Gamma Thickness Measurement
Quick Start
Gamma gauges just work - as long as you can get accurate measurements value for the substrate then for substrate + coatings. The question is how accurate the computed value will be if the detector has a nominal 0.25% error. For a thin coating on a thick substrate the error will be large!
Gamma gauge
There is very little to say about GBS - Gamma Backscatter measurement. A radio-isotope source emits gamma rays which are scattered by anything that gets in their way in a linear fashion. So twice the amount of "stuff" in the way gives twice the backscatter, with no dependence on whether that stuff contains, say, TiO2 rather than straightforward organic compounds.
The method works by measuring backscatter so is a single-sided measurement device. The unit is small.
The big issue is that there is no way to distinguish coating from substrate so you need two sensors. The first measures the thickness of the substrate, the second measures the thickness of coating+substrate. If this is a scanning system then ideally the downstream head is synchronised to scan exactly the same part of the web, to avoid errors from variable substrate thickness.
Assuming that the substrate is of constant thickness then the only issue is accuracy. And that's what the app calculates. Suppose that the gauge has an accuracy of 0.25%. Then there's an error in measuring the substrate and an error in measuring substrate+coating. When these errors are summed (actually, the square root of the sum of the squares) then for thin coatings on thick substrates the errors can be high.
The strengths and limitations are clear:
- For very thin coatings there isn't enough differential gamma scattering compared to the substrate.
- High variability in substrate thickness requires smart correlation of measurement points.
- The method is "universal" so is ideal for those producing a big variety of different coatings on different substrates.
- Calibration is necessary but not too arduous.
- Although the devices are provably safe, the idea of "radiation" spooks some users.
Summary
- Accuracy: Broad limits of high accuracy with the obvious issues of low accuracy if coating is much thinner than the substrate or if there are uncontolled substrate thickness variations.
- Size: Small
- Safety: Safe, but "radiation" spooks some people
- Scanability: Easy to scan, and to use in spot mode
- Cost: Medium
- Wet/Dry: Works on both
- Big limitations: Thin coatings are difficult; requires two scanning stations.
