Application Notes on Thickness Gauges.

Thickness gauges are used to make precise dimensional cross section measurements on a wide variety of coatings and materials including steel, plastic, glass, rubber, ceramics, paint, electroplated layers, enamels, pavement, multi-layer deposits, etc. There are many mechanical, nondestructive and destructive techniques available to accomplish this task: IR or nuclear gauges, eddy current, magnetic particle, laser, ultrasonic, coulometric, and X-ray, are only a few of the many techniques.
Beta, IR or nuclear gauge testing involves the absorption of x-ray, infrared or Beta particle radiation to measure the thickness of materials or coatings. On nonmetallic materials such as paper or plastic films or webs, radiation is transmitted through the material and a radiation or Geiger-Muller detector is located on the other side to measure radiation levels. On coated metallic materials, the radiation or Geiger-Muller detector is located on the same side and backscattered radiation is measured.
Coulometric instruments use an electrochemical process to etch away a plated or metallic layer at a predetermined rate. The amount of time to remove the plated layer provides an indication of coating thickness. Coulometric measurement is a destructive technique.
Eddy current, penetrating radar and other electromagnetic thickness gauge techniques are used to detect or measure flaws, bond or weld integrity, thickness, electrical conductivity, coating thickness, detect the presence of rebar or metals. Eddy current is the most widely applied electromagnetic NDT technique. The eddy current method is also useful in sorting alloys and verifying heat treatment. Eddy current thickness gauges use an electromagnet to induce an eddy current in a conductive sample. The response of the material to the induced current is sensed. Since the probe does not have to contact the work surface, eddy current testing is useful on rough surfaces or surfaces with wet films or coatings.
Laser thickness gauges include methods such as laser shearography, magneto-optical, holographic interferometry or other optical techniques to detect flaws, residual stress or measure thickness.
Magnetic particle or current flow uses an external magnet magnetizes the part. Magnetic poles created at flaws, cracks or other discontinuities attract magnetic particles. The magnetic particles are fine iron oxide particles (0.125 to 60 microns) with a high permeability (easily magnetized) and low retentivity (ability to stay magnetized). Three methods are typically applied: dry non fluorescent, wet non fluorescent and wet fluorescent.
Mechanical gauges physically contact a sample to measure thickness using a gap and/or comparison to a known dimensional standard or master. Micrometers and calipers are common types of mechanical gauges used for dimensional gauging.
Ultrasonic instruments use beams of high frequency acoustic energy that are introduced into the material and subsequently retrieved. Thickness or distance calculations are based on the speed of sound through the material being evaluated. The most widely used of all UT techniques is the pulse-echo technique.
Thickness gauges using penetrating X-rays or gamma rays to capture images of the internal structure or a part or finished product. The density and composition of the internal features will alter the intensity or density of these features in the X-ray image. Densitometers are used to quantify the density variations in the X-ray image. Penetrameters or other X-ray thickness gauge references are located with the part during imaging for sizing of internal cracks, pores, defects or other features.
Sebastian

G.M. Technical

Nunes Instruments

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Coimbatore. 641012.

Tamil Nadu

India,

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