December 9, 2011

NOTE ON ELCOMETER

The coating thickness on magnetic substrate is determined by means of electrical probe placed on the coating and developing electromagnetic field in the base metal.

The potential variation of these fields is a function of distance between the probe and base metal and is measured in microns. This requires smooth surface wherein the probe is placed. It is not been used for checking of zinc coating because of uneven surface. The requirement of zinc coating is in GSM which can not match with microns which is only indicative. This is the reason that all the standards refer zinc coating thickness measurement online by x-ray fluorescent method & off line by chemical method.

Difference in zinc coating between surfaces (top & bottom) :-

It is inherent in hot dip galvanizing process, the coating mass is not always equally distributed on both the product surfaces. So as per the National & International Standards, the coating mass of at least 40% of the value for the single spot test exists on each surface of the product. E.g. in 275 gsm triple spot test (including both the surfaces), the corresponding single spot test is 235 gsm (including both the surfaces) and the single spot test per surface shall be min 94 GSM. (i.e. 40% of single spot test including both the surfaces)

Difference in zinc coating is due to the uneven movement of the sheet between the two nozzles which is used for wiping of extra zinc when comes out from the pot.

Practical aspects of the above process can be viewed when someone will personally present on the spot.

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