Measuring on rough surfaces with the PosiTector 200.
Coatings applied over non-metal substrates such as wood, concrete and plastic tend to have some surface texture. This roughness often gives the illusion of poor instrument repeatability.
Measuring coatings with rough surfaces can be a challenge for any measurement method, and ultrasonic testing is no exception.
By design, the PosiTector 200 is well equipped to handle these situations.
In this video we discuss two techniques for measuring coatings with rough surfaces.
Rough or textured coating surfaces are often the result of the brushing, rolling or spraying application method.
Sometimes texturing is deliberately produced for reasons such as
Microscopically, you can see the rough surface of this red coating on MDF.
Under greater magnification, the surface texture on this red coating looks like hills and valleys. The dark lines in the photo are scale markers from a destructive Tooke or Paint Inspection Gage.
The PosiTector 200 is factory optimized to measure most coatings right-out-of-the-box. Simply turn it on, and measure.
Rough or textured surfaces may cause the instrument to occasionally display LOW measurement values, typically under 50 microns, 2 mils, depending upon the amount of roughness.
In this example, repeated measurements are within a few microns of each other. The small variations in individual thickness results are due to substrate roughness; however, the effects of coating roughness are more dramatic.
On lightly textured surfaces, using more ultrasonic gel will reduce the number of times low results appear.
But on heavily textured surfaces, even sufficient gel won’t prevent low values from occurring.
When measuring ultrasonically, gel is applied to propagate ultrasound into the coating.
When the PosiTector 200’s probe is brought down, it rests on the peaks and attempts to measure the 134 micron thickness.
These low values result from the ultrasonic pulse reflecting off the surface [36 micron arrow appears] of the coating rather than off the substrate.
The best way to prevent these occasional low values is with a simple change to the instrument’s Low Range setting.
PosiTector 200 B models have a default measurement range of 25 to 760 microns.
The Low Range value is the minimum thickness the PosiTector 200 will report.
The High Range value is the maximum thickness the PosiTector 200 will report.
The default high range value rarely needs to be changed.
But a properly adjusted Low Range value is the key to successfully measuring rough coatings.
In our example, we want the instrument to ignore the 36 microns surface roughness, so we change the “Low” range value to something greater than the default 25 microns.
Now, within the new measurement range of 50 to 760 microns, the 36 micron result is ignored, and the next loudest echo (value) is reported.
By increasing the Low Range we effectively tell the gage to ignore the effects of surface roughness because the PosiTector 200 can only report thickness values within its measurement range.
Let’s go over that again.
The first measurement result is lower than expected due to surface roughness, so we need to adjust the Low Range value.
Access the PosiTector’s menu by pressing the center button, and scroll down to set up.
Press the center button to select, then scroll down and select the Graphics option. Press the minus button to exit the menu.
A chart is now displayed on the screen, with several buttons across the top of the screen. This Graphics mode is available on PosiTector 200 Advanced models.
Use the UP or DOWN button to highlight the Lo option. Then, using the PLUS button, increase the Low limit to a value greater than the surface roughness.
Since the 36 micron measurement result is no longer within the measurement range, the gage immediately reports the next loudest echo, the one that correctly represents the paint thickness.
Measure again with the new Low Range value.
Repeated measurements confirm that the PosiTector 200 is no longer influenced by the surface texture.
Here’s how the process of changing the Low Range value would look on the PosiTector 200 B Standard model.
From the menu, select the ‘Set Range’ option from the Cal Settings submenu. The current high and low range settings are displayed above the current reading. Press the PLUS button to increase the low limit to a value greater than the surface roughness.
Since the 33 micron measurement result is no longer within the measurement range, the gage immediately recalculates thickness using the next loudest echo. The thickness result immediately changes to the correct value of 127 microns.
When the Low Range value is raised beyond the roughness, the PosiTector reports the next loudest echo, the one that correctly represents the paint thickness.
The second rough coating issue an inspector has to cope with is apparent lack of repeatability, something that every coating thickness gage must deal with. Notice how a series of measurements are not always the same. Close, but not perfectly repeatable.
This occurs with any measurement technique. The degree of roughness can vary over a surface and the instrument measures depth a little differently depending upon if the probe is placed on the top of a roughness peak or lower down in a valley.
Here’s another cross section image showing the subtle surface roughness.
International organizations such as ISO, ASTM and SSPC have a universal solution for this.
Simply average a series of measurements.
All PosiTector 200 models have the ability to do this easily.
Press the center button to enter the gage menu, and select the MEMORY submenu. On Advanced models, select ‘New Batch’. On Standard models, select ‘On’.
The display makes room for important statistical values including the number of measurements at the top right of the LCD.
Now, as measurements are taken, a running average is displayed at the top left, along with minimum and maximum values.
The large, easy-to-read LCD provides valuable, real-time information with each additional measurement. Outliers, or bad readings caused by operator error, can be deleted with the minus key.
After 8 [arrow] measurements are taken, ranging from a high of 137 [arrow] to a low of 113 [arrow] microns, the average thickness is determined to be 125.8 [arrow] microns.
To recap, when measuring on rough surfaces: