The Principles of Metrology

Anyone who operates instruments will use terminology that refers to metrology. The definition of metrology in encyclopedias and branch dictionaries is a science of measurements covering all aspects concerning theory and practice.

The obvious theoretical analysis referring to measurements will not be discussed here. This article aims to present definitions, acceptance criteria for measuring equipment and applications of this knowledge through the demonstration of measuring procedures.

Operators of measuring equipment will be familiar with the concept that quality measuring instruments are required to provide reliable measuring results that can be trusted. Until recently, this definition has been used in the application of quality norms without any further analysis. However, operator knowledge of measuring instruments has now changed, so much so that the supervision of measuring instruments is now mandatory.

Due to its requirements on measuring equipment and measurements, there are two main approaches to metrology in a laboratory: legal metrology and scientific and technological metrology.

Legal Metrology

The International vocabulary on legal metrology defines it as a science of measurements referring to activities arising from legal requirements with reference to measurements, measuring units and measuring methods performed by competent bodies. Each country may have different subjects covered by legal metrology. The legal metrology service is a competent body responsible for operating legal metrology. Providing uniformity of measurements is the primary subject of legal metrology.

Up until 2001 in Poland, Central Office of Measures defined legal metrology regulations on measuring instruments and published these in Official Newsletters on Measures and Tests. While these regulations were strict, they were not compatible with EU legal regulations. On 11th May 2001, a new regulation called “Measures Law” was issued, regulating most laws applicable to measuring instruments (Dz. U. 2001 No. 63 point 636 with alter changes).

Issued by a Ministry, a valid executive law applies to issues of the economy along with metrological requirements covering specific measuring instruments.

In EU member countries, measuring instruments are currently subject to legal metrological control as specified in new approach directives 90/384/EC, on non-automatic weighing instruments, and 2004/22/EC. However, this only refers to instruments that are currently on the market. As of 30th August 2002, regulations on conformity assessment systems have been implemented into Polish Legislation (Dz. U. 2002 No. 166 point. 1360 with later changes).

Measuring instruments introduced to the EU market are subject to conformity assessment in accordance with European law. These can be performed by a notified body (in Poland it is Central Office of Measures) or by a manufacturer.

It can be considered a breakthrough in Polish mentality that manufacturers have now been granted the right to assess conformity. Previously, such activities could only be carried out by national offices. It is considered a “new approach” for Polish manufacturers who implement and certify quality systems to have been granted some of the rights and competences that are discussed in EU new approach directives.

Clear requirements for the introduction of a product to market are outlined by this regulation. Previously, it was either a notified body or a manufacturer who could issue a declaration of conformity.

This declaration has now been replaced with a so-called initial verification document. There is still the issue of subsequent verification.[2] That is the verification of a measuring instrument after its initial verification or conformity assessment, covering an obligatory periodical verification or verification after instrument repair. Free choice is granted to member countries by the EU. It simply states that member countries should supervise legal metrology of measuring instruments in use.

A schema of conformity assessment procedure and sequential verification of a measuring instrument.

Figure 1. A schema of conformity assessment procedure and sequential verification of a measuring instrument. Image Credit: Radwag Balances & Scales

Measuring instruments used in specific applications are covered by legal metrological control (legal metrology). These applications are defined in Regulation of Measures in Article 8 point 1:

  • In health care, life and environment protection
  • In protection of public safety and order
  • In protection of consumer rights
  • On collection of payments, fees, taxes, non-taxed budget fees, on determination of discounts, fines, revenues and compensations, and also on determination and collection of charges and renders
  • On custom control
  • In trade

The requirements are described in further detail in terms of specific measuring instruments within the regulations on measuring instruments. Where non-automatic measuring instruments are concerned, the requirements for determination of mass are specified by the regulation of Ministry on Economy, Labour and Social Policy of 3rd October 2003:

  • In trade
  • For calculation of trade charges, customs, taxes, bonuses, discounts, fines, revenues, compensations and other similar forms of payments,
  • In the application of other legal regulations and expert opinions and issues in court trials
  • In medical practice to weigh patients to monitor, diagnose and treat,
  • For the preparation of medicines on prescription in chemists and for analyses conducted in medical and pharmaceutical laboratories,
  • For calculation of fares in direct trade and in packaging of goods.

A clear definition is given to each measuring instrument through general application from the Regulation.

New approach Directives (90/384/EC – non-automatic weighing instruments and 2004/22/EC – other measuring instruments) define the requirements of applications in terms of conformity assessment. Polish legislation implements these requirements in applicable regulations.

Requirements for non-automatic weighing instruments, introduced to market-based on conformity assessment, are defined by the Regulation of Ministry of Economy, Labour and Social Policy of 11th December 2003. The Attachments to Ministry of Economy Regulation of 18th December 2006 determine the areas on application and the requirements for measuring instruments covered by Directive 2004/22/EC (MID).

Any instrument currently in use that was introduced to the market before the 1st May 2004 is not covered by any of the new approach directives. For such instruments, the corresponding regulations of ministry on economics are applicable.

Polish legislation is not uniform when it comes to measuring instruments. Changes in legislation were supposed to be completed by 31st December 2007, but these only came into play during the second half of January 2008. Authorization for sequential verification of a measuring instrument is a very important issue.

Since 29th March 2005, the Ministry of Economy has outlined a valid regulation. However, this only covers a small group of measuring instruments. A system of sequential verification authorization forms has been around for many years in EU countries. Several measuring instruments in use in Poland, including weighing instruments, are subject to legal control. However, they are not listed with instruments that can be controlled with authorization forms. If granted an authorization to perform sequential verification, the customer should not be made to pay additional costs due to an instrument being out of operation. The issue of authorization for sequential verification is justified by many more arguments.

The Ministry of Economy’s impending new regulations will hopefully benefit both measuring instrument operators and citizens. This would bring Polish regulations closer in line with those present in countries of the “old EU”. Access to EU regulations and solutions has been made much easier since Poland became a member, including the ones referring to metrological supervision. These standards can provide a solution and customer satisfaction for those who are citizens of EU and, therefore, should be applied in Poland.

Scientific and Technological Metrology

Measuring instrument operators also have contact with metrology not concerning legal regulations, commonly known as “scientific and technological metrology”. One basic activity referring to this type of metrology is the calibration process.

Not regulated by legal metrology, calibration is a voluntary activity. However, its results may refer to requirements determined by legal regulations.

Operators are well aware of the importance of calibration results and of working in accordance with the norms of quality systems. Results obtained from test procedures have become more objective since knowledge on measurements and their errors have become more widespread.

The decision to implement a specific quality system governed by international standards (ISO 9001, ISO 17025 etc.) means also accepting the requirement to calibrate all measuring instruments.

All competent laboratories will perform calibration of measuring instruments. Accreditation by a national accrediting body, for example, The Polish Accreditation Centre, is the highest form of confirmation of laboratory competence.

Verification is not a counterpart of calibration or vice versa. As such, organizations that have implemented quality systems need to calibrate measuring instruments independent of their verification.

The table below presents the differences between legal metrology and scientific and technological metrology.

Table 1. Comparison of legal metrology with scientific and technological metrology. Source: Radwag Balances & Scales

. .
System of legal metrology Metrology system based on requirements from norms
Obligatory Voluntary
Based on legal regulations Based on norm regulations
Verification Calibration and / or checking
Requires in cases described by legal regulations Reference to international or national standard and determination of measuring uncertainty
Errors referred to verification unit of an instrument Errors referred to reading unit of an instrument
Tests performed by an authorized body of legal administration Tests performed by competent laboratory (if possible accredited one)
Document issued: verification markings, verification certificate Document issued: calibration certificate or checking certificate

 

Systems of legal metrology are, in many cases, obligatory and outlined in specific legal acts. Confirmation of specific requirements through an activity of legal metrology is considered verification of a measuring instrument. Based on international requirements and national norms, systems of scientific and technological metrology are voluntary. However, if an organization has voluntarily implemented quality norms and works in accordance with specific quality norms, then it should adhere to these requirements.

The determination of errors of a measuring instrument is the most important difference between verification and calibration. For verification, errors of a measuring instrument are referred to as verification unit. For calibration, errors are determined based on a reading unit. However, in some instances, the measuring instrument verification unit is n times bigger than their reading unit. This is often the case with very basic measuring instruments in laboratories and industry, for example, a weighing instrument (balance/scale).

Measuring instruments should be handed in for periodical calibration. They should be checked internally, to ensure that their indications are correct and there are no defects.

Legal acts, such as Measures Law, determine whether a specific instrument is subject to verification procedure.

The focus should be placed on monitoring measuring equipment through periodical verification and checking procedures aimed at calibrating instruments and checking that results adhere to international and national standards (point 4.6 of ISO 9001 norm). Such procedures are generally sufficient in organizations that implement quality systems. Metrological supervision does not need to be introduced where operators of measuring instruments do not have or do not need to implement quality systems.

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Last updated: Nov 28, 2019 at 6:34 AM

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