advantages of ultrasonic thickness gages over flaw detectors for corrosion thickness measurements

Advantages of Ultrasonic Thickness Gages over Flaw Detectors for Corrosion Thickness Measurements

Greg Bauer | Product Specialist, Ultrasonic Thickness Gages January 11, 2017

Advantages of Flaw Detectors § More versatile

–  Straight beam, angle beam, scanning

§  Includes specialized software for sizing defects: –  Distance Amplitude Correction (DAC) –  Time Varied Gain (TVG) –  Distance/Gain/Size (DGS)

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Instrument Priority §  Flaw detectors

–  Accuracy of signal amplitude

§  Thickness gages –  Consistency and accuracy of the time of flight measurement

§ Use the right tool for the job

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Why were flaw detectors used for measuring thickness? § History of ultrasonic (UT) equipment

–  Ultrasonic instruments were released in the late 1940s –  Digital thickness gages released in the 1960s

–  No waveform = less confidence –  Thickness gage with on-board waveform released in 1980s

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Additional Information from Live A-Scan

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Waveform 1 Waveform 2

Detection of backwall Detection of noise

Additional Information from Live A-Scan

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Waveform 1 Waveform 2

Heavily corroded backwall Cleaner backwall

Advantages of Thickness Gages over Flaw Detectors § Size and cost

§ Ease of calibration and operation

§ More consistent and accurate time of flight measurement

§ Specialized features or software for special applications

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Thickness Gage Measurement Advantages §  Auto zero compensation

§  V-path correction

§  Auto probe recognition

§  Detection algorithms and digital signal processing (DSP)

Auto Zero Compensation

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Why are dual element transducers used for corrosion applications?

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V-Path Correction When Using Thickness Gages

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Sample

TX RX

Angular Sound Path

True Thickness

Thickness Measurement Error When Using a Flaw Detector

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Auto Probe Recognition § Optimizes setup and receiver gain

–  Easier and faster setup for the user

§ Recalls transducer V-path correction –  Accurate measurements over large thickness ranges

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V-Path Builder on Olympus 38DL PLUS® Thickness Gage

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Step 3 Step 4

Step 1 Step 2

Flaw Detector Detection Mode — Flanking/Edge Gate

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Detection point at 60 dB Detection point at 54 dB

§  Thickness reading changes 0.011 in. (0.280 mm) from a 6 dB change

Flaw Detector Detection Mode — Peak Gate

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Signal from the pit Majority of signal not from thinnest area

Corrosion Thickness Gages Use Algorithms and Digital Signal Processing (DSP)

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Detection Threshold

Thickness Gage Application Advantages §  Temperature compensation

§  Electromagnetic acoustic transducer (EMAT)

§  Internal oxide

§  Encoded B-scan

§  Echo-to-echo measurement

§  THRU-COAT® technology

High-Temperature Applications

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§ Setup adjustments –  Increase gain –  Highest update rate –  Auto zero –  Temperature compensation

Olympus 38DL PLUS Thickness Gage Temperature Compensation § Sound velocity changes with temperature

–  Approximately 1% for every 100 °F (55 °C) change in temperature for carbon steel

–  V0: Sound velocity at calibration –  T0: Temperature at calibration (user inputted) –  T1: Temperature at measurement (user inputted) –  k: Temperature coefficient (user inputted)

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Olympus 38DL PLUS Thickness Gage Temperature Compensation

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Boiler Tube Applications §  Two solutions

–  Measuring tube wall thickness using EMAT –  Measuring tube wall and internal scale thickness using 38DLP-OXIDE

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Olympus 38DL PLUS Thickness Gage — Measurements with the E110-SB EMAT § Electromagnetic acoustic transducer (EMAT)

–  Requires external oxide scale to measure tube wall thickness –  No couplant required –  Fast and easy

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§ Measure boiler tube wall thickness and internal scale thickness –  Minimum scale with M2017: 0.010 in. (0.250 mm) –  Minimum scale with M2091: 0.006 in. (0.150 mm)

–  Shear wave couplant (SWC-2) required

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Olympus 38DL PLUS Thickness Gage — Optional 38DLP-OXIDE Software

§ Enables the 38DL PLUS thickness gage to perform encoded B-scans –  Data is transferrable –  Less expensive

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Olympus 38DL PLUS Thickness Gage — Optional 38DLP-EBSCAN Software

Distance traveled

§ Echo-to-echo –  Used to ignore coatings and measure only the true metal thickness

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Measuring Thickness on Painted Metal Applications

Auto echo-to-echo Manual echo-to-echo

§ Measuring thin materials –  About 0.080 to 0.100 in. (2.0 to 2.5 mm) minimum

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Echo-to-Echo Limitations

0.080 in. (2.0 mm) sample 0.040 in. (1.0 mm) sample Valid echo-to-echo measurement No separation, false reading

§ Samples with a heavily corroded back surface –  No multiple backwalls

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Echo-to-Echo Limitations

§  THRU-COAT technology –  Used to measure paint thickness and steel thickness at the same time

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Measuring Thickness on Painted Metal Applications

§ Measure thinner samples –  Minimum of about 0.040 in. (1.0 mm)

§ Measure thicker samples § Measuring painted samples with corroded back surfaces

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Advantages of THRU-COAT Technology

Echo-to-echo THRU-COAT technology

§ Used in standard, THRU-COAT, and echo-to-echo mode §  Limitations

–  Rough or textured surfaces –  Coating has to be at least 0.005 in. (0.127 mm) –  Cannot be used for high temperatures

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THRU-COAT Transducers (D7906-SM, D7906-RM, D7908)

Thickness Gage Feature Advantages §  Doubling warning

§  Gain optimization

Thickness Gage Feature — Doubling Warning § Definition of doubling

–  Gage detects the second backwall reflection and misses the first backwall signal

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Dual Element Transducers — Measuring Thick and Thin Samples

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Thick sample Thin sample

Calibration Doubling Warning

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Potentially wrong echo detected!

CAL VEL on thick CAL ZERO on thin

Doubling Issue with Dual Element Transducers § How to correct the doubling issue

–  Increase gain so the first backwall is above the 20% detection threshold

Waveform 1 Waveform 2

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Gain Optimization Calibration § Advantages

–  Increase useful life of transducer –  Help prevent calibration and measurement errors

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Default gain of 66 dB for this D790 Default gain of 59 dB for this D790

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