1. 磁性仪器通常用于测量钢基材上的干涂层厚度. Non-magnetic operating equipment for non-ferrous metal substrates. All coating thickness measuring instruments shall be calibrated and coating thickness measured in accordance with ASTM D1186 or ASTM D1400.

2. Determining the thickness of each coating in a multi-coating system should be a checkpoint, especially when the type of each coating is different. 例如, if an inorganic zinc primer/epoxy intercoat/polyurethane topcoat system is specified, each layer should be measured to ensure the correct thickness, as the coating thickness meter will not give the thickness of the individual layers on the applied coating. When measuring multilayer coatings with a nondestructive instrument, the average thickness of the first layer must be determined before applying the second layer. 明显地, the result measured after applying the second coat will be the total thickness of the two coats, and the specific thickness of the second coat can only be determined by subtracting the average thickness measured by the first coat.

干涂层厚度的无损测量

3. Where practical conditions permit, it is recommended that a method be provided to indicate that the coating thickness is too thin or too thick so that the coating builder can make appropriate repairs. Possible methods include brushing a light coating of the same color, a compatible brush marker, chalk, or other material that can be easily removed. Crayons or incompatible spray paint should not be used.

4. The coating thickness measurement is carried out to reasonably ensure that the specified thickness is achieved. 然而, it is impossible to measure every square inch of surface. Both ASTM D1186 and D1400 specify that when using coating thickness measuring instruments, five separate point measurements should be made in each 9.3 square meter (100 square feet) area. Each point measurement consists of taking the average of three instrument readings in a 12.7 毫米 (1/2 英寸) diameter circle.

5. Non-destructive dry film thickness measuring instruments are divided into three basic categories: magnetic tension meter, electromagnetic probe and eddy current probe. Destructive dry film thickness measuring instruments are handled separately.

(1) Magnetic tension meter

A magnetic tensiometer usually consists of a lever that passes through the center of a dial that holds a coil spring. The dial is located at the fulcrum of the lever. One end of the spring is connected to the lever and the other end is connected to the dial. One side of the lever contains a permanent magnet, while the other end contains a counterweight.

The spring tension overcomes the magnet’s attraction to the substrate and lifts the magnet off the coated surface. The spring tension is calibrated so that the point at which the magnet separates from the surface can be equivalent to the distance between the magnet and the surface. This distance is converted to mil (or micron). The thickness reading represents the gap between the magnet and the substrate. 然而, this reading may be affected by voids, rust, embedded impurities, rust, ETC. 所以, a thorough visual inspection is required during the work process to ensure that the coating is applied to a clean surface and is not contaminated during the drying process.

干涂层厚度的无损测量

Magnetic tension coating thickness gauges must be calibrated before, during and after each use to ensure accurate measurements. The calibration method is described in ASTM D1186. The known thickness of the plastic gasket is placed directly on the polished surface and the instrument is then adjusted to be as close to the thickness of the gasket as possible. One or two gaskets representing the actual field coating thickness should be selected so that calibration can be carried out within the range of use. The user must remember that the accuracy of the magnetic tension meter is between 5% 和 10% of the reading, 与规定厚度的轻微偏差不构成拒收的理由.

干涂层厚度的无损测量

(2) 电磁探头

ASTM D1186 中描述了用于铁表面的电磁探针测厚仪. 这些仪器还使用非磁性校准 (塑料) 使用前的垫片方法. 腐蚀控制行业中使用的大多数涂层测厚仪都是这种类型. 它们是准确的 (±3% 至 5% 读数, 取决于制造商), 提供厚度的数字显示, 并消除用户对模拟刻度的解释. 此外, 此类仪器通常配备微处理器，用于存储测量数据并对存储的数据进行统计评估, 包括平均值, 标准差, 最高和最低读数, 以及获得的总读数. 该数据通常可以下载到计算机和/或打印机，并且可以节省与手动测量文档相关的大量时间, 平均, 并确定厚度范围. 然而, 因为这些仪器是电子的, 它们比机械张力计更脆弱，更容易受到现场损坏. 此外, 这些仪器对粗糙表面更敏感.

干涂层厚度的无损测量

ASTM D1400 中描述了涡流探头测厚仪，用于测量应用于有色金属基材的非导电涂层的厚度. 探头由金属内的交变磁场激励，并通过修改探头线圈的电气特性进行测量. 这些变化的程度由探头与基材的距离决定，并在测厚仪上显示为涂层厚度. 涡流探头测厚仪是利用有色金属基体上的塑料垫片以类似于电磁探头的方式进行校准. 有些仪器将电磁探头和涡流探头功能组合到一个单元中.

(3) 防范措施

Some precautions are required when using any instrument with magnets. The magnet is exposed, so it is easy to attract iron filings, steel pellets or particles. 使用过程中, the magnet must be cleaned, otherwise it will lead to incorrect readings. If the instrument is used on a soft coated film, a plastic gasket can be placed on top of the surface to prevent the magnet from deforming the coating. The distance from the edge of any surface to the instrument must not be less than 1 英寸; The presence of residual magnetism in the structure of the measured coating can have a false effect on the reading. Dial type instruments (magnetic tensile thickness gauges) also present an additional problem of “human error” during use, as it is easy to continue turning the dial after the point at which the magnet is lifted from the surface, which will result in incorrect thickness measurements. 所以, it is necessary to ensure that the dial is stopped at the point where the magnet is unadsorbed from the surface.

6. The inspector shall record the date and location of the measurement, the type of coating tested (primer, intermediate coat, top coat), the product and the type of measuring instrument used, the calibration method and whether the inspection area conforms to the specifications.