3. Role of Measurement Uncertainty

As per GUM (JCGM 100:2008), uncertainty growth due to drift should not cause total uncertainty to exceed acceptable limits before recalibration. Calibration intervals must be shortened if drift contributes significantly to uncertainty between calibrations. 

4. Common Calibration Interval Models

Time-Based Intervals 

Fixed intervals (for example, annual calibration) are acceptable only when supported by historical performance data. Time-based intervals are simple but must still be justified. 

Usage-Based Intervals 

Intervals based on cycles, hours, or number of measurements are appropriate when wear or drift correlates strongly with use. Examples include torque tools and mechanical gauges. 

Condition-Based Intervals 

Calibration is triggered by defined conditions such as environmental exposure, relocation, overload, or maintenance. This approach is effective when drift is event-driven rather than time-driven. 

Performance-Based (Statistical) Intervals 

Intervals are determined by ongoing performance monitoring. Calibration occurs when drift trends approach predefined limits. This is the most technically robust approach but requires disciplined data management. 

5. Documentation and Compliance

ISO/IEC 17025 requires justification and documentation of calibration intervals, including: 

• Historical data 
• Risk assessments 
• Uncertainty budgets 
• Review records 

This documentation supports audit readiness and technical accountability. 

6. Example Application: Digital Pressure Gauge

Instrument: Digital pressure gauge (0–10,000 psi) 
Application: In-process pressure verification 
Requirement: ±0.05% of reading 

Findings: Over four calibration cycles, drift increased gradually, leading to a failed calibration at 12 months. 

Decision: Using the control chart and staircase methods (ILAC-G24 §6.2, §6.3), the interval was reduced to 9 months. 

Result: No further failures observed in subsequent cycles; interval change was documented and justified based on risk and uncertainty analysis. 

7. Conclusion

Calibration interval determination is a dynamic, evidence-based process integral to laboratory quality systems. International standards encourage laboratories to go beyond fixed schedules by implementing methodologies that reflect: 

• Actual equipment performance 
• Measurement uncertainty contributions 
• Operational and environmental risks 

A combination of statistical tools, risk models, and sound metrological judgment allows organizations to strike an optimal balance between measurement confidence and operational efficiency. 

About Richard J. Bagan, Inc. 

Richard J. Bagan, Inc. is an ISO/IEC 17025–accredited calibration laboratory specializing in dimensional, force, torque, and materials testing instrumentation. Our metrology services support manufacturers, laboratories, and regulated industries across the United States. 

For gage block calibration, uncertainty evaluation, or consultation on dimensional standards management, contact our laboratory team.