Uncertainty of A&D Pipette Accuracy Testers - A&D COMPANY, LIMITED - #1

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Consideration of the Gravimetric Method and its Measurement Errors 1/8 The Uncertainty of Volume Calibration using A&D Pipette Accuracy Testers (Considerations of the Gravimetric Method and its Measurement Errors) 1. Volume Measurement by the Gravimetric Method For calibration of capacity meters such as pipettes, ISO8655-6 recommends the gravimetric method, in which the mass value of the distilled water dispensed from a capacity meter is measured with a balance and then converted to a dispensed quantity (volumetric value). No method can measure directly the physical quantity of a minute volume. Therefore, the most common and precise method is to measure the mass value of distilled water, whose physical properties are known, using a balance and then convert the mass to a volumetric value. Conversion from a mass value to a volumetric value involves the temperature of the distilled water and the barometric pressure as parameters. However, the variation of the measurement results due to barometric fluctuation is negligible, and in practice, it will suffice to set and use a representative value (fixed value) of the location of measurement. Consequently, the equipment to be controlled at the time of volume calibration will be the balance and the thermometer. What follows is concrete consideration of the uncertainty of volume calibration. 2. Uncertainty Components in Volume Calibration by Gravimetric Method In the gravimetric method, the mass value of the distilled water dispensed from a capacity meter such as a pipette is measured using a balance, and the measured mass value is then converted to a volumetric value. Therefore, the uncertainty of volume calibration can be divided into “2-1 Components concerning the mass measurement using a balance,” and “2-2 Components concerning the mass-to-volume conversion.” 2-1 Components Concerning the Mass Measurement using a Balance Errors in the mass measurement lead directly to errors after the mass-to-volume conversion. There are errors due to the balance and errors due to the measurement method. Errors due to the balance will be considered from the product specifications (3-1 Uncertainty based on the performance of the balance). As for errors due to the measurement method, it is necessary to consider the evaporation of the distilled water during the mass measurement (3-2 Uncertainty based on evaporation). 2-2 Components Concerning the Mass-to-Volume Conversion The density of distilled water is approximately 1 g/mL but varies depending on the temperature (water temperature). Meanwhile, in order to measure the mass of an object precisely, it is necessary to correct for buoyancy since the balance is calibrated using a weight (density 8000 kg/m3). Therefore, these factors (density variation of distilled water due to temperature change and correction for buoyancy) must be considered when converting the measured mass value of the distilled water to a volumetric value. The conversion factor used to calculate the volume of distilled water from its mass is called the