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May 6 - May 12, 2013

How does the concave or convex curvature of a surface affect the dry film thickness reading?

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Selected Answers

From Tom Swan of M-TEST on June 21, 2013:
I am assuming the question refers to how the DFT readings are effected  when using a Magnetic DFT gauge. On a flat surface, the magnetic field is spherical and decreases by the cube of the distance from the surface being measured. Once the strength of the field is determined (reason for 2-point calibration), it is easy to calculate the DFT if you know the strength of the magnetic field at any given point. In theory, it will work for any DFT thickness, but in reality, it is determined by the size of the coil in the probe generating the field. That is why DFT probes get larger as the thickness they will measure gets greater. As far a curvature, picture a fixed- ize bubble on a flat surface, As you bend the surface away from the bubble (convex), the bubble begins to flatten out. Assuming the bubble was 3 mils tall when you started, as you begin to bend the surface, the top of the bubble gets closer to the surface decreasing the distance to 2 mils or 1 mil, even though the size of the bubble did not change. On a convex surface the magnetic field is bent towards the surface making the measured distance less than it actually is. On convex surfaces the field flattens. On a concave surface just reverse what I just said. The magnetic field bulges out. By doing a 2-point calibration, you let the meter know what the shape of the magnetic field is so it can judge the distance based on both the shape and the strength of the magnetic field. Since the magnetic field decreases at the cube of the distance to the surface, small differences in the magnetic field strength can magnify errors based on curvature. That being said, except with relatively small objects such as small piping, the errors are generally pretty small.

From Mark Puckett of Orfanos Contractors, Inc. on June 21, 2013:
You need to contact the gauge manufacturer of the specific instrument you are using with the details of what you actually measuring.It would depend on the type of gauge and probe utilized and the orientation of the gauge, among other factors, as well as the substrate and coating or lining being measured.

From Karen Fischer of Amstar of WNY on June 12, 2013:
I believe the question is asking how the curvature affects the READINGS taken of the ry film thickness, not how the curvature affects the paint. Seems to me that logically, it would depend on the degree of curvature and how "flat" the Type 2 Gage can rest on the surface. The ability of the probe region of the gage to "rock" on a convex surface or be held further away from the pinpoint on a concave surface is likely to affect the accuracy of a reading.

From Gerald Burbank of Burbank on June 10, 2013:
Depending on the curvature, convex surfaces can cause wet coatings to pull away from the peak of the curve. The steeper the angle, and the less viscous the coating, the greater the effect that surface tension will have (as with edges), by thinning the wet film at the peak. Conversely, fluids will tend to gather in the trough of a convex surface due to surface tension along the edges. As stated in Mr. Tibbetts' post, other factors come into play as well. I imagine that ambient conditions and gravity plays a role as well. Gravity would tend to offset some of the effects of surface tension on a convex curve where the peak is inverted. Estimating the degree to which wet coatings would tend to flow toward or away from the peaks and troughs of a curved surface would need to take into account all of these variables.

From Ken Tibbetts of K C Tibbetts Company on June 6, 2013:
It all depends on the method of coating application--dipping, hand, robot, electrodeposition, etc. Of course, there's substrate temperature, basic material from which the substrate is made, the coating itself, size of object, atmospheric conditions, and lots more. There is no one right or wrong answer to the question. Give me details and I'll give a specific answer,

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Tagged categories: Coating / Film thickness; Dry Film Thickness (DFT)

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