Matt Morrissey, Product Manager, Weighing Technology Calibration-approved belt scales according to MID Industries around the world use belt scales every day to weigh solid materials on conveyors gravel in an aggregates quarry or coal unloaded from a ship for a power plant, just to name a couple. If the material loaded from trucks and wagons or loaded/unloaded from ships is to be billed based on the belt scale data, the scale needs an approval and acceptance according to the Measuring Instruments Directive (MID) 2004/22/EC in Europe. In the US, the approval agency is NTEP, in Canada, Measurement Canada, and around the world a number of other agencies in various countries. Belt scale suppliers must prove their scale s measurement accuracy to agencies through comparative weighing operations. The notified agency then confirms the accuracy with a declaration of conformity. After this initial acceptance, the accuracy has to be proven again every two years. So, what s OIML? The International Organization of Legal Metrology (OIML) is an organization that internationally governs the legal aspects of metering. It issues measuring instrument recommendations, which are recognized in many countries and impact national standards DIN 8132 in Germany, for example. An OIML certificate confirms that a product such as a belt scale conforms to OIML recommendations. This includes test procedures for MID approval as well as for the initial acceptance of the installed unit. Although it is not a calibration approval, it serves as the basis to obtain one. siemens.com/weighing 1
Parts of a certified belt scale system When inspectors examine a scale for certification purposes, they must inspect each piece of equipment that makes up the belt scale system: The belt scale: The MID approval applies to the single idler belt scale MSI and the dual idler belt scale MMI-2. The model to be used is determined by the required accuracy class and the application data such as flow rate, belt speed and the conveyor characteristics. Specific requirements also apply to the belt conveyor. The integrator: Based on the load and belt speed indication, the Milltronics BW500 integrator calculates the respective flow rate and totalizes it, e.g. for truck loading. In the MID approved version parameters and calibration data are locked via a certification switch to avoid any modifications. The switch is located on the main board. Once the enclosure is sealed it cannot be accessed anymore. The MID version supports communications via Smartlinx, e.g. Profibus DP. The speed sensor: The SITRANS WS300 speed sensor measures the rotations of the tail pulley or bend pulley (provided as an option with adaptor for speed sensor). The Milltronics BW500 integrator uses its input to calculate the belt speed. Totalizer and limit indicators The enclosure includes two indicator lights for alarms when 100% of maximum rate is exceeded or for below 20% of maximum rate, as well as an additional totalizer. Whether or not the totalizer is resettable depends on the agreement with the relevant Office of Weights and Measures. The lights can be controlled via the Milltronics BW500 relay outputs and the totalizer via the totalizer output. Select a resolution of 10 or 100 kg, depending on your application. Alibi printer Delivery notes are usually printed by a DCS or PC. Despite this, each weighing operation must be logged directly from the integrator using an alibi printer. Either a continuous paper or a delivery note printer can be used for this. The documents from the alibi printer must be conserved in case the receiver doubts the customer's delivery note. Preparation of the "calibration acceptance" Request for conformity assessment: When commissioning the belt scale, it is recommended that users perform material tests comparable to the acceptance to make sure that the required measurement accuracy will be achieved. The relevant Siemens technician will then file an application for a conformity assessment (calibration acceptance) with the notified body. This notified body may be any Office of Weights and Measures. Ideally, users would choose the closest one; however, in some European countries it may be necessary to choose a notified body in a neighbor country. As soon as possible, a date should be arranged with the customer, the Office of Weights and Measures and Siemens TechSupport for the acceptance, but only after having determined the measurement accuracy of the belt scale using material tests. It should also be clarified in advance who will assume the costs for the calibration officer, the calibration vehicle and the Siemens technician for the commissioning. Accuracy classes There are 3 accuracy or calibration classes (see table). The calibration error limits are the admissible measurement errors (±) for acceptance and recalibration. The maximum errors permitted in service apply to regular operation. CARRYING IDLERS SKIRTBOARDS HOPPER BELT SCALE HEAD PULLEY TAIL PULLEY IMPACT IDLERS RETURN IDLERS BEND PULLEY VERTICAL GRAVITY TAKE-UP SNUB PULLEY Part of a certified belt scale system, the speed sensor measures rotations on either the tail pulley or bend pulley, as seen above. 2
Becoming familiar with how belt scales are certified is beneficial to anyone using weighing technology In practice, Class 2 is for excavated earth like sand, and Classes 1 and 0.5 are for higher-grade materials, such as grains or coal. Minimum totalized load Based on maximum rate, belt speed, and belt length, the minimum loading amount, or minimum totalized load can be obtained. It corresponds to the minimum amount required for a material test. Weigh this minimum amount specifically during the commissioning of the belt scale and preparative tests. The smallest test load is the bigger of the following values: 2% of the totalized load with maximum rate during one hour The totalized load with maximum rate over one belt revolution Maximum live load (maximum load) / 3000 * 200 (Cl. 2) or * 400 (Cl. 1) or * 800 (Cl. 0.5) Example: For calibration class 1, 300 t/h maximum rate, 2 m/s belt speed and 300 m belt length (unrolled belt), the minimum load is 12.5 t. Calibration acceptance Verification of the truck scale: The calibrated truck scale used for the material tests must be verified before the belt scale acceptance using calibrated weights according to the minimum load (see above). Depending on the calibration officer, this verification can be performed either on the same day or within a maximum of two weeks before the belt scale acceptance. It includes the verification of linearity and measurement accuracy. For the reference measurements, the display resolution should be increased to one kilogram. This may require a password. Zero verifications At the beginning of the calibration, diverse zero verifications are performed. Please ensure the totalizer dropout of the Milltronics BW500 is deactivated set P619 to "0". By doing so, negative loads are subtracted from the current count. Before the verifications, run the conveyor for about 30 minutes to warm up the belt and ensure that it is empty; the duration is about three belt revolutions. From lighter loads to extremely heavy aggregates, the Siemens Milltronics MSI belt scale can be certified for a number of uses in industry. 3
The Siemens Milltronics MMI belt scale, the Milltronics BW500 integrator, and the SITRANS WS300 speed sensor are all part of a certified belt scale system. The subsequent zero and material tests are described in DIN 8132. This directive is based on the OIML R50 which is the basis for the MID approval. Error limits for zero setting With the conveyor belt running empty, the maximum admissible deviation is 0.05% (for Cl. 0.5), 0.1% (Cl. 1) or 0.2% (Cl. 2) of the totalized load for the maximum rate over the measured period. Example Cl. 1: 350 t/h over 133 sec => totalized load 12.9 t => admissible totalization maximum of 13 kg Speed of response zero display A test load is now applied to each Milltronics MSI belt scale. It is 0.05% (for Cl. 0.5), 0.1% (Cl. 1) or 0.2% (Cl. 2) of the maximum load. With the test load applied and conveyor running empty, there must be a detectable difference compared to the unloaded belt. Example Class 0.5 with Milltronics MMI: 700 t/h, 1.67 m/s, 2.4 m weigh area => load = 140 kg => test load = 140 g (= 70g per MSI) Verification of zero setting for small totalized loads If the minimum totalized load does not meet more than three belt revolutions at maximum rate, the totalized values from the verification Error limits for zero setting at the beginning as well as the minimum and maximum value of the verification may deviate a maximum of 0.18 % (Cl. 0.5) or 0.35 % (Cl.1) of the totalized load at maximum rate over the measured period. Accuracy tests The totalizer dropout in the Milltronics BW500 may be activated again for example, 2% (setting P619 = 2 ). Also run the conveyor for 30 minutes to warm up the belt. The linearization function of the Milltronics BW500 can be used. An adjustment is allowed, but all verifications have to be repeated in that case. Material Tests The following tests are performed with constant belt speed (± 5%). For varying or adjustable belt speeds additional material tests are required, see below. The following material tests are performed: Two inspection pairs (total of four) with 20-35% of maximum rate Two inspection pairs (total of four) with approximately 100% of maximum rate One inspection pair (total of two) with approximately 50% of maximum rate The admissible deviations from totalizing are: Maximum permissible tolerance for Class 0.5: ± 0.25 % for all comparative measurements Maximum permissible tolerance for Class 1: ± 0.5 % for all comparative measurements Maximum permissible tolerance for Class 2: ± 1.0 % for all comparative measurements Additional material tests with variable belt speeds: When the belt speeds vary (deviation > ± 5%), the tests must be performed for each belt speed (e. g. 0.5 m/s, 1.31 m/s or 1.68 m/s adjustable). For adjustable belt speeds, three additional individual verifications are required, at each of the three feed rates. During www.siemens.com/weighing 4
Two Milltronics MSI belt scales mounted in tandem make the Milltronics MMI system, with trade-approved accuracy of ± 0.25%. each verification, the belt speed must be changed over its entire range (e.g. adjustable between 0.5 m/s and 1.68 m/s). Nameplates On a Siemens belt scale, next to the nameplate, the scale has an adhesive label with the relevant application data according to MID requirements. After all of this, the belt scale is certified and ready. Becoming familiar with how belt scales are certified is beneficial to anyone using weighing technology helping to decrease some of the mystery that surrounds these crucial weighing devices. A certified scale After successful verification, a yellow sticker (Office of Weights and Measures ID number) and a red sticker ("calibration stamp") are applied to the integrator, belt scale, junction box, and, if applicable, to the limits value/totalizer display. The Siemens technician then receives a conformity declaration from the Office of Weights and Measures stating that the installed conveyor belt scale meets the metrological requirements of the MID directive 2004/22/EC. Finally, the regional Siemens technician, as the introducer of the belt scale, also provides the customer with an equivalent conformity declaration. It includes the name of the notified body having performed the conformity assessment. Recalibration Recalibrations are required after two years, but the recalibration procedure is identical to the initial calibration, i.e., even the truck scale must be verified. Therefore the recalibrations of belt scale and truck scale should be combined, if possible. Siemens AG Process Industries and Drives Östliche Rheinbrückenstraße 50 76187 Karlsruhe Germany Subject to change without notice Available as pdf only Siemens AG 2016 The information provided in this article contains merely general descriptions or characteristics of performance which in case of actual use do not always apply as described or which may change as a result of further development of the products. An obligation to provide the respective characteristics shall only exist if expressly agreed in the terms of the contract. All product designations may be trademarks or product names of Siemens AG or supplier companies whose use by third parties for their own purposes violate the rights of the owners. www.siemens.com/weighing 5