RECENT ILC ACTIVITY IN ROMANIAN MASS MEASUREMENTS

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1 The Fifth International Proficiency Testing Conference Timisoara, Romania () th 8 th September, 0 RECENT ILC ACTIVITY IN ROMANIAN MASS MEASUREMENTS Adriana Vâlcu, Sterică Baicu National Institute of Metrology, Bucharest, Romania adriana.valcu@inm.ro; sterica.baicu@inm.ro Abstract ISO/IEC 70, [], requires that laboratories have quality control procedures for monitoring the validity of tests and calibrations undertaken. This monitoring may include the participation in interlaboratory comparisons (ILC) or proficiency testing (PT) programs. By interlaboratory comparisons it is determined and continuously monitored the individual performances of laboratories that perform calibrations, thus identifying potential problems for which corrective actions are initiated. In Romania, at an interval of two years, the Romanian Bureau of legal Metrology (BRML) organizes interlaboratory comparisons in different areas. For mass field, an ILC of mass standards was part of interlaboratory comparison program for Six standard weights (nominal values: 00 g, 0 g, 0 g, g, 00 mg and 0 mg) used as travelling standards, were circulated in two separate loops between twenty two BRML laboratories, mass Laboratory of National Institute of Metrology (INM) acting as reference and pilot laboratory. The paper presents in tabular and graphical form: laboratory s mass results, uncertainty declared by participants for each weight, the normalized errors obtained for each laboratory, with respect to the INM. Analysis of the performed results belonging to participating laboratories shows that about % of the total reported results are discrepant. Key words Interlaboratory comparison (ILC), normalized error (En), measurement uncertainty. INTRODUCTION In Romania, at an interval of two years, the Romanian Bureau of legal Metrology (BRML) organizes interlaboratory comparisons in different areas.

2 For mass field, an ILC of mass standards was part of interlaboratory comparison program for Six standard weights (nominal values: 00 g, 0 g, 0 g, g, 00 mg and 0 mg) used as travelling standards, were circulated in two separate loops between twenty two BRML laboratories, mass Laboratory of National Institute of Metrology acting as reference and pilot laboratory. The Weights were treated as F accuracy class, even if, for weights of 00 mg and 0 mg the condition.3. from [] was not met. A preparatory document - technical protocol - (containing measurement instructions, transport and use) was drawn up by INM to inform the participating laboratories with requirements of interlaboratory comparison. CIRCULATION SCHEME, TIME SCHEDULE Interlaboratory comparison was organized using a circular scheme, with two petals, Figure. Including the RL, the first petal covered eleven laboratories whereas the second one covered twelve. Fig. Circulation scheme Initial planning of the period for performing measurements was from June 03 to May 0. Subsequently, in the second petal, laboratories no. and requested a delay for completing the measurements. For this reason the last calibration performed by the reference laboratory was at the end of July 0. Each participant had approximately weeks to carry out the measurements. 3 REFERENCE VALUES The weights used as itinerant standards were selected by Mass Laboratory and belong to measuring range between 00 g and 0,0 g as follows: 00 g, 0 g, 0 g, g, 00 mg and 0 mg. Specifications for the artifacts, in accordance with [], are shown in Table.

3 Table. Specifications for artifacts Parameter Nominal Value Weight 00 g 0 g 0 g g 0, g 0,0g Accuracy class F F F F F F Material brass brass brass brass nickel silver nickel silver Density (kg/m 3 ) u ρ (kg/m 3 ) The reference values x ref and U ref for itinerant standards were determined by reference laboratory at three different times: at the beginning (June 03), at the middle (November 03) and at the end of interlaboratory comparison (July 0). For each itinerant standard was calculated the average of the three values obtained. MEASUREMENT INSTRUCTIONS Before starting the intercomparison, the participants received information concerning the nominal value of the weights as well their estimated density. Each participating laboratory determined the conventional mass of the transfer standards and their associated uncertainty. All weighing were performed in air. Measurements were carried out after adequate acclimatization at the mass laboratory, as described in [] for weights of accuracy class F. In addition, instructions were given how to handle, store and transport the weights. For confidentiality purposes, for each laboratory it was assigned a code number ( to 3), the reference laboratory having the code. No other detailed calibration instructions were given to the laboratories. Regarding the method of calibration, laboratories have opted for method of substitution (Borda), using one or more standards. In order to not influence the mass value of the weights during the intercomparison, the participants were informed to not wash the weights, the only accepted method of cleaning being the use of a brush that accompanied the travelling standards. REPORTING RESULTS BY PARTICIPANTS Each participating laboratory determined the conventional mass and associated uncertainty for each itinerant standard. The measurement results were sent to the reference/pilot laboratory in a report where a list of the equipment used for comparison, method used, laboratory environmental conditions, condition of the transfer standard surface and uncertainty budget were included. Also, details on the standards used were required, in order to prove traceability of laboratory s measurement results. The participants were requested to: - specify the uncertainty budget in sufficient detail including: weighing process, reference weight, air buoyancy correction and mass comparator; - report the final results in a calibration certificate. RESULTS OF INTERCOMPARISON. Generalities 3

4 Laboratories presented the results in accordance with the requirements of the technical protocol that accompanied itinerant standards, with the following comments: - some of the participants (,, 8 and ) did not present the final results in a calibration certificate; - laboratory no. has not reported the result for 0 mg weight in the calibration certificate; - laboratories no. and have not sent the primary data.. Presentation of results The measurements results obtained in the interlaboratory comparison are summarized in Table. The results are presented exactly in the form that was sent by the participants. In this table, the first three rows represent the results obtained by the reference laboratory at all the three different times: at the beginning, at the middle and at the end of interlaboratory comparison. The fourth row represents the average of these three results. Table. Deviation (D) from nominal mass and expanded uncertainty (U) for each weight reported by the participants The criterion used for evaluating the performance of laboratories was the normalized error E n, which takes into account both the result and its uncertainty. The normalized error E n is given as [3]: E n U x lab lab x ref U ref Where:

5 E n x lab x ref U ref U lab = normalized error; = result of measurement carried out by a participant laboratory; = the comparison reference value; = measurement uncertainty reported by reference laboratory; = measurement uncertainty reported by participant laboratory. In the calculation of E n values, the reference values were obtained from the arithmetic mean of the measurements performed at three different times, according to Table. The E n data for each laboratory is presented in the Table 3 and provides supplemental information concerning the measurement capability of the participating laboratories []. Table 3: Normalized deviations E n from reference values Nominal Value 00 g 0 g 0 g g 00 mg 0 mg X ref,033 0,77 0,00 0,003-0,0-0,03 U ref 0,300 0,00 0,00 0,030 0,0 0,008 Laboratory s code Normalized deviation E n -0,7-0, -0,33 0,3-0, -0, 3-0,3 0,0-0,0 0, 0,83 0,0-0, 0,07-0, -0,,0,0-0,7 0,03 0,3 -,0 0,,3-0,3-0, 0,77 0, -0,3-7,0 7-0, -0, -0,0 0, 0,3-0,8 8-0, -0,3-0, 0, 0, 0,7 -,8 -, -0,77-0,03 0,7-0,3 0-0,0 0,0 0,0-0,3 0,3 0,3 0,8-0,3-0,30-0,0-0, 0,0-0,3 0,07 0, 0,0 0,3 0, 3-0, 0, 0,0-0,07-0,0 0,7-0,03-0, 0,00 0, 0, 0,0-0,03-0, -0,0 0,, 0,0 7 0,0 0,07-0,0-0,03 0, 0, 8 0,00-0, 0,0-0,8 0,7 0, -,8 -, -0, 0,7 3,7 3,8 0-0,3 0,07 0, 0,0 0,3 0, 0,0 0,07 0,00 0,0 0,3 0,8 0,33-0, 0,00,8-0,0 -,8 3-0,03 0,7 0, 0, 0, -0,0 E n > E n < - In the Figures to 7 are presented in graphical form (for each weight) the results of measurements reported by participating laboratories.

6 00 g weight,0,70,0 0, ,0-0,30-0,80 -,30 Fig. The difference between participant s results and reference value and uncertainty for 00 g weight 0 g weight 0,0 3 0,0 3 0, ,00-0,0-0,0 Fig.3 The difference between participant s results and reference value and uncertainty for 0 g weight

7 0 g weight 0,0 0,0 0,30 0,0 0,0 0, ,0-0,0-0,30 Fig. The difference between participant s results and reference value and uncertainty for 0 g weight g weight 0,30 0, 0,0 0, 0,0 0,0 0, ,0-0,0 8-0, Fig. The difference between participant s results and reference value and uncertainty for g weight 7

8 00 mg weight 0,00 0,00 0,000-0,00-0,00-0,0-0, ,0-0,300 Fig. The difference between participant s results and reference value and uncertainty for 00 mg weight 0 mg weight 0,00 0,00 0,000-0,00-0, ,0-0,00-0,0-0,300 Legend: o Fig.7 The difference between participant s results and reference value and uncertainty for 0 mg weight Reference mass value; Measurement uncertainty U (k=) reported by reference laboratory; Result of measurement carried out by a participating laboratory; Measurement uncertainty U (k=) reported by a participating laboratory. Figure 7 presents a complete image, a centralization of normalized errors obtained from participants results, for all the weights. 8

9 Normalized errors g 0 g 0 g g 00 mg 0 mg Analysis of results Fig. 7 Centralization of normalized errors As general observations it can be mentioned the following: - Lab : in the handwritten tables are some components of uncertainty that are not in the printed tables; - Lab : the characteristics of the balances from the annex of the technical protocol are not in accordance with the primary data; - In Table it can be seen that laboratories 8,, and have some results where the number of decimal places of conventional mass is not consistent with that of calibration uncertainty; - Laboratories codified with numbers,,, 8, 0, 3, 8, and presented some results with an excessive number of decimal places; - Laboratories,,, 3, 8, and reported an uncertainty less than /3 of the maximum permissible error, which led (for some weights) to a non compliance with index value within the [-, ]. From the Table 3 it can be seen as following: -for values of the normalized deviations in the range [-...+], laboratory i is compatible with reference laboratory and is considered able to perform calibration of class F standard weights in the analyzed domain with corresponding measurement uncertainties []; - participating laboratories (, 3, 7, 8, 0,,, 3,, 7, 8, 0,, 3) have obtained all results consistent with those of pilot laboratory for F accuracy class. Table presents a centralization of uncertainty components reported by each participant laboratory. Thus, can be also viewed the deficiencies of each participating laboratory in reporting the expanded uncertainty.

10 Table : The measurement uncertainty components of each participating laboratory and deficiencies observed by the reference/pilot in reporting of results * the two components were listed in the same column because the participants have differently noted them. o ) for some balances this component should be reported; o ) it was not properly calculated; o 3) even is used the same balance, this component it was not introduced in the uncertainty budget for all the weights; not applicable; v these components should be introduced in the budget table even if their effect was negligible. 7 CONCLUSIONS The results of interlaboratory comparison show that approximately % from the total results are discrepant. Fourteen participating laboratories (, 3, 7, 8, 0,,, 3,, 7, 8, 0,, 3) have obtained all results compatible with those of the reference laboratory for class F. Two laboratories ( and ) have only one incompatible result, four laboratories (,, and ) have two and laboratory no. has four inappropriate results. Table presents the weights for which these laboratories have obtained inconsistent results. Given the participants' comments, as their proposed corrective actions, the reference laboratory / pilot made the following proposals: - validation of the calculation file used; - for ensuring the quality results, it is recommended replicated calibrations using the same or different methods; - a better checking of input data (environmental conditions, measuring instruments used, etc.); - reducing the time between calibrations of measuring instruments (where applicable); 0

11 - it is also recommended that the number of performed measuring cycles not be in excess (see section. C..3 of []). By performing this ILC it was also checked the correctness of reporting measurement results, the method of calculating the mass and associated uncertainty and also in establishing the degree of compatibility between the participants results and results of measurements obtained by the reference laboratory. REFERENCES [] ISO/IEC 70:00(E): General requirements for the competence of testing and calibration laboratories. [] OIML R - Weights of classes E, E, F, F, M, M-, M, M-3 and M3, 00. [3] SR EN ISO/CEI 703, Conformity assessment General requirements for proficiency testing. [] Adriana Vâlcu. Interlaboratory Comparison of Five Standard Weights of Class F in Several Romanian Laboratories. SPRINGER (MAPAN Journal), 7, -.