Abstract In order to minimize background and facilitate decontamination we built a cylindrical ionization chamber using ultrahigh-vacuum metallic seals. Sapphire and pure alumina insulators isolated the collector electrode from the chamber. The chamber and inlet lines are bakeable to 450°C and withstand pressures up to 3 atm. The unshielded background of a new chamber was 2 fA. The operating voltage plateau of the chamber was determined at 20 and 764 torr. We calibrated output current vs total tritium content, using tritiated methane and a tritium-argon mixture, both by increasing the pressure of a constant-concentration standard and by diluting the standard gas mixture at vonstant pressure. Theoretical calculations of efficiency of beta-energy absorption in spherical vessels filled with several gases are presented, as well as experimentally determined chamber efficiencies for methane and argon.