AVS 46th International Symposium
    Vacuum Technology Division Thursday Sessions
       Session VT-ThM

Paper VT-ThM6
Contraflow Leak Detectors with Improved Sensitivity Under High Helium Background Conditions*

Thursday, October 28, 1999, 10:00 am, Room 610

Session: Outgassing, Leaks, and Mass Flow Controllers
Presenter: C. Dong, Old Dominion University
Authors: C. Dong, Old Dominion University
G.R. Myneni, Thomas Jefferson National Accelerator Facility
G.A. Rooks, Varian Vacuum Technologies
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The leak detection sensitivity of the contraflow leak detectors is limited in general by its high operating pressure. Helium is known to be trapped in the fore pump of the leak detector and contributes to the large background signal particularly after leak checking large leaks (~1.0e-4 Pa. l/s). The trapped background helium can be effectively removed by purging the system with dry nitrogen. Such nitrogen purge is found to shorten the clean up period of leak detector by 90%. A novel method was developed to detect small leaks in large background helium signal. This method includes the lowering of pumping speed and adjustment of leak detector parameters such as the reduction of emission current and lowering the zero offset to minimize the background helium signal. We are able to leak check 3.0e-8 Pa. l/s and smaller leaks in a background signal of 4.0e-6 Pa. l/s of helium with 20 % accuracy. * This work was supported by U.S. DOE Contract No. DE-AC05-84ER40150 and Varian CRADA No. SURA-97-S002