Rationale
A wet film thickness gauge is a tool used to measure the thickness of a liquid coating that has not yet cured on a substrate. Its working principle is based on geometric measurement, through the preset toothed structure on the gauge body touches the surface of the wet film, and the scale between the first wetted tooth and the unwetted tooth is read to determine the wet film thickness. This measurement is crucial for controlling the final dry film thickness, estimating material consumption, and ensuring that the coating performance meets the design requirements. The relationship between wet film thickness WFT and dry film thickness DFT can be approximated by the formula: DFT = WFT × (solid volume fraction). The solid volume fraction is provided by the coating supplier.
Instrument structure
Common wet film thickness gauges are mainly divided into two structures: comb tooth type and wheel type. Comb tooth thickness gauges are usually single-row or double-row tooth structures, with tooth heights increasing or decreasing along the gauge body, and are made of stainless steel or plastic. The wheel thickness gauge usually contains three concentric wheels, of which the middle wheel is an eccentric wheel, and its surface has a scale to judge the thickness by rolling the wet marks. Both types need to ensure that the teeth or rims are clean and free of wear to ensure measurement accuracy.
| Comb tooth thickness gauge | Simple and portable in structure, suitable for flat or curved surfaces |
| Wheel thickness gauge | The wide measuring range is suitable for continuous measurement, and the operation requires smooth scrolling |
Preparation
Environmental conditions and coating conditions must be checked before measurement. Ambient temperature and humidity can affect coating fluidity, so it is recommended to operate in the construction environment specified by the coating supplier. Ensure that the coating is evenly applied and that the surface is free of air bubbles, impurities, or visible sagging. Clean the measuring teeth or rims of the thickness gauge to avoid residual old coatings affecting readings. If measuring metal surfaces, make sure there is no magnetic interference. At the same time, prepare a soft cloth and appropriate solvent for cleaning to clean the gauge in time.
Measurement steps
First, hold the side edge or handle of the thickness gauge and avoid touching the measuring teeth with your fingers. Place the thickness gauge vertically on the surface of the wet film to ensure that all tooth tips are in contact with the coating at the same time. For the comb tooth type, press the gauge body into the wet film with stable pressure until it touches the substrate, and then lift it vertically to observe which of the two adjacent teeth is wet and the longer teeth are not, and the corresponding scale is the thickness of the wet film. For the wheel type, place the gauge wheel lightly on the wet film, roll it slowly for half a week and then lift it, and check the scale corresponding to the boundary between wet and unwet on the eccentric wheel.
Each measurement position should be repeated two to three times, and the average value should be taken to reduce the error. The measurement point should cover representative areas of the coating, such as the center, edges, etc., and avoid obvious defects. When recording measurement data, the measurement location and environmental conditions should be indicated.
Influencing factors
Coating rheology has a direct impact on measurements. High viscosity paints can cause unclear tooth marks, and low viscosity paints may result in low readings due to flow. Surface roughness affects the contact between the teeth and the substrate, and rough surfaces require more measurements. The non-vertical operating angle or improper lifting speed may cause the coating to drag and produce errors. Too high a temperature may accelerate the volatilization of the solvent and change the coating state. In addition, the manufacturing accuracy and tooth wear of thickness gauges need to be checked regularly.
| Coating rheology | Viscosity affects the clarity of tooth marks and the stability of readings |
| Surface roughness | Rough surfaces need to be measured at multiple points to get an average value |
| Operation method | Vertical placement and stable lifting reduce errors |
| environmental conditions | Temperature and humidity affect coating curing and flow |
Maintenance and calibration
Immediately after each use, use a soft cloth dipped in appropriate solvent to clean the gauge to avoid curing residue of the coating. Store in a dry and corrosion-free environment to prevent collision deformation. Regularly check the scale accuracy of the thickness gauge, which can be calibrated using standard thickness sheets or sent to professional institutions for calibration. If the teeth have visible wear or deformation, they should be considered for replacement. The wheel thickness gauge needs to check whether the wheel axle rotates smoothly.
Notes:
Appropriate protective gloves should be worn during operation to avoid skin contact with uncured paint. When cleaning with solvents, it should be done in a well-ventilated place, away from fire. Waste paints and cleaning materials are to be disposed of in accordance with local environmental regulations. Clean the instrument in time after measurement to prevent cross-contamination.
Cited Literature
ASTM D4414 - Standard Practice for Measuring Wet Film Thickness Using a Notch Gauge
ISO 2808 - Determination of paint film thickness for colored and varnished paints
GB/T 13452.2 - Colored paints and varnishes - Determination of paint film thickness