Two tape measurement interlaboratory comparisons

Measurement tapes are classic and widespread length measuring instruments that are used in everyday life for many applications, including in commercial transactions. Because of their importance, the accuracy classes of these instruments, for example, are regulated throughout Europe in three accuracy classes in Annex X of the European Measuring Instruments Directive 2014/32/EU (MID) [1]. In Germany, traceability and testing are the responsibility of the weights and measures offices for commercial use. For industrial use in the context of quality management, these instruments are often traceable to laboratories accredited by the German Accreditation Body (DAkkS). Internationally, many National Metrology Institutes also offer the calibration of measuring tapes, whose measurement results also influence the local market due to the globally distributed production chains. It is important that all these measurements are comparable. For this purpose, the temperature of the calibration (20.0°C) as well as the tension force to be applied (depending on the tape, typically 50 - 100 N) are defined. Nevertheless, large sources of uncertainty remain with the various, partly very complex methods and with the influence of the measuring persons, which cannot be neglected. In order to ensure comparability, PTB was conducting two interlaboratory comparisons as a pilot laboratory: a national one within the framework of the German Calibration Service DKD and an international one within the framework of the European metrology organisation EURAMET.

In the national DKD interlaboratory comparison, 15 German laboratories participated, including DAkkS-accredited laboratories and those seeking such accreditation, as well as three calibration offices. The measurement uncertainties of these laboratories are adapted to the requirements of the MID (Measuring Instruments Directive) with an average of about 50 µm and a length-dependent increase of about 20 µm/m. The measurements of this comparison were carried out in autumn 2021 and spring 2022. Divided into two rings, each laboratory had to calibrate two commercial Class I tapes, one with painted, and one with etched lines. The evaluation of this comparison has been completed, and a confidential draft of the final report has been circulated within the group of participants. It is due to be published in the course of next year.

In the international intercomparison EURAMET.L-S27, 16 institutes were participating that belong to the European regional metrology organisation EURAMET, but also five institutes located on other continents [2]. With their more complex measuring equipment, these institutes typically achieve expanded measurement uncertainties (coverage factor k=2) of about 15 µm and a length-dependent increase of a few micrometres per metre tape length. As a measuring task, two precision measuring tapes of 10 and 50 m length, as well as a commercial measuring tape of class I were to be calibrated. The values were collected and centrally evaluated by PTB. The intercomparison started on 01 January 2018. The effects of the Corona pandemic and the associated restrictions in the international movement of goods made it considerably more difficult to carry out this elaborate experiment. Thus, the tapes were held in the customs of a country for several months in spring 2020. In the meantime, however, all measurements have been completed and a first internal draft of the evaluation, the so-called Draft A, of the final protocol has been prepared and distributed. The final report of this project will also be published in the course of next year.

Even if the concrete results will not be published until next year, a first interim conclusion can be drawn. The measuring tapes proved to be pleasingly stable in the course of the intercomparisons. Many laboratories were able to confirm their claimed measurement uncertainties in the comparisons. However, in individual cases the correct application and transmission of the clamping force seems to cause problems. Also, the influence of the line quality of the tape to be measured on the achievable reproducibility should not be underestimated. The corresponding repeatability should be explicitly determined experimentally as a type A uncertainty according to the "Guide to the Expression of Uncertainty in Measurement" (GUM) [3], whereby a slight variation of the reading location appears to be sensible in the case of automatic line acquisition.

Two test standards of the international intercomparison for scale calibration EURAMET.L-S27 in a carrying case.

References:

[1] Directive 2014/32/EU (2014) on the harmonisation of the laws of the Member States relating to the making available on the market of measuring instruments data.europa.eu/eli/dir/2014/32/oj
[2] Measurement of Steel Tapes of 10 m and 50 m - EURAMET [visited on 2022-11-09]
[3] JCGM (2008) Evaluation of measurement data – Guide to the expression of uncertainty in measurement, JCGM 100:2008 www.bipm.org/en/committees/jc/jcgm/publications