25.02.2009
At present the OIML Recommendation R50 for belt weigher is being revised and adopted to current technical developments. Amongst others it is the objective to treat belt weighers with regard to EMC (electromagnetic compatibility) and testing against software aspects in the same way as other types of weighing instruments.
Belt weighers (BW), also named ”continuous totalising automatic weighing instruments”, are used for weighing bulk material. Due to their comparably low accuracy they are normally used for weighing cheap products such as sand and gravel. They form a special category of weighing instruments because they do not allow the determination of mass within the classical meaning. Instead of weighing a constant load being on the load receptor the development of the load is determined when the loaded belt is running over the load receptor. Thus the load is summed up over the belt length / time elapsed. The load receptors are normally subsequently installed into the conveyor belts. Either separate load receptors are used, on which belt pulley bearings are mounted, or the load receptors are originally designed as a belt pulley load receptor. This special kind of weighing instruments requires special test procedures, which are laid down in OIML R50 [1] of 1997. R50, however, is obsolete with regard to current EMC requirements (EMC = electromagnetic compatibility) and with regard to latest requirements for testing software, especially with systems based on open platforms (e.g. PC). Thus, R50 was decided to be revised even in 2007. The main part of the work had been finished in 2008 and the previously final draft will be submitted for voting to the corresponding working group in March 2009.
In addition to adapting EMC and software requirements to the standards of more recent OIML recommendations, e.g. R76/2006 [2] (nonautomatic weighing instruments), R51/2006 [3] (automatic catchweighers) and R107/2007 [4] (automatic discontinuous totalisers), a supplementary, higher accuracy class had been introduced. To achieve this accuracy class in practice it was necessary to introduce a new technical device to determine the belt profile over the total length of the belt, that is of the different dead loads of the empty belt. Furthermore, the modular approach, which is already used for all other automatic weighing instruments, was adopted for belt weighers. Belt weighers could not be tested as a whole even before now, e.g. on their susceptibility to influence factors (temperature, humidity, etc.). For that reason normally a so-called simulation setup was tested, composed of the metrologically relevant modules of a belt weigher (indicator, load receptor, displacement transducer). Because the weighing instrument yet works dynamically and an integration of the load over the belt length is required, respectively, there will only be a limited use of the modular approach. In the past PTB used it for load cells that were nearly identical in outer design and of which the inner structure (bending beam, shear bending beam, etc.) was the same. These were allowed to be alternatively used as substitute to the load cells of the simulation setup, however, under additional conditions to be observed, as defined in the type approval certificates.
The revision of R50 allows future extended use of the modular approach. At the same time introduction of a higher accuracy class opens the way to using these weighing instruments for weighing more expensive materials. Adoption of EMC and software requirements from recently published OIML Recommendations is a step forward to uniform treatment of all types of weighing instruments. This standardisation facilitates using modules that have already been tested according to another updated OIML recommendations and thus reduces the efforts for repeating tests.
[1] OIML R50-1 (1997), "Continuous totalizing automatic weighing instruments (belt weighers) , Part 1: Metrological and technical requirements - Tests", OIML, Paris
[2] OIML R76-1 (2006) "Non-automatic weighing instruments. Part 1: Metrological and technical requirements - Tests", OIML, Paris
[3] OIML R51-1 (2006) "Automatic catchweighing instruments. Part 1: Metrological and technical requirements - Tests", OIML, Paris
[4] OIML R107-1 (2007) "Discontinuous totalizing automatic weighing instruments (totalizing hopper weighers). Part 1: Metrological and technical requirements - Tests", OIML, Paris
Contact person:
Karsten Schulz, FB 1.1, AG 1.13, E-Mail: karsten.schulz@ptb.de