1.Why is multiple sampling necessary?
Thickness Variation in the Length Direction (Longitudinal)
Raw Material Variation: The thickness of the cold-rolled substrate may vary slightly during the rolling process.
Zincing Process: On a continuous hot-dip galvanizing production line, the strip runs at high speed. Fluctuations in the molten zinc in the zinc pot, the pressure and stability of the air knife (equipment controlling zinc layer thickness), etc., can all cause slight unevenness in the coating thickness along the length direction.
Thickness Variation in the Width Direction (Transverse)
Center-Edge Difference: This is the most common variation. Due to the "edge effect," the left and right edges of the strip are usually slightly thicker than the center of the galvanized layer. Standards even allow thicknesses exceeding the upper limit in specified areas at the edges.
Sheet Shape Influence: If the strip has waviness or warping, it will affect the distance between it and the air knife, thus causing transverse coating thickness unevenness.
2.How should the sampling location be selected?
Longitudinal: Take at least one sample from the beginning and one from the end of the coil. For critical applications or long coils, additional samples may be taken in the middle.
Transverse: On the removed sample plate, measure at least three points along the width: the left side, the center, and the right side. Sometimes more points may be measured (e.g., two points at least 40 mm from the edge and the center point).
3.What is the measurement method?
Measuring Tools: Use a micrometer with an accuracy of 0.01 mm. Calipers cannot be used because they lack sufficient precision, and the measuring surface may press against both the substrate and the zinc coating simultaneously, leading to inaccurate readings.
Measurement Contents:
Total Thickness: Directly measure the total thickness of the galvanized sheet (substrate + zinc coating).
Substrate Thickness: Measure the thickness of the steel substrate after removing the zinc coating at the measurement points using electrolytic or chemical methods.
Zinc Coating Thickness: Calculated by (total thickness - substrate thickness), or measured non-destructively on the sample using a professional magnetic thickness gauge or eddy current thickness gauge. These instruments are commonly used in production lines and laboratories.
4.How is the result determined?
Compare the data from all measurement points with the nominal thickness and permissible deviation specified in the standard or protocol.
When making a judgment, it is usually not based on the average of all points, but rather on the requirement that the thickness at each measurement point must not be lower than the minimum permissible value specified in the standard, and that the average value meets the requirements. Special tolerance ranges may be allowed for the thickness of edge areas.
5.Why is measurement necessary?
Monitor the stability of the production process to ensure consistency across the entire coil. Monitor the uniformity of galvanizing, especially the difference between the center and the edges.