ARTICLE OF INTEREST • Gating Techniques for Ultrasonic Thickness Testing Using Flaw Detectors
Gating Techniques for Ultrasonic
Thickness Testing Using
Flaw Detectors
By Paul Holloway, P.Eng, MASc, CGSB UT3 MT2, President, Holloway NDT & Engineering Inc.
Purpose
The purpose of this article is to provide guidance on settings and methods,
in particular the careful use of gating, to ensure accuracy of thickness testing
on corroded steel and other metallic components. Specific applications include
boiler tubes, tank floors, piping and vessels where the testing is performed
from the OD or top surfaces, inspecting for metal loss due to corrosion on the
opposite side.
Introduction
Modern ultrasonic thickness gauges offer a distinct advantage over
traditional flaw detectors in ease of setup and features such as gain
optimization and V-path correction. However, in many cases, flaw detectors
are still used for thickness measurement. While both types of instruments
require careful operation and a fundamental understanding of ultrasonic
theory, much of what happens automatically in a thickness gauge must be
performed manually with a traditional flaw detector.
Traditional Approach
measure the same point on the waveform at all times. An ultrasonic waveform
is a series of peaks and valleys, seen using the unrectified RF mode where
the positive and negative lobes of each cycle can be clearly identified. With a
fully rectified A-scan, the negative lobes are flipped-up alongside the positive
lobes, making it difficult to tell them apart. To achieve accurate results, a fully
rectified signal is not ideal.
Gate mode (Edge): The following example illustrates the typical
variations in measurement using edge mode. Fig. 1 shows a typical rectified
A-scan with a basic 5 MHz dual element transducer. In this case, Gate 1 is
triggering near the top of the lobe and Gate 2 is triggering near the bottom,
resulting in a difference of 0.005 in.
Fig. 2 illustrates the zoom function used to show the difference in triggering
points more clearly. Because the signal is sinusoidal, using edge mode near
the top of the lobe will result in a higher reading (the measurement point is
further to the right) than at the bottom. This behaviour is inherent to edge
mode, and makes this mode less than ideal when high accuracy is desired.
It should be pointed out, however, that while it is possible to use edge mode
accurately by zooming in to carefully identify the correct lobe while manually
The most common mistake made when it comes to thickness reading is
a blind allegiance to the approach of “crank up the gain, make the backwall
as loud as possible and go for it”. Little attention is paid to frequency, pulse
or receiver settings, gate mode, or precisely where on the waveform the
measurement is made. This may be sufficient on clean surfaces without
any corrosion, but can produce significant error when dealing with irregular
backwall surfaces. As well, this sort of simplified approach doesn’t put into
practice much of the theory learned in training, and doesn’t differentiate the
NDT technician from anyone else with a pulse.
Effect of Some Standard Settings
28
CINDE Journal • Vol.37 • No.3 • May/June 2016
Fig. 1: Fully rectified waveform with G1 and G2 set on edge
www.cinde.ca
...continued on page 30
t
The following lists a few of the standard settings and the drawbacks
of each.
Full rectification: For maximum accuracy, it is essential to calibrate and
ARTICLE OF INTEREST • Gating Techniques for Ultrasonic Thickness Testing Using Flaw Detectors
Fig. 2: Zoomed in view showing difference in triggering points
Fig. 3: RF mode signal
adjusting the gate level to a consistent percentage of the height, this is
cumbersome to perform for every reading.
Optimal Techniques
The following illustrates some techniques to better control variations in
readings and improve measurement accuracy.
Use two gates: Activating a second gate allows us to fine tune the
velocity and zero offset such that the first backwall reading (G1) and the echoto-echo reading (G2-G1) are the same. The ability to accurately measure
echo-to-echo is useful to subtract the lift-off produced by coatings. However,
it is of little use when sizing small pits as they may be too small to produce
reliable multiple echoes. Both gates should be set to the same measurement
mode and polarity.
Gate mode (First Peak): The gate measurement mode can be set to
edge (aka flank), peak or first peak. As shown previously, using edge mode
is fraught with variations when the signal changes amplitude. Using the peak
position of the signal is preferred because it stays relatively stable in time.
However, the waveform contains many peaks and setting the mode simply to
peak means measuring the position of the highest peak crossing the gate,
and the highest one may not always be the one you want. Setting the mode
to first peak will isolate the first lobe.
Use RF mode to confirm waveform characteristics: A typical
thickness waveform is shown in Fig. 3. In this case, an Olympus D790 probe
(dual crystal 5 MHz, 11 mm dia.) was placed on the 0.300 in. step of a 5-step
steel wedge. Using RF mode, we can differentiate the negative and positive
lobes in each cycle.
We take note of the fact that the first significant signal is negative, and will
use only the first negative lobe during calibration and testing. The first positive
lobe could also be used, but this article continues assuming the first negative.
Gate polarity (Negative): Since the positive half of the waveform is
not going to be used, there’s no point in considering it during measurement.
Setting gate polarity to negative will cause only the negative lobes to
trigger the gate.
A-scan display (Half-wave Negative): Once the signal characteristic
is confirmed and the gates are observed to trigger on the peaks of the first
negative lobes, there’s no need to look at the positive half of the signal. By
switching to half-wave negative (Fig. 4), we can isolate the only part we’re
interested in, providing maximum screen height to detect small pit signals.
30
CINDE Journal • Vol.37 • No.3 • May/June 2016
Fig. 4: Half-wave negative view
Measurement display: Because both gates are being used, we need
to ensure the measurements displayed include the thicknesses in Gate 1 and
Gate 2 as well as the difference between the two (G2-G1).
Calibration
Using a reference standard that spans the thickness range being measured,
the velocity and zero offset values can be set. This can be done manually
or with an auto-calibration function. If performed using auto-calibration,
the values may need to be fine-tuned to ensure the echo-to-echo reading
matches the reading from Gate 1.
Adjust gain to keep the peak signal onscreen: If the peak of the
signal is beyond 100%, it runs the risk of saturation. Some instruments will
operate beyond 100% screen height, but only to a certain point. Once the
signal is saturated, the equipment can no longer discern the peak, so it begins
to register smaller and smaller values. Always adjust the gain so that the peak
being measured exceeds the gate level but remains below 100% screen height.
Use both gates to calibrate: You may choose to simply use Gate 1
and the auto-calibration function, but using two gates allows us to perform
a calibration quickly as well as set up a corresponding echo-to-echo
measurement at the same time. First, set Gate 1 and Gate 2 over the first and
second backwalls (Fig. 4). Then using the G2-G1 reading, adjust the velocity
such that this reading equals the reference thickness. Then adjust the zero
offset such that Gate 1 also equals the reference thickness.
www.cinde.ca
ARTICLE OF INTEREST • Gating Techniques for Ultrasonic Thickness Testing Using Flaw Detectors
Effect of V-Path: The V-path is a characteristic of dual element
transducers where the elements are angled slightly to emit sound from one
side and receive it on the other. This angle is referred to as the “roof angle”.
Many thickness gauges will recognize the probe type and compensate
for V-path internally. However, most flaw detectors don’t include V-path
correction as a standard feature. While the calibration may appear accurate
at the thin and thick limits chosen for calibration, there is inherently some
error incurred on the points in between. This is more pronounced when
taking readings over long distances and with higher roof angles. After
calibration, the accuracy of the intermediate points should be checked to
confirm the effect of the V-path error.
Finally, it should be pointed out that while calibration is critical for determining
the zero offset, calibration for velocity is a bit of a misnomer. The material
velocity of your step wedge is probably not the same as the test piece, and
unless you have a machined and accurately verified thickness sample of the
exact material you’re inspecting (and you probably don’t), this is the one
variable we really can’t do too much about. However, performing a complete
calibration on a known reference standard is good practice and verifies that
your machine is operating properly.
Fig. 5: Peak-Mem indicating area of interest in front of first backwall
Measurement Settings
While performing the actual inspection, the following settings and functions
can aid in detection.
Set reference and scanning gain: When scanning for pitting, there
may be many locations that are perfectly clean. We need to ensure we can
accurately observe the backwall signal as well as identify pitting. This may
mean adjusting the gain up and down repeatedly during inspection, and some
flaw detectors have a quick adjustment button to toggle between the current
scanning gain and the reference gain. The reference gain should be suitable
for measuring a clean backwall, whereas the scanning gain should be high
enough to detect small pitting, but will be limited by the noise level.
Peak-Mem or Envelope function: Activating the Peak-Mem or
Envelope function to capture a “memory” on the A-scan is an extremely
useful tool when scanning for corrosion. It is difficult to keep your eyes on
the probe to ensure sufficient overlap between passes, while at the same
time monitoring the screen for a small pit signal. By activating this function,
it allows you to keep your eyes on your probe, looking back at the screen
every few seconds to see if a pit signal has appeared. The previous area can
then be investigated further. Fig. 5 shows a small pit signature found during
scanning, while Fig. 6 shows the result of final sizing after prove up. Use of
Peak-Mem is preferred over use of the Minimum Thickness reading, as small
pits may not necessarily reach the gate level.
Other Sources of Error
In addition to the proper use of the equipment as shown above, a welltrained and competent inspector should be able to recognize and account for
a number of variables such as those shown below:
• Temperature: calibration should be performed at the same
temperature as the test surface, or use the temperature
compensation feature of your instrument (if equipped)
• Coating: if taking readings through coatings, first make sure they
are well-adhered. Subtract the average coating thickness from your
Fig. 6: Small pit located with 0.280 in. remaining wall
Fig. 7: Lower frequency = greater difference between lobes
first echo readings by either using a coating thickness gauge on a
few locations or use echo-to-echo to get the real metal thickness
on a sample of uncorroded areas then subtract that from a sample
of first echo measurements
• Surface curvature: When possible, use a curved reference
standard. When taking measurements, make sure the acoustic
barrier (the dividing line) on the face of a dual transducer is
perpendicular to the axis of curvature (i.e. the line goes across
www.cinde.ca
CINDE Journal • Vol.37 • No.3 • May/June 2016
31
ARTICLE OF INTEREST • Gating Techniques for Ultrasonic Thickness Testing Using Flaw Detectors
•
Fig. 8: A slight change in finger pressure resulting in a 0.005 in. difference
the pipe, not along it). This will ensure the shortest sound path
distance.
• Frequency: The rule of thumb is “use the highest frequency you
can get away with”. Higher frequency means more attenuation and
noise, as it’s more susceptible to the microstructure of the steel.
But using a higher frequency also provides improved sensitivity
and resolution, critical in locating and sizing pits. A benefit of using
a higher frequency is the shorter wavelength results in a smaller
difference in reading when different points on the waveform are
measured. This can be seen in Fig. 7 where two points on a 2.25
MHz waveform result in considerably more measurement variation
(0.008 in.) than with the 5 MHz waveform in Fig. 2 (0.005 in.).
The same can be said when using peak mode, when the signal
drops below the gate and the subsequent lobe is measured.
• Zooming in too far: Use the zoom function carefully to avoid
missing something off to the left of your screen.
• Gate position: Watch the screen for small pits that show up before
the start of your gate and adjust accordingly.
• Saturation: Even with peak mode, if the signal goes too far off
screen (the limit will depend on your instrument), the measurement
point will default to the leading edge of the first wavefront that’s
“out of reach”. This essentially defeats the purpose of using peak
mode, which is why it’s important to keep the signal onscreen
similar to a shear wave weld inspection.
• Probe face wear: When performing a corrosion scan, particularly
with a repetitive scrubbing motion, the probe face can wear down
alarmingly fast. On boiler tube scans, this may make a perfectly flat,
brand new probe look “cupped” by lunch. These probes can still
be used, but may not sit flat on a regular step wedge and may be
difficult to calibrate, which is just one reason why the geometry of the
reference standard should match the test piece. Probe face wear
should be monitored carefully, and calibration checks performed at
regular intervals.
• Scanning speed: Pits can be small and disappear off the screen
in the blink of an eye. To quote Ferris Bueller: “If you don’t stop and
look around once in a while, you could miss it”.
32
CINDE Journal • Vol.37 • No.3 • May/June 2016
Couplant thickness / pressure: Fig. 8 shows what can happen
when slight changes in finger pressure or probe orientation change
which lobe the gate is triggering on. In this case, a 0.005 in. error
has occurred. Flipping back and forth between lobes can happen
regardless of gate measurement mode, so the signal must be
monitored closely.
Conclusion
Ultrasonic thickness testing is considered an “entry-level” task, but requires
far more insight than simply turning on an instrument and following the
onscreen calibration instructions. Many factors must be taken into account in
order to achieve accurate and repeatable ultrasonic thickness measurements.
References
1.
NDT.net - October 1997, Vol.2 No.10, Theory and Application of Precision Ultrasonic
Thickness Gaging, Kenneth A. Fowler, Gerry M. Elfbaum, Karen A. Smith and Thomas
J. Nelligan
2.
TNT (The NDT Technician), Vol.3 No. 4, Focus: Ultrasonic Thickness Measurements
at High Temperatures, Nisbet, Ronald T.
3.
ASTM E797-10, Standard Practice for Measuring Thickness by Manual Ultrasonic
Pulse-Echo Contact Method
4.
Materials Evaluation, Vol.63 No.9, Ultrasonic Thickness Gaging Accuracy in Tubulars,
pp: 915-918, Wood, Brian K.
www.cinde.ca
Paul Holloway is a Professional Engineer and CGSB
Level UT Level 3 and MT Level 2. He is the President
of Holloway NDT & Engineering Inc., a company
specializing in ultrasonic testing field services, NDT
training and consulting, and engineering services related
to the construction, manufacturing, petrochemical and
power generation markets. He received his Master
of Applied Science degree in Mechanical Engineering
from the University in Waterloo in 2004. He is also an
active member of the CSA W59 committee. Paul drinks
Starbucks French Roast, black.
Paul Holloway
Georgetown, ON Canada
paul@hollowayndt.com • www.hollowayndt.com
(905) 703-0201