Positive Material Identification (PMI). PMI is a way to test the chemical composition of weld metal and base metal. There are three different types of portable instrumentation that you can use to check the chemical composition of a metal. Optical Emission Spectrometry (OES), X-Ray Fluorescence (XRF), and Laser Induced Breakdown Spectroscopy (LIBS).
Before you start any production welding, you should always test your weld metal to ensure it’s what you purchased. Just because a spool of wire has a sticker on it that says ER316L, doesn’t guarantee you that your product is ER316L. Everyone makes mistakes, so make sure you are using the correct welding consumable.