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Proposals and points of discussion for the planned revision of the OIML R60

Proposals and points of discussion for the planned revision of the OIML R60. Sascha Mäuselein Oliver Mack. Table of contents. Test procedures and requirements Test procedures and requirements for load cells equipped with electronics Load transmission Test and environmental conditions

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Proposals and points of discussion for the planned revision of the OIML R60

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  1. Proposals and points of discussion for the planned revision of the OIML R60 Sascha Mäuselein Oliver Mack

  2. Table of contents • Test procedures and requirements • Test procedures and requirements for load cellsequipped with electronics • Load transmission • Test and environmental conditions • Definitions, further requirements anddocumentation • Quality of the certificate

  3. 1. Test procedures and requirements • Continuous acquisition of the LC signal during Creep Test and determining dead load output return - DR • Time interval for the DR • Creep criterion for the CC(30-20) value • Additional test of eccentric load • Excitation voltage • CH test • Extension of temperature range • Settling effects of the load cell and load transmission

  4. 1.a) Continuous acquisition of the LC signal during Creep Test and determining DR Background: To determine the creep error and DR a continuous and periodic record of the indicating instrument is required. The characterization of creep effect and DR due to single measurement results on selected and fixed time intervals is not sufficient to characterize the individual behaviour of load cells. Problem: Several load cells fulfill the requirements according to R60 if creep and DR are characterized by single measurement results at fixed time intervals as given in Table 6 (see R60, 5.2.3). But with the additional information acquired from a continuous acquisition of the measurement data, the load cell may not meet the requirements. Proposal: Continuous acquisition of measurement data is state of the art. So a continuous acquisition of the LC signal during the Creep Test (creep error and DR) should be acknowledged ( by replacing the second sentence in chapter A.4.2.8 by “Continue to record periodically thereafter, at least one value in at most five seconds.” and by adding this sentence in chapter A.4.3.10.)

  5. 1.b) Time interval for the DR Background: To determine minimum dead load output return (DR) one value of the indicating instrument at a specified time is recorded. Problem: By taking just one value it is not possible to decide if the output is stable or not. Often load cells show a strong creep behavior within a few minutes after a load change that has to be estimated. Proposal: The requirements of the DR should be kept during a time interval from the time specified in Table 6 (see R60, 5.2.3) and time specified in Table 6 (see R60, 5.2.3) multiplied by a factor of 5. This requirement demands a continuous acquisition of the LC signal during the creep test (see point 1a.)

  6. 1.c) Creep criterion for the CC(30-20) value Background: The difference between the reading obtained at 20 minutes and the reading obtained at 30 minutes shall not exceed 0.15 times the mpe (see 5.3.1.1). Problem: The definition of mpe is not clear in this context. The definition of mpe for accuracy tests of LCs is given table 5. In 5.3.1 a smaller mpe for creep (0.7 ·mpe) is defined. The criterion evaluates the decay of creep effects to archive a stable measurement signal after 30 minutes. Admittedly if the mpe (see table 5) for accuracy tests is taken into account the criterion defined in 5.3.1 is not sufficiently high. Proposal: The mpe for creep defined in 5.3.1 should be used as criterion. The last sentence in 5.3.1 should be changed in: The difference between the reading obtained at 20 minutes and the reading obtained at 30 minutes should not exceed 0.15 times the 0.7 times the absolute value of the mpe given in table 5.

  7. 1.d) Additional test of eccentric load Background: OIML R76 (2006) for weighing instruments defines eccentricity tests (see R76, A.4.7 and 3.6.2). Equivalent tests for load cells are not designated. Problem: Single Point load cells are used as a module within weighing instruments. Currently, eccentricity tests are not provided for load cells. Proposal: If a load cell is to be used as single point load cell, the manufacturer has to define the maximum allowable dimension of the platform. Eccentricity tests comparable to OIML R76 (2006), A.4.7 and 3.6.2 should be added in the OIML R60 after chapter 5.4 and in A.4 respectively C.2, as optional test.

  8. 1.e) Excitation voltage Background: Most load cell manufacturers define a range of excitation voltage of DC or AC within the LC can be used. Problem: There is currently no assurance that a LC fulfills the criteria of R60 within the whole range of excitation voltage as defined by the manufacturer. As shown in the diagrams that follow, the excitation voltage affects the creep behaviour of load cells and shall be taken into account. At left: Results of a creep test of the same LC at 40°C and excitation voltage of 5V. In this instance the LC passed the test. At right: excitation voltage was changed to 10Vand in this test the LC failed DR

  9. 1.e) Excitation voltage For some special LCs with more than one output channel the signal shape and the carrier frequency of the excitation voltage affect the test result. Up to now there have been no test procedures defined in OIML R60 which consider the effects of a variation in the excitation voltage. Proposal: The manufacturer defines a recommended excitation voltage and whether AC or DC should be used for tests according to OIML R60. The admissible excitation voltage of the LC is limited to a range of ±30%. For larger ranges of exitation voltage, additional tests (accuracy and creep) are required.

  10. 1.f) CH test Background: After humidity testing, the load cells are removed from the humidity chamber. For a long period of time afterwards, the load cell must be maintained in standard atmospheric conditions to attain temperature stability (normally 1 to 2 hours). Problem: No maximum time interval is specified in which the measurement has to be started after removing the load cell from the humidity chamber. It has to be taken into account that the effects of humidity could not be determined accurately if this period of time is too long. Proposal: After removing the load cell from the humidity chamber it should be required that the tests must be carried out within 12 hours.

  11. 1.g) Test for hysteresis Background: Hysteresis is an important criterion to characterize a strain gauge load cell. Problem: There is currently no criterion defined in OIML R60 to characterize hysteresis effects. Proposal: The maximum hysteresis of the LC should be determined at all tested temperatures. The maximum hysteresis should not be greater than 0.5 mpe.

  12. 1.h) Extension of temperature range Background: There are increasing applications for load cells under legal control with an extended temperature range. Accordingly it should be possible to extend the temperature range during testing, for instance from -40°C to +70°C. Problem: Which temperature should be used for the accuracy and creep test? Is it sufficient to test at a minimum and maximum temperature and at 20°C? Is it possible to define a temperature range which does not include standard atmospheric conditions of 20°C? Should the tests be carried out with more than three temperatures if an extended temperature range is defined by the manufacturer? Proposal: The accuracy and creep tests have to be carried out with the maximum and minimum temperature of the temperature range and a temperature as close as possible to the midpoint of the temperature range.

  13. 1.i) Settling effects of the load cell and load transmission Background: OIML R60 has no requirements concerning settling effects of the load cell and the load transmission. Problem: Experience offers that by repeating the temperature test several times, some load cells change their characteristics. This may be observed when a load cell fails the initial two temperature tests but then passes during a third test under identical metrological end environmental conditions. Proposal: For taking into account settling effects and long time effects the measurement results of the first and second accuracy test at reference temperature should be compared. An adequate repeatability criterion has to be defined. The minimum dead load output (MDLO, see OIML R60, A.4.1) between the first measurement and the second measurement at reference temperature usually at 20°C should be limited (for example 0.5v). Up to now there is no criterion defined for the repeatability of MDLO at reference temperature.

  14. 2. Test procedures and requirements for load cells equipped with electronics • New requirements of EMC for digital LCs • Software requirements for digital LCs • Span stability • Characteristic line of digital LCs

  15. 2.a) New requirements of EMC for digital LCs Background: The requirements of EMC for weighing instruments have changed (OIML R76 (2006), B.3). Problem: There is a discrepancy between the requirements for weighing instruments according OIML R76 (2006) and the requirements for load cells equipped with electronics according OIML R60 (2000). Proposal: The requirements and test procedures for EMC tests as defined in OIML R76, B.3 for non-automatic weighing instruments should be integrated in OIML R60.

  16. 2.b) Software requirements for digital LCs Background: Generally the metrological characteristic of a digital LC is realized by software with adjustable LC parameters in principle. This software can be fixed but it is also possible to change software components or the whole software via download. Problem: Some digital LCs offer the possibility to change the LC parameters via interface with a terminal or indicator or by incorporating a software download. It is expected that in future more functionality is implemented in digital LCs. The load cell becomes more and more an intelligent senor with functionalities of a weighing instrument. Proposal: A topic concerning software requirements for digital LCs should be added in the OIML R60. The requirements concerning software, software security and documentation of software should reflect the requirements in OIML R76 and the guide WELMEC 2.3 "Guide for examination software" and software guide WELMEC 7.2. An OIML R60 certificate for a load cell is only valid if a change and / or download of software and new parameters are not possible. The software has to be documented completely with all functions and interfaces included.

  17. 2.b) Software requirements for digital LCs Remark: Is it meaningful in general to certify digital load cells according to OIML R60? There are no standardized data interfaces. These types of load cells only work together with particular, specialized indicators or terminals. Would it not make sense to test and certify these two modules (load cell and indicator) together as complete weighing instrument or weighing module? This problem becomes more complex if you take into account that digital load cells with software offer intelligent functions which strongly affect the output signal. Additionally, state of the art digital load cells permit the change of software via download. The requirements on software security for legal metrology according to OIML D31 or OIML R76 e.g. do not apply to the load cell and are only verifiable with an associated indicator. In this case the test procedure equates more or less with the requirements in OIML R76 applicable to a complete weighing instrument.

  18. 2.c) Span stability Background: The OIML R60 requires a span test which includes all applicable tests. The test duration must not exceed 28 days. Problem: The requirement of “all applicable tests” includes temperature tests as well as EMC tests. In combination with cyclic humidity test (OIML R60, A.4.5) consisting of 12 temperature cycles of 24-hour duration each, the span stability test is hardly practicable within 28 hours. There are discrepancies between the test procedures for span stability according OIML R60 and OIML R76, B.4. In OIML R76 the performance tests include only temperature tests and, if applicable, the damp heat test. Other performance tests (e.g. EMC) may be performed. Proposal: The included tests for span stability should also be listed exemplarily in OIML R60. Furthermore, the EMC tests should either be excluded as described in OIML R76 or EMC tests should be included and the maximum test duration should be increased to 40 days.

  19. 2.d) Characteristic line Background: Digital compensation algorithms of digital LCs for linearization generally work with few measuring points. Problem: If the accuracy tests are performed on limited measurement points, the requirements of OIML R60 must be fulfilled. Qualified statements about the metrological characteristic of the LC are not possible due to the limited criteria. It is conceivable that the non-linearization leads to discontinuities of the characteristic line or that the requirements are mismatched between the measurement points of the compensation algorithms. Proposal: The manufacture has to describe the compensation algorithm in principle and give information about the measurement points. The accuracy tests of the load cell shall carried between these measurement points for compensation. Additionally the accuracy tests can be carried out with any preload.

  20. 3. Load transmission Background: In OIML R60 no defined requirements concerning load transmission devices exist. Problem: For some LCs the load transmission has a strong influence on the test result. Influences due to the design characteristics of the force transmission device and the variation of the clamping torque and the material of the load transmission device could lead to different test results. The figure shows the test results for a LC with ball joint (left hand side) and a single point load introduction (right hand side). Results of a pattern test for the same LC. Left hands: ball joint load transmission, the LC passed the test. Right hand side: The same LC with single point load transmission, the LC failed the test.

  21. 3. Load transmission As shown in some cases the load introduction is a necessary condition of a load cell to fulfill the requirements of R60. In these cases manufacturers have to postulate special and fixed load transmission devices for the OIML certificate of conformity. This means that the specification of the load transmission device (design, clamping torque, material) is an essential part to fulfill the requirements according R60. Proposal: A manufacturer should describe all possible designs or principles of load transmission devices for a load cell or load cell family (technical drawing or in principle drawings, range of torque, material specifications) with which the LC is in conformity with the requirements defined in R60. If the load transmission is not critical for the load cell the manufacturer should declare that a defined standard load transmission can be used. Our proposal is a new chapter added between chapter 4 and 5 of R60 which deals with the topic of load transmissions (see WELMEC 2.4 “Guide for Load Cells”).

  22. 3. Load transmission X.1 Standard load transmission devices The following tables identify different types of LCs, (compression, tension, etc.) and typical load cell mounting devices suitable for them. The symbols below classify the mobilitybetween one point of contact on the load cell and its counterpart on the load receptor or mounting base.

  23. 3. Load transmission

  24. 3. Load transmission X.2 Load transmission devices for the certified LC X.2.1 Standard load transmission devices The manufacturer has to declare which load transmissions defined in X.1 should be used with the LC. Additional restricting information for example a specified clamping torque or specified materials used as load transmission has to be declared by the manufacturer if necessary. X.2.2 Special load transmission devices Load transmission devices not defined X.1 has to be specified in detail. X.3 Load transmission devices for the test The test laboratory is free to decide which of the valid load transmissions is used for the test. The load transmission with the highest requirements should be chosen. For load transmissions not defined X.1 the test laboratory is free to decide if one or more load transmissions have to be tested. X.4 Load transmission devices valid with the OIML certificate of conformity The OIML certificate of conformity is valid for the load transmissions, if applicable with restrictions, as declared by the manufacturer and written in the Report of the OIML

  25. 4. Test and environmental conditions • Temperature stability • Thermal gradients within the load cell • Time of stable conditions before starting the test • Change of environmental conditions

  26. 4.a) Temperature stability Background: The maximum difference between the temperatures during the test is fixed to 2°C. Nothing is written about cyclic changes of temperature. Problem: For some LCs the test results differ in dependence of the used temperature chamber because the requirements are not fixed sufficiently exact. For some LCs cyclic changes of temperature in the range of 2°C lead to a different test result compared to more stable temperature conditions. The stability of state of the art temperature chambers is better than 0.2°C. Proposal: To use common temperature conditions the environmental conditions should be specified in the following which corresponds with state of the art temperature chambers: The maximum difference between the temperatures during the test should not exceed a value of 0.5°C.

  27. 4.a) Temperature stability Remark: Is it functional to start a discussion about the relevance of the criteria “Stability of Temperature”? On the one hand it is essential to define a criterion for the stability of temperature to ensure a high reproducibility of the test results. On the other hand a high stability of temperature is contrary to the normal environmental conditions of a weighing instrument. So there are also good reasons to define a variation of temperature during the test under which the load cell has to fulfil the requirements. This should be a point of discussion. The figure below shows an example for the influence of temperature gradients. The diagram shows both the ambient temperature between 39°C and 40°C and MDLO of a LC expressed as change of the verification intervals v under a relative humidity of 30 % and 85 % as a function of time. Under 40°C temperature conditions and a relative humidity of 85 % the investigations document a strong correlation between a temperature variation of 1°C and MDLO. With a change in the order of 2v the mpe of the LC is failed by far. Under similar temperature fluctuations in the same range of 1°C but under reduced relative humidity conditions of 30% MDLO passed the requirements clearly. Since temperature fluctuations in a range of 1°C are common environmental conditions of a weighing instrument e.g. used on a farmer’s market a LC showing the characteristics of the figure is not qualified for legal metrology. On the other hand it is questionable if this characteristic is detectable under optimised laboratory conditions with temperature und humidity fluctuations as small as possible.

  28. 4.a) Temperature stability Ambient temperature between 39°C and 40°C and MDLO of a LC expressed as changeof the verification intervals v under a relative humidity of 30 % and 85 % as a function of time.

  29. 4.b) Thermal gradients within the load cell Background: There is no criterion defined in R60 for spatial thermal gradients within the load cell R60 gives only the information that no significant thermal gradients within the load cell should be introduced by the loading system. But the word “significant” gives space for interpretation. Problem: Test machines always induce spatial thermal gradients between mounting plate and load introduction of the LC. The spatial thermal gradients are an individual characteristic of the test machine for instance, if the load is realized with masses outside of the temperature chamber. Also the load transmission devices and integrated thermal barriers for example, affect the spatial thermal gradient. This leads to different results for diverse LC testing machines if the LC is influenced by thermal gradients. Reproducibility is not warranted. Proposal: The maximum value of the thermal gradient between mounting plate and load introduction of the LC should be fixed to a value of 0.5°C.

  30. 4.b) Thermal gradients within the load cell Remark: Is it functional to start a discussion about the relevance of the criteria “Thermal gradients within the load cell? On the one hand it is essential to define a criterion for the thermal spatial gradient to ensure a high reproducibility of the test results. On the other hand a spatial thermal gradient optimized under laboratory conditions and as small as possible is contrary to the normal environmental conditions of a weighing instrument. So there are also good reasons to define a variation of spatial temperature gradients under which the load cell has to fulfil the requirements. This should be a point of discussion.

  31. 4.c) Time of stable conditions before starting the test Background: A stabilization period for the LC is required, as recommended by the manufacturer (A.3.2.6). And before starting the test a stable output is required (A.4.1.5 / A.4.2.5). Problem: These two requirements could lead to a misunderstanding. The first requirement is given by the manufacturer and normally defined as short as possible. The second requirement is in the response of the test laboratory and normally essentially long to guarantee a temporally temperature stability as much as possible. Furthermore the word “stable” gives space for interpretation. Proposal: The criterion stable output should be a specified as output within 1/10 of the LC verification interval for at least 30 minutes for the accuracy test (chapter A.4.1.5) and for at least 60 minutes for the creep test (chapter A.4.2.5).

  32. 4.d) Change of environmental conditions Background: Tests shall be performed under stable environmental conditions. The ambient temperature is deemed to be stable when the difference between extreme temperatures noted during the test does not exceed one fifth of the temperature range of the LC under test, without being greater than 2°C. Problem: A change of environmental conditions from time of stabilization to time of measurement could affect the test result. For example, the strength of the recirculation air within the climatic chamber has an influence on the test result of some LCs. To reduce this influence the strength of the recirculation air is often reduced after the stabilization period and before the measurement. This affects the thermal equilibrium conditions and is an individual characteristic of a temperature chamber. Reproducibility between different chambers is not warranted. Admittedly a load cell should fulfill the requirements under all defined environmental conditions and thus also under recirculation air conditions. Proposal: It should not be allowed to change the strength of the recirculation air during the time of temperature stabilization and measurement. If the LC is influenced by the recirculation air of the climatic chamber a wind deflector should be used. The manufacturer should make a reference that this LC only can be used within a closed body without air circulation.

  33. 5. Definitions, further requirements and documentation • Definition of a load cell family • Definition of minimum LC requirements • OIML Report • Picture of the LC mounted in the experimental setup

  34. 5.a) Definition of a load cell family Background: The certification of load cells according to OIML R60 allows concentrating load cells in one load cell family to reduce the number of tests. One of the requirements for load cells of one load cell family is the “same shape” of the load cells. Problem: The interpretation of the requirement "same shape" is not consistent and leaves room for interpretation. While some accept the outer dimensions, PTB assumes that the area of thin places, the geometry in the area of fixing and the realization of the load introduction all have metrological relevance and thus have to be taken into account for the definition of “same shape” and a load cell family. Examples for load cells with identical outer dimensions but different geometries are shown below. Do these load cells have the same shape and thus belong to one load cell family?

  35. 5.a) Definition of a load cell family A) Difference of geometry in the area of thin places (e. g. round or oval drilling) B) Difference of geometry in the area of fixing / load introduction (e. g. groove, base, offset) C) Difference of geometry in the inner of fixing / load introduction (e. g. drilling, thread, dropping) Proposal: Some examples for the definition “same shape” should be fixed in the new R60. Examples A, B, C do not offer the same shape.

  36. 5.b) Definition of minimum LC requirements Background: In OIML R60 there are metrology requirements defined for load cells. Problem: Some LCs fulfill the metrology requirements and recommendations defined in OIML R60 under optimized laboratory conditions but they are not suitable for practical use in weighing instruments due to lack of robustness. Proposal: Minimum LC requirements of robustness (e.g. for cable connections) and minimum constructive requirements should be defined to avoid failures of LCs and to guarantee a minimum standard for the practical use of LCs in weighing instruments.

  37. 5.c) OIML Report Background: An OIML R60 certificate of conformity includes associated Test Reports but not an OIML Report. Problem: Not all significant technical information (e.g. technical drawings or pictures, material properties, cable connections, specifications of the load transmission) is given in the OIML R60 certificate of conformity or the test reports. Especially the identification of certified LCs was not well-defined or possible in all certificates in the past. Neither in the Test Report nor the certificate technical documentation was referenced or available. The subsequent identification of certified LCs is not possible neither for owner of a certificate nor the issuing authority. Proposal: Implementation of a Report (comparable to the Report in R76) with essential technical information of the LC which may include: a technical drawing and picture, geometries, material, cable connection, load introduction, mounting moments and all essential information given in the data sheet.

  38. 5.d) Picture of the LC mounted in the exp. setup Background: In the Test Reports less information of the mounting of the LC within the experimental setup and of the load introduction during the test is given. Problem: For some LCs the load transmission has a high influence on the measurement result. It should be possible to reconstruct the test conditions. Proposal: Implementation of pictures in the Test Report which shows the LC mounted in the experimental setup and documents the installation situation of the LC under test.

  39. 6. Quality of the certificate • Problem “Golden Sample”

  40. 6.a) Picture of the LC mounted in the exp. setup Background: There are no requirements defined in OIML R60 to prevent that certificates of conformity for OIML R60 are issued for golden samples. Problem: There is reason to believe that load cells from production often failed the requirements for which they are certificated. The responsibility of manufacturers to meet the requirements specified in the data sheets is not necessarily satisfied and guaranteed. In Europe the Measurement Instrument Directive 2004/22/EC (MID) demands that manufacturer of measuring instruments concerned declare the conformity to type based on quality assurance of the production process. Likewise in MID, Annex D, No. 1 it is written: “Declaration of conformity to type based on quality assurance of the production process is the part of the conformity assessment procedure whereby the manufacturer fulfills the obligations laid down in this annex and ensures and declares that the measuring instruments concerned are in conformity with the type as described in the EC-type examination certificate and satisfy the appropriate requirements of this Directive”. A manufacturer of measuring instruments according to MID has to give the declaration of conformity also for modules of the measuring instrument and thus also for load cells.

  41. 6.a) Picture of the LC mounted in the exp. setup Up to now only automatic weighing instruments are covered by MID. But similar requirements will be implemented in all other directives and also in the directive 2009/23/EC for non-automatic weighing instruments (NAWI). This may not be in the scope of the technical requirement in OIML R60 but requirements for production monitoring of load cells should be demanded and defined by OIML. Otherwise the significance of OIML R60 certificates is at risk and could be put in question in the future in Europe.

  42. 6.a) Picture of the LC mounted in the exp. setup • Proposal: • It is not realistic to implement procedures for the quality assurance of the production process of load cells. The following suggestions make it more difficult to obtain certification through the evaluation of only a golden sample: • A label with all essential information (Emax, class, Y, Z, temperature range, etc.) should be required to be permanently fixed on the load cell. An accompanying document is insufficient. • Manufacturers must provide their own measurement results for the load cells that have received certificates and describe the test facilities used for tests. As far as possible the results have to fulfill the requirements of OIML R60. The manufacturer’s results and his facilities used for testing certified load cells should become part of the OIML Test Report. • The time period for which the OIML certificate of conformity is valid should be limited. Currently, the certificates are valid for unlimited time. A limitation of this time interval (e.g. 5 to 10 years) would be useful to check for changes in the load cell caused by innovations in design, new production processes, or materials used in the manufacturing process. At a minimum, manufacturers should provide their own measurement results for the load cells that have received certificates and describe the test facilities used for tests.

  43. Thank you for your attention Proposals and points of discussion for the planned revision of the OIML R60 Sascha Mäuselein Oliver Mack

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