How to Set Calibration Intervals (and When to Calibrate Early)

“How often should we calibrate?” is the most common question TEQ receives. The honest answer is that the interval is the user’s decision, and it should be based on evidence.

ISO/IEC 17025 makes this explicit. A calibration laboratory may not put a recommended calibration interval on a certificate or calibration label unless the customer has asked for it (clause 7.8.4.3). The laboratory sees the instrument for one day. The user sees how it is used every day.

Starting point for a new instrument

For a new instrument with no history, a starting interval is normally based on:

• the manufacturer’s recommendation
• the ratio between the instrument’s accuracy and the tolerance you need
• how often and how hard it is used
• the environment: dust, vibration, temperature swings, corrosive atmosphere
• the consequence of a wrong measurement, whether that is product safety, legal metrology or a customer requirement

One Year is common for many instruments. It is a convention, not a rule.

Adjusting the interval with evidence

Once calibration history builds up, the interval should be reviewed. ILAC-G24 / OIML D 10 describes several methods:

• Control chart method. Plot the error at the same calibration points over time and estimate when drift will reach the tolerance limit.
• In-use time. Base the interval on operating hours or number of cycles rather than calendar time.
• In-service checks. Use regular checks against a check standard to decide when full calibration is needed.

A simple rule many laboratories use: if the last three calibrations at least all found the error well inside the tolerance (for example, under half of it), the interval can usually be extended. One out-of-tolerance result is a reason to shorten it and investigate.

Events that call for calibration before the due date

• Before first use of a new or repaired instrument
• After any adjustment or component replacement
• After a drop, overload, overpressure or electrical surge
• After relocation. This matters especially for balances, which must be recalibrated or at least checked after being moved.
• When intermediate checks show unexpected results
• Before a critical measurement campaign, validation or qualification

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