Overview - What ISO/TR 20461:2023 covers
ISO/TR 20461:2023 is a technical report that gives detailed guidance on the evaluation of measurement uncertainty for volume measurements of piston-operated volumetric apparatus (POVA) using a gravimetric method. It supports and expands the gravimetric procedures in the ISO 8655 series (notably ISO 8655‑6 and ISO 8655‑7:2022, Annex A) and follows the principles of ISO/IEC Guide 98‑3 (the GUM). The report describes the mathematical model for converting balance readings to delivered volume, the relevant corrections (evaporation, thermal expansion, pressure), and worked examples for uncertainty of the mean delivered volume and for the uncertainty in use of a single delivered volume.
Key technical topics and requirements
- Measurement model and formulas: Relates balance indications, air and water density, evaporative losses and POVA thermal expansion to delivered volume; includes the Z correction factor for pressure/temperature effects.
- Uncertainty evaluation procedure: Follows ISO/IEC Guide 98‑3 steps - identify input quantities, estimate expected values and standard uncertainties, determine sensitivity coefficients, combine uncertainties, and calculate expanded uncertainty using an appropriate coverage factor.
- Uncertainty component groups:
- Gravimetric measuring system: weighing uncertainty, temperature, water density (e.g., Tanaka formula), air density, weights density, air‑cushion effects.
- POVA characteristics: cubic expansion coefficient, resolution, setting accuracy.
- Liquid delivery process: repeatability (experimental standard deviation), reproducibility, handling.
- Practical additions: Examples of calculations, Annex A (approaches to estimate uncertainty in use for a single delivered volume) and Annex B (volume corrections due to pressure changes).
Practical applications - who uses this standard
- Calibration laboratories performing POVA calibration and traceable volume measurements.
- Metrology specialists, quality managers and instrument manufacturers who need to estimate and report measurement uncertainty for pipettes and piston-operated dispensers.
- Laboratories implementing ISO 8655 procedures and seeking a compliant uncertainty evaluation consistent with ISO/IEC Guide 98‑3.
- Regulatory and accreditation bodies assessing calibration methods and conformity.
Related standards
ISO/TR 20461:2023 is an essential reference for accurate, traceable gravimetric volume measurement and for producing robust uncertainty budgets for piston‑operated volumetric apparatus.