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Separate the quantity from the display string
The source dates shown in this guide identify documentation checks on 22 October 2026, not original publication dates. Start with the measured quantity, source value, exact unit string, and the field or instrument that produced it. A label such as “weight,” “rate,” or “temperature” does not by itself identify a dimension. Preserve prefixes, exponents, denominator terms, and any annotation such as dry, wet, gross, net, gauge, or absolute.
| Field | Worksheet value | Stop if |
|---|---|---|
| Quantity | Domain name and definition | Label is only a display string |
| Raw value | Original numeric representation | Rounded value replaced it |
| Source unit | Exact spelling and expression | Unit inferred from column name |
| Canonical unit | Controlled identifier or unresolved | Alias changed meaning |
| Context | Basis, temperature, pressure, or denominator | Context is absent |
| Precision | Declared digits and resolution | Precision invented after conversion |
The BIPM, (opens in a new tab) base-unit page is useful for checking SI names and symbols, but it is not a substitute for the producer’s unit contract. The UCUM, (opens in a new tab) notation can preserve a machine-readable expression, while a human-readable label can remain in a separate display field.
Record the conversion as a derivation
A conversion record should identify input unit, output unit, factor or formula, reference source, effective date if the factor changes, rounding mode, and the artifact that performed the operation. For affine conversions, keep the offset separate from the scale. For compound units, show how numerator and denominator dimensions change. If a conversion depends on a context such as temperature or currency date, that context belongs in the record.
| Conversion case | Record | Do not infer |
|---|---|---|
| Scale only | Factor, source, significant digits | More decimals mean more accuracy |
| Affine | Scale, offset, and order of operations | Multiply without offset |
| Compound | Each dimension and exponent | Convert only the numerator |
| Contextual | Reference state and effective time | One factor works for every context |
| Unknown | Unresolved unit and owner question | Guess from a familiar abbreviation |
Keep the raw value and converted value in separate columns. A conversion is a new derived artifact, not a correction to the source. If a reviewer changes a factor or rounding rule, record a new derivation and retain the prior result.
Keep precision, rounding, and missingness visible
Rounding can be introduced by the instrument, serialization, conversion, display, or final report. Record which stage applied it. Do not infer uncertainty from the number of decimal places, and do not convert an absent value into zero. A value that is numerically exact under a formula can still carry limited source precision.
- Capture the raw numeric token and unit before parsing.
- Normalize the unit expression without erasing the source spelling.
- Resolve dimension and contextual assumptions.
- Apply the stated formula with a named factor source.
- Record rounding mode, output precision, and missing-value policy.
- Compare only values whose quantity, basis, and unit contract match.
Use a synthetic conversion worksheet
Synthetic unit lineage rows
row | quantity | raw | source_unit | target_unit | operation | status
fixture-1 | length | 2.5 | m | cm | scale from named table | derived
fixture-2 | rate | 4 | m/s | km/h | compound scale | derived
fixture-3 | temperature | 20 | fixture-scale | K | context missing | hold
fixture-4 | mass | blank | kg | g | no value | missing
lineage: keep raw token, unit expression, factor source, rounding, and reviewer note| Test | Expected result | Hold condition |
|---|---|---|
| Alias | Source spelling retained and mapped | Alias treated as universal |
| Compound | Numerator and denominator checked | Only one dimension converted |
| Affine | Offset recorded separately | Scale applied alone |
| Precision | Rounding stage identified | Digits treated as uncertainty |
| Missing | Missing state preserved | Blank changed to zero |
This fixture uses invented values and a deliberately unresolved unit. Add a prefixed SI unit, a unit with a denominator, a conversion that needs a reference date, and a value with an explicit missing marker. The worksheet should make an unresolved conversion visible instead of silently emitting a number.
Keep conversion evidence separate from measurement conclusions
Unit lineage can show how a number was represented and which assumptions were applied. It cannot establish calibration, accuracy, safety, ownership, or suitability for a downstream purpose. Preserve source and derived values together and ask the domain owner to interpret them.
Pair this guide with decimal precision and numeric exports for representation checks and the data provenance log for transformation history. After a supported non-health scope, authority to describe the collection, and a controlled measurement packet are established, the qualified appraisal request path may be a preparation step. It does not convert units, validate measurements, create a quote, match a buyer, or grant a license.
Takeaways
- Record quantity, raw value, source unit, context, and precision before conversion.
- Keep canonical unit identity beside the original unit string.
- Treat each conversion as a derivation with a named factor or formula source.
- Separate rounding and missingness from measurement accuracy.
- Do not infer scientific meaning from a familiar unit label alone.