ISO/DIS 18491:2025(en)

ISO TC 44/SC 10

Date: 2025-02-11

Secretariat: DIN

Welding and allied processes — Measurement of arc energies

© ISO 2025

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Contents

Foreword 4

Introduction 5

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 General 2

5 Welding equipment and measuring devices 3

6 Determination of arc energy 3

7 Measurement parameters 4

7.1 General 4

7.2 Arc voltage 4

7.3 Welding current 4

7.4 Total instantaneous energy or average instantaneous power 4

7.5 Welding speed and length of the run 5

8 Calibration and validation of measuring instruments 5

Annex A (informative) Range of power source types and measurement methods 6

Annex B (informative) Conversion factors 8

Bibliography 9

Foreword

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This document was prepared by Technical Committee ISO/TC 44, Welding and allied processes, Subcommittee SC 10, Quality management in the field of welding.

This second edition cancels and replaces the first edition (ISO/TR 18491:2015), which has been technically revised.

The main changes are as follows:

— update of normative references in Clause 2;

— update of vocabulary throughout this document in Clause 3 "Terms and defintions", Clause 4 "General", Clause 5 "Welding equipment and measuring devices", Clause 6 "Determination of arc energy", Clause 7 "Measurement parameters", Clause 8 " Calibration and validation of measuring instruments" and in Annex A "Range of power source types and measurement methods";

— addition of a new Annex B "Conversion factors";

— update of the Bibliography.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html. Official interpretations of ISO/TC 44 documents, where they exist, are available from this page: https://committee.iso.org/sites/tc44/home/interpretation.html.

Introduction

Welding “arc energy” or “heat input” are fundamental values used to manage the consistency of weld metal and heat-affected zone properties. ISO 15614 requires the measurement of the welding energies implemented during a welding procedure qualification, but it does not give details about the methods to be used. Likewise, during the construction of a welded assembly, inspectors should make sure that the welding energies comply with the Welding Procedure Specification (WPS). But again, the details of the methods to be used are not specified. As a consequence, there may be a lack of consistency between the methods used to measure the welding energies during welding of the test piece and the methods used during the manufacturing process. This is a potential source of error which could have implications on the safety or quality of a welded component or structure.

Moreover, the latest technological breakthroughs in microprocessors and electrical power manipulation have yielded welding power sources and control systems that are capable of generating complex waveforms. However, these control systems and waveforms increase the difficulties related to voltage and current intensity measurements, as their values are manipulated at frequencies which can reach thousands of Hertz. The measuring instruments generally used by inspectors, such as TRMS clamp meters, can no longer be relied on to correctly measure the welding energy since differences exceeding 30 % with respect to the true energy can sometimes be found.

This document provides guidance on how to accurately measure welding energy and calculate heat input, both in the case of traditional welding systems and those that employ complex waveforms.

Welding and allied processes — Measurement of arc energies

This document specifies the measuring of parameters needed to calculate arc energies for arc welding processes.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 4063, Welding, brazing, soldering and cutting — Nomenclature of processes and reference numbers

ISO 17662, Welding — Calibration, verification and validation of equipment used for welding, including ancillary activities

ISO/TR 17671 (all parts), Welding — Recommendations for welding of metallic materials

ISO/TR 25901‑1, Welding and allied processes — Vocabulary — Part 1: General terms

ISO/TR 25901‑3, Welding and allied processes — Vocabulary — Part 3: Welding processes

ISO/TR 25901‑4, Welding and allied processes — Vocabulary — Part 4: Arc welding

IEC 60974‑14, Arc welding equipment – Part 14: Calibration, validation and consistency testing

For the purposes of this document, the terms and definitions given in ISO/TR 25901-1, ISO/TR 25901-3, ISO/TR 25901-4 and ISO/TR 17671 (all parts) and the following apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at https://www.electropedia.org/

3.1

power

product of arc voltage and welding current

3.2

energy

product of arc voltage and welding current and arc time

3.3

arc energy

E

product of arc voltage and welding current divided by welding speed 

or 

energy (3.2) divided by length of the run 

or 

power (3.1) divided by welding speed

Note 1 to entry: Arc energy is an abbreviation of arc energy per unit length.

Note 2 to entry: Arc energy is a measure of the amount of energy generated by the welding arc per unit length of the run. Not all the energy created by the arc is absorbed by the weld; some is lost to radiation, some to convection and some to the slag. The percentage loss is welding process dependent. For measurement purposes, this loss is ignored. When it is necessary to use arc energy to quantify its effect on a material, the arc energy is multiplied by a thermal efficiency factor k (see ISO/TR 17671, all parts). The term "heat input" is then generally used instead of arc energy.

3.4

waveform controlled welding

welding process modification of the voltage and/or current wave shape to control characteristics such as droplet shape, penetration, wetting, bead shape, or transfer mode(s)

3.5

total instantaneous energy

TIE

welding energy determined by summing the product of current and voltage measurements made at rapid intervals which capture brief changes in the welding waveform

3.6

average instantaneous power

AIP

welding power determined by averaging the product of current and voltage measurements made over time at rapid intervals which capture brief changes in the welding waveform

3.7

run out length

length of a run produced by the melting of a covered electrode

[SOURCE: ISO/TR 17671‑2:2002, 3.2]

The arc energy is determined as shown in Formulae (1), (2), and (3). The symbols shown in Table 1 are used for all formulae.

Table 1 — Symbols of terms used

Formula (1) provides the arc energy using welding current and arc voltage.

(1)

Formula (2) gives the arc energy using total instantaneous energy.

(2)

Formula (3) gives the arc energy using average instantaneous power.

(3)

The following equipment shall be used:

For current measurement: Clamp meter or displayed value of average welding current.

For voltage measurement: Voltmeter or displayed value of average arc voltage.

For time measurement: Stopwatch or displayed or documented weld sequence time.

For distance measurement: Steel rule, tape measure or displayed value of automation device.

For welding speed measurement (as an alternative to a measurement based on distance and time): Displayed value of automation device.

For average instantaneous power: Wattmeter or displayed value of AIP.

For total instantaneous energy: Wattmeter or displayed value of TIE.

Three methods can be used to determine arc energy:

— Method A: by calculation using welding current, arc voltage, and welding speed [Formula (1)];

— Method B: by calculation using instantaneous energy measurements and length of the run [Formula (2)];

— Method C: by calculation using instantaneous power measurements and welding speed [Formula (3)].

For non-waveform-controlled welding, method A, B, or C can be used indifferently.

For waveform-controlled welding, method B or C should be used as method A can introduce errors up to 70 %.

All pulsed welding processes, for example, pulsed gas metal arc welding, are waveform-controlled welding processes. Power sources that are sold as synergic, programmable, or microprocessor-controlled are generally capable of waveform-controlled welding. If any doubt exists on whether waveform-controlled welding is being performed, the welding equipment manufacturer should be consulted.

For multi-arc welding, the arc energy is measured for each arc.

Table A.1 and Figure A.1 provide the range of power source types and measurement methods that support production welding based on power source type and measurement method used to weld the test piece.

When method A is used, measure arc voltage (7.2), welding current (7.3), and welding speed (7.5).

When method B is used, measure total instantaneous energy (7.4) and length of the run (7.5).

When method C is used, measure average instantaneous power (7.4) and welding speed (7.5).

The voltage values should be measured as close to the arc as practical in order to prevent the measurement error due to voltage drop in the welding cables.

NOTE The voltage measurement is affected by cable size, length, and connection quality and has to be taken into account during measurement.

The position of the connectors shall be recorded in the WPQR to provide consistency of the measurement during production. The first one should be connected to the earth clamp on the work piece and the second one should be connected as described in Table 2.

Table 2 — Preferred arc voltage measurement locations

Guidelines for proper measurement of voltage are given in IEC 60974-14.

NOTE Measurement for energy should be the same from WPQR to production weld. One should pay attention to the voltage drop in the current path and to the compensation that some power source manufacturers.

The current measurement with external meters or using the displayed current of the welding equipment shall include all ground connection cables or current lead cables, if multiple ground connection cables or current lead cables are used. It is inaccurate to measure the current intensity on a single cable and multiply by the total number of cables, or to mix the ground connection cables and current lead cables in one measurement.

For measurements of welding current below the recommended range of the clamp meter, the cable may be looped through the clamp, in which case the measured value is divided by the number of loops.

Measurement of total instantaneous energy or average instantaneous power should utilize a welding power source or external meter that is capable of calculating and displaying energy or power.

The sampling and calculation frequency is typically a multiple of the welding waveform frequency and shall be at least 5 kHz.

When using external meters, the same guidelines as 7.2 and 7.3 apply.

These meters are often identified by the terms “true energy”, “true power”, or “power factor”.

Meters identified by the terms “kVA”, “DC power”, or “average power” do not generally meet these requirements.

To determine the welding speed, a run is deposited and the time elapsed between arc starting and arc extinguishing is measured. After completion of the run, its length is measured (see Figure 1). The welding speed is obtained by dividing the length of the run by the elapsed time.

For mechanized and automatic welding, the welding speed can also be determined by measuring the time required to cover a distance without making a weld run or by using meters that are part of the equipment.

Key

Figure 1 — Length of the run

The length of the run corresponds to the distance between the start of the run and the middle of the crater at the end of the run, as illustrated in Figure 1.

Calibrate or validate measuring equipment according to ISO 17662.

1. 
   (informative)
   
   Range of power source types and measurement methods

Table A.1 and Figure A.1 provide the range of power source types and measurement methods that support production welding based on power source type and measurement method used to weld the test piece.

Table A.1 — Range of power source types and measurements

NOTE A waveform-controlled power source can also be able to provide non-waveform controlled welding.

Figure A.1 — Logigram for qualification and production measurement method

NOTE Contact the powersource manufacturer to get additional information about the considered waveform-controlled welding process.

1. 
   (informative)
   
   Conversion factors

Example: To convert kJ/mm in J/cm the conversion factor is 10 000. It involves that 1 kJ/mm = 10 000 J/cm.

Bibliography

[1] MELFI T. New code requirements for calculating heat input. Weld. J. 2010, pp. 61–63

[2] Institut de Soudure report No. 32988, Study of the various methods of welding current measurement to establish a specification for welding procedure qualifications and calibration of indicators - July 98 - Study funded by CETIM