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Most probes are rated for use between - 20 °С and + 60 °С, at temperatures below - 20 °С specially designed probes may be required and contact time should be limited as recommended by the manufacturer.

For measurements above 60 °С a high temperature probe is required and the couplant shall be designed for use at the test temperature.

It is also recommended that when using А-scan equipment it should have a "freeze" mode to allow the operator to assess the signal response. The probe contact time shall be limited to the minimum time necessary to achieve measurement as recommended by the manufacturer.

In the measurement of thickness in hazardous atmospheres there shall be strict compliance with prevailing safety regulations and standards.

In explosive atmospheres the probe, cable and equipment combination shall be classified as intrinsically safe and relevant safety certification and or documentation shall be checked and completed prior to use.

In corrosive atmospheres the couplant shall not react adversely with the environment and shall retain its acoustic properties.

7 Instrument setting

All instrument setting shall be carried out with the same equipment as that which will be used for the measurements. Instrument setting shall be carried out in accordance with the manufacturer’s instructions or other valid standards or procedures.

It should be noted that this clause covers only the setting of the instrument (in service), the verification of the equipment is not considered, but can be performed according to the design specification.

Ultrasonic instruments do not measure thickness; they measure time-of-flight. The thickness is calculated by the application of a factor which is the sound velocity of the material.

d~vxt/n (1) where

d is the thickness;

v is the sound velocity;

t is the measured time;

n is the number of transits through the test object (see Figure 1).

7.2 Methods

The method for setting of the instrument shall suit the measuring mode and the equipment and probe in use. The setting shall be carried out under comparable operating conditions as those of the measurement instrument.

The tables in Annex В give guidance on the selection of methods for setting instruments.

Differences exist between calibrating a digital thickness instruments (types 5.1 a) and b)) and an А-scan instrument (type 5.1 c)).

See also 5.1 a) and 5.1 b).

Many digital thickness instruments can be used in measurement modes 1, 2 and 3. The setting of the instrument can be achieved in either of two ways:

• adjust the displayed reading such that it agrees with the measured known dimensions of the series of reference blocks;

• adjust or set the material velocity on the instrument to agree with the known velocity of the test object.

See also 5.1, c).

Reference to EN 583-2 shall be made for information regarding the time base setting of an А-scan instrument.

When using mode 1 with an А-scan instrument the horizontal time base is set such that the transmission pulse indication and the first back-wall echo from the reference block are displayed at convenient positions on the screen to agree with a screen graticule or the digital display.

When using mode 2 with an А-scan instrument adjust the transmission pulse indication such that it is off the screen and the interface echo is at zero on the graticule. Then adjust the first back wall echo to be at the mark relating to the known thickness of the reference block.

When using mode 3 with an А-scan instrument, adjust the first back-wall echo to be at the mark relating to the known thickness of the reference block. Then adjust the n^th back-wall echo to be at the mark relating to the n times the known thickness of the reference block. When measuring the test object, the zero point of the graticule will correspond to the surface of the test object. The object thickness is equal to the position of the n^th back-wall echo divided by n. n is normally in the range 2 to 10. See Figure 2.

Mode 4 can only be used with an А-scan instrument. The instrument shall be set up to operate in through transmission mode according to the manufacturer’s manual. A transmission pulse indication should be available to represent the zero time pulse, set this to align with the zero on the graticule and the received pulse is set to align with a known thickness on the graticule.

E1 E2 E3 En

Key

A transmit/receive probe

В test object

C sound path travel time

D transmission pulse indication

E1 to En backwall echoes

Figure 2 — Instrument setting for mode 3

7.3 Check of settings

Checks of the settings of a thickness measuring system shall be carried out with a reference test piece:

1. on completion of all measurement work;

2. at regular intervals during the work session;

3. if probes or cables are changed;

4. if material types are changed;

5. if the material or equipment temperature changes significantly;

6. if major operating controls are adjusted or considered altered;

7. at other intervals as directed by specific procedural instructions.

8 Influence on accuracy

The cleanliness of the piece affects its thickness measurement. Inadequate surface preparation may lead to inconsistent results.

Adhering dirt and scale shall be removed by brushing before measurement.

Roughness interferes with the estimate of thickness (overvaluation) and modifies the coefficients of reflection and transmission at the interface.

In circumstances where there is significant roughness the sound path is increased and the contact surface is reduced. The measurement uncertainty increases with decreasing thickness.

If the surface opposite the input surface (back-wall surface) is rough the acoustic signal can be deformed, this can result in measurement error.

Scanning on an irregular surface with a contact probe necessitates the use of a thick couplant layer. This may create beam distortion.

When using modes 1, 2 or 4 the couplant layer transit time may be included in the reading, which will result in an additive error equivalent to three to four times the actual couplant depth. In this case it is not possible to determine the minimum remaining wall thickness.

The coupling medium should be chosen to suit the surface conditions and the irregularities of the surface to ensure adequate coupling.

Temperature modifies the sound velocity (in both the material and in any delay path and face of the probe) and also the overall acoustic attenuation.

As for all measurements, if maximum accuracy is required then the temperature variation and effect upon the following additional items has to be considered:

• references: standards, gauges, test-blocks;

• apparatus: equipment, probes, etc.;

• process and methods: couplant, object under test.

Sound velocity decreases with increase in temperature in most metals and plastics, whereas it can be seen to increase in glass and ceramics.

The influence of temperature on the velocity of sound in metals is normally insignificant. The longitudinal (compressional) wave velocity in most steels decreases by approximately 0,8 ms'¹ °С'¹.

The influence of temperature on plastics is significant. For acrylic, which is normally used for probe delays, the coefficient is - 2,5 ms'¹ °С'¹. Compensation for this shall be applied.

Apparent increase of the material thickness (or even apparent decrease in the case of heat treated material) can be seen when cladding (constitution, composition, thickness, cladding process, number of layers, etc.) is not taken into account.

The measurement accuracy required shall dictate whether the plating should be considered.

For example, with the instrument calibrated for steel:

— Steel 1 mm at v = 5 920 ms'¹;

Zinc 20 pm at v = 4 100 ms’¹;

— Actual thickness 1 mm + 20 pm = 1,02 mm;

(

(2)

' * ^l0~¹) ₊ * И = I.738^-’ _s

5 920 4100

1.738"⁷ x 5 920 = 1,029 mm (3)

• Measured thickness 1,029 mm;

• Deviation 0,009 mm.

Cladding thickness can be measured. Measurement accuracy depends on the same parameters as the measurement of the base material.

8.1.4 Non-metallic coating

When measuring through coatings, errors will occur as a result of the differing sound velocities of the coating and the test object.

For example, with the instrument calibrated for steel:

• Steel 1 mm at v = 5 920 ms'¹;

• Paint 100 pm at v = 2 100 ms'¹ (this is a generic value and not indicative of a type);

• Actual thickness 1 mm + 100 pm = 1,1 mm;

• (' ^x '.d ₊ (^|0° * И . 2,165"’ s (4)

5 920 2100

2,165”⁷ x 5 920 = 1,282 mm (5)

• Measured thickness 1,282 mm;

• Deviation 0,182 mm.

It can also be difficult to obtain the desired measurement if the coating material is:

• similar in acoustic properties to the test piece material;

• of a significant thickness compared to that of the test piece.

Key

A probe

В coating or plating

C increased sound path through coating

D sound path travel time

E metal

Figure 3 — Increased sound path through coating

8.1.5 Geometry

The opposite walls of the test object (piece) should be parallel within ± 10° otherwise measurement can be difficult or erroneous. This is due to deformation or lack of back-wall echoes due to “spatial integration".

In this case the small contact surface area between the probe and the test piece can reduce the effectiveness of the couplant and in turn the signal quality. The probe shall be aligned to the centre of curvature of the test object. These factors will affect measurement performance by giving poor acoustic transmission and repeatability.

The probe face shall always allow adequate coupling. Small radii require a small probe diameter.

Accurate measurement depends on material homogeneity throughout its thickness. Local or general changes of composition will result in changes of velocity compared to that of the material of reference blocks and therefore subsequent measurement errors.

True equipment resolution is the smallest increment of the quantity being measured that can be recognised by the system. E.g. digital thickness instruments may display an apparent resolution of 0,001 mm but only be capable of measuring with a resolution of 0,01 mm. An А-scan instrument (type 5.1 c)) has no stated or assumed thickness resolution, it depends on a number of factors, e.g. digitising speed, screen resolution (pixel number in the x and у axes) and time base setting.

Equipment resolution is influenced by the choice of probe type and frequency.

Higher probe frequencies provide greater thickness resolution than lower frequencies do. This is basically because the higher frequency pulses offer a sharper and more definite timing edge. This is particularly noticeable on A-scan instruments.

The range of the equipment is that range of thickness that the system can practically measure. The number of digits on the display of a digital instrument only infers a range of numbers that can be displayed.

Instruments will have a minimum thickness that they can measure. This is generally independent of probe frequency and application. The maximum thickness that can be measured is usually governed by probe frequency and/or application (material conditions, etc.).

The probe will dictate a measurement range independent of the instrument. Generally, the minimum range of a probe is controlled by its frequency and the velocity of the material being tested. A probe shall be chosen such that its minimum measurable thickness is below the minimum thickness to be measured.

As a guide it can be assumed that a probe cannot measure less than one whole wavelength at the velocity in question.

A = v// (6)

where

Л is the wavelength;

f is the probe frequency;

v is the sound velocity.

Probe frequency also dictates the maximum thickness that can be measured. A high frequency probe will have less penetrating power than a lower frequency one.

Consideration should be given to the type of material in question as this also has an affect on measuring range.

The selection of probe frequency is controlled by the range of material thickness to be measured and also by the type of material.

The measuring system shall be selected such that its measuring range properly covers the thickness of interest. In the case of an А-scan instrument (type 5.1 c)) the range setting shall be such that it suits the desired resolution at that range without switching ranges.

It is recommended that instrument settings are checked at both ends of the thickness range to be measured.

The evaluation is dependent on several parameters and the method of calculation.

The most important parameters are shown in C.1.

Two basic methods are shown in C.2.

9 Influence of materials

The material of the object to be measured may influence the selection of technique to be applied for ultrasonic thickness measurement.

Forged or rolled metals normally have a low attenuation and a constant and well-defined sound velocity. These materials are easily measured using standard procedures described in Clause 4.

Material composition including alloying elements and impurities, and its manufacturing process will affect grain structure and orientation and therefore homogeneity.

This can cause localised variation of velocity and attenuation in the material, resulting in erroneous measurements or in extreme cases the loss of readings.

In anisotropic materials velocity is not necessarily the same in different planes and the structure may cause variations in beam directions. This will result in erroneous readings. Materials, which are rolled or extruded, particularly austenitic steel, copper and its alloys, lead and all fibre-reinforced plastics are examples of this.

To minimise the risk of error, setting of the instrument shall be carried out in the same plane as the measurement.

Acoustic attenuation is caused by energy loss through absorption (e.g. rubber) and by scattering (e.g. coarse grains). This effect can cause a reduction of signal amplitude or a signal distortion.

Castings generally exhibit attenuation through absorption and scattering resulting in lack of or erroneous readings.

High attenuation through absorption alone may be found in plastics.

Poor attention to surface conditions will result in either inability to obtain measurements or erroneous measurements.

If the surface is coated measurement may only be achieved through the coating, provided it has good adhesion to the material. When measurements are made through coating, multiple echo technique shall be used, mode 3 (see Clause 4).

If only a single echo can be achieved due to bad reflection or high attenuation, the coating thickness equivalent shall be known and be subtracted from the single echo reading, see 8.1.3 and 8.1.4.

Where neither of these conditions can be met, the coating shall be removed, provided this is allowed.

Surface roughness for example caused by wear or corrosion highly influences the coupling conditions and the measurement accuracy. Extreme surface roughness may preclude measurement modes 2 and 3 (Clause 4) leaving single echo technique, mode 1, as the only alternative.

The resulting measurement values may not be considered more accurate than the surface condition allows. This is illustrated in Figure 4 showing a probe bridging a surface cavity. A measurement recorded in this position will include the equivalent of the couplant layer thickness.

Key

A probe

В test object

C sound path

D couplant

Figure 4 — Sound path through couplant layer

Ultrasonic thickness measurements are frequently related to service-induced material loss by corrosion or erosion. These mechanisms produce different types of reflecting surfaces. When performing ultrasonic thickness measurements with the purpose of detecting material loss and/or measuring remaining wall thickness, it is necessary to have a knowledge of the type(s) of material loss to be expected, and to apply a procedure adapted to this specific type of wear, corrosion or erosion.

In industries such as oil/gas, power generation, energy distribution, storage and transport of products, the corrosion mechanisms are frequently linked to vessels and pipes made of ferrous materials such as rolled steel plates, seamless pipe and welded assemblies.

The following types of corrosion in steel vessels and piping components are to be considered when selecting the ultrasonic technique to be applied:

• uniform corrosion;

• pitting;

• deposit attack;

• crevice corrosion;

• galvanic corrosion;

• flow induced corrosion;

• weld zone corrosion;

• combinations of two or more of the above types of corrosion.

The illustrations in Annex A show important shapes and distributions of ultrasonic reflectors to be considered. Annex A also proposes technical data to be applied for detection and measuring.

10 Test report

Taking into account any specific requirements agreed at the time of enquiry and order the following information shall be recorded:

1. Operator’s name;

2. operator’s qualification details;

3. operator's company details;

4. dates of first and last measurement in this report;

5. location/site details;

6. equipment type and serial number;

7. probe type description (including element size/frequency) and serial number;

8. reference block details, if applicable;

9. couplant type;

10. equipment measuring method/mode;

11. material type;

12. instrument setting details, i.e. method;

13. general description of plant/structure/parts under inspection including definition of surface conditions, e.g. coated/insulated/rough/smooth/shot-blasted;

n) details of company/agency requiring and purpose of survey;

15. reference to applied standard or specifications;

16. operator’s signature.

10.3 Inspection data

1. Measurement pattern descriptor;

2. measurement point location descriptor/identifier;

3. original thickness, if applicable;

4. allowable tolerances (where known);

5. measurement results (table and/or map);

6. diminution as percent or actual, if applicable;

7. supporting drawings showing locations of discontinuities;

8. visual inspection/condition comments;

supporting drawings/sketches showing measurement locations.Annex A

(informative)

Corrosion in vessels and piping

A.1 General

Corrosion in components such as vessels and piping can be caused by different mechanisms. Table A.1 gives some guidance regarding the types of ultrasonic reflectors, which may occur with the different corrosion mechanisms and some guidance regarding the ultrasonic techniques recommended for measurement of the remaining material thickness.

A.2 Measurement of general corrosion

A.2.1 Instrument

For general corrosion, digital display instruments may be used. If the instrument does not give reliable readings due to difficult surface conditions, inclusions in the material or heavy coating, an А-scan instrument should be used.