🔩 Ultrasonic thickness determination — four modes of measuring the pulse transit time

Materials and NDT PN-EN ISO 16809

Overview

**Status and scope of the editions.** The current edition is PN-EN ISO 16809:2025-12 "Non-destructive testing — Ultrasonic thickness determination", which according to the PKN card gives the principles for determining the thickness of metallic and non-metallic materials using the contact technique **or the immersion technique**, based exclusively on the measurement of the transit time of ultrasonic waves. It replaced the edition PN-EN ISO 16809:2019-08 (implementing ISO 16809:2017), now withdrawn. The description below corresponds to the content of the edition ISO 16809:2017, whose preview was available, and covers only the contact technique; it does not describe the immersion technique — a laboratory applying the current edition should check its requirements in the standard itself.

The standard specifies the principles for ultrasonic thickness measurement of metallic and non-metallic materials by direct contact, based exclusively on the measurement of the transit time of ultrasonic pulses. The thickness of a component or structure is determined by accurately measuring the time a short pulse generated by a transducer needs to pass through the thickness of the material once, twice or several times; the thickness is calculated by multiplying the known sound velocity in the material by the transit time and dividing by the number of passes of the pulse through the wall.

The task of thickness measurement is divided into two fields of application with different conditions and techniques: measurements during manufacture and measurements of the residual wall thickness in service. Very often measurements are made on corroded surfaces, for example of storage tanks and pipelines.

The accuracy of the measurement depends on the accuracy of measuring the transit time, on the method of determining the time (zero crossing, flank to flank, peak to peak), on the mode selected — in the multiple-echo mode the accuracy is higher than in the single-echo modes — and on the frequency used, since higher frequencies give a more accurate time measurement. Depending on the thickness and material, frequencies from 100 kHz with the through-transmission technique for highly attenuating materials up to 50 MHz for thin metal sheets are used. The measurement consists in determining the transit time and calculating the thickness assuming a constant sound velocity — if the velocity along the pulse path is not constant, the accuracy is seriously impaired.

Method principle

A short ultrasonic pulse generated by a transducer passes through the thickness of the material and is reflected from the opposite wall. The thickness is calculated as the product of the known sound velocity in the material and the transit time, divided by the number of passes of the pulse through the wall. The standard describes four modes: mode 1 measures the time from the excitation pulse to the first back-wall echo, minus a zero correction taking into account the thickness of the probe wear plate and the couplant layer (single-echo mode); mode 2 measures the time from the end of the delay line to the first back-wall echo (single-echo mode with a delay line); mode 3 measures the time between successive back-wall echoes (multiple-echo mode); mode 4 measures the transit time of the pulse from the transmitter to a receiver placed on the opposite wall (through-transmission technique).

Applications

• Wall thickness measurement during the manufacture of metallic and non-metallic products
• Measurement of the residual wall thickness of storage tanks and pipelines in service
• Assessment of corrosion and erosion losses on corroded surfaces
• Grid measurements on the surface of a component to assess the distribution of losses
• Thickness measurement of components accessible from one side only

Key parameters

Standard

Standard number

PN-EN ISO 16809:2025-12

Title (PL)

Badania nieniszczące — Ultradźwiękowe wyznaczanie grubości

Title (EN)

Non-destructive testing — Ultrasonic thickness determination

Step-by-step procedure

1. Operator qualification

   The operator must have basic knowledge of the physics of ultrasound and detailed knowledge of and training in ultrasonic thickness measurement, and must know the product and the material. Qualification according to ISO 9712 or equivalent is recommended; for categories III and IV under the Pressure Equipment Directive the personnel must be approved by a recognised third-party organisation.

2. Checking the object

   The object must permit propagation of the ultrasonic wave, and there must be free access to each measuring point. The surface of the measured area is to be free of dirt, grease, lint, scale, welding flux and spatter, oil and other substances interfering with the test.

3. Surface preparation

   Provide a clean and even contact area at least twice the probe diameter — poor contact causes loss of energy and distortion of the signals and of the sound path. Remove loose parts and non-adhering coatings by brushing or grinding. On corroded surfaces grind the contact area over an area at least twice the probe diameter and clean it of corrosion products, taking care not to reduce the thickness below the permissible value.

4. Selection of mode and equipment

   On the basis of knowledge of the material, the geometry, the expected thickness and the required accuracy, select the instrument, the probe and the measurement mode (guidance in Annex D of the standard). For highly attenuating materials provide the through-transmission technique from 100 kHz, for thin sheets frequencies up to 50 MHz. With dual-element probes apply compensation of the V-path error, and on curved objects select a probe with a contact surface much smaller than the diameter of the object.

5. Calibration of the system

   Calibrate the measuring system on one or more specimens or reference blocks representative of the object measured, i.e. of comparable dimensions, material and structure. The thickness of the reference blocks or their steps should cover the range of thicknesses measured; the thickness or the sound velocity of the reference block must be known.

6. Measurement through a coating

   If the surface is coated, the coating must adhere well to the material, otherwise it must be removed. When measuring through a coating its thickness and sound velocity must be known, unless mode 3 (multiple echoes) is used.

7. Carrying out the measurements

   Apply the couplant and carry out the measurements. When the measurement is to cover an area of the component, plan the spacing between measuring points: the spacing should be uniform, a grid is recommended, and its density should be chosen to balance confidence in the result and the effort required.

8. Evaluation and recording of results

   Take into account that the result depends on the sound velocity assumed — with a variable velocity along the pulse path the accuracy is seriously impaired. Record the measured values together with the location of the points, the measurement mode, the probe used and the sound velocity assumed.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The couplant must not pose a hazard to the health of the operator or harm the object and the equipment — selection in accordance with the safety data sheet
• Grinding of corroded surfaces — safety glasses, respiratory protection, beware of sparks at installations containing flammable products
• Risk of reducing the wall thickness below the permissible value with overly aggressive grinding — careful surface preparation
• Measurements on tanks and pipelines in operation — work permit, safety harness, work at height and in confined spaces
• Power supply of instruments in field conditions — insulation check, residual current protection

Frequently asked questions