INFLUENCE OF THE FILAMENT POTENTIAL WAVE-FORM ON THE SENSITIVITY OF GLASS-ENVELOPE BAYARD-ALPERT GAUGES

Nonlinearities of approximately 10%-15% in the sensitivity of glass en velope Bayard-Alpert gages (BA gages) have been observed in the pressu re range 10(-7)-10(-2) Pa. These nonlinearities were studied in modifi ed BA gage tubes with platinum coatings on their inner glass surfaces by measuring the equilibrium potential of the platinum coating as a fu nction of pressure. The sensitivities of the gage systems (gage tube p lus controller) were found to depend on the inner surface potential, a nd this potential was found to depend on pressure and on the details o f the filament potential waveform provided by the gage controller. It was found that the nonlinearities could be minimized by holding the in ner surface to a fixed direct-current (dc) potential, by modifying the alternating-current filament potential wave form, or by using a contr oller that provides a noise-free dc filament-heating current.