What is PMI test? (Positive Material Identification)

The PMI (positive material identification) test proves which alloy constituents exist in the material.

There are various common test procedures. With optical emission spectrometry (OES) in accordance with DIN 51008-1 and -2, an arc is generated between the thermowell surface and the test equipment, and the spectrum of this arc enables the alloy’s elements to be identified – both qualitatively and quantitatively. A characteristic feature of this procedure is the fire mark that is left on the workpiece.

A test procedure which doesn’t damage the surface is X-ray analysis; during the X-ray the atoms of the thermowell material are energised until they radiate themselves.

The wavelength and intensity of the emitted radiation is again a measure of the alloy’s constituent elements and their concentrations.

Following a successful PMI test / positive material identification test, the thermowell is marked with “PMI”.

Author: Carl Ellis