Selection of Rectification

When selecting signal rectification, factors such as filters, frequency, transducer type, transducer construction, dampening, specimen material, and damage mechanism should be considered. Once an inspection setup has been chosen, technicians should view the RF signal to determine rectification. It is crucial for technicians to ensure the accuracy of their inspections, regardless of the chosen rectification, and to use discretion when making this selection.

Points to Consider for Each Polarity:

• Width and amplitude of the first half cycle of each polarity. The highest possible amplitude for the first half cycle is desirable.
• Ratio between the first half cycle’s amplitude and the total signal amplitude.
• Uniformity of the leading edge of the first half cycles.

Why Half+ Rectification is Undesirable:

The first positive half cycle is generally wider, less uniform (lower leading edge gradient), and has a lower amplitude compared to the first negative half cycle. The difference between the first half cycle and total signal amplitude is usually greater. Wider, low-amplitude signals result in thickness readings that change more with increased amplitude, due to variations in the leading edge gradient. This change diminishes as amplitude increases, leading to a larger discrepancy between thickness readings at 80% FSH (Full Screen Height) and scanning amplitudes.

Pros of Full Wave (POFW):

1. Easier to identify “corrosion walk.”

Cons of Full Wave (COFW):

1. The first positive half cycle may not break the gate in every calibration step due to its low amplitude (typically 2-10% above the gate threshold) and may not break the gate at all thickness levels when the signal is at 80% FSH.
2. Step 1 is highly dependent on transducer selection, making it difficult to identify which transducers will encounter issues.
3. Greater variation in thickness readings between scanning dB and 80% FSH.
4. Incipient stage pitting may not break the gate at reference dB due to the low amplitude of the first positive half cycle.
5. The first positive half cycle can be misinterpreted as “noise,” causing technicians to adjust or grease their transducer shoes to eliminate it.
6. Lack of space between half cycles when adjusting gate start can make it challenging to eliminate noise.

Pros of Half-Wave Rectification (POH-R):

1. The first half cycle is closer to the total signal height.
2. Issues with bringing the first half cycle vs. total signal height to 80% FSH for measurement are reduced.
3. The first half cycle consistently breaks the gate across all calibration steps when using a wide range of transducers, as it is typically 15% or more above the gate threshold.
4. Thickness reading changes when adjusting amplitude are significantly reduced.
5. Less frequent adjustments to the gate threshold are needed to eliminate noise.
6. Signal isolation from noise can be achieved by placing the gate start between half cycles.

Cons of Half-Wave Rectification (COH-R):

1. Technicians must understand the differences in signal characteristics between rectifications, which may require additional training for proficiency in scanning with half-wave rectification.

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Learn more about Ultrasonic Inspection during our training courses offered at the National Inspection Academy. Visit our website for more details: https://Nationalinspection.org/services/ultrasonic-testing/

About the Author

Dave Arnett has been working in the skilled trade of nondestructive testing since 2008. He is currently an IRATA level III rope access tech and holds level II certifications in all major inspection methods. Dave has spent his career traveling to Australia working offshore as well as working in the remote oil field of Prudhoe Bay, Alaska. https://www.linkedin.com/in/david-arnett-211743156/