ESSENTIAL VARIABLESAn essential variable is a variable that will influence or change the mechanical or metallurgical properties of the welded joint - changes affecting the procedure approval. Any change in an essential variable requires a new welding procedure specification (WPS).Essential variables include: wall thickness, joint design, process, materials, consumables, welding position, direction, heat input (voltage, amperage, travel speed), heat treatment.APPROVING THE PROCEDUREWhen the data has been collected, the procedure must be validated by producing and testing a trail weld.If the procedure is to be used on a fabrication, which has been designed to meet the requirements of a code, the test weld is done under the supervision of an independent witness. The detailed arrangements for the test are subject to agreement between the contracting parties.A number of British Standards make cross-reference to another standard which covers approval testing. Other codes of practice include their own weld procedure / welder approval information. In general they include a standard format, which can be used to report the results of an approval test.Range of approval. (extent of approval, scope of approval)Provides a working range over which certain variables may alter without requiring a new welding procedure or welder approval.Variables include thickness (e.g. 1/2 down to 2x above), diameter (e.g. 1/2 down to 1/2 above) materials (different materials can be covered), position, process, parent plate group, and consumables.Re-approval of a welding procedure is necessary if there is a change of any of the essential variables or considerable defect re-occurrence.ABBREVIATIONSWPS welding procedure specification - an approved and accepted welding procedure; an authorised document.PQR procedure qualification records - proof the procedure works - record of tests undertaken to qualify procedure.WAC welder approval certificate - required to ensure a particular welder is capable of