Accuracy in Metrology: How to Measure It and Why It Matters

Accuracy is one of the most important—and often misunderstood—concepts in quality control and dimensional inspection. It is frequently confused with resolution or repeatability, but it plays a unique role in ensuring your measurements truly reflect reality.

In this post, we explain:

• What accuracy means in metrology

• How to evaluate and determine it

• How it differs from other performance specifications

• Why accuracy is critical to your inspection process

• Steps you can take to improve and validate it

What Is Accuracy in Metrology?

Accuracy is how close a measured value is to the true or nominal value. For example, if a part should be 100 mm long and your system reports 100.01 mm, the accuracy error is 0.01 mm.

ISO Definition: Accuracy = Trueness + Precision

• Trueness refers to how close the average of many measurements is to the true value.

• Precision (repeatability) is how consistently you get the same result.

Example: If your scanner always reports 99.97 mm for a 100 mm block, it is precise but not accurate. Both accuracy and precision are necessary.

How Is Accuracy Determined?

Accuracy is often tested using certified artifacts with known dimensions such as:

• Length bars

• Step gauges

• Sphere probes and nests

• Grid plates

These artifacts are traceable to international standards (e.g., NIST, PTB), allowing your device’s output to be compared against the true value.

Common Verification Techniques:

• Gauge Block Checks — measure known lengths and compare results

• Sphere Probing Test — check spatial accuracy with calibrated spheres

• Laser Interferometry — high-end calibration for CMMs and laser trackers

• Volumetric Accuracy Testing — evaluates errors across the full measurement volume

Tip: Perform tests across your entire working volume, not just one corner.

Accuracy in Different Devices

Laser Trackers: Accuracy often expressed as (±µm + µm/m). For example, ±15 µm + 6 µm/m means a 15 µm base error plus 6 µm per meter measured. Sensitive to environmental drift, target reflectivity, and alignment.

Portable Arms: Accuracy depends on joint encoder quality, mounting stability, and probing technique. Typical ranges are ±0.020 mm to ±0.060 mm.

3D Scanners: Accuracy linked to scanner technology (blue light, laser, photogrammetry), resolution, and calibration. Often verified by scanning reference objects and comparing results to CAD models.

Accuracy Is Not Resolution

Term	What It Tells You	Common Misunderstanding
Accuracy	How close your measurement is to the true value	Trusting flawed results
Resolution	Smallest detail the system can detect	Overengineering or underusing tools
Repeatability	How consistent repeated measurements are	False confidence in faulty results

High resolution without accuracy is like using a fine magnifying glass on a blurry map.

Factors That Affect Accuracy

1. Temperature: Most materials expand or contract with heat. For example, a 1-meter steel beam expands about 0.012 mm per °C. Always measure in temperature-controlled environments and use thermal compensation features when available.

2. Mounting and Fixturing: Unstable fixtures introduce movement or distortion. Use certified granite tables or heavy-duty carts and maintain consistent probe angles and part positions.

3. Calibration and Drift: Measurement devices can drift due to internal electronics warming up, environmental factors (floor vibration, lighting), and operator handling. Log accuracy checks regularly to identify trends before they impact production.

How to Improve Measurement Accuracy

• Use certified artifacts regularly, not just during annual calibration—perform weekly checks if possible.

• Establish warm-up protocols; most devices require 10–30 minutes to stabilize thermally.

• Train operators on proper techniques, including probing angles, scanning speeds, and alignment strategies.

• Validate software settings like filtering, smoothing, and alignment methods to avoid distortions.

• Use redundant checks by measuring from multiple perspectives and comparing results.

Why Accuracy Matters for Your Business

• Pass/Fail Decisions: Tight tolerances (often ±0.05 mm or less) mean a 0.02 mm error can cause good parts to be rejected.

• Traceability: Compliance with ISO 17025 and AS9100 requires reliable measurement evidence.

• Reputation: Delivering inaccurate data damages customer trust and business credibility.

“When your measurements define million-dollar decisions, accuracy isn’t optional—it’s everything.”

Call to Action: Get Accuracy You Can Trust

Are you confident in your system’s accuracy? Need help validating or improving your results? At SS Metrology Solutions, we provide:

• On-site or virtual consultations

• Accuracy audits with traceable artifacts

• Operator training on probing, scanning, and alignment best practices

• Support with calibration verification and ISO 17025 compliance

Contact us today to schedule a consultation and ensure your measurement data meets your highest expectations.