow) the drag is given by Stokes' solution above 1.1 K and by ballistic roton and phonon drag below 0.7 K. At larger velocities we find a sharp transition to turbulent drag which varies with the square of the velocity above a temperature independent threshold velocity.

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Turbulent flow of superfluid 4He at mK temperatures around an oscillating microsphere is known to be unstable at low driving forces, switching intermittently between turbulent and laminar phases. The lifetimes of the turbulent phases are exponentially distributed, and the mean lifetimes grow exponentially with the square of the driving force. These experimental results are attributed to statistical fluctuations of the density L of the vortex line length. As a result, a normal probability distribution of L2 is found having a standard deviation of 2.9 x 10(14) m(-4) and a spectral bandwidth Deltaomega approximately 13 s(-1).