ILAC-G24 (aligned with OIML D 10) is the internationally accepted guidance document for determining and reviewing calibration intervals. ISO/IEC 17025:2017 requires the interval to be justified and set by the laboratory.
Factors Affecting the Interval
- Manufacturer recommendation and manufacturer accuracy specification
- Frequency of use and ambient conditions
- The instrument’s stability/drift characteristics
- The criticality (risk) of the quantity being measured
- The trend in previous calibration data
- Cost of calibration and cost of replacing the instrument
ILAC-G24 Methods
The guide defines five different methods for setting the interval:
- Method 1 — Automatic adjustment: if the deviation is within the limit the interval is extended, if exceeded it is shortened
- Method 2 — Control charts: statistical process control is used
- Method 3 — Characteristic properties: a table value by instrument type
- Method 4 — “Calendar time” + use: both time and number of uses
- Method 5 — Black box (in-use testing): periodic intermediate checks
Practical Starting Values
If there is no historical data, the manufacturer’s recommended interval or industry practice can be a starting point:
- DMM, electrical calibrator: 12 months
- PT100, thermocouple: 12–24 months (depending on usage intensity)
- Micrometer, caliper: 12 months
- Gauge blocks: 24–60 months
- Balance: 12 months + monthly internal checks
- Reference weight (E2): 24–36 months
Important
The interval is not static. Every calibration result must be analysed; the drift trend must be monitored, and the interval shortened or extended with justification.
Intermediate Checks
To maintain confidence between calibrations, intermediate checks must be performed — for example a repeatability test for a balance, a reference voltage reading for a DMM, or an ice-water point check for a thermometer.