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