Measurement plays an important role in all walks of life and certainly in all sectors of industry. Any deviation from the true value is ultimately reflected on the quality of the end product. The precise, accurate and reliable measurement (ISO, 2013) data is a mandatory requirement of national and international level of quality systems. The organisation for international standardisation (ISO) has laid down guidelines to achieve universally acceptable high-quality measurement results in all fields like chemical, physical, engineering, environmental etc (ISO, 2005). The national standards in India are defined and involve all stakeholders for emphasis on adherence to quality standards in all spheres of activities. The primary objective is promoting and protecting the interests of the nation and its citizens. In accordance with the international norms and conditions, the result of the product is obtained based on specific methods that are stable, accurate and precise. Figure 1 shows the schematic for accurate and precise measurement.
Standard in Measurements
In metrology, the measurement result of an unknown value can be obtained only by comparing it with a known value. A standard (NCC, 2013) is an object, system or a unit of known quantity which can be compared with other measurement units. Basically, a standard is used for comparison or a reference point to evaluate an unknown thing. Use of standards for all measurements is mandatory for globally acceptable quality control of an end product. According to ISO’s guidelines (ISO, 2015), “Quality assurance is to provide adequate confidence that a product, process, or service will satisfy the given quality requirements” and quality control is “the operational techniques and activities that are used to satisfy quality requirements which is an important part of the quality control of the Quality assessment”.
Certified reference material (CRM) activity has two aspects. The first is the preparation of these materials and second is their characterisation. Consider first the production of CRM. CRMs have to be produced over carefully controlled production conditions to assure maximum homogeneity. Further, these should be produced in sufficient quantity to assure long term integrity of the measurement system. CRMs have to cover a wide industrial spectrum. Therefore, it will not be possible for a single organisation to produce all of them and it should naturally be taken up by specialised agencies. In the Indian context for example, a certified reference material for metal industry may be prepared by the National Metallurgical Laboratory, for the cement industry it may be Cement Research Institute and so forth.
For the characterisation and measurement of the product’s properties, an organisation like the Indian Standards Institution could lay down the specifications (ISO, 2017). The National Physical Laboratory, New Delhi being the custodian of the national standards for physical measurement should characterise and give the reference data for these materials. It should stock them and issue them with characterisation certificate, listing their properties and accuracies.
The CRMs, therefore, serve several valuable purposes. These are routinely used in engineering and industrial quality control. In research they may be used for the calibration of research apparatus and methods. Indeed many research groups often wish to verify the working of their apparatus by using these materials. It may be used for round robin intercomparison between different laboratories. One aspect of this round robin system is to send a particular CRM to one particular laboratory, which after measurement sends this CRM to the second laboratory and so on. The last laboratory sends it back to the first laboratory, which rechecks the measurement for stability. This activity automatically leads to compatibility between the measurement laboratories and their traceability to national standards.
Quality Control | Use of Standards
The CRMs and their corresponding reference data are primarily required for three purposes:
To perform comparative measurements in which the measured property value is given directly in terms of the standards.
To perform apparatus checks, thus basing the performance of the apparatus on the standards.
To train new persons for high level of reliability and expertise.
Values measured using a comparison with respective CRMs/BNDs are accurate and reliable. Thus, the use of CRMs/BNDs helps in the calibration of analytical equipment and validation of test methods, quality control in measurement, ensures reliable measurements and provides traceability with international measurement systems (SI unit).
Quality Control | Types of Standards
It is difficult to achieve the true value of a measurand by any means. Nevertheless, a high level of accuracy is attainable by using a level of standards using reference materials . There are many types of standards available which help in calibrating the instrument and evaluating the measurand value. The standards enable strengthening of industry through quality testing of products and processes making them internationally competitive.
Reference materials are those that are sufficiently homogenous and stable with respect to one or more specified properties, which have been established to be fit for their intended use in a measurement process.
Certified Reference Materials (CRMs)
A certified reference material is one that is characterised by a metrologically valid procedure for one or more specified properties. It is accompanied by a certificate that provides the value of the specified property, its associated uncertainty and a statement of metrological traceability.
Bhartiya Nirdeshak Dravyas (BND®)
CRMs have been trade marked as Bhartiya Nirdeshak Dravyas (BND) by CSIR-National Physical Laboratory. It is intended to disseminate traceability throughout the world for an analytical measurement of certified parameter in various sectors. The production of BNDs and its implication on quality is shown in Figure 3.
The concept of traceability plays an important role as it provides accurate measurement with associated measurement uncertainty through an unbroken chain of measurements. The confidence in the accuracy and reliability of the measurement arises when the result is linked with a SI unit. The CRMs developed at NPLI are traceable to SI units. Verification of specified properties of the developed CRMs is being done using analytical equipment with proper validated method and primary reference materials (traceable to SI unit).
Metrological traceability property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty[JCGM 200:2008, 2.41 (VIM3)]. Figure 4 shows the pyramid for the traceability chart.
CRMs in the Indian and international scenario
Extensive use of CRMs is seen by most of the users, viz., industries, academic institutes, R & D organisations and various sectors in science and technology in India (ISO, 2017). However, they face great difficulties in the procurement of SI traceable CRMs of various matrices including aqueous elemental CRMs and so are completely dependent on international CRM producers. To minimise the difficulties of importing CRMs, CSIR-NPL, being the National Metrology Institute (NMI) embarked on its mission three decades ago to meet the demand of CRMs towards the preparation of indigenous certified and disseminated from SI traceable CRMs. The CRMs procured from the international market are too costly, with difficulties in procurement and completely dependent on the storage condition and shelf-life period.
NIST, USA has more than 1,300 Standard Reference Materials (SRMs) in industrial materials, environmental analysis and health measurements in science and metrology. BAM, Germany produced 400 CRMs from a variety of fields, including iron and steel, environment, foodstuffs, polymers etc. KRISS, Korea produced CRMs for chemical composition, bio-analytical materials, and industrial and engineering materials. The European Union has produced around 800 CRMs in the fields of food and feed analysis, environmental analysis, engineering and health application. NMI, Japan has developed CRMs for industrial materials, high purity inorganic materials, environmental materials, polymer materials and Gas CRMs.
Dissemination of BNDs
To meet the country’s requirement, CSIR-NPL has developed two strategies. The first development of BNDs at its own and the second is promotion of Reference Materials Producers (RMPs) by validation of the method and providing traceability to SI units.
CSIR-NPL & RMPs
CSIR-NPL has started its mission towards the dissemination of BND® through the Reference Material Producers (RMPs) in which NPL will establish the measurement traceability of the developed BNDs. A broad range of reference materials has to be prepared by CSIR-NPL and RMPs for each property and it must cover the range of measurement encountered in practice. Let us take the example of thermal conductivity. CRMs would be required to span the conductivities from low conductivity insulating materials to very high conductivity pure metals. This covers a range of one million or more. The CRM data for these reference materials must be obtained over the range of temperatures required by the users. Obviously, to be of use, the data should be as accurate as the state of art level permits in the field.
A similar practice will be adopted for the petroleum industry in order to ensure the highest standard quality. In this regard, the recently signed MoU between CSIR-NPL and HPCL, India will boost the CRM market in petroleum sector.
NPL has embarked its vision to disseminate measurement traceability using the development of BNDs through RMPs in various commodities like fuels, food, textiles, cement, biological fuels, instruments, ores and minerals, as depicted in Figure 5. CSIR-NPL aims quality control for the entire by implementing its mission and vision.
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International Organisation for Standardization (ISO), 2013. 5725-1:1994, Accuracy (trueness and precision) of measurement methods and results – Part 1: General principles and definitions, Available at: https://bit.ly/2Mwe59u
International Organisation for Standardization (ISO), 2015. ISO 9001:2015, Quality management systems—Requirements, Available at: https://bit.ly/2rmX7vQ
International Organisation for Standardization (ISO), 2015. ISO Guide 30, Reference materials—Selected terms and definitions, Available at: https://bit.ly/2BCSirD
International Organisation for Standardization (ISO), 2017. ISO Guide 35, Reference materials — Guidance for characterization and assessment of homogeneity and stability, Available at: https://bit.ly/2wfY3FQ
International Organisation for Standardization (ISO), 2017. ISO/IEC 17025:2017(EN), General Requirements for the Competence of Testing and Calibration Laboratories, Avilable at: https://bit.ly/2zDA6Zs
National Caliberation Center (NCC) NOAA, 2013. News from the JCGM-WG1 ISO/IEC Guide 98-1:2009 (JCGM/WG1/104), Uncertainty of measurement -Part 1: Introduction to the expression of uncertainty in measurement, Available at: https://bit.ly/2wp80ky