Calibration and Metrology: Traceability, Uncertainty, Zero & Span

Calibration and Metrology

These terms define the reliability and "trustworthiness" of a measurement result.

Traceability

The property of a measurement result whereby the result can be related to a reference (usually national or international standards like SI units) through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.

• Simply put: Being able to prove that your 1kg weight is actually 1kg because it was checked against a standard, which was checked against a master standard, and so on.

Uncertainty (Measurement Uncertainty)

A parameter characterizing the dispersion of values that could reasonably be attributed to the measured quantity. It tells us the "doubt" in the measurement.

• No measurement is perfect. A result is reported as "100.00 g ± 0.01 g", where 0.01 is the uncertainty.

Zero & Span

The two fundamental points for calibrating a linear instrument.

• Zero: The output when the input is at its minimum (e.g., 0 kg -> 4 mA).
• Span: The output when the input is at its maximum (e.g., 100 kg -> 20 mA). Adjustment of these two points is the most common form of calibration.