How to Measure Preheat

How should preheat be measured?

AWS D1.1 Structural Steel Welding Code, Section 5.6 states: Preheat and all subsequent minimum interpass temperatures shall be maintained during the welding operation for a distance at least equal to the thickness of the thickest welded part, but not less than 3 in. [75mm] in all directions from the point of welding.

In general, when preheat is specified, the entire part should be thoroughly heated so the minimum temperature found anywhere on that part will meet or exceed the specified preheat temperature.