Measurements with a Modulated Bayard–Alpert Gauge in Aluminosilicate Glass at Pressures below 10−12 Torr

The background conditions achieved in a simple ultrahigh-vacuum system with an envelope of aluminosilicate glass (Corning 1720) have been studied. Pressures were measured by a dynamic method and also with a modulated Bayard–Alpert gauge. Good agreement between the two methods was obtained using a modified analysis for the modulated gauge. A total pressure of ${\text{8 × 10}}^{\text{−13}}$ Torr (equivalent nitrogen) was measured with the whole system at room temperature, and one of ${\text{7 × 10}}^{\text{−15}}$ Torr (equivalent nitrogen) was measured with a portion of the system immersed in liquid helium. A discussion is given of variations in the residual modulation factor of the gauge in the presence of extremely low gas pressures.