Measurement and unit control
Why a sound engineering calculation begins with traceable measurements and deliberate unit conversion.
ENGINEERING PRINCIPLE
A numerical model is only as reliable as the quantity, unit, datum and accuracy of each entered value.
reported value = measured value × stated conversion factorVariables and units
- quantity = physical property being measured
- unit = agreed scale
- datum = reference condition
- uncertainty = credible range around a measurement
Worked example
A 1,000 mm measured length is exactly 1.000 m and 39.3701 in. The conversion is straightforward; deciding the datum, tolerance and measurement method is the engineering work.
Calculation method
- Record the measured quantity, unit and reference condition.
- Convert using a stated authoritative factor.
- Retain precision that is justified by the measurement, then check the result against the drawing or specification.
Practical applications
- Drawing interpretation
- Test data preparation
- Supplier-data comparison
Review boundary and reference
- Do not imply greater accuracy than the measurement supports.
- Avoid mixing nominal, actual and tolerance-limit values in the same calculation.
Formula and displayed units independently checked for the stated conditions. This is not a code-compliance design method.
Source: NIST Special Publication 811, Guide for the Use of the International System of Units (SI).