What Does Tooke Gauge Mean?
A Tooke gauge is a destructive testing tool used to determine the overall thickness, or the individual layer thickness, of a dry paint or coating in accordance with ASTM D4138. The gauge works by making a small incision (about the width of a pencil) through the coating down to the substrate level. The triangular V-shaped groove is then observed through a microscope on the gauge where an adjustable scale is used to measure the thicknesses of the coating layers. Coating an object to the proper dry film thickness reduces the likelihood of corrosion.
Corrosionpedia Explains Tooke Gauge
Although coating thicknesses are typically measured using non-destructive tools, these devices have their limitations. For instance, due to their method of operation, non-destructive tools can only measure coating thicknesses on non-metallic substrates. Furthermore, non-destructive methods cannot distinguish between different coating layers (e.g., top coat, intermediate layer, primer, etc.). A Tooke gauge must be used when the thickness of individual layers needs to be determined, such as for a failure analysis.
The Tooke gauge consists of a gauge body, a battery compartment, tungsten carbide cutting tips, a focus adjustment tool and a microscope with a scale etched into the lens. When the incision is made, the lines on the Tooke gauge’s scales are aligned with the various layers of the coating. Since the resulting cut is in a V-shape (or more specifically, an equilateral triangle), basic trigonometry is used to calculate the thickness of each coating layer.