Introduction
On-site measurement of coating wet film thickness is a key part of coating process quality control. As a simple and economical measurement tool, wet film comb gauge is widely used in painting sites in construction, shipbuilding, automobile, industrial anti-corrosion and other fields. This paper aims to standardize the operation process of wet film comb gauges, ensure the accuracy and repeatability of measurement results, and provide a reliable technical basis for coating operations.
Measurement principle
Wet film comb gauges are typically made of metal or plastic with teeth on either side at different heights. When measuring, press the comb gauge vertically into the wet film coating to be measured to observe which teeth are stained with the coating. Wet film thickness is between the last stained tooth height and the first unstained tooth height. The principle is based on direct contact measurement, and its accuracy is affected by the manufacturing accuracy of the comb gauge, the operation technique and the coating state.
Preparation before operation
Thorough preparation before measurement is the basis for obtaining reliable data. The operator needs to make sure that the coating is applied evenly and that the surface does not have obvious sagging or drying. Choose a wet film comb gauge with a suitable range, and its maximum range should be slightly higher than the estimated wet film thickness. At the same time, check whether the tooth surface of the comb gauge is clean and damage-free, and ensure that the technical information of the coating to be tested is understood, especially its applicable measurement methods and environmental conditions.
Standard operating procedure
Standardized measurement steps can effectively reduce human error, and the specific process is as follows:
1. Select measurement points: Select at least three measurement points in areas with uniform and flat coatings, avoiding edges, corners, and surface defects.
2. Clean the comb gauge: Use appropriate solvents and a soft cloth to clean all tooth surfaces of the comb gauge to ensure no old coating remains.
3. Perform the measurement: Place the comb gauge perpendicular to the surface of the substrate and steadily and quickly press the end of the teeth into the wet film until it touches the substrate. After holding for about 1 to 2 seconds, lift the comb gauge vertically.
4. Reading recording: Immediately observe the tooth surface and record the thickness value corresponding to the last tooth contaminated with the wet film and the thickness value corresponding to the first uncontaminated tooth. Wet film thickness is between these two values, usually the average is taken as the reported value.
5. Cleaning and retesting: Clean the comb gauge immediately after each measurement and repeat steps 3-4 near the same area to obtain representative data for that point.
Influencing factors and control
The measurement results are influenced by a number of factors and need to be controlled in operation:
| Substrate surface roughness | Rough surfaces may result in high readings, and it is recommended to measure on a smooth surface or perform multi-point statistics. |
| Operating angle and speed | Non-vertical insertion or slow lifting speed can scratch the coating, causing distorted readings. |
| Rheological properties of coatings | Highly thixotropic coatings can cause differences in readings and need to be measured within the time specified by the coating. |
| environmental conditions | Temperature and humidity affect the curing speed of the coating, and the environmental parameters at the time of measurement should be recorded. |
| Comb gauge accuracy and calibration | Regularly calibrate or verify the comb gauge according to relevant standards. |
Data Logging and Processing
All measurement data should be recorded by the system. The record should include: measurement location, wet film thickness reading range, ambient temperature and humidity, measurement time, and operator. For a single measurement area, the thickness average and range of multiple measurement points should be calculated to evaluate the uniformity of the coating. If the measured value continues to exceed the range of process requirements, timely feedback should be provided to adjust the coating parameters.
Safety and maintenance
Wear appropriate personal protective equipment during operation and avoid contact with uncured paint. Immediately after use, the wet film comb should be thoroughly cleaned with the recommended solvent, checked for wear or deformation of the teeth, and stored in a dry, corrosion-free environment. It is recommended to establish instrument management records and check their status regularly.
Conclusion
Following a standardized operating procedure is key to ensuring effective wet film comb gauge measurement results. Through systematic pre-preparation, strict process control, and complete data logging, accurate wet film thickness data can be provided for quality control of the coating process, helping to optimize construction efficiency and the protective performance of the final coating.
References
1. International standard ISO 2808:2019, Colored paints and varnishes - Determination of paint film thickness.
2. National standard GB/T 13452.2, Determination of paint film thickness of colored paints and varnishes.
3. Industry Technical Manual, "Coating Inspection and Measurement Technology", published by the Protective Coatings Association.
4. Compilation of general operating guidelines for instrument manufacturers (based on publicly available technical data).