MEASUREMENT ACCREDITATION of Electrical Calibration Laboratories 

By:
Lawrence Bernard Cronin
Consultant LBCS
13 Homelea, Orpington, Kent, BR6 6LT, England, U.K.
Tel: +0044 (0) 1689 818893
Fax: +0044 (0) 1689 810542
E-mail lbcronin@globalnet.co.uk 

Abstract

Reliable measurement underpins most of the activities of mankind, including manufacturing, service industries and the health service. For example, measuring instruments are used throughout manufacturing processes to ensure the quality of the product is satisfactory. It is essential for international trade that all measuring instruments throughout the world should be based on a compatible system of measurement; calibration and traceability achieve this. Traceability is the process by which measurements are related through an approved chain of comparisons to national and international standards. This paper describes the techniques used in the assessment of electrical measurement laboratories so as to provide assurances that the chain of measurement traceability is unbroken. 

Introduction

The importance of measurement accreditation can be seen from the efforts made by 5500 or so European organisations, which are currently accredited to ISO/IEC Guide 25 or the European Standard EN 45001 [now replaced by ISO/IEC Standard 17025], to demonstrate that they were exercising control over their measurement operations.

The importance of a fully integrated calibration system within the quality control arrangement was not realised by all organisations for a number of years. This prompted the introduction of procedural standards to provide guidance for quality control and calibration requirements. This has included the introduction of ISO/IEC Guide 25 and EN 45001, which have been adopted by many national accreditation schemes, as the criteria for assessment of calibration and testing laboratories. Schemes of this kind have developed in the U.K. since the 1960s

The growth in the number of registrations to the quality assurance ISO 9000 series of Standard has led to a more widespread understanding of the importance of reliable measurements, particularly in the area of traceability of measurements.

Traceability is defined in the BS Vocabulary of Metrology PD6461: Part 1:1995 as:

‘Property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.’

Recent Background to Reliable Measurement Systems

The subject of measurement has a long and distinguished history, however, the need for reliable measurement systems has grown rapidly over the last thirty years and this increase is continuing due to the number of companies seeking registration to ISO 9000 series of Standards. The introduction of the ISO Standards for quality systems have all played important roles in raising the overall status of measurement in the industrialised world. In the UK alone there are over 50,000 companies registered to comply with ISO 9000; in Europe and elsewhere there is a growing emergence of ISO 9000 registrations as young certification bodies start to develop and registrations increase.

National Measurement System

Ensuring confidence in the accuracy of measurement is of paramount importance and it is the National Measurement System (NMS) of a country that underpins this confidence. The NMS is the technical and organisational infrastructure that ensures a consistent and internationally recognised basis for measurement. In the UK the NMS has two central objectives:

-to enable individuals and organisations in the UK to make measurements competently and accurately; also to demonstrate the validity of such measurements.
-to co-ordinate the UK's measurement system with the measurement systems of other countries.

The National Physical Laboratory (NPL) plays a central role in the organisation of the NMS in the U.K. The principal task of NPL is to develop and disseminate national measurement standards, and most of its resources are devoted to this activity.

In the U.K. calibration and testing laboratories, particularly those that have been accredited by the United Kingdom Accreditation Service (UKAS) and its forerunners, the British Calibration Service (BCS) the National Testing Laboratory Accreditation Scheme (NATLAS) and the National Measurement Accreditation Service (NAMAS), play a vital part in the NMS. UKAS-accredited Calibration and Testing laboratories, some 90 percent of which are in the private sector, provide a service to a wide range of customers in British industry. The UKAS-accredited calibration laboratories constitute the main channel for the dissemination of measurement standards, which are essential for R&D, quality control, sales specifications for products and regulatory purposes. 

European Co-operation for Accreditation [EA]

The body which co-ordinates the accreditation of calibration and testing in Europe is the European co-operation for Accreditation [EA].  The EA was formed in November 1997 by the merger of EAC [European Accreditation of Certification] and EAL [European Co-operation for Accreditation of Laboratories]. The U.K. member of EA is UKAS, which has accredited over 2000 laboratories in the U.K., including over 550 calibration laboratories.

The merger of EAC and EAL into EA was seen as being cost-effective and that it should strengthen the voice and influence of the Western European accreditation bodies in the accreditation arena in Europe and the rest of the world. This is now especially important for the future due to the growing expansion of accreditation bodies now emerging world-wide.

Accreditation Standards

The Standard for measurement accreditation is the ISO/IEC Standard 17025 “General requirements for the competence of testing and calibration laboratories”. This standard has two main sections covering Management and Technical requirements.

Laboratories meeting the ISO/IEC Standard 17025 requirements for calibration and testing activities also comply with the relevant requirements of the ISO 9001 standard.

Essential Aspects of Measurement Accreditation Criteria

There are five aspects which, backed by an efficient measurement audit service, are essential requirements for measurement accreditation. These are:

- staff 
- equipment 
- accommodation and environment 
- documentation [Quality Manual and Measurement Procedures 
- traceability

Staff

Laboratory staff must have academic and/or professional qualifications as well as metrological training, experience and skills relevant to their role. Technical qualifications provide confidence that a person has the knowledge to resolve complex technical problems, which often occur in measurement. In some cases a sound background in solving measurement problems and guidance by experienced metrologists will be enough.

The level of experience and the necessary skills laboratory staff need depend on the level of accreditation sought. Staff is assessed on the ability to perform measurements within the scope of accreditation and not on whether they have the skills to undertake state-of-the-art measurements. It is important to note that approved signatories are approved for a particular accreditation, which is not transferable.

Equipment

Equipment must be capable of supporting and maintaining the scope of measurement accreditation. The laboratory must maintain an inventory of all equipment used to support its measurement capabilities. Each item must be uniquely identified and individual case history maintained providing information such as:

- maintenance record
- compliance checks
- calibration history

One of the major uncertainty contributions in uncertainty budgets for measurement accreditation is drift. This must be based on actual data and not only on manufacturer’s specifications. Laboratories must maintain documented performance data of equipment to analyse and predict the uncertainty contribution for drift. If a reference standard becomes faulty, to maintain existing the accreditation its replacement must be capable of satisfying the actual calibration performance of the original.

Accommodation and Environment

a] Accommodation

The requirements for laboratory accommodation will vary slightly from one measurement field to another, but are basically the same. In the mass calibration field, for example, the laboratory must be a separate identifiable space not shared with other activities. The area must also be large enough and easily accessible. A ground floor or basement is a good location for reducing the effects of vibration and maintaining temperature control.

Direct sunlight should not enter the laboratory or shine on standards, instruments, or equipment being measured or to be measured. Windows should be minimal size, face north and be double-glazed.

It is important that the laboratory is clean and free from fumes. Staff must have working space where results can be written out and where certificates of calibration can be generated. There must be adequate cupboard space to hold standards and calibration equipment and filing cabinets to hold laboratory documents.

b] Environment

The environmental control must be such that at no time are reference standards and other equipment subjected to conditions likely to affect their accuracy. The ambient temperature and relative humidity levels (with tolerances set for laboratory) must be stated in quality procedures. In most measurement fields environmental monitoring equipment must be provided to continuously record temperature and humidity conditions within the laboratory. In the mass calibration field, where air buoyancy correction may be necessary a suitable barometer must also be provided.

Other areas, which may require attention, are: dust control, vibration, acoustic noise, noxious fumes, magnetic fields and electro-magnetic interference. Lighting must also be adequate for the purpose. The power supplies to the laboratory should be appropriately controlled and monitored. Where purity of waveform or stability of voltage is important, means must be provided to ensure that supply aberrations do not affect measurements. The laboratory must also have an effective mains earthing system.

BS 7789:1995 (Design of measurement laboratories) is recommended for further reading. It gives guidance on design criteria and considerations for the siting, construction, environment, supply of services and equipping of measurement laboratories.

Documentation

a] Quality manual

This should cover all items, which are relevant, with the main emphasis on quality policy, quality procedures, contract review, staff responsibilities, quality audits and reviews.

b] Measurement procedures

These are necessary for each measurement parameter for which accreditation is being sought to ensure uniformity and continuity of measurement is maintained. They describe how the measurement capability is achieved and include, scope of measurement, equipment to be used, diagrams, method and uncertainty budgets. Each area and what it should include are:

-diagrams: e.g. for electrical measurement the requirements of guarding, earthing and any loading considerations that may be necessary 
-methods: techniques used, precautions taken and any formulae needed. Reference to manufacturers’ handbooks is acceptable but the date and issue must be clearly stated
-uncertainty budgets: all relevant uncertainty contributions associated with the measurement system, e.g. for electrical measurement, importation uncertainty of reference standards, drift, temperature coefficients, linearity, resolution, frequency and thermal effects, and short term stability.

Estimations for uncertainty contributions should be based on historical and factual evidence wherever possible. The UKAS guidance document for calibration uncertainties of measurement is M 3003 Edition 1. This complies with ISO’s Guide to the Expression of Uncertainty in Measurement.

The major task of a measurement accreditation assessor is to undertake a full technical evaluation of the measurement capability described in the measurement procedure. This requires in-depth studies of techniques used, and of historical and factual evidence provided by the laboratory. It also requests practical measurements conducted in the laboratory for at least the most sensitive and demanding parameters to be accredited.

Traceability

Traceability for accreditation means that a laboratory must have its reference standards calibrated by approved sources. These include the National Physical Laboratory (NPL) in the UK, the National Institute of Standards and Technology (NIST) in the USA and the Physikalisch-Technische Bundesanstalt (PTB) in Germany, plus other national authorities where accreditation bodies have international agreements with. Traceability is also accepted from accredited laboratories whose measurement capabilities are appropriate for another laboratory’s accreditation needs.

Measurement audits

These are the means of providing the practical proof. Most organisations seeking, or having obtained measurement accreditation from a national accreditation authority within Europe must participate in measurement audit exercises.

A measurement audit provides supporting evidence for a claimed and/or accredited measurement capability. By sampling the product of the laboratory’s quality system and its operation it also provides partial evidence that other aspects of the calibration system are likely to be sound. There are various methods but all are similar in one respect. A suitable instrument is given to the laboratory with specific instructions on the measurements to be made and the results to be reported.

An accredited laboratory will have its equipment or transfer standards calibrated by a higher echelon laboratory in the national measurement system. The traceability obtained together with essential in-house measurements should enable the laboratory to establish and support its existing uncertainty budget.

If such practices gave complete assurance that 95 per cent of all measurements from a calibration laboratory would be within specified limits, further assurances would not be needed. Unfortunately, there are many factors which can introduce errors and which may not be apparent during in-house checks or day-to-day measurements.

 A list of some of these factors for electrical equipment is given below. If all aspects of the assessed system are strictly adhered to and in-house comparisons are frequently carried out then these undetected factors should be kept to a minimum: 

- standards may drift by unsuspected amounts or be damaged in use and/or transit.
- environmental control may be inadequate for some measurement methods
- incorrect measurement procedures may have been used. 
- a failure in the maintenance of calibration equipment or incorrect items used for a
   calibration procedure. 
- design and construction problems in measuring instruments or design and
   reliability problems in the development of measurement systems. 
- power supplies may have become unsuitable due to distortion and/or stability. 
- earthing, screening or loading may not be in accord with agreed procedures
- connections between items of equipment may not be in accord with agreed 
  procedures. 
- operators may be inadequately trained or supervised.
- uncertainties of measurement may be incorrect. 

This list is not exhaustive but can in general be reduced to four main categories, quality of equipment, quality of measurement procedures, quality of environmental controls and quality of personnel. There are a number of basic methods for carrying out measurement audit exercises, the first is the circumferential audit, the second is generally termed a radial audit and the third is a continuing process method. Further information on measurement audits is given in ISO Guide 43, parts 1 and 2.

Schedules of Accreditation

A laboratory obtaining accreditation is issued with a Schedule of accredited calibrations or tests by the accreditation body. The Schedule is the formal statement of the range of activities for which a laboratory has been accredited. The Schedule is produced by the accreditation body in consultation with the assessor(s) involved in the assessment and the accredited laboratory; it is based on the information provided and agreed at the assessment. The best measurement uncertainties are quoted in the Schedule. The capability is expressed as an uncertainty at a given confidence level. Any comments or notes that are needed to clarify or qualify the measurements carried out are included. Extracts from two Schedule layouts are shown in Tables 1, 2 and 3. 

TABLE 1 Part of UKAS Permanent Laboratory Accreditation Schedule for NPL No. 0478.

Measured Quantity and Range or Instrument	Frequency	Best Measurement Capability Expressed as an Expanded Uncertainty (±)*	Remarks
DC RESISTANCE 0.1 mW 1 mW 10 mW 100 mW 1 W 10 W 25 W 100 W 1 kW 10 kW		3.0 ppm 1.5 ppm 1.4 ppm 0.7 ppm 0.06 ppm 0.05 ppm 0.05 ppm 0.05 ppm 0.05 ppm 0.06 ppm	) ) ) 4 terminal resistors at temperatures between ) 17 °C and 25 °C and 1 mW dissipation ) ) ) )

TABLE 2 Part of UKAS Site Accreditation Schedule for Wavetek Ltd., No. 0183 SI.

Measured Quantity and Range or Instrument	Frequency	Best Measurement Capability Expressed as an Expanded Uncertainty (±)*	Remarks
DC VOLTAGE 100 mV   1 V   10 V   19 V   100 V   1 kV		5 ppm   2.7 ppm   2.1 ppm   2.4 ppm   3 ppm   3 ppm	This accreditation is restricted to a transportable computer controlled Multifunction Transfer Standard that is normally sent unaccompanied to the client's facility.  The client is responsible for the initial set up of the system using the procedure provided.  The client must apply to the laboratory using a special application for test form, which is available on request.

Table 3 Part of UKAS Permanent Laboratory Accreditation Schedule for the Ministry of Defence, Calibration Centre, Bolkiah Garrison, Negara Brunei Darussalam No. 509.

Measured Quantity and Range or Instrument	Frequency	Best Measurement Capability Expressed as an Expanded Uncertainty (±)*	Remarks
Mass	Nominal value (g) 20 000   10 000    5 000    2 000    1 000       500       200       100         50         20         10          5           2          1          0.5          0.2          0.1          0.05          0.02          0.01          0.005          0.002          0.001	(mg) 20   10    5    2    1    0.5    0.2    0.1    0.06    0.05    0.04    0.03    0.024     0.020    0.016    0.012    0.010    0.008    0.006    0.005    0.004    0.004    0.004

*The Expanded Uncertainty is given for k=2, providing a level of confidence of approx. 95%. 

It is normal policy that a laboratory’s Schedule be defined in terms as precisely as possible. This will enable a client to establish accurately and unambiguously the range of calibrations and tests covered by a laboratory’s accreditation. The capabilities shown in a schedule are for a laboratory’s Best Measurement capability.

Certificate of Calibration or Test Report
  
An accredited certificate or report gives assurance that:
  
- the work has been carried out to the highest standards; 
- the laboratory issuing the certificate has been stringently assessed by independent 
  experts; 
- the agreed or specified methods and procedures have been followed; 
- the measurements are traceable to national and international standards.

Only a laboratory that holds accreditation can issue an accredited certificate or report. The accreditation body normally lists these laboratories and the services for which they have accredited in a directory of accredited laboratories.
  
Laboratories can only issue certificates or reports for activities, which are covered by their accreditation. Laboratories apply for accreditation for specific tests or calibrations, and are assessed for that work. If a laboratory meets the requirements, it will be accredited for those areas assessed. 
  
The Need for Certificates of Calibration and Test Reports
  
Accredited certificates and reports fulfil the following needs: 
  
- Commercial: Many buyers specify accredited reports or certificates for tests on 
   products and material, to justify at least some important aspects of the supplier’s sales 
   specifications.   
- Technical: Manufacturers need to be sure that components will meet stringent 
   specifications, which will save them production costs and improve the quality of the 
   final product. 
- Legal: The European Community increasingly requires that products sold in the EC, 
   including the UK, must comply with EC requirements regarding independent 
   assessment of products or systems. Such assessment will be carried out by bodies 
   approved for this purpose by Member States according to agreed criteria. 
  
Conclusions
  
This paper has described some of the major fundamentals of measurement accreditation. If the criteria of measurement accreditation are strictly adhered to, then customers of accredited laboratories should have sufficient assurance that their equipment has been measured to acceptable standards. 

Acknowledgement

The author wishes to thank all his past and present metrology colleagues for their support in the presentation of this paper. 
  
Biography
  
Lawrie Cronin has been associated with metrology, inspection and QA for over 35 years, working with the U.K. Ministry of Defence [MOD], The British Calibration Services [BCS], The National Measurement Accreditation Service [NAMAS], The United Kingdom Accreditation Service [UKAS] and Industry. In his time with the above organisations, he has been Head of the DC/LF electrical standards and calibration laboratories at the MOD’s Services Electrical Standards Centre [SESC] and Principal Officer for electrical measurement with UKAS. He now runs his own company, LBCS, providing consultancy services in metrology, inspection, quality assurance and measurement accreditation. He is a Fellow of the Institution of Electrical Engineers and a member of the Institute of Acoustics.