Originally published in Welding & Metal Fabrication, 2001, Vol. 69, No. 4, May, pp 10-12 by DMG World Media UK Ltd.
Eur Ing Geoff Melton is Section Manager - Arc welding at TWI and chairman of BSI committee WEE/6 and CENELEC TC26A
The second edition of BS7570 Validation of arc welding equipment [1] , was published last year and is now being proposed as a draft International Standard. It is therefore opportune to review the basic principles of validation and to discuss the changes in the new edition.
The need for validating (or calibrating) arc welding equipment gained momentum in the nineteen eighties [2] . In a number of industry sectors, particularly offshore and power generation a need was identified to have in place a system to ensure that the correct welding parameters were used and documented. Although this requirement is often imposed for manual welding it becomes of increasing importance for mechanised and automated welding. Fabricators required equipment to be validated so that different equipment, maybe in different parts of the country would give reproducible results. At that time, the design of welding power sources was such that they were inaccurate and susceptible to drift. Most equipment was thyristor based and controlled by analogue circuitry. For pulsed welding, many of the important parameters were inadequately calibrated to give reproducible results. Consequently companies and organisations produced in house procedures for bringing their equipment into 'calibration'. But there were no agreed specifications as to what accuracy the equipment should or could be calibrated to. Furthermore, there was a lack of knowledge of the correct principles for calibrating equipment and many so called 'calibrated' power sources simply had their open circuit voltage (no-load voltage) checked against the value on the rating plate!
It was with that background that BSI was asked to produce a standard for 'calibrating' welding equipment. The task was passed to WEE/6 the committee responsible for arc welding equipment. The committee set out to produce guidance on how to calibrate equipment and to specify accuracy levels. It soon became apparent that calibration was not the most appropriate word to use and the word 'Validation' was chosen, meaning the operation to verify that the equipment complies with the operating specification. In 1992, BS75770 Code of practice for the validation of arc welding equipment [3] , was published and by 1996 it became the equivalent European Prestandard ENV 50184.
This standard addressed the need identified in EN729 Weld Quality Systems [4] to calibrate inspection, measuring and test equipment and is now being used by many countries throughout Europe, particularly Denmark, Sweden, Holland and Belgium.
After a few years it became apparent that a revision of BS7570 was required and BSI WEE/6 initiated the drafting, by forming a panel of people with experience of using the original standard. Last year, the revised standard BS7570:2000 was published. This revised standard is reviewed and compared to the original 1992 version, below.
Accuracy of welding equipment
Validation is the procedure of demonstrating that the equipment conforms to the operating specification. Therefore, before validation can be carried out it is necessary to establish the specification, i.e. to what accuracy the equipment is to be validated. This will depend on the design of the equipment, the specification to which it was manufactured or a specification required by the user. Generally, the manufacture does not state values for accuracy, but if the equipment is manufactured to a recognised National, European or International standard, as it should be, a level of accuracy is defined for welding current and arc voltage. However, these are the minimum levels of accuracy and are usually insufficient for mechanised welding. Therefore, for more advanced equipment, the manufacturer may produce to a higher specification or it may be possible for the user to validate to a higher level of accuracy than in the product standard.
Since 1989 the relevant International standard for arc welding equipment construction has been IEC 974-1:1989 Safety requirements for arc welding equipment Part 1. Welding Power Sources. This standard was published in Europe as EN 60 974-1:1990 [5] and the equivalent British Standard was BS 638 Part 10:1990. This standard has been revised and has now become BSEN 60 974-1:1998 [6] . Prior to 1989, arc welding equipment would have been built to National Standards, e.g. in the UK BS638 Part 1,2 or 3.
Each of these standards specifies the required accuracy for indications of current and voltage and the accuracy class of analogue meters. However, the state accuracy is different for each standard, as shown in table 1, so it is important that the relevant construction standard is known. The standard should be marked on the rating plate of the equipment. The revision of BS7570 takes into account the latest developments in the construction standard BSEN 60 974-1:1998.
Table 1 - Validation accuracies for standard grade power sources
| Quantity | Accuracy |
|---|
Current and voltage
BS 638-10:1990 |
±10% of the true value |
Current and voltage
(BS EN 60974-1:1998) |
±10% of the true value, between 100% and 25% of the maximum setting |
| ±2.5% of the maximum setting, below 25% of the maximum setting |
| Analogue meters |
Class 2.5 |
Digital meters
Current
Voltage |
±2.5% of maximum rated welding current
±2.5% of no-load voltage
OR
according to the manufacturer's specification |
None of the above mentioned standards specifies the accuracy for digital meters, nor was this specified in BS7570:1992. This caused problems of interpretation as more and more equipment is appearing with digital meters fitted. Consequently, this is proposed in BS7570:2000 as ±2.5% of maximum rated welding current or no-load voltage, unless it is specified by the manufacturer. It should be noted that digital meters are generally significantly more accurate than this but ±2.5 % takes into account the accuracy of the shunt or current transducer.
In BS7570:1992 a second higher grade of accuracy, grade 2 or precision grade was introduced to meet the requirements of mechanised welding. However, in practice this grade was rarely used and not adopted by power source manufactures. So, in the revise standard the proposed accuracies for grade two have been removed from the body of the standard and included in an informative annex. Consequently, a reference to equipment validated to BS7570:2000 implies that it has been carried out to the standard grade.
The accuracy of wire feeding for MIG/MAG or TIG filler is not specified in any constructional standards, but often wire feed speed is specified in welding procedures, particularly of US origin. BS7570:2000 therefore recommends accuracies of ±10 % for standard wire feeders and ±2.5% for precision grade.
Validation methods
BS7570:2000 states that 'Unless specified otherwise, e.g. by the welding procedure, all power source controls should be validated. If meters are fitted, these should be validated in preference to other methods of setting the relevant parameter'. In practice, validation of meters is usually carried out, although it is possible to validate other controls.
Essentially, validation involves connecting a resistive load bank across the output of the power source. The load bank is adjusted to regulate the power source output. Calibrated instruments are used to measure the voltage and current and these values are compared to the power source settings or meter readings. Although current can be measured anywhere in the circuit it is important to measure voltage by connecting the calibrated meter at the same place as the equipment being validated. It is also important to validate like with like, e.g. a calibrated true reading rms meter should be used to validate another true rms meter. Detailed validation procedures are recommended in the standard, which specifies the number of measurements that should be taken. With the exception of mechanised TIG, an arc load is not recommended as the welding parameters will fluctuate making it difficult to take an accurate reading.
The values chosen for the resistance of the load bank depend on the welding process and are specified by linear relationships between voltage and current, known as the conventional load lines, Figure 1. For example for a TIG welding power source the load should be varied to maintain the following relationship; voltage = 10 + 0.04 x current, e.g. at 100A the load should be adjusted to give 14V. The conventional load lines for each process are reproduced from the equipment construction standard in BS7570:2000 and are the same as used in the design of power sources. The first edition of BS7570, detailed various methods of constructing a load bank, but this has been deleted and a number of purpose built load banks are now commercially available, Fig 2. Different values of resistance can be switched in or out and calibrated integral meters may be fitted. Although in the UK, anyone may carry out a validation procedure it is important that traceability back to National standards can be demonstrated.
To carry out a validation procedure a knowledge of the different types of equipment is required and guidance is given in BS7570:2000. If power source settings are to be validated it is essential to understand the difference between a flat characteristic as used for MIG/MAG and a drooping characteristic as used for TIG/MMA. For MIG/MAG the voltage control should be validated, but for TIG/MMA the current control should be validated.
Consistency checks
BS7570:2000 introduces the principle of consistency checks, for controls that are not marked in absolute units, but are used to set the welding parameters. A good example of this is the voltage tappings on a MIG/MAG power source, which is often marked as coarse A,B,C and fine 1,2,3 etc. this type of control can be validated by carrying out an initial characterisation, measuring the voltage at each setting. Then, during a re-validation these values can be compared to the new measurements to check that the settings have not drifted out of the prescribed specification.
Pulsed welding parameters
Although the need to validate pulse welding parameters is identified, these requirements now form an informative annex to BS7570:2000. Unless specified otherwise, pulse parameters should be accurate to ±5%. However, in practice, pulse parameters for digitally controlled power sources should be far more accurate, but it may be a struggle to comply for analogue control. Methods of validating pulse parameters are considered to be generally outside the scope of the standard because knowledge of the power source control circuitry and specialist instrumentation is generally required and is most likely to be carried out by equipment manufacturers.
Labels and certificates
The labelling requirements are much clearer in the new standard. Equipment should be labelled 'Passed' or 'Failed', together with the date the label was issued, the name of the authority issuing the label and the make, model and serial number of the equipment. A validation certificate should also be issued, whether the equipment passes or fails and this should also include the results obtained as well as other details relating to the equipment and tests.
References
- BS7570:2000, Code of practice for the validation of arc welding equipment.
- Melton G B. Validation of arc welding equipment. Welding and Metal Fabrication, Jan/Feb 1993, 61(1) 42-46.
- BS7570:1992, Code of practice for the validation of arc welding equipment (withdrawn).
- BSEN 729:1995 Quality requirements for welding - Fusion welding of metallic materials.
- BS 638 Pt 10:1990, Arc welding equipment, power sources and accessories - Part 10 specification for safety requirements for arc welding equipment: welding power sources (withdrawn).
- BSEN 60974-1:1998 Arc welding equipment - Part 1. Welding power sources.
Acknowledgements
The author wishes to acknowledge the work of BSI Committee WEE/6 which was responsible for preparing BS7570:2000. The content of this article is representative of the personal views of the author and are not necessarily those of BSI. Extracts from BS7570:2000 are reproduced with the permission of BSI. Copies of this and other standards can be obtained by post from BSI, 389 Chiswick High Road, London W4 4AL.