The bottom line
Ultrasonic coating thickness measurement works on non-metallic substrates (concrete, wood, plastic) by measuring echo time through the coating. X-ray fluorescence (XRF) measures thin metallic coatings on metallic substrates by analysing fluorescent emission. They target different applications and rarely overlap.
How each works
Ultrasonic
A high-frequency sound pulse travels through the coating, reflects off the substrate interface, and returns to the probe. The gauge converts echo time to thickness using the coating’s sound velocity. Defined in ASTM D6132.
X-ray fluorescence (XRF)
An X-ray beam excites atoms in the coating, which emit characteristic fluorescent radiation. The intensity of the fluorescent signal correlates with coating thickness. Defined in ASTM B568.
Comparison
| Aspect | Ultrasonic | XRF |
|---|---|---|
| Substrate | Non-metallic (concrete, wood, plastic, FRP) | Metallic (steel, aluminum, copper) |
| Coating | Any organic coating (paint, epoxy, urethane) | Metallic coatings (zinc, nickel, chrome, tin) |
| Range | 18 μm – 15 mm | 0.01 μm – 50 μm (typical) |
| Accuracy | ±(1% + 1 μm) | ±(5% + 0.1 μm) |
| Radiation safety | None required | X-ray shielding required |
| Portability | Fully portable | Benchtop or portable with safety enclosure |
| Cost | $200-$3000 | $5,000-$50,000+ |
Common misconception
- “XRF is always more accurate” — XRF excels at very thin metallic coatings, but for organic coatings on non-metallic substrates, ultrasonic is the only viable option.