Calibration of Weighing Instruments Conference Centre, Centurion August 1st Denis LOUVEL, MT-SI denis.louvel@mt.com Calibration of Weighing Instruments Measurement Uncertainty
Agenda 1 What is the most important? 2 Risk management 3 Uncertainties related to a weighing 4 Physics behind a weighing result 5 Safe Weighing Range 6 7 8 9 To shift Highlight Click the arrow, press shift key and move top or bottom For internal use - Confidential
Product or balance? What is the most important the product or the balance? Do you need a compliant product or a compliant balance? For internal use - Confidential
Product or balance? What is the most important the product or the balance? Do you calibrate the balance for the balance? Do you calibrate the balance for the product? For internal use - Confidential
Product or balance? What is the most important the product or the balance? Are you sure that the content of your current calibration certificate helps? For internal use - Confidential
Agenda 1 What is the most important? 2 Risk management 3 Uncertainties related to a weighing 4 Physics behind a weighing result 5 Safe Weighing Range 6 7 8 9 To shift Highlight Click the arrow, press shift key and move top or bottom For internal use - Confidential
Risk management Preferential regime? We can assess the balance uncertainty since more than 20 years. Nevertheless, the classic verification for a balance is to compare the error against the maximum permissible error: Error Maximum permissible error (MPE) This assessment is the same as requested by the Legal Metrology For other measuring instruments (e.g. thermometers) the uncertainty is always taken into account in assessing compliance of the instrument: Error + Uncertainty Maximum permissible error (MPE) It seems that balances are entitled to a preferential regime and can be assessed according to outdated criteria For internal use - Confidential
Risk management Decide what is the best for you Knowing that a balance has an error smaller than the MPE, that's good. Knowing the measurement uncertainty is much better. Ignore uncertainty when you weight the product is risky. For internal use - Confidential
Agenda 1 What is the most important? 2 Risk management 3 Uncertainties related to a weighing 4 Physics behind a weighing result 5 Safe Weighing Range 6 7 8 9 To shift Highlight Click the arrow, press shift key and move top or bottom For internal use - Confidential
Uncertainties related to a weighing 3 uncertainties, not only 1 3 uncertainties are involved when you are weighing your product: The uncertainty of the error of indication The uncertainty of the balance The uncertainty of the product weighed on the balance For internal use - Confidential
Uncertainty of the error of indication All these properties contribute to the uncertainty of the error of indication In the first level, parameters below are taken into account for the determination of the uncertainty of the indication error U(EI): Repeatability of the weighing Resolution of the balance (at zero and on load) Load eccentricity Ambient temperature during calibration (sensitivity coefficient of the balance to temperature) Standards weights (uncertainty + durability) Basically, every balance has one important nominal property, the so-called readability. Readability accounts for the rounding error of a digital indication. This balance property is not measured but its contribution to the overall measurement uncertainty is given by the number of digits after the comma and thus is a fixed value. (click) A digital weighing instrument has four important measurement properties that contribute to the overall measurement uncertainty. These properties are The sensitivity The nonlinearity The eccentricity, or corner load And the repeatability These properties are briefly explained in the next slides. There are other properties besides these four measurement properties and the readability as a nominal property, but only these contribute significantly to the overall measurement uncertainty of the respective weighing system.
The accreditation domain The balance has not only 1 uncertainty, but one per load The accreditation covers only the uncertainty of the error of indication For internal use - Confidential
Uncertainty of the Balance All these properties contribute to the balance uncertainty A second level is necessary to take into account how the balance is used, installed, set: the use or not of the internal adjustment device the correction or not of the errors of indication EI the temperature range in the room where the balance is used the eccentricity With these situations, we can define the balance uncertainty from which the user obtains the uncertainty for each weighing he carries out. Basically, every balance has one important nominal property, the so-called readability. Readability accounts for the rounding error of a digital indication. This balance property is not measured but its contribution to the overall measurement uncertainty is given by the number of digits after the comma and thus is a fixed value. (click) A digital weighing instrument has four important measurement properties that contribute to the overall measurement uncertainty. These properties are The sensitivity The nonlinearity The eccentricity, or corner load And the repeatability These properties are briefly explained in the next slides. There are other properties besides these four measurement properties and the readability as a nominal property, but only these contribute significantly to the overall measurement uncertainty of the respective weighing system.
The accreditation domain No information for u(IP) The accreditation covers only the uncertainty of the error of indication The accreditation never covers the uncertainty of the balance For internal use - Confidential
Uncertainty of the weighing of the final product All these properties contribute to the balance uncertainty A last level is necessary to finally know the influence of the balance on the product: the uncertainty of the balance the correction or not of the buoyancy effect With this information, we can finally assess the product conformity. Basically, every balance has one important nominal property, the so-called readability. Readability accounts for the rounding error of a digital indication. This balance property is not measured but its contribution to the overall measurement uncertainty is given by the number of digits after the comma and thus is a fixed value. (click) A digital weighing instrument has four important measurement properties that contribute to the overall measurement uncertainty. These properties are The sensitivity The nonlinearity The eccentricity, or corner load And the repeatability These properties are briefly explained in the next slides. There are other properties besides these four measurement properties and the readability as a nominal property, but only these contribute significantly to the overall measurement uncertainty of the respective weighing system.
The accreditation domain No information The accreditation covers the uncertainty of the error of indication The accreditation never covers the uncertainty of the balance The accreditation doesn't help you to calculate the buoyancy effect (ncpa) and the uncertainty of the weighing result, U(M) For internal use - Confidential
Agenda 1 What is the most important? 2 Risk management 3 Uncertainties related to a weighing 4 Physics behind a weighing result 5 Safe Weighing Range 6 7 8 9 To shift Highlight Click the arrow, press shift key and move top or bottom For internal use - Confidential
Physics Behind A Weighing Result The balance is adjusted / verified / calibrated with weights and provides indications as if the weighed object has a density identical to the weights All balance users know the buoyancy effect (Archimedes' principle), but none of them applies a correction. They all think that the weight displayed by the balance is the true mass of the object and no correction is necessary. Unfortunately, this is totally wrong and this can question the quality of the weighing result. When balance is adjusted with standard weights, both equilibriums carried out to weigh an object (at zero and on load), leads to the following relation: 𝑀×(1 –𝑎/𝑟)=(𝑥–𝐸𝐼)×(1–𝑎/𝑟0) Users of balances do not apply a correction related to the buoyancy effect. The correction is not necessary as long as it doesn't jeopardize the compliance of the product. Before to decide to neglect the correction, it is necessary to know it. For internal use - Confidential
Air Buoyancy Effect It may be neglected The relative air buoyancy decreases when the density of the object increases. Close to 8,000 kg/m3, it becomes null and becomes negative for greater densities. According your weighing requirement, decide if a correction is necessary or not. For internal use - Confidential
Agenda 1 What is the most important? 2 Risk management 3 Uncertainties related to a weighing 4 Physics behind a weighing result 5 Safe Weighing Range 6 7 8 9 To shift Highlight Click the arrow, press shift key and move top or bottom For internal use - Confidential
Calibrate the process, not the balance Contact Microsep about the safe weighing range of your balance The uncertainty of the balance should be used at least to define the safe weighing range Make sure that your balance is suitable for your process For internal use - Confidential
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