Screening FAQs

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Screening – Basics

What is GUL Screening and what is it used for?

GUL Screening is normally used to rapidly and cost effectively locate and classify areas of damage on a pipeline. Since waves are sent along the pipe from the sensor location, 100 percent coverage (even of inaccessible areas) can be obtained.

Originally designed for detection of corrosion under insulation (CUI), the application of GUL Screening has expanded into many other areas. It is also widely used to inspect for corrosion at pipe supports, at air/soil interfaces, in buried sections, on overhead lines, and many more.

Please see the Screening Overview page or Screening Case Studies page for more information.

GUL Screening can locate corroded areas

What is the Wavemaker® pipe screening system?

Wavemaker® is the product name of the guided wave system that GUL Screening utilises

The system comprises of 3 primary components: a sensor (the transducer ring), a Wavemaker® instrument, and a computer running the analysis and reporting software. Details of these can be found on the Wavemaker System page.

What is the difference between LRUT and GUL screening?

All three names refer to the same general inspection concept. However, we feel that GWT and GUL Screening most accurately describe the method and system being used. More information is available on the Terminology page.

What is the range of pipe sizes and thicknesses that can be inspected using GUL Screening?

GUL Screening has solutions for pipes with diameters from 0.75 inch (19.05 mm) and above. The applicable pipe wall thicknesses range from 3 mm to 40 mm (1/8 to 1.57 inch). We have a range of transducer ring types and sizes to meet your inspection requirements. Please see our PDF Product Catalog for standard products or contact us separately for custom sizes.

Which types of pipe and material is GUL Screening applicable to?

GUL Screening can be used on almost all metal pipes. However, it currently cannot be used on plastic pipes.

See the Applicable Materials page for more information.

Screening – Technology

What are guided waves?

Guided waves are those waves whose propagating characteristics are greatly influenced by the geometry of the structure in which they exist and not just the mechanical properties of the material. Since the structure ‘guides’ the waves, the waves are very sensitive to any change in the geometry of the structure (for example a loss of section caused by corrosion). Therefore, even at low frequencies, very small changes cause reflections that can be detected and analyzed. This allows for 100 percent coverage even far from the sensor. More in-depth information can be found on the What are guided waves? page.


Why do we use the T(0,1) torsional wave in GUL Screening?

GUL screening uses the T(0,1) torsional mode instead of the alternatives for several reasons including:

  • It has better signal to noise (SNR) so it can find smaller defects
  • It can be used over a wide frequency range to balance sensitivity vs. range
  • It is NOT sensitive to liquid loading
  • It is sensitive to a very wide range defect types

More detailed information is available on the Why Torsional page.

How does GUL Screening work?

The torsional T(0,1) guided wave is generated within the pipe, via a ring of transducers. The guided wave will travel away in both direction from transducer ring at a speed of approximately 3,250 m/s (10,662 ft/s). The waves will be reflected from both pipe features (e.g. welds, supports, flanges) and defects (e.g. corrosion, erosion, mechanical damage), causing the signal to travel back towards the transducer ring. GUL Screening works by collecting and analysing the properties of these reflected waves (e.g. time of arrival, torsional and flexural contents).

Interaction of T(0,1) with a girth weld.

Interaction of T(0,1) with a localised defect.

What is the A-scan in GUL Screening?

The A-scan is a plot that shows the reflected guided wave signals against the pipe axial distance. The black and red signals are the torsional and flexural components of the reflected signal. The x-axis represents the relative distance of the signal from the transducer location. In other words x = 0 m is the transducer ring location and the positive and negative distances represents the forward and backwards direction from the transducer ring.

What is the Distance Amplitude Correction (DAC) curve?

The DAC curves provide a quantitative link (or calibration curve) between the signal amplitude and cross-sectional area change (CSC) within the pipe. so that we can ultimately quantify the extent of the damages in terms of pipe CSC. An accurate DAC curve is required to call the severity of defects in pipes

The figure above shows an A-scan with the Weld and Call DACs labelled. In most inspections, the Weld DAC is established by using either weld size parameters or via the Absolute Calibration method. This results in a calibrated Call DAC, which can be used to estimate defect severity and determine end of inspection test range.

The Call DAC is the threshold level that is used to determine the defect severity if found. Typically, it is set between 1% and 6% CSC according to the sensitivity requirement of the inspection.

What is Absolute Callibration?

Absolute Calibration is GUL’s patented method of calibrating the DAC curves. Put simply, this method uses reverberation signals from pipe features, such as welds or flanges, to rapidly and accurately calibrate the DAC curves.

What is Unrolled Pipe Display is GUL Screening?

What is Unrolled Pipe Display in GUL Screening?

The Unrolled Pipe Display is a colour plot that shows the guided wave reflection amplitude as a function of axial distance and circumferential position. This plot is generated by using a proprietary algorithm that enables total focusing of the guided wave signals at all locations (within the valid inspection range), analogous to Total Focusing Method in phased array UT.

Using post processing to produce the unrolled display avoids the need to collect additional data making the inspection faster.

How does GUL Screening differentiate between a pipe feature and a defect?

The systems sends and received multiple guided wave modes and frequencies. Different types of pipe features and defects reflect the guided wave modes and frequencies differently. These allow characteristics allow a trained inspector to infer the type of feature that caused the reflection. There are several Level 1 and 2 GUL Training (GULT) courses that are designed to equip inspectors with the skills and knowledge to confidently and competently perform this task for different application areas.

The inspector will typically also use their knowledge of the site, drawings, and various visual clues to confirm any reflections that have been identified and provide further information about the integrity of the pipe.

What is the defect detection sensitivity for GUL Screening?
(What is the smallest defect that you can reliably find?)

The detection sensitivity can be adjusted by using different transducer modules and software configurations. The HD (high definition) rings generally can see smaller defects (for example small pits) than the standard modules. However, the inspection range is reduced, especially if the pipe is generally corroded (a lot of energy is reflected back from each of the small (not always important) corrosion locations) or has a thick coating. Therefore, the inspector will normally choose a setup based the type of defect that is expected.

On clean pipes where localized pitting is the main concern, 1% cross-sectional area change (CSC) would be a standard detection level target. However, for buried pipes where gross external corrosion is the main concern, a 5% CSC will normally be used that will allow the inspection range to be extended. Once a result has been collected, it can be easily analyzed to mark the relative noise levels and the height of any reflections that can be reliably detected above this noise level. The absolute calibration method allows this to be accurately converted into an equivalent cross sectional changes level for that test so that the sensitivity achieved for each test can be known and validated.

What is the defect location accuracy for GUL Screening?
(How precise will the location of my defect be?)

If measuring from the transducer ring (sensor) location, the accuracy of defect location is ±100 mm (approx. 4 inches) for the typical variations in pipe temperature and the typical inspection ranges. By measuring the relative distance from the closest visible feature, defects can be located much more precisely.

How is noise defined in GUL Screening?

There are 2 types of noise considered in GUL Screening, namely coherent and incoherent noise. Coherent noise is not random and can be attributed to the pipe condition, corrosion, scales and other features that causes unwanted guided wave scattering. Incoherent noise is random (i.e. can be reduced by signal averaging) and is attributed to background and electrical noise.

How do we estimate defect severity?

After identifying a defect in the result, the defect can be ranked using user defined categories (the defaults are Minor, Medium and Severe). The classification is determined on the depth of defect, which is roughly estimated by considering the total cross-sectional area change (CSC) and distribution (circumferential extent) of this loss and assuming a defect shape. The inspector will use the relative heights of different guided wave modes (compared to the DAC curves) to determine the classification.

How is the inspection range defined?

The inspection range can either be determined by the intersection of the Call and Detection Threshold (DT) or by limited by certain pipe features such as flanges or elbows. The DT is typically defined as 6 dB above the noise level.

What is the typical inspection range for GUL Screening?

In ideal circumstances, the test range can be more than 200 meters (656 feet) in each direction. However, this may not always be the case as the inspection range can be limited by attenuation due to pipeline properties (coatings, surrounding environment and existing pipe metal condition) or features such as bends or flanges.

The figure below provides a guide to the typical and average inspection range for GUL Screening.

What are the Dead and Near Field Zones in GUL Screening and how long are they?

The dead zone is indicated by the green area in the A-scan. Data is not available in this area because the transducers are still transmitting the signal and not receiving any (pulse-echo configuration).

The near field zone is the gray area next to the dead zone in the A-scan. Data is available in this region, but should not be used for defect detection because the system is transitioning from a transmit to a receive mode, which affects the received amplitudes.

The size of the dead zone and near field zone are a affected by the frequency of the guided wave signal (higher frequencies = smaller zones). It can be estimated that the length of dead zone and near-field zone are 0.275 m (1 foot) and 0.9 m (3 feet) per sensor direction respectively.

Screening – Applications

What are the typical applications for GUL Screening?

GUL Screening can be used for detection of:

  • Corrosion under insulation (CUI)
  • Erosion
  • Corrosion under pipe supports (CUPS)
  • Touch Point Corrosion (TPC)
  • Pitting Corrosion
  • Sludge
  • Internal Corrosion
  • Weld Defects

GUL Screening can be used to inspect:

  • Straight, bare pipes
  • Road crossing pipes
  • Buried pipes
  • Slug catcher
  • Pipe penetrating Earth Walls
  • Overhead pipes
  • Tunnel pipes
  • Pipes with Concrete Anchor Supports
  • Risers
  • Caissons
  • Anchor Bolts
  • Rail

What are the common pipe fittings and recommendations?

Pipe Feature
Can we use data past this feature for GW Inspection?
Can a 5% CSC defect be reliably detected on/at this feature?
GW Inspection Comments
Girth Welds
Yes – observe for increased flexural content.
Important features for DAC calibration.
Spiral Welds
Yes, if welds are grounded and flush with pipe surface.
Pipe with this feature can be inspected but be wary of coherent noise level.
Simple Support
Yes – observe for increased flexural content.
Use unrolled pipe display to determine if reflection is at 6 o’clock. Reflection values will depend on pipe stress at the support location.
Yes – observe for increased flexural content
Coherent noise caused by trailing echoes from the feature. Higher frequencies may reduce this effect.
Clamped Supports
Yes – for relatively loose clamps.
Yes – for relatively loose clamps.
It may be necessary to loosen clamps during GW inspection for best performance.
Welded Supports
Yes, if coherent noise reduced with higher inspection frequencies +good SNR.
Use higher inspection frequency to reduce coherent noise from this feature.
Concrete Anchor Supports
Use higher inspection frequency to reduce coherent noise from this feature.
Guided waves do not propagate past this feature due to screw-type fitting.
Pipe Elbows
No – For short radius elbows and relatively small diameter pipes.
For short radius elbows (< 3D bends) and relatively small diameter pipes (< NPS 12), the Inspection data becomes less sensitive and reliable after the second bend weld.
Pipe Caps
Guided waves do not propagate past this feature due to 100% discontinuity.
Depends on defect location and properties of the branch. Generally no – if same diameter
Depends on defect location and properties of the branch. Generally no – if same diameter.
Reflection/noise would depend on ratio of opening diameters. Guided wave would typically propagate past this feature and continue inspection in the direction of the pipe where the transducer ring is attached. The branch distorts the 2nd weld, making it non-symmetric.
Guided waves do not propagate past this feature due to rubber seal and 100% discontinuity.
Concentric Reducers
Yes – but not recommended.
Guided waves can propagate past this feature, but generally inspection past this should be treated with caution. Any indication should be followed up by additional guided wave test locations on the smaller diameter pipe after the reducer.
Guided waves do not propagate past this feature.
Branch (Tee)
Depends on defect location and properties of the branch. Generally no – if same diameter
Depends on defect location and properties of the branch. Generally no – if same diameter.
Reflection/noise would depend on ratio of opening diameters. Guided wave would typically propagate past this feature and continue inspection in the direction of the pipe where the transducer ring is attached. The branch distorts the 2nd weld, making it non-symmetric.
Guided waves do not propagate past this feature due to screw-type fitting.
Guided waves do not propagate past this feature.
Link Seal
Little-to-moderate reflection at the interfaces; potentially some attenuation within this feature.

What is the operating temperature for GUL Screening?

  • Transducer Rings: From -40°C to +350°C (-40 to 662°F).
    The operating temperature varies across different transducer ring types.
Transducer Ring TypePipe Operating Temperature Range
EFC Inflatable, HD Inflatable,
HD Solid, Solid
-40°C to +150°C
Inflatable, HT Solid, HT‑HD Solid
-40°C to +350°C
  • LEMO Connectors: -40°C to +482°C (-67 to 662°F); according to equipment/component.
  • LEMO Cables: -40°C to 482°C (-76 to 662°F); according to equipment/component.

What thickness of coating or paint is suitable for effective GUL Screening?

GUL Screening is most effective when used on pipes with coatings or paint thicknesses of up to 1 mm (3/64 inch).

Do you have any case studies avaliable?

Yes. Please see link below:
GUL Case Studies

How fast is data collection?

Using the default data collection procedures, data collection with the Wavemaker G4MINI typically takes 2 minutes.

What are the relevant specifications and standards?

  • BS 9690:2011 Non-destructive testing – Guided Wave Testing
  • ASTM : E2775 -2011 Standard Practice for Guided Wave Testing of Above Ground Steel Pipework Using Piezoelectric Effect Transduction
  • ISO/DIS 18211.2 Non-destructive testing – Long range inspection above ground pipelines and plant piping using guided wave testing with axial propagation
  • ASME : Article 18 Guided Wave Testing Method for Basic Piping

Can the Claw or other rings be used to inspect finned tubes or pipes with welded fins?

GUL Screening is currently not applicable to finned tubes or pipes due to the relatively large coherent noise generated by the fins’ reflections.

How do we deal with reducers and 90 degree bends during GUL Screening inspection?

Reducers can often be inspected through, depending on the relative size change. For example, NPS14:NPS12 (DN350 to DN300) has little effect, whereas NPS4:NPS2 (DN100 to DN50) would result in a more significant signal loss.

Bends are similar—there is always some loss of signal, and the tighter the bend, the greater the loss. Data analysis allows the loss level to be quantified so that the level of sensitivity obtained around the bend is known.

Can we perform Corrosion Mapping using GUL Screening?

GUL Screening generates an Unrolled Pipe Display, which appears very similar to a conventional corrosion map. However, it shows cross‑sectional area change (CSC) and does not directly map wall thickness. Nevertheless, it will locate areas of wall loss and we are able to estimate remaining wall for these areas.

Can we inspect pipes with concrete inner lining?

Yes. To estimate the inspection range, we would need to know the thickness of the concrete lining and the pipe wall thickness, as well as the information on the condition of the concrete bond to the pipe, or if bonded at all.

Can GUL Screening be used to measure stress in pipes?

GUL Screening is not designed for measuring stress in pipes.

Screening – Hardware

How many transducer rings (and sizes) do I need to perform GUL Screening on pipes?
Can inflatable rings be combined? | Combining Rings | Large Diameter Pipes | Combined Ring Size Chart

Compact Rings from 6 to 24-inch diameter

GUL Screening require a specific transducer ring size for a specified nominal pipe size; a single ring size can tolerate small differences in the equipment’s intended nominal pipe diameter. For example, to inspect an NPS 8 pipe, you would need a NPS 8 Compact ring. The photo below shows a range of Compact ring with different sizes.

Two transducer rings can also be combined to form one larger ring. For example, a 30″-Compact and a 26″-Compact can be combined to inspect NPS 60 (DN 1500) pipes.

Testting a 70-inch diameter Pipe
Compact Ring Combine Size Chart
These combinations
are not recommended.
Long arm required on
at least one ring.


EFC, HT, HD Ring Combine Size Chart
These combinations
are not recommended.
Long arm required on
at least one ring.



What is a Long Arm or Long Curved Arm?

A long arm is an accessory for the inflatable transducer rings. It is required when combining rings to inspect larger diameter pipes in order to ensure the combined ring can successfully close and be secured on to the pipe. In some cases, the long arm can also be used to allow one ring to inspect pipe sizes slightly larger than the designed inspection size. For example, a 28″ EFC ring can be used to inspect an NPS 30 (DN 750) pipe.

For more info, you can refer to our spare parts for inflatable rings webpage.

What is the radial clearance of the transducer rings?

Transducer ring typeRadia Clearance


38 mm
Inflatable EFC, HD and HT


63 mm
Solid ring


76 mm
HT Solid, HD Solid and Claw


50 mm


25 mm

Calibration – How long does the calibration certificate last? How can I renew it?

Calibration is recommended for the Wavemaker G4mini (or older generation system) and its calibration certificate is valid for 3 years. The transduction system must be shipped back to GUL annually for calibration.

What is the warranty period for GUL Screening equipment?

Most of our equipment is supplied with a one year warranty. It is possible to purchase an extended warranty for some sorts of equipment (for example the electronics). Please note that the warranty only allows for normal wear and tear of the equipment; it does not cover misuse of the equipment such as large drops or immersion in water.

What is the maximum LEMO cable length which can be connected between the transducer ring and the Wavemaker system?

Typically, we offer cable lengths of up to 25 meters, however cable lengths can be as long as 100 meters.

How many transducer rings can we connect to one Wavemaker G4mini at a time?

In order to inspect large diameter pipes, two rings can be joined together using the 16 to 8 Channel Converter Box. However, the setup is only for testing a single pipe at a time (Connecting rings on multiple pipes at the same time is really only relevant for connecting multiple gPIMS to an autonomous data collector).

I am having trouble with installation or operation of my GUL Screening transducer rings or Wavemaker G4mini. Who can I contact?

For technical enquiries, please contact us at

Screening – Software

Where can I download the WavePro software from?

For more information, please see GUL Software Updates webpage.

What is TRUNK?

The TRUNK is GUL’s cloud platform that provides data storage and analysis services for our customers.

Can we perform remote firmware upgrade of the G4mini Base to G4mini Full?

No, remote upgrading is not possible. In order to upgrade the firmware of a G4mini Base to a Full, the equipment would need to be returned to GUL(UK).

Can we export the generated report and incorporate it in any other software?

The GUL Screening results can be exported out to a variety of formats including Microsoft Word, Excel or PDF; export into other formats can be done upon request and discussion.

I am having trouble with installation or operation of my WavePro software. Who can I contact?

For technical enquiries, please contact us at

Screening – Training

What are the relevant GUL Screening training courses and their requirements?

Typically, a Level 2 GULT qualified and certified operator is required to perform GUL Screening; Level 1 operators can assist a Level 2 operator(s). For more information on training, please see GUL Training webpage.

Screening – Repair

I think my equipment is damaged. What do I do?

Step (1) : Download the Repair Request Form.

Step (2) : Fill up the required details in the form.

Step (3) : Email it to

Step (4) : A GUL member will contact you once they have received and processed your repair request form.

After investigation of the equipment, we will normally provide a quotation for the repair and ask for a PO (and in some cases payment) before proceeding with the repair. We seek to repair all items within two weeks of receiving them in our office.

However, this may be extended if we do not have all of the required parts on hand, there are a lot of items to be repaired, it is a holiday week in the UK, or there are delays in getting approval for the repair.

Do you offer data analysis and review services?

Yes, GUL offer consultancy services on reviewing GUL Screening data. For a price quotation, please email your enquiry to

Where can I find more information on Repair and Services?

For more information, please see GUL Repair and Servicing webpage.

Screening – Marketing

Where can I find out more about the GUL Screening?

Check out the GUL Screening webpage.

Is there a GUL Screening demonstration video?

Is there a GUL Screening webinar / live demo video?

What do I do if I cannot find an answer to my question on this page?

Please send us a message on our Contact Us page. We will be happy to help.

Where can I find the Material Safety Data Sheet (MSDS) for the Wavemaker G4mini?

For the G4mini battery MSDS, please see Battery MSDS.

Where can I find the Declaration of Conformity (CE DoC) for the Wavemaker G4mini?

For the G4mini CE DoC, please see CE DoC.

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