The positive material identification test – PMI test – serves as a proof of the alloying constituents present in the material, confirming the melting analysis of the material contained in the 3.1 certificate.
There are two different test procedures common for thermowells:
PMI test: X-ray fluorescence analysis of the thermowell stem
1. PMI test: X-ray fluorescence analysis (XRF)
X-ray fluorescence analysis uses X-rays to stimulate the atoms of the thermowell material to produce natural radiation, without damaging the surface of the metal. Pummel and intensity of the emitted radiation is, in turn, a measure of the alloy’s constituent elements and their concentration.
PMI test: Spectrographic analysis on a WIKA model TW10 flange thermowell
2. PMI test: Spectrographic analysis (OES)
With spectrographic analysis (“Optical Emission Spectroscopy”), an arc is generated between the thermowell surface and a tungsten electrode in the test instrument, and the spectrum of this arc enables the alloy’s elements to be identified – both qualitatively and quantitatively.
For the OES, it does leave a characteristic burn mark of approx. 5 mm diameter on the workpiece.
Burn mark from the arc on a workpiece