Method of measuring low pressures within evacuated, sealed glass tubes

During the heating of evacuated and sealed glass tubes, small volumes of gas are usually evolved causing a pressure rise. A simple method of measuring these gas pressures by manometer without the necessity of opening the tube is described. The method requires that mercury and a small inner glass tube be sealed in the tube. Internal pressure changes occurring when an evacuated vitreous silica tube was heated up to 550°C are described. The maximum pressure reached in the tube was 2.5 torr.