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Conference “Load Cell Tests by OIML R60” Buenos Aires, June 7th to 11th, 2010

Conference “Load Cell Tests by OIML R60” Buenos Aires, June 7th to 11th, 2010. Oliver Mack Physikalisch-Technische Bundesanstalt (PTB) Braunschweig www.ptb.de. Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology.

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Conference “Load Cell Tests by OIML R60” Buenos Aires, June 7th to 11th, 2010

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  1. Conference “Load Cell Tests by OIML R60” Buenos Aires, June 7th to 11th, 2010 Oliver Mack Physikalisch-Technische Bundesanstalt (PTB) Braunschweig www.ptb.de Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Oliver Mack:Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology INTIBuenos Aires 1 / 25

  2. Introduction • Modules of a weighing instrument: • Indicator • Analog or digital data processing device • Weighing module • Terminal • Primary display • Load cell • Test procedure for load cells (LCs): • OIML Recommendation R60 (Edition 2000) • WELMEC Guide 2.4 (Edition 2001) • Aim of the presentation: • Overview of unconsidered abnormalities • Start a discussion for a revision of R60 and WELMEC 2.4

  3. Test procedures according to the recommendation OIML R60 Accuracy tests (at 20°C, -10°C, 40°C, 20°C): Reproducibility, linearity, hysteresis, sensitivity Creep forward and creep recovery (at 20°C, -10°C, 40°C): Creep effects and zero point return Influence of humidity and barometric pressure Require additional tests

  4. Verification interval v: Classification according to the recommendation OIML R60 Classification: Class D (ordinary accuracy weighing instruments): up to 1000 v Class C (balance for trade purposes): up to 10000 v Class B (precision balance): up to 100000 v Class A (special accuracy weighing instrument): more than 50000 v Maximum permissible error (mpe): pLC: apportionment factor, generally pLC = 0.7

  5. Single-Point LC Beam LC with ball joint Uncovered according OIML R60: • Eccentricity effects (covered acc. OIML R76) • The material of the force transmission Abnormalitiies of LCs – uncovered acc. OIML R60 - Load transmission - WELMEC 2.4: The load transmissions for beam LCs may be applied

  6. Accoring to OIML R76 for weighing instruments the tests are carried out on 5 positions of the plattform Mpe for class C3 and the LC-error expressed in verification Intervals v as a function of the load for a 8 kg-load cell Abnormalitiies of LCs – uncovered acc. OIML R60 - Eccentricity effects -

  7. Abnormalitiies of LCs – uncovered acc. OIML R60 - Eccentricity effects - Result: Depending on the position where the load is applied the mpe for the LC is signifiantly exceeded. But in combination with the modular concept it is not taken into account sufficiently. Consequence: PTB carries out additional eccentricity tests according to OIML R76 if single point LCs are tested acc. to OIML R60.  Procedure not harmonised

  8. Single-Point LC Abnormalitiies of LCs – uncovered acc. OIML R60 - Material of the load transmission - Uncovered: Neither OIML R60 nor WELMEC 2.4 regularize the kind of material used for the load transmission e.g. by a declaration of the manufacturer. Example: Single-point LC made of stainless steel with aload transmission made of the same material or aluminium.

  9. Requirements on load cells acc. OIML R60 - Temperature effect on MDLO - Relevance: Temperature effect on the output signal under a minimum dead load Emin (e.g. Emin = 0 kg). Criteria fulfilled: If MDLO of a LC over a defined temperature range does not vary by an amount greater than a minimum verification interval vmin for any change of ambient temperature of 5°C for class C. Definition vmin: Smallest interval into which the LC measurement range can be divided and thus the smallest difference of mass which can be measured by the LC. Criteria given by a ratio: Y = Emax / vmin

  10. Requirements on load cells acc. OIML R60 - Temperature effect on MDLO - Stainless steel Temperature effect on MDLO in v for a class C3-LC and the mpe for Y = 20000 as a function of the ambient temperature

  11. Abnormalitiies of LCs – uncovered acc. OIML R60 - Material of the load transmission - Aluminium • Load transmission made of • Stainless steel: Y = 20000 • Aluminium: Y = 4500 Temperature effect on MDLO in v for a class C3-LC and the mpe for Y = 20000 as a function of the ambient temperature

  12. Abnormalitiies of LCs – uncovered acc. OIML R60 - Eccentricity effects - Result: Even if the requirements for load transmission acc. WELMEC 2.4 are fulfilled, the geometry of the force introduction and the materials used for it strongly affect metrological characteristic of a LC. Consequence: Concrete descriptions of the load transmission if necessary and a definition of applicable material should be discussed for a coming revisions of both OIML R60 and WELMEC 2.4.

  13. Abnormalitiies of LCs – uncovered acc. OIML R60 - Effects of temperature gradients - Frame: No concrete temperature gradients (spatial and temporal) are specified in OIML R60. Simply a temperature stability of 2°C within a accuracy test cycle has to be fulfilled. PTB-Requirements: A temporal temperature stability of < 0.2°C / hA spatial temperature gradient of < 0.2°C / 10 cm Point of discussion:Makes this sense ?

  14. Abnormalitiies of LCs – uncovered acc. OIML R60 - Effects of temperature gradients - Time stability of MDLO: Time stability of MDLO: Time stability of MDLO and mpe in v for class C3 as well as the ambient temperature as a function of time

  15. Abnormalitiies of LCs – uncovered acc. OIML R60 - Effects of temperature gradients - Time stability of MDLO: Time stability of MDLO and mpe in v for class C3 as well as the ambient temperature as a function of time

  16. Conclusion:Consequences for coming revisions • Experience of the current issues of OIML R60 and WELMEC 2.4 are presented. • There are abnormalities of LCs which may strongly affect the metrological characteristic of LCs but have not been taken into account yet. • In particular the influence of force introduction, eccentricity effects, temperature gradients, excitation voltage and drying effects of the sealing are presented. • Need of further investigations • The effects are not considered in OIML R60 or WELMEC 2.4 and should be discussed in a coming revision.

  17. Conclusion Thank you for your attention

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