This document gives requirements and guidance on the accuracy, installation, commissioning, calibration and verification of automatic level gauges (ALGs) both intrusive and non-intrusive, for measuring the level of petroleum and petroleum products having a vapour pressure less than 4 MPa, stored in pressurized storage tanks. This document gives guidance on the use of ALGs in custody transfer application. This document is not applicable to the measurement of level in caverns and refrigerated storage tanks with ALG equipment.