Sampling procedures in legal metrology

Working group 8.42

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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 OIML R 87), for conformity assessment when placing utility meters on the market (see EU-Directive 2014/32/EC and OIML G 20) and subsequent verification of meters at predefined periodic intervals.

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 K. 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  (Klauenberg 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 (K. Klauenberg and C. Elster 2018).

 

 

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.

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 “GM-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.