Ask ten quality managers how often equipment should be calibrated and most will say "annually." It's a sensible default, and for much laboratory equipment it's exactly what auditors expect to see. But an interval you can defend in an audit isn't a habit — it's a decision, made for reasons you can point to. Here's how to make it.

Start with what's mandated

Some intervals aren't yours to choose. Testing standards, project specifications and accreditation requirements often set maximum periods between calibrations. Compression testing machines used for concrete testing, for example, must be verified at intervals set by the relevant testing standards, and pre-stressing work commonly requires jacking systems to hold a current calibration certificate before they're used on site. If a standard applies to your work, its interval is your ceiling — you can calibrate more often, never less.

Then weigh the risk

Where no standard dictates the interval, the question becomes: what happens if this instrument drifts out of tolerance between calibrations? Every result it produced since the last good calibration is now in doubt. For a balance that checks incoming goods, that might mean re-checking a day's deliveries. For an autoclave in a medical setting, or a machine whose results certify structural concrete, the consequences are of a different order. The more it would cost you to discover an instrument was wrong, the shorter its interval should be.

Factors that shorten the interval

  • Heavy or harsh use. Instruments used constantly, transported between sites, or exposed to dust, vibration and temperature swings drift faster than bench-bound equipment.
  • A history of drift. If calibration keeps finding an instrument near or beyond its tolerance limits, the interval is too long for that instrument — regardless of what the schedule says.
  • Critical measurements. Equipment whose results feed directly into product release, compliance reports or safety decisions deserves a margin of caution.
  • Repairs and events. Any repair, adjustment, relocation or suspected damage — a dropped load cell, a jarred balance — should trigger recalibration immediately, whatever the calendar says.

Factors that can support a longer interval

Stable instruments in controlled environments, with a documented history of passing calibration well within tolerance, can sometimes justify extended intervals — but only when supported by evidence and by regular in-house checks between calibrations. A daily check weight on a balance, or a reference thermometer check on an oven, gives you early warning of drift and strengthens the case for the interval you've chosen.

The audit test

Whatever intervals you set, record the reasoning. "Annually, per manufacturer recommendation and standard practice" is defensible. "Six-monthly, due to site transport and criticality of results" is better. What auditors question isn't the number — it's a number with no reasoning behind it.

If you'd like help setting intervals for your equipment, we do this every day. For EasyCAL subscribers, the portal also tracks due dates automatically, so once an interval is set, nothing slips through. Get in touch and we'll help you build a schedule that holds up.