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Sampling procedures in legal metrology

Working group 8.42

Topics

Overview

Sampling procedures are used in industry, medicine and for election forecasts, among others. In legal metrology sampling plans are applied to verify the quantity of product in prepackages (see Opens external link in new windowOIML R 87), for conformity assessment when placing utility meters on the market (see Opens external link in new windowEU-Directive 2014/32/EC and Opens external link in new windowOIML G 20) and subsequent verification of meters at predefined periodic intervals.

 

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Research

Sampling procedures for surveilling measuring instruments

Regular inspections are a key element of metrological quality control. One aim is, that 95 % of the utility meters fulfil conformance standards at all times. To guarantee this with high confidence and without inspecting every meter, PTB working group 8.42 develops procedures for regular sampling inspections. 

  1. PTB working group 8.42 has developed a new statistical method, which combines standard sampling plans with a reliability function.  (See figure and Opens external link in new windowK. Klauenberg and C. Elster 2017)
  2. In addition to developing this simple and generally applicable method, the mathematical assumptions for more efficient sampling plans were researched and published  (Opens external link in new windowKlauenberg et.al. 2018).
  3. The complementary methods 1) and 2) can be combined. One option offers high efficiency via small sample sizes and requires evidence that additional quality criteria are fulfilled. The alternative offers high flexibility via few assumptions. Both methods are readily realized by look-up tables (Opens external link in new windowK. Klauenberg and C. Elster 2018).

 

 

Schematic illustration of the inspection of (water) meters based on sampling, such that at least 95% of the devices fulfil conformance standards in the future

The aim of ongoing research is to develop new statistical approaches for a more efficient, more flexible and at the same time reliable surveillance of the quality of utility meters. Specifically, the benefit of additional information (so-called prior knowledge) shall be investigated when repeating reliability inspections and when implementing these in shorter time intervals. The methods of Bayesian experimental and adaptive design which are developed and investigated, shall identify new possibilities for repeated statistical quality control and at the same time provide the basis to suggest alternative sampling procedures, among others for section 35 of the Measures and Verification Ordinance.

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Application

Statistical procedure enables enforcement of a change in the Measures and Verification Ordinance

Subsequent verification of utility meters is legally regulated in section 35 of the Measures and Verification Ordinance in Germany. To enforce the respective new regulation, the verification authorities have published the new administrative regulation “Opens external link in new windowGM-VA SPV”. This regulation is based on the developed statistical procedures described above and on consultations carried out by PTB.

Starting 2019, a new procedure has entered into force which – by means of regular sampling inspections – will guarantee with high confidence that 95 % of the in-service measuring instruments fulfil the legal requirements at any time. Inspections carried out before then did not allow any statements about the quality of utility meters between such assessments. The new statistical procedure thus offers better consumer protection for water, electricity, gas and heat meters.

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Committee work

PTB working group 8.42 supports the verification authorities in implementing the new section 35 of the Measures and Verification Ordinance.

Subgroup "Statistics for the Measures and Verification Act"

In order to support legal metrology in statistical questions, the PTB Subgroup "Statistics for the Measures and Verification Act" was established. As a first task, the subgroup has initiated the development of sampling plans for utility meters in order to implement section 35 of the new Measures and Verification Ordinance. Representatives of verification authorities, testing laboratories, conformity assessment bodies, utility companies and associations of the utilities gas, water, heat and electricity met with PTB experts to deliberate on new sampling plans which comply with the increased requirements of section 35, to discuss these sampling plans and to make them public. 

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Publications

K. Klauenberg;C. Elster
OIML Bulletin, LIX(3),
10--16,
2018.
K. Klauenberg, R. Kramer, C. Kroner, J. Rose;C. Elster
Quality and Reliability Engineering International,
2018.
K. Klauenberg;C. Elster
Metrologia, 54(1),
59--68,
2017.
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