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

Paper VT+MS-ThM10
How Stable are Spinning Rotor Gauges

Thursday, November 6, 2003, 11:20 am, Room 323

Session: Reproducibility, Precision, and Accuracy of Vacuum and Process Measurements
Presenter: R.F. Chang, National Institute of Standards and Technology
Correspondent: Click to Email
The spinning rotor gauge is an excellent transfer standard in the pressure range of 0.0001 to 10 Pa (10@super -6@ to 0.1 torr) because of the remarkable stability exhibited by its accommodation coefficient. The stability comes from the fact that the accommodation coefficient depends mainly on the rotor surface properties of roughness and cleanliness, and does not change as long as these surface properties remain the same. Therefore, as common sense might dictate, one must avoid altering the rotor surface properties mechanically or chemically by not scratching the rotor surface or exposing it to corrosive agents. It is important that the accommodation coefficient remain constant when a spinning rotor gauge is moved from one laboratory to another such as in an inter-laboratory comparison or proficiency test. The level of confidence of the agreement between two laboratories is limited by how much the accommodation coefficient may have shifted in transit. For example, to transfer a spinning rotor gauge from one vacuum chamber to another, one must remove and reinstall the suspension head. During this procedure, the rotor comes into contact with the inner wall of the vacuum housing (thimble) and may be scratched. Sometimes the rotor is removed from the thimble for shipping, which requires additional handling of the rotor. By measuring the accommodation coefficient before and after various handling and cleaning procedures, we have quantified their effects on the accommodation coefficient. The results and impacts on gauge calibrations, including some surprises, will be presented and discussed.