Bayard-Alpert hot-cathode ionization gauge tubes employ a heated filament for the source of electrons. The Agilent 571 Bayard-Alpert Type Standard-Range Ionization Gauge Tube offers high performance over a pressure range of 2 x 10-10 to 1 x 10-3 torr. The 571 gauge withstands long periods of degassing or accidental exposure to atmospheric pressure while at operating temperature and will still recover its original characteristics. A reference gauge is also available, sealed off at ≤ 5 x 10-6 torr for use in testing a gauge controller