This standard specifies ultrasonic acceptance levels 2 and 3 for full penetration welded joints in ferritic steels, which correspond to ISO 5817 quality levels B and C. An acceptance level corresponding to ISO 5817 quality level D is not included in this standard as ultrasonic testing is generally not requested for this weld quality. These acceptance levels are applicable to testing carried out in accordance with ISO 17640. ISO 11666:2010 applies to the examination of full penetration ferritic steel welds, with thicknesses from 8 mm to 100 mm. It can also be used for other types of welds, materials and thicknesses above 100 mm, provided the examinations have been performed with necessary consideration of the geometry and acoustic properties of the component, and an adequate sensitivity can be employed to enable the acceptance levels of ISO 11666:2010 to be applied. The nominal frequency of probes used in ISO 11666:2010 is between 2 MHz and 5 MHz unless attenuation or requirements for higher resolution call for other frequencies. The use of these acceptance levels in conjunction with frequencies outside this range needs to be considered carefully.