This is a ready-to-use worksheet. It turns “the balance passed its calibration” into two separate, documented answers: how much doubt surrounds a reading from this balance, and how small a sample this balance can weigh before that doubt swallows the result. Work top to bottom and replace every <<FILL: ...>> placeholder. A filled specimen follows the template. This content is general educational reference, not legal or regulatory advice; verify the current edition and clause numbers of every cited standard before you rely on it.
Instrument and applicability
Complete a separate worksheet, or a separate operating-load row, for each distinct region of the range the balance is actually used at. A budget built near full scale does not describe performance near zero.
Step 1: Repeatability
Repeatability is the single most informative input here, because it reflects the balance as it performs today, worn parts, siting, and drift included, not as it performed when new.
Step 2: Reference weight uncertainty
If the certificate states no coverage factor, do not assume one; obtain a certificate that states it, or get the issuing lab to confirm it in writing.
Step 3: Linearity
Step 4: Display resolution
Step 5: Any additional component (optional, state if included)
Step 6: Combine the components
Step 7: Apply the coverage factor
State the coverage factor explicitly every time you report an expanded uncertainty. A number without its coverage factor cannot be compared to anything.
Step 8: Derive the minimum weight
The factor of 2 is deliberate: it turns the repeatability standard deviation into an approximate 95% two-sided bound, the same convention used across published USP <1251> and balance-vendor minimum-weight guidance. Always round the result up, never down, and never weigh below it on this balance for this application.
Step 9: Consistency and approval
Acceptance criteria
This worksheet is complete when: the budget includes at minimum repeatability, reference weight uncertainty, linearity, and resolution, each with a real traceable source, not an assumed value; the combined and expanded uncertainty are calculated by the stated method, not estimated; the coverage factor is stated alongside the expanded uncertainty; the minimum weight is derived from the balance’s own current repeatability, rounded up; and the minimum weight is both posted at the instrument and reflected in any method or record that specifies a sample weight for it.
Filled specimen
The following works the calculation through for a 220 g analytical balance, instrument BAL-1102, QC laboratory. Numbers are illustrative; replace them with your own measured values.
Reading this specimen: the balance’s expanded measurement uncertainty is about plus or minus 0.00042 g around a reading near 100 g, and no sample below 0.25 g should be weighed on it where a 0.10% relative accuracy is required. A method elsewhere in this lab that calls for a 0.1 g aliquot on this same balance is now a documented mismatch, not a surprise found later.
Common inspection findings this worksheet prevents
- A measurement uncertainty statement that exists only as a line on a calibration certificate, with no record of how it was built or what it includes.
- A minimum weight copied from the balance’s manual rather than derived from its own current repeatability.
- A method or batch record that specifies a sample weight below the balance’s documented minimum weight, discovered only after the fact.
- An expanded uncertainty reported with no stated coverage factor, so the number cannot be checked or compared.
- The same uncertainty budget reused for years without a repeat after a repeatability shift, a relocation, or a requalification.
How to adapt this worksheet
- Complete one worksheet per balance or instrument per operating load that matters to your processes; a balance used at both 5 g and 200 g needs the calculation at both ends if both are used for GMP decisions.
- Set your house target relative accuracy (commonly 0.10% absent a tighter process-specific need) in the governing SOP and apply it consistently across the fleet.
- Where a linearity check or an additional component (drift, buoyancy, eccentricity) is routinely significant for your equipment, add it as its own row rather than folding it into another component.
- File the completed worksheet with the calibration record for that instrument, and post the resulting minimum weight at the instrument itself.
- Confirm the current edition and effective date of USP <41>/<1251> and the coverage-factor convention in the Guide to the Expression of Uncertainty in Measurement before you rely on them in a procedure.
