Wet film thickness gauge measures the wet film thickness of varnish.

Measurement principle

A wet film thickness gauge is a mechanical measuring tool used to measure the thickness of a coating in its liquid state. Its working principle is based on eccentric wheels or toothed structures on both sides of the gauge, with a fixed difference between its maximum and minimum radial dimensions. When the gauge rolls over the surface of the wet film, the teeth of the instrument first contact the substrate and subsequently leave a mark on the wet film. The operator reads the scale marked on the first tooth that touches the wet film, which is the thickness of the wet film. The measurement process does not rely on electrical or optical principles, but only by physical contact, so its accuracy depends heavily on the operator's specification and instant readings.

Procedure:

A standardized measurement process is the basis for reliable data. First, the varnish coating to be measured should be measured immediately after application to avoid thickness changes caused by solvent volatilization. Place the thickness gauge perpendicular to the surface to be measured, ensuring that the tips of all teeth are in contact with the wet film at the same time. Then, roll the gauge on the surface of the wet film at a steady speed for half a week to a week. Check the gauge immediately after rolling to see which tooth end is the first to be varnished. The scale corresponding to this tooth is the wet film thickness value. Multiple measurements and averages are often taken in areas adjacent to the coating to improve the representativeness of the results. After the measurement is completed, the gauge should be cleaned in time to prevent the drying of the residual paint film from affecting the subsequent measurement accuracy.

Influencing factors

The reliability of measurement results is influenced by a variety of factors. The rheological properties of the varnish are one of the key factors, and highly thixotropic coatings can rebound as the gauge teeth roll over, resulting in high readings. The surface roughness of the substrate can also introduce errors, where the gauge teeth may not stably contact all points. Consistency in manipulation, such as rolling speed and applied pressure, needs to be stable. Environmental conditions such as temperature and humidity can affect the viscosity and leveling of the varnish, which in turn affects the form of the wet film. There is also uncertainty in the measurement itself, mainly due to instrument manufacturing tolerances, reading parallax, and coating inhomogeneity. Typically, wet film thickness gauges have a measurement uncertainty range of between ±5% and ±10% of the nominal value.

Wet film thickness vs. dry film thickness

One of the main purposes of measuring wet film thickness is to estimate the final dry film thickness. The conversion relationship between the two depends on the volume solids of the varnish. Volumetric solids refers to the percentage of dry film volume to the initial wet film volume after the varnish has cured. The conversion formula is as follows:

DFT = WFT × VS / 100

Among them, DFT stands for dry film thickness, WFT represents wet film thickness, and VS represents the volumetric solid portion of varnish (expressed as a percentage). For example, if a varnish with a volume solids content of 50% is measured to be 100 microns, the dry film thickness is estimated to be about 50 microns. This conversion provides critical process control metrics for coating application, helping to ensure that the final coating meets the design thickness requirements.

Application scenarios

Wet film thickness measurement is widely used in industrial coating, automotive repair, wood finishing and anti-corrosion engineering and other fields of process control. In varnish construction, real-time monitoring of wet film thickness can adjust the spraying parameters in time to avoid defects such as insufficient coverage and uneven gloss caused by too thin film thickness, or sagging, slow drying, and increased internal stress caused by too thick film thickness. A number of domestic and foreign standards standardize the measurement method of wet film thickness, providing a unified operating basis for the inspection work of different industries.

Notes:

Although wet film thickness gauges are easy to use, they need to be aware of their technical limitations. This tool is not intended for measuring coatings that have started gel or where thixotropy is particularly strong. For very thin layers (less than 25 microns) or very thick layers, there may be a lack of range or accuracy. The measurement results reflect the instantaneous thickness of the measurement point and do not represent the uniformity of the coating over the entire area. Therefore, it is often used as an aid to process control rather than as a basis for final acceptance. The evaluation of the final coating performance should be based on the measurement results of dry film thickness and related performance tests.

Conclusion

As an economical and fast on-site measurement tool, wet film thickness gauges play an important role in the varnish application process. By understanding how it works, following standardized operating procedures, and fully considering the impact of coating characteristics and environmental factors, operators can effectively monitor the coating process and ensure the desired quality of the coating. Combined with dry film thickness measurement and other performance tests, a more complete coating quality evaluation system can be constructed.

References

International Organization for Standardization. ISO 2808: Colored and varnished paints - Determination of paint film thickness.

American Society for Testing and Materials. ASTM D1212: Standard for Measurement Method for Wet Film Thickness of Organic Coatings.

National Standardization Administration of China. GB/T 13452.2: Determination of paint film thickness for color paints and varnishes.