A Flowrate Calibrator for Cryogenic Helium

Abstract

A new calibration module of the absolute type, designed and built at IMGC allows flow meters to be calibrated in cryogenic helium to an estimated accuracy of ± 0.5 %.

The system, which generates flowrates up to 320 cm³/S, is based on the axial displacement of a membrane bellows completely immersed in a LHe bath and operated from outside the cryostat. The working bath is contained in a vacuum-tight closed volume, mostly made of OFHC copper and surrounded by a cooling bath; in this way it is possible to work in supercritical helium at absolute pressures up to 0.5 MPa, with the temperature set at any stable value between 2.2 and 4.2 K simply by controlling the pressure of the two baths. In addition, quasi-stable temperature down to λ-point can also be maintained in the working bath at pressures higher than that of liquid-vapour equilibrium. Direct calibrations are also possible in SFHe down to 1.5 K under equilibrium conditions. The double He bath allows long-duration tests to be executed, since the temperature can be maintained at the desired value by simply regulating the pumping speed over the external cooling bath, with the amount of helium in the working bath remaining unchanged during testing. The volume generated by the bellows vs. its axial displacement was accurately measured with water at room temperature and corrected for LHe temperature; owing to the intrinsic, although slight, non-linearity of this relationship, a polynomial fitting was used to evaluate the actual volume displaced during each test.