1 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Oliver Mack Physikalisch-Technische Bundesanstalt (PTB) Braunschweig 1 / 25 Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology INTI Buenos Aires Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Conference “Load Cell Tests by OIML R60” Conference “Load Cell Tests by OIML R60” Buenos Aires, June 7th to 11th, 2010

2 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients 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 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Difference between force transducer calibration and load cell testing Force transducer and load cells are Idential in construction design measurement technique Vary in the field of application metrological boundary consitions

4 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Difference between force transducer calibration and load cell testing Force transducer calibration (e.g. acc. ISO 376): Calibration certificate for a individual transducer with specified uncertainties (e.g. non-linearity, hysteresis, ect.) Load cell certification (e.g. acc. OIML R60) Test certificate for a load cell family or group of same material, design, measurement technique, same method of construction and set of specifications Based on the measurement results of one or several test pattern The specifications garantied in the data sheet of the manufacturer (keyword: results of „golden samples“)

5 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients 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

6 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Classification: Class D (ordinary accuracy weighing instruments): up to 1000 v Class C (balance for trade purposes): up to v Class B (precision balance): up to v Class A (special accuracy weighing instrument): more than v Verification interval v: Classification according to the recommendation OIML R60 Maximum permissible error (mpe): mpep LC x 0.5vp LC x 1.0vp LC x 1.5v class C0v  m  500v500v  m  2000v2000v  m  10000v p LC : apportionment factor, generally p LC = 0.7

7 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Requirements on load cells acc. OIML R60 - Repeatability - Repeatability error: Determined from: the maximum difference of three measurement cycles (for class C) for each temperature separately. The criteria is fulfilled if the maximum difference between identical load steps is not greater than the absolute value of mpe for than load

8 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Requirements on load cells acc. OIML R60 - Load cell error - Load cell error is defined as the difference between the output signal of a load cell under load and a straight line drawn between the minimum load output and the LC-output at a load of 75% of the measurement range taken on increasing the load at 20°C. Consequence: Several load cell errors such as non-linearity, hysteresis and zero point return are combined in one measurement curve

9 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Requirements on load cells acc. OIML R60 - Load cell error - Mpe for class C3 and the LC-error expressed in verification Intervals v as a function of the load for a 200kg-load cell

10 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Requirements on load cells acc. OIML R60 - Time depending effects - Creep forward: Criteria fulfilled if the change of the LC-output between loaded (  90% E max ) and initial load (  E min ) of the LC does not exceed 0.7 times the mpe for the applied load Definition: Dead load output return (DR): Difference between the outputs under initial load (  E min ) before creep forward and 10seconds up to 60 seconds after creep forward. Criteria: DR shall not exceed half the value of a LC verification interval definied by a ratio Z with Z  n max DR  E max / (2 · Z)

11 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Requirements on load cells acc. OIML R60 - Time depending effects - Error limits for creep and DR (Z=5000) as well as creep and DR of a C3-LC expressed as change of v and as function of time

12 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Abnormalitiies of LCs – uncovered acc. OIML R60 - Load transmission - Single-Point LC WELMEC 2.4: The load transmissions for beam LCs may be applied Uncovered according OIML R60: 1.Eccentricity effects (covered acc. OIML R76) 2.The material of the force transmission Beam LC with ball joint

13 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients 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 - Accoring to OIML R76 for weighing instruments the tests are carried out on 5 positions of the plattform

14 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients 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

15 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients 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. Single-Point LC Example: Single-point LC made of stainless steel with a load transmission made of the same material or aluminium.

16 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Requirements on load cells acc. OIML R60 - Temperature effect on MDLO - Relevance: Temperature effect on the output signal under a minimum dead load E min (e.g. E min = 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 v min for any change of ambient temperature of 5°C for class C. Definition v min : 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 = E max / v min

17 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Requirements on load cells acc. OIML R60 - Temperature effect on MDLO - Temperature effect on MDLO in v for a class C3-LC and the mpe for Y = as a function of the ambient temperature Stainless steel

18 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Abnormalitiies of LCs – uncovered acc. OIML R60 - Material of the load transmission - Temperature effect on MDLO in v for a class C3-LC and the mpe for Y = as a function of the ambient temperature Load transmission made of Stainless steel: Y = Aluminium:Y = 4500 Aluminium

19 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients 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.

20 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Abnormalitiies of LCs – uncovered acc. OIML R60 - Effects of temperature gradients - Point of discussion: Makes this sense ? 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 / h A spatial temperature gradient of < 0.2°C / 10 cm

21 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients 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 Time stability of MDLO:

22 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients 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

23 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients  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. Conclusion: Consequences for coming revisions

24 / 24 Introduction Eccentricity effects INTI Buenos Aires Oliver Mack: Recommendation For The Revision Of Test Procedures For Load Cells In Legal Metrology Material effects Test Procedure Conclusion Outlook Temperature gradients Conclusion Thank you for your attention T h a n k y o u f o r y o u r a t t e n t i o n