Maintaining calibration equipment between calibrations

ISO/IEC 17025 clause 6.4 requires procedures for handling, transport, storage, use and planned maintenance of equipment, and intermediate checks where needed to maintain confidence in performance. For temperature equipment the plan is short but must be followed.

A minimal plan

• Reference PRT: ice-point check before each use or weekly; record and chart the value; investigate a shift of more than the certificate uncertainty.
• Dry block: annual re-characterisation of gradients; insert inspection for wear and contamination.
• Liquid bath: fluid level, contamination and viscosity checked; fluid change interval defined.
• Readouts: zero and reference-resistor checks.
• Loggers: battery replacement schedule; clock check against a reference time.
• All: transport cases, temperature limits for storage, and a rule that a dropped reference is checked before reuse.

A control chart of ice-point readings is the single most informative maintenance record a temperature laboratory keeps.

A maintenance plan you can actually run

Clause 6.4 requires procedures for handling, transport, storage, use and planned maintenance, and intermediate checks where they are needed to maintain confidence. The failure mode is a plan so elaborate nobody follows it. This one fits on a page.

The ice-point chart earns its place

Of everything in that table, the control chart of ice-point readings is the single most informative record a temperature laboratory keeps. It costs a few minutes a month and it does three jobs: it detects a drifting or damaged reference long before the annual certificate does, it provides the evidence for lengthening or shortening the calibration interval, and — on the day an out-of-tolerance finding triggers a look-back — it locates when the change began, turning a twelve-month impact assessment into a one-month one.

The unglamorous failures

• Batteries. A logger or handheld that dies mid-measurement takes the data with it, and rechargeable cells fail gradually rather than suddenly. Replace on a schedule, not on failure.
• Well inserts. They wear, corrode and pick up debris, and a worn insert opens the air gap that dominates a dry block's radial term. Inspect, and re-characterise after replacement.
• Connectors and leads. A corroded terminal produces a lead-resistance imbalance that a three-wire connection converts straight into a temperature offset.
• Transport. A reference PRT carried in a van is the commonest cause of a sudden ice-point shift. Use a proper case, and check the ice point after any journey.
• Bath fluid ageing. Ethanol takes up water and its freezing point rises; oil oxidises and thickens; water grows things. All three change the uniformity you characterised.

Record the checks even when they find nothing. A file containing only the month a problem appeared reads as though the checks began that month, and it cannot support an interval decision.