Coupling Check Requirements And Differences

Coupling Check Requirements And Differences

ASTM E273-15 does not have clear specifications when it comes to coupling checks. It does however state that 100% volumetric inspection of the weld and entire pipe length shall be made by the NDE system.
This gives rise to a very important issue. How do we know that the ultrasound beam made it to the inspection area and if it can return to the ultrasonic probes?
All serious NDE system manufacturers therefore provide a coupling check during inspection. Below we will cover the main methods used in achieving coupling check and why some methods are better than others.
The main method used by IIS Ltd and a few other NDE system manufactures is called Pitch-Catch. It is the most robust way to assure the ultrasound beam made it to the inspection area and back to the probes. This setup is illustrated in the picture attached.

Pitch-Catch Coupling Check Benefits
- Requires two opposing probes used in pulse-echo for inspection and pitch-catch for coupling check. One probe sends a pulse and its pair across the weld listens for this pulse.
- Coupling Gates/Alarms/Paint indicate when coupling signal is to low or not at all.
- In addition, at IIS Ltd we also use a flow meter on the main coupling lines. Should the flow decrease, the test head lifts to save the probes from excessive heat damage. This also triggers the above alarms and is used as another layer of protection. It saves customers thousands of dollars should a coupling pump fail, or lines get clogged.
- Oil on the pipe, as well as excessive rust or dust, attenuates or blocks the coupling signal triggering an alarm.
- Aging or damaged transducer will have an attenuated or missing coupling signal triggering an alarm.
- Misaligned transducers can have a significantly attenuated coupling signal, triggering a coupling alarm as well.
- This technique also provides valuable monitoring of the transducers.

Unfortunately, not all NDE system manufacturers use a Pitch-Catch coupling check. Below you will see why other methods aren’t as robust and effective at ensuring coupling check is performed.
The most common coupling check is performed using an internal reflector in the probe holder. Usually this is very common when using focused probes. A percentage of the beam energy gets reflected from the pipe surface (entry point) back inside the probe holder. Using a reflector, this beam can be reflected back to the probe, and the signal produced gets monitored.

Internal Reflector Coupling Check Issues
- By using this method, we essentially check that the ultrasound beam made it to the top of the pipe and back to the transducer NOT to the weld.
- Oil on the pipe can significantly attenuate or block the beam from reaching the weld. However, in this case the coupling signal might not even be lost or reduced at all.
- Depending on the design of the transducer holder and reflector, air bubbles may accumulate on the reflector causing a false coupling alarm.
- Aging or damaged transducer can still provide enough signal for a coupling check but not enough for a full inspection.
- Misaligned transducers will still produce a coupling signal as this signal comes from the internal probe reflector and not another transducer across the weld.

Other NDE systems also perform coupling checks using the following methods which are even worse and should be avoided.
- Monitoring the interface signal from the surface of the pipe. This has all the issues that the internal reflector coupling check method has.
- Monitoring the Main Bang (initial pulse) signal. This method should not be allowed at all. Even with a damaged transducer the Main Bang signal is still strong.

Knowing the above, you have a better understanding of your system and it helps you troubleshooting and maintain the system.
At the end of the day, if you want to test a weld using ultrasound, make sure that the beam made it there and back successfully. And be sure this is recorded during your inspection in order to show an API auditor that you truly tested the entire weld volumetrically and through the entire pipe length. 

Daniel Lucaci

    • Related Articles

    • Using Focused vs Non-Focused Transducers

      When presented with a quote for a new NDE system, let’s say for a tube and pipe application, ask the NDE equipment manufacturer if they plan to use Focused or Non-Focused transducers and why. There are clear advantages for each type of transducer, ...