Whi

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aper Influences on Mass Calibration

Minimizing uncertainties

Contents

1 Introduction 2

2 Balances vs. comparators 2

3 Thermal stability 3

4 Vibrations 3

5 Air drafts 3

6 Magnetism 4

7 Air buoyancy 4

8 Density measurement 6

9 Conclusion 6

This white paper will be of interest to anyone involved in mass calibration activities, from National Metrology Institutes (NMI's) to private calibration laboratories of any size.

In mass calibration, making accurate measurements and minimizing the uncertainty of such measurements is paramount. However, the extent to which physical influences can influence daily mass calibration are often under-estimated. This can lead to uncertainties which do not fulfil the requirements of the applied regulations. By analyzing multiple influence sources, the main driver for performance and thus correction steps to improve the combined measurement uncertainty can be defined.

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tion 1. Introduction

In mass calibration, minimizing uncertainties is critical and multiple physical effects can affect the measurement. The main influencing factors are the comparator balance itself, thermal stability, vibrations, air drafts and magnetism. The impact of static factors can be reduced by making appropriate choices with regards to equip-ment and environment.The variable influence of air buoyancy effects must be corrected, if limitations are exceeded. However, it is recommended to apply corrections at all times, to ensure the real conventional mass is reported and not the uncorrected conventional mass, which could generate balance calibration errors at a later stage.To enable these calculations to be made, air and artifacts densities must be known to sufficient accuracy, and then applied in mathematical calculations to correct for these physical effects and thus generate the mass in conventional mass values. To specify the ideal selection of equipment, an uncertainty analysis should be carried out. This analysis should determine the main influencing source of uncertainties and therefore the correction required.

2. Balances vs. ComparatorsBalances and comparators are influenced by the same physical effects, but these effects are often undetectable on a typical balance. This is because the influence of the physical effects is often lower than the resolution of the balance. However, with comparators, the smallest effects can be detected due to their very high resolution and stability. Examples of manual and robotic mass comparators (figure 1).

Figure 1: Examples of manual and robotic mass com-parators. From left to right: XPE56C (52 g / 0.1 g); XPE26003LC (26.1 kg / 1 mg); a5XL (6.1 g / 0.1 g)

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3. Thermal StabilityConvection caused by temperature differences between the artifact and the environment, such as sun irradiation or sub-optimal room heating, generates force changes applied to the artifacts. Therefore, this force influences the mass readings, which results in instability of the reading or incorrect measurements.The extent of the convection influence can exceed the limitation of regulations resulting in invalid mass values being used in subsequent calculations. By enhancing the temperature stability of artifacts and environment, these effects can be reduced.

4. VibrationsVibrations generated by a multitude of external sources can increase the stabilization times of high resolution balances or comparators and prevent reliable readings from being obtained. By installing sensitive instruments in an appropriate environment, for example on rigid stone tables and in low vibration areas of the building, such as the basement, comparator performance is significantly improved and stabilization times are reduced. Specific settings on the balance models can also be adapted to improve the performance.

5. Air DraftsAir, with its significant matter of 1.2 kg/m3, generates forces by interfering with surfaces. These forces applied on a weighing pan generate mass changes of a significant extent.As air drafts do not have a continuous stable flow, the variation of forces generates instability in the balance readings. To reduce this influence, mechanical protection of the balance weighing pan is typically employed so that the air draft is decreased to a non-influencing extent. The occur-rence of the air currents within a laboratory should be considered such that no direct air flow is directed on top of a high accuracy balance. It is preferable to have an air flow pillar in front of the balance to reduce the influence of the operators body heat. Balances should not be installed in

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tion close proximity to swinging doors, as air turbulences and drafts will have

an influence on the measurement. Sliding doors offer the advantage of preventing air pulses and drafts. In addition, it is recommended to avoid placing balances or sensitive measuring devices in an area of the labora-tory which experiences a high volume of foot traffic, in other words many people passing by frequently.

6. MagnetismArtifacts containing ferrites may interact with other magnetic sources. To eliminate this risk, it is recommended to install balances and compara-tors in locations without ferrites in the near vicinity. In weights handling, ferrites are also not allowed, as interaction is very likely and cannot be eliminated and damage of weights is possible. Minimum distances to fer-rites are recommended to reduce the influences of magnetic interference on mass measurements.

7. Air BuoyancyIn mass calibration, the effects of air buoyancy must be also considered. In addition, the assumption that different materials have different densi-ties must/should be made, as the tested artifacts are commonly made of stainless steel or lower grade steels. Different densities at identical mass lead to different weights volumes, which are affected differently by the ambient air.

For weight calibration, the mass has to be defined according to the weights density convention of 8000 kg/m3, air density of 1.2 kg/m3, and temperature of 20C.

Weights densities differing from the conventional values in combination with air density differing from the conventional value of 1.2 kg/m3 must be corrected, to result in the conventional mass (see Fig. 2).

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7.1. Factors influencing air buoyancy

Air density is one of the factors which influence air buoyancy. Air density reduces with altitude by 1.2 % per 100 m. This results in the maximum air density deviation of 10% being exceeded at 830 m above sea level. A weight's conventional mass is directly affected by this air density change, if the weights density deviates from the conventional value of 8000 kg/m3. The more the density of the weight deviates from 8000 kg/m3, the stron-ger the influence on the air buoyancy correction.

7.2. Extent of influences

According OIML R111, the air buoyancy influence shall not be more than 25% of the maximum permissible error. [2]Two weights are compared at an altitude of 840 m according to OIML R111: E1 1 Kg weight, density 8048.25 kg/m3 E2 1 Kg weight, density 7840.65 kg/m3

This altitude results in an air density of approximately 1.070 kg/m3, and an air buoyancy effect of 0.425 mg, which is 26% of the maximum permissible error (MPE) of 1.6 mg of the E2 1kg weight.As this exceeds the 25% limit imposed by OIML R111, a correction must be applied.

Commonly, it is strongly recommended to apply air buoyancy correction each time, as influences are significant. This also maintains standardized procedures for intercomparable measurement results.

Figure 2: Effect of different densities on weight volume and air buoyancy

Density Volume Buoyancy

113.63 cm38800 kg/m3

7200 kg/m3 138.88 cm3

Displaced air

volume

Displaced air

volume

Figure 3: Example calculation of the air buoyancy influence

7.3. Air buoyancy uncertainties

To calculate the air buoyancy effects, the air density and artifacts densities must be known at high accuracy to enable low measurement uncertainties.The accuracy of the artifacts density contributes more to the air buoyancy correction uncertainty than the ac-curacy of the air density. Therefore, it is necessary to know the density of artifacts accurately to allow an air buoyancy correction with sufficient accuracy. The combination of the uncertainty influences must be calculated individually for each artifact to find the specific uncertainties.

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tion 8. Density Measurement

To enable low air buoyancy correction uncertainties, the densities of air and weights must be known to a certain level of accuracy. For low accuracy mass calibrations in lower altitudes, the density is estimated according to materials or defined by the supplier material specifications. The air density is measured with medium accuracy climate sensors. For higher accuracy measurements, or where the combination of air density and weights densities becomes more significant, the density of the weight must be measured as well, to reduce the overall air buoyancy correc-tion uncertainty. Weights density is measured according OIML R111 [2] methods A3 or A2, both based on buoy-ancy force measurement of volumes in known liquids. The weights are immersed in well-known liquids and the buoyancy force is measured and deducted from the mass. The mass difference in combination with the liquid density results in the volume and density of the weight. With this measurement technology, density is measured highly accurate and reducing the air buoyancy correc-tion uncertainty to lowest levels. To even improve the accuracy, the buoyancy force is measured differentially to mass references to reduce the mass measurement uncertainty.

9. ConclusionIn mass calibration, various external influences can have an effect on weighing performance and therefore accu-racy of measurement. This can lead to a disruption of measurement quality of an unacceptable extent. In order to achieve reliable and accurate mass measurement, environmental influences should be minimized. To fulfill the requirement of all mass values being stated in conventional mass, environmental influences and physical effects must be corrected with appropriate measurement technology. The weights densities and the air density must be known to a sufficient level of accuracy to enable lower uncertainties in the uncertainty of air buoyancy correction. This improves the overall measurement accuracy as a result. By taking into consideration all the physical influences which can affect weighing accuracy, improving laboratory climatic conditions and applying air buoyancy correction, the mass calibration capabilities of a specific labo-ratory can be improved. This leads to generation of more reliable calibration results, and full compliance with industry regulations.

References

1. "Weights of classes E1, E2, F1, F2, M1, M12, M2, M23 and M3", Weights, T. 9. Paris, France (2004).2. OIML R111 - TC 9/SC 3, (2004)

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