Establishing the correct calibration interval is a core requirement under ISO/IEC 17025:2017 Clause 6.4. Setting intervals too short results in unnecessary testing downtime and expenses, while excessively long intervals risk undetected out-of-tolerance (OOT) conditions.

1. The 5 Recognized ILAC-G24 Methods

The International Laboratory Accreditation Cooperation (ILAC-G24) recognizes five established methods for determining calibration intervals:

  • Automatic or 'Stepped' Adjustment: Adjusting the next calibration interval based on whether the instrument was found In-Tolerance or Out-of-Tolerance.
  • Control Chart Method: Tracking drift over successive calibrations (e.g. measuring R0 drift in SPRT probes).
  • In-Use Time Logging: Calibrating based on accumulated operational hours rather than calendar months.
  • In-Service Testing ('Black Box' Checks): Using intermediate check standards between formal annual accredited calibrations.
  • Statistical Reliability Models: Using Weibull or Poisson distributions for large fleets of identical instruments.

2. Implementing Intermediate Check Standards

Maintaining intermediate check standards—such as using the DB Therion Dry Block and DB Metron PRT for monthly single-point Ice Point (0°C) verifications—allows laboratories to safely extend formal recalibration intervals while maintaining full audit compliance.