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How the Echo-to-Echo and THRU-COAT® Techniques Measure Up for Corrosion Inspection

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One way inspectors monitor for corrosion is by measuring the remaining wall thickness of pipes or other metal structures. However, when measuring the remaining wall thickness of metal pipes, tanks, ship hulls, or other painted or coated structures, you have options.

Remaining Wall Thickness

In many industrial and petrochemical maintenance applications, it’s necessary to measure the remaining thickness of metal that’s subject to corrosion. Oftentimes, these metal pipes and parts are coated with paint or a similar non-metallic coating. With conventional ultrasonic thickness gages, the presence of paint or similar coatings will cause measurement errors, typically increasing the apparent metal thickness by more than twice the thickness of the paint due to the paint's much slower sound velocity. Two solutions to this problem are available for the 38DL PLUS® and 45MG (optional) thickness gages: echo-to-echo measurement and THRU-COAT® measurement. Each technique has its strengths and weaknesses, so understanding both will help you pick the right solution for your application.

Figure 1. 38DL PLUS thickness gage.

Figure 2. THRU-COAT measurement technology.

Figure 3. Echo-to-echo measurement.

Echo-to-Echo Measurement

Echo-to-echo thickness measurement utilizes the well-established technique of timing the interval between two successive back wall echoes that represent successive round trips of the sound wave through the test material (Figure 4). In situations involving painted metal, these multiple back wall echoes occur only within the metal, not in the coating, so the interval between any pair of them (back wall echo 1 to 2, back wall echo 2 to 3, etc.) represents only the metal thickness, and the coating thickness is cancelled out.

The advantages of using the echo-to-echo technique include:

  • Works with a variety of common transducers
  • Often works through rough-surfaced coatings
  • Can be performed at high temperatures up to approximately 500 °C (930 °F) with appropriate transducers

The limitations of the echo-to-echo technique include:

  • Requires multiple back wall echoes, which may not exist in severely corroded metals
  • Thickness range may be more limited than with THRU-COAT measurement

Figure 4. How echo-to-echo measurement works.

THRU-COAT Measurement

THRU-COAT measurement uses patented software to identify the time interval represented by one round trip in the coating. This time interval is used to calculate and display the coating thickness. By subtracting this interval from the total measurement, the gage can also calculate and display the thickness of the metal substrate.

The advantages of using THRU-COAT technology over the echo-to-echo technique include:

  • Works over a wide range of metal thicknesses, typically from 1 mm (0.04 in.) to greater than 50 mm (2 in.) in steel
  • Requires only one back wall echo
  • May measure minimum remaining metal thickness more accurately when there is pitting in the metal

Some of the disadvantages of using THRU-COAT technology are:

  • Coating must be non-metallic and at least 0.125 mm (0.005 in.) thick
  • Coating surface must be relatively smooth
  • Requires you to use one of two special transducers
  • Can only be used on surfaces with a max temperature of 50 °C (125 °F)

Check out this video or application note to learn more about echo-to-echo and THRU-COAT measurement.

February 7, 2017
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