This part of ISO 4266 gives 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 part of ISO 4266 gives guidance on the use of ALGs in custody transfer application. This part of ISO 4266 is not applicable to the measurement of level in caverns and refrigerated storage tanks with ALG equipment.