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PUBLISHED BY THE ACADEMY OF SCIENCES OF ALBANIA JNTS JOURNAL OF NATURAL AND TECHNICAL SCIENCES 2016 (1) XXI (41) 2 2016/Vol. XXI (1) JNTS Traceability of mass unit in the republic of Albania 3 TRACEABILITY OF MASS UNIT IN THE REPUBLIC OF ALBANIA Luljeta DISHA, Dëfrim BULKU General Directorate of Metrology DPM, Sector of Mass and Density, Tirana, Albania Antoneta DEDA Department of Physics, Faculty of Natural Sciences, University of Tirana, Albania ABSTRACT Traceability is the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. It is of the greatest importance for applications demanding substantial accuracy - and modern technology provides a huge range of them - it is surprisingly easy to inadvertently end up with the wrong values through inadequate attention to the detail of traceability. Inaccurate or wrong measurement might lead to wrong decisions with economic consequences. National Metrology Institutes (NMI) such as the Mass Laboratory at the General Directorate of Metrology (GDM), Tirana, Albania have the prime responsibility of being the source of traceability of the highest metrological level to the SI, or if this is not yet feasible, to other internationally agreed references, for metrology users in their country and adequate to their needs. This comprises the development, maintenance and dissemination of national measurement standards traceable to the SI, or when this is not (yet) possible, to other internationally agreed references. A major part of this work is the international recognition of these measurement standards and of the calibration and measurement reports and certificates issued on the basis of internationally assessed and approved Calibration and Measurement Capabilities (CMC) in accordance with the rules laid down in the CIPM, MRA. Mass Laboratory has been part of some Key Comparisons organized by other National Metrology Institutes of the highest level. BIPM has the mission of establishing world- wide uniformity of measurements. The kilogram is the only SI unit still based on a physical object. The present paper provides some information about the method used to carry out and disseminate traceability of mass unit in Albania along with the results of a Key Comparison. Keywords: mass, metrology, weight, calibration, weighing, key comparison, uncertainty 4 Luljeta DISHA, Dëfrim BULKU, Antoneta DEDA 2016/Vol. XXI (1) 1. INTRODUCTION A proper national measurement system is of vital importance for all the countries, as it helps meeting the challenges and requirements of local and foreign markets. The concept of measurement traceability is a key one for understanding how a properly organized national measurement system provides greater assurance of accurate measurements (VIM 2008). On one end of the traceability chain are the routine measurements related to manufacturing, commerce and environmental protection and at the other end are the (NMIs), responsible for the most accurate possible realization of the SI units of different measurement and their dissemination. Currently, the Directorate of Scientific and Industrial Metrology (DSIM), GDM, Tirana, Albania has the prime responsibility of being the source of traceability of the highest metrological level to the SI to the international level, via a system of national and reference standards and provides technical support in the measuring of various physical quantities in the territory of the Republic of Albania. Key comparisons are intended to demonstrate a high degree of equivalence of national measurement standards. Once this demonstration of equivalence is accomplished, the traceability chains established independently by each NMI provide a basis for acceptance of measurement results. There are realized different Key Comparisons from Mass Laboratory of DPM in the framework of IPA 2008 project supported by European Commission and some other supported by European Association of National Metrology Institutes (EURAMET) and the degrees of equivalence resulting from these comparisons are within their respective standard uncertainties showing that the Key Comparison is a success for Mass Laboratory. Mass Laboratory has been even a Pilot laboratory in one of these international comparisons. 2. MATERIALS AND METHODS Mass Laboratory at the General Directorate of Metrology (GDM), Tirana, Albania has the prime responsibility of being the source of periodic calibrations at the national level and collaborates with the National Institute of Metrological Research (INRIM), Italy and Physikalisch-Technische Bundesanstalt (PTB), Germany. The latter provide their traceability of Mass Unit to the BIPM. Here, calibration of weights in free air involving the substitution method which consists in comparing of two mass standards of the same nominal mass value is applied. One of the mass standards is used as reference one, that it is traceable to International Unit of Mass and the other one is the test mass, which value has to be determined by using mass comparators. Calibration results and proper JNTS Traceability of mass unit in the republic of Albania 5 measurement uncertainty of the test weight are stated in the calibration certificate (ISO/IEC 1702, 2005) by documenting the traceability to the national standards, which realize the unit of measurement according to SI. The conventional mass of the test weight is calculated from the formula (OIML R 111-1 2004): mct m cr(1 C ) m c (1) where: mcr – Conventional mass value of the reference weight, which is taken from its last calibration certificate mc - is the average weighing difference observed between the test and reference weights which is calculated with formula: 1 n mc m ci (2) n t1 The conventional mass difference, mc between the test weight and the reference weight of a cycle, i, is calculated with formula: mci l i m cr C i (3) Ci – Air buoyancy correction calculated with formula: 1 1 Ci() a 0 (4) t r where: a – Air density which is calculated with formula: 0.34848p 0.009( hr ) exp(0.061 t ) a (5) 273,15 t 6 Luljeta DISHA, Dëfrim BULKU, Antoneta DEDA 2016/Vol. XXI (1) 3 p - the pressure, hr - the relative humidity,t - the temperature, in °C, 0 = 1.2 kg/m air density, t – density of test weight, r - density of reference weight. The standard uncertainty of air density uρa is calculated by: 2 2 2 2 a a a ua u f u p u t u hr (6) t hr -4 where: uf = 10 , standard uncertainty of approximation formula for the calculation of air density, up - standard uncertainty of the pressure probe, ut - standard uncertainty of the temperature probe, uh - standard uncertainty of the relative humidity probe, The true mass of the test weight is calculated from the formula (Schwartz et al., 2007). mTRW m X m (7) where CVVa () T R (8) where: VT – is the test standard volume and VR- is the reference standard volume. Then the formula will be as follow: ' mT m R a () V T V R m W (9) ' where: mW - is the corrected value for air buoyancy For a cycle ABBA (r1t1t2r2) we take the corresponding readings in the weighing instrument display: Ir11, It11, It21, Ir21,...Ir1 n, It1 n, It2 n, Ir2 n (/)IIII l t1 i t 1 i r 2 i t 2 i (10) i 2 where i= 1,…n-number of cycles ABBA which depends on the class of weights. Usually true mass term is used for NMI measurement level, while conventional mass term is used for secondary level other than NMI measurement level (VIM, 2008). JNTS Traceability of mass unit in the republic of Albania 7 Uncertainty calculation in mass measurement Calculating and expressing uncertainty is important to anybody wishing to make good quality measurements. The uncertainty calculations are based on (GUM, 2008). The expanded uncertainty - U, of the conventional mass of the test weight is as follows: U k uc (11) where: k- Coverage factor, in this case, k = 2 and represents confidence level H” 95% uc - combined uncertainty 2 2 2 2 uc u w u() m e u b u ba (12) where: uw – Standard uncertainty of the weighing process, (Type A), obtained by statistical means. s2 u (13) w n s – Standard deviation of the mass difference, for n – number of cycles A-B- B-A and is given by formula: n 1 2 s() mi m (14) n 1 i1 where: m - Difference of conventional mass of a weight comparison m - Average difference of conventional mass u() mr - Standard uncertainty of the reference weight (taken from Calibration Certificate) u() m u() m r (15) r k ub– standard uncertainty of the air buoyancy correction (Type B) 8 Luljeta DISHA, Dëfrim BULKU, Antoneta DEDA 2016/Vol. XXI (1) 2 u2 u 2 u m r t [ m ( )]2 r t b r cr a 0 4 4 (16) r t r t Even if the air buoyancy correction is negligible, the uncertainty contribution of the buoyancy effect may not be negligible and shall be taken into account. uba – Standard uncertainty of the balance The recommended approach to determine this component is to test the mass comparators at reasonable time intervals and use the results from the test in the uncertainty calculations. 2 2 2 2 uba u s u d u E u ma (17) us - Uncertainty of the sensitivity of the balance, this will be included, if the balance is calibrated with a sensitivity weight of mass ms and of standard uncertainty u(ms), if not, the uncertainty contribution due to sensitivity is normally negligible. ud – Uncertainty due to the display resolution of a digital balance For a digital balance with the scale interval, d, the uncertainty due to resolution is: d d u 2 2 d (18) 3 6 uE- uncertainty due to the eccentric loading: d 1 D d u 2 (19) E 2 3 where: D is the difference between maximum and minimum values from the JNTS Traceability of mass unit in the republic of Albania 9 eccentricity test.d1 - is the estimated distance between the centres of the weights, d2 - is the distance from the centre of the load receptor to one of the corners.

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