Before knowing the Variables first we need to know about WPS and why it is needed in the welding process. The WPS stands for Welding Procedure Specifications, it has a parameter, which is a written record of real-time experiments and documented as WPS for future use with help of WPS’s parameters, the welding is done while production hours and the results will be the perfection in the weld. The parameters in WPS are also said to be variables and it is three types;

• Essential Variables.
• Supplementary Variables.
• Non-Essential Variables.

With the help of these three variables the WPS is written and without these three we can’t write a WPS as well so these three Essential, supplementary, and non-essential plays are crucial roles in forming a WPS.

1. Essential Variables: 

If any change in essential variables the whole WPS should be requalified, because the change in essential variables also reflects the changes in mechanical properties, so requalification of WPS is mandatory.

2. Supplementary Essential Variables: 

If any change in supplementary Essential Variables for example if any impact in toughness, then the toughness properties will change, and if in the welding toughness is the main criterion. Then the supplementary essential is assumed as essential variables and WPS is requalified.

3. Non-Essential Variables: 

If any change in Non-Essential Variables simply we can change the WPS with no need for requalification.

For each welding process, a separate list of essential, supplementary, and non-essential variables is mentioned in ASME SEC IX.