AVS 50th International Symposium
    Vacuum Technology Thursday Sessions
       Session VT+MS-ThM

Paper VT+MS-ThM11
Practical Procedures for the Frequency Corrections of the Spinning Rotor Gauge Residual Drag

Thursday, November 6, 2003, 11:40 am, Room 323

Session: Reproducibility, Precision, and Accuracy of Vacuum and Process Measurements
Presenter: J. Setina, Institute of Metals and Technology, Slovenia
Correspondent: Click to Email
Spinning rotor gauge (SRG) uses a magnetically levitated steel ball as sensing element to measure low gas pressure, which is determined from the decay of the rotational speed of the rotor caused by the momentum transfer to the surrounding gas molecules. In addition we also have a small, gas pressure independent component to the measured SRG signal. This is called a residual drag (RD), and the main sources are eddy currents induced in the ball by asymmetries in the magnetic suspension field and eddy currents induced in surrounding metallic components by the rotating component of the ball's magnetic moment. In general, the RD depends on the ball rotational speed. The SRG operates the ball in a pre-selected frequency window, usually from 405 to 415 Hz, and the RD changes during the gas pressure measurement as the ball speed changes. The frequency dependence can be observed as saw-tooth variation of readings during continuous operation at constant pressure. For accurate measurements the frequency dependence of the RD has to be considered also. The commercial SRG controllers do not have the ability to take into account the frequency dependence of the RD and to make automatic on line corrections. The corrections have to be done separately by the user. We will describe our methods to determine the frequency dependence of the residual drag and procedures to perform the corrections to the pressure readings. The RD and its frequency dependence are unpredictable in magnitude for a given suspension of the rotor. Both can change considerably when the rotor is re-suspended. It is our experience that the frequency dependence remains reproducible during uninterrupted suspension, if vertical alignment or position of the suspension head stays well fixed. It is our experience also, that the behavior of the RD of the same ball is different in various suspension heads of different SRG controllers.