Coating thickness is tested by using a magnetic thickness gauge. There are three types of magnetic thickness gauges: pencil-style, banana-style, and electronic. Electronic thickness gauges are the most common and the most accurate. Information on how to measure the thickness with magnetic gauges is contained in ASTM E376, and the minimum coating thicknesses and sampling requirements are contained in the appropriate specification for the product (ASTM A123, A153, or A767).  A single measurement of coating thickness is not sufficient to evaluate whether the galvanized coating adheres to the specification (ASTM A123, A153, A767) requirements.  Each specification calls out sampling size and averaging techniques to develop a true picture of compliance.

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Alana Hochstein


When taking measurements using a magnetic thickness gauge, the measurement is for only one side of the beam.  For items galvanized according to ASTM A123 (structural beams),  the coating thickness requirements listed in Table 2 are for one-side only.

Please note the minimum required coating thickness for all structural beams equal to or greater than 1/4-inch (6.4mm) which are galvanized to ASTM 123 is 100um for one side (ref. Tables 1&2, ASTM A123).  A coating thickness of 180 microns is not a current requirement within ASTM A123 for any material.

Kalyan Banerjee

The coating thickness means one side or both side of structural beam. Client asked for 180 micron DFT.

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