through paint or other similar coatings
Measuring the remaining wall thickness is not a problem
In many industrial and petrochemical maintenance processes, the remaining wall thickness of those corrosion-susceptible metals is measured through one or more layers of paint or similar non-metallic coatings. When measuring a material with a lacquer or similar coating using a conventional ultrasonic thickness gauge, a measurement error occurs: the measured thickness value will often be more than twice the actual thickness of the metal, because the sound velocity of the coated material is much slower than that of the metal.
There are two solutions to this problem:
Echo-to-echo measurement and penetrating coating measurement.
Due to their good detection of rough surfaces and pitting defects, dual element transducers have long been the standard industrial equipment used in corrosion measurement applications. For this reason, these transducers are generally recommended for all common corrosion gauge applications. In some applications, such as the inspection of metallic materials with smooth lacquer layers, a single element delay line transducer may be recommended due to the need for high measurement accuracy.
Paint thickness measurement related theories
The longitudinal wave sound velocity in steel is generally about 5,900 m s (0.).2320 in s), while the velocity of sound for lacquer and similar coatings is generally lower than 2,500 m s (0.).1000 in/µs）。Conventional ultrasonic thickness gauges, when measuring the entire thickness of a metal material with paint, mistakenly measure the paint layer using the velocity of sound in steel, which means that the paint layer is at least 2 percent thicker than it actually is35 times (the ratio of two velocities of sound). In the case of thick coatings with strict tolerances, errors due to inaccurate paint thickness can have a serious impact on the overall measurement results. The solution to this problem is to measure the paint thickness and then subtract it from the overall thickness measurement.
The echo-to-echo thickness measurement method is a well-established technique that simply measures the propagation time of a sound wave between two successive backwall echoes. The two successive backwall echoes represent the two successive round-trip sound paths completed by the sound wave propagating through the material being measured. When measuring a metal material with a lacquered layer, these multiple backwall echoes occur only inside the metal and are not generated on the paint layer, so the period between each pair of adjacent echoes (e.g., backwall echoes 1 to 2, backwall echoes 2 to 3, etc.) only represents the thickness of the metal, but does not include the thickness of the paint layer.
Through-coating measurements are carried out using a software method that has been obtained to discern the time period represented by the round-trip sound path of the sound beam in the paint layer. This time period is used to calculate the coating thickness and display the coating thickness on the screen of the instrument. By subtracting this time period from the overall thickness measurement, the gauge can also calculate the thickness value of the metal and display it on the screen.
Advantages of both technologies
Advantages of echo-to-echo technology:
A variety of common probes can be used.
Materials with rough surface coatings can often be inspected.
Inspections can be done at high temperatures, and with the appropriate probe, temperatures up to around 500 or 930 F can be operated.
Advantages of the Penetrating Coating Function:
It can measure a wide range of metal thicknesses, generally ranging from 1 mm (0.).040 inches) to 50 mm (2 inches) or more.
Only one backwall echo is required.
When measuring materials with pitting defects, the minimum remaining thickness of the metal can be measured with a high degree of accuracy.
Measuring range through the coating
The through-coating function uses the D7906-SM or D7908 probe and can only measure non-metallic coatings, e.g. 012 mm (0.)005 inches) thick or thicker paint or epoxy materials. If you are measuring a coated steel part, but the thickness of the coating is not displayed on the instrument screen, the thickness of the coating is lower than the minimum thickness required by the through-coating function, or the thickness of the coating cannot be measured by the through-coating function. However, in many cases, an ultrasonic thickness gauge can measure the thickness of the coating, subtract the thickness of the coating from the overall thickness value, and display the thickness of the steel material on the screen. You can also try using the echo-to-echo feature to measure, or you may need to remove the coating before taking a measurement.
The thickness of the metal that can be measured in the through-coating mode generally ranges from about 1 mm (0.).040 inches) to 50 mm (2 inches) and above, depending on the acoustic properties of the metal and the conditions of the inner surface of the sample.
Echo-to-echo measurement range
In echo-to-echo mode, the range of thickness that can be measured depends on the transducer selected and the type of thickness gauge used, as well as the acoustic properties of the metal being measured and the conditions of the surface being measured. Because echo-to-echo measurements require at least two backwall echoes by definition, this feature cannot be used in some cases, for example, in heavily corroded, very rough, highly scattering, or highly attenuating materials, where a second multiple backwall echo cannot be generated. In this case, the through-coating function should be used if possible.