How to Measure Blast Surface Profile A Complete Guide
If you are working on a job for blasting and painting your surface profile is critical. Improper surface preparation is one of the most common reasons for coating failure. Depending on who you are doing painting and blasting work for they may even require documentation of your surface profile. Whether for documentation reasons or for ensuring your surface preparation has been done correctly it is critical to ensure you measure your profile correctly. The two ASTM standards that govern the measurement of blast profile include D 4417, Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel and D 7127, Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument. These standards are important as they will distinguish where different tools fall in measurement of blast profile.
ASTM D4417 – 11 Methods of Measurement of a blast surface profile
This standard describes three different ways to measure surface profile of a blasted surface. One is a visual/tactile method and the other two ways involve measuring the peak to trough (the depth of the blast profile).
The first method of field measurement is the visual comparison of the blast profile to other pre made comparative blast surfaces using multiple sample areas to determine the underlying blast profile. This can be one of the quickest and easiest ways of obtaining an estimate of a products blast profile. This can be done with a surface comparator tool. The challenge with this is that it your testing is based on visual inspection and relies on the inspectors blast profile measurement experience. This tool is typically replaced if anything happens to it though there are methods of ensuring its calibration if needed.
A second method of measuring a blast profile is a digital gauge depth micrometer. This tool uses a probe that is pressed onto the surface profile and measures the height from the peak to depth of the blast profile. This tool reduces risk of inspector error and can provide quick results though you will need to take the average of a few different measurements to ensure accuracy. This tool can be calibrated to ensure proper reading using gauge blocks which are premade to a certain setting for calibration purposes.
A third method is the use of a replica tape. A replica tape is placed on the blast surface and rubbed with a tool to recreate the surface profile in the replica tape. The benefit to this method is it provides a physical document of your blast profile which can help if issues arise with a coating system down the road. The challenge is it can take a little bit of time to learn how to use it to get accurate, repeatable results and it is a one use per tape measurement where the others can be used on multiple projects after they are purchased. The replica tape has a small window in in where the actual blast pattern is transferred to the tape for measurement. Depending on the intended depth of the profile different tapes may be needed as different tapes have different ranges which they can effectively detect a blast profile.
D7127 – Methods of Measuring Blast surface profile
In addition ASTM D4417 – 11 provides an alternative way to measure the blast profile. D7127 allows for the use of a stylus instrument that measures the rt which is the distance between the peak and trough of a blast pattern as well as the peak count in the blast section that is being assessed. This equipment may be a bit more involved than the depth micrometer but adding the number of peaks and troughs may provide a bit more detail about the blast profile obtained. It can also be calibrated to ensure ongoing proper results.
General Blast Profile measurement Tool Considerations
As a general rule you want to ensure that you routinely perform the relevant calibration tests for your blast profile tool to ensure it is properly working. You also have to limit the tools to any exposure of the blast environment due to blast environments having a lot of media and other particles flying around while blasting.
Ultimately proper measurement of a blast profile is critical to ensure you have successfully prepared your substrate for a coating system. Failure to do so can result in premature failure of a coating. By using a tool that has been certified to measure blast profile you can be more certain that you are preparing your surface properly and in some cases meet documentation requirements that may be in place for your painting and blasting job.