PMI Testing

Positive Material Identification

PMI

The Positive Material Identification (PMI) is an analytical technique that has the aim to clearly recognize a material. In mechanic industry it is an identification testing of metal alloys needed to prevent the misuse of parts, materials and welding electrodes that could cause extensive damage.

Two techniques are mainly used:

  • Optical emission (OES): it is based on the observation of the light emitted during the metal vaporization from a surface by a controlled electric discharge. In the plasma state, the atoms composing the first material surface layers generate photons; the light beam has wavelengths distinctive of the element atomic number. An optical prism separates the light coming from the sample into its components (lines) and a detection system quantifies the intensity of some bright lines, which are correlated with the concentrations of the elements that have generated the spectrum;
  • Energy Dispersive X-ray Fluorescence (ED XRF): it uses small X-ray tubes of very low power, which are directed towards the test piece. The X response signals coming from the sample are analyzed by a fluorescence discrimination system. An ED XRF system is typically lighter and smaller than a compact OES instrument.

At Metalprove S.r.l. both types of PMI instruments are available and are used daily at the customer premises, depending on the needs or specifications to be fulfilled. It has to be noticed that some elements such as carbon, boron and silicon are invisible to the XRF technique and can be measured only by a OES instrument. The optical emission spectrometer, however, leaves small traces on the test surface because of the discharges, and it is therefore not recommended on items with good surface finish.

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