Calibration of Ultra Microbalances in the Submilligram Range

Abstract

The paper presents a calibration methodology for electronic ultra microbalances intended for measurements in the load range below 1 mg, which is not covered by the requirements of the OIML R111 document or the EURAMET cg-18 guideline, resulting in a significant gap in ensuring measurement traceability for sub-milligram measurements. Mass standards with nominal values of 0.5 mg and 0.2 mg were produced from AISI 316L steel using the mechanical subdivision method, and an original Sub-Milligram Linearity Calibration Method was proposed, extending the EURAMET cg-18 procedure — including its Appendix C on linearity — to the range below 1 mg. Calibration was performed using the substitution method against a 1 mg reference standard on a Mettler Toledo ultra microbalance, in two loading sequences (ascending and descending), with no significant effect of hysteresis found. The maximum value of |ΔL(m)| at the calibration points was determined (0.113 µg for the 0.2 mg point), and an uncertainty budget compliant with EURAMET cg-18 was developed, taking into account instrument resolution, repeatability, the uncertainty of the reference standard, and environmental influences. It was shown that the expanded uncertainty of the sub-milligram standards (approx. ±1.50 µg, k = 2) is dominated by the uncertainty of the intermediate 1 mg standard rather than by the indicating instrument itself. The proposed methodology may serve as a basis for developing future guidelines for calibrating ultra microbalances below 1 mg.