Ultrasonic Thickness Gauge through Coating (Paint Barrier Thickness Measurement) Principle Introduction
Introduction to the principle of ultrasonic thickness gauges through coatings (thickness measurement through paint): The longitudinal speed of sound in steel is representative of 5.900m/s (0.2320in/us), but in paint or similar coatings the speed of sound is typically less than 2.500m/s (0.1000in/us). Conventional ultrasonic equipment will incorrectly measure the coating at the speed of sound of steel when measuring the total thickness of metal with a lacquer layer, meaning that the coating will show a value of at least 2.35 times (the ratio of the two speeds of sound) its true thickness. In cases involving thick coatings and tight tolerances, this error introduced by the coating can be a significant fraction of the total thickness measurement. The solution to this problem is to measure or calculate the thickness in such a way ---- that the coating component is removed ---- from the measurement.
Echo-echo measurement simply applies the well-established technique of a time interval between two adjacent bottom surface echoes that represents the continuous round trip travel time of the sound wave through the test material. In the case of those with coated metals, these multiple echoes can only occur in the metal and not in the coating, so the interval between any pair of echoes (bottom echo 1 to 2, bottom echo 2 to 3, etc.) represents only the thickness of the metal after the coating thickness has been removed. Through-coating measurements use a software to determine the time interval represented by one round trip in the coating. This time interval is used to calculate and display the coating thickness, and by subtracting this time interval from the total measurement, the instrument can also calculate and display the thickness of the metal substrate.
Each of these techniques has advantages and disadvantages and consideration should be given to which method is best for a particular application:
Measurement Through Coatings Advantages:
1, can measure a wide range of metal thicknesses, representative, in steel, from 1mm to 50mm.
2, only one echo is required
3, more accurate measurement of the minimum thickness remaining in pitting situations.
Disadvantages of measuring through coatings:
1, The thinnest coating is 0.125mm.
2, The surface of the coating should be relatively smooth
3, Maximum surface temperature is about 50°C or 51.67°C.
Echo-echo measurement Advantages:
1, can use a variety of common probe work
2,Often can penetrate the rough surface coating work
3, with the appropriate probe can be close to 500 ℃ or 498.89 ℃ high temperature work
Echo-echo measurement disadvantages:
1, the need for multiple bottom echo, in severely corroded metals may not exist multiple bottom echo
2, Thickness range is more limited than through-coating measurements
