The AxoTek calibration interval calculator helps you decide how often to calibrate an instrument or standard using the methods of the OIML D 10 / ILAC-G24 guide, the S2 statistical method of NCSL RP-1 and the examples from the paper "Determination of calibration intervals" presented at CENAM's 2004 Metrology Symposium. Use it to set the initial interval of a new instrument and to review it with its calibration results.
What is a calibration interval?
It is the maximum time (or hours of use) allowed between two successive calibrations of an instrument. An interval that is too long increases the risk of using an out-of-tolerance instrument without knowing it; one that is too short multiplies the cost: calibrating twice as often as needed costs twice as much, not counting the time the instrument is out of service.
ISO/IEC 17025 requires a calibration programme that is reviewed and adjusted as necessary, and ISO 9001 requires measuring equipment to be calibrated or verified at specified intervals. Neither sets the number of months: the laboratory must choose a method, document it and show that it works.
How to choose the initial interval
According to OIML D 10, the first decision is made by experienced staff considering:
the manufacturer's recommendation;
the expected extent and severity of use;
the influence of the environment (temperature, humidity, vibration);
the required measurement uncertainty and the maximum permissible error (MPE);
adjustments or changes made to the instrument;
the influence of the measured quantity (for example, thermocouples at high temperature);
published data about the same or similar instruments.
As a reference, CENAM compiled initial intervals published by NIST, Nordtest, DIN and laboratories: 12 months for multimeters, balances and barometers; 6 months for Bourdon gauges and pressure transducers; 24 months for hygrometers and force transducers; up to 60 or 120 months for steel tapes and rules. The calculator includes that table in method 1.
Keeping a fixed interval without review "is not recommended": after the first calibrations, use one of the review methods.
The 5 review methods of OIML D 10
1. Automatic adjustment or "staircase" (calendar time). At each calibration, if the as-found error is within, for example, 80 % of the MPE, the interval is extended; if it is outside the MPE, it is reduced. Simple and fast, but it is wise to set a minimum and a maximum.
2. Control chart (calendar time). The deviation of significant points is plotted against time, the drift is calculated and from it the optimum interval. It is the most reliable, but it needs data and processing.
3. "In-use" time. Same as the previous methods, but the interval is measured in hours of use with an elapsed-time indicator. Useful for thermocouples at extreme temperatures, dead-weight testers or gauges subject to mechanical wear; not for passive standards or instruments that drift on the shelf.
4. In-service checking or "black box". Critical parameters are checked frequently with portable equipment; if anything is outside the MPE, the instrument goes to a full calibration. It gives maximum availability of the instrument.
5. Other statistical approaches. For groups of identical instruments, the fraction still in tolerance after a given time is analysed (for example, the S2 method of NCSL RP-1).
Formulas used by the calculator
Drift (methods 2 and 3): Drift = Deviation / (t₂ − t₁). With more than two calibrations a least-squares line is fitted.
Interval from drift: Interval ≤ Tolerance / |Drift|.
Reliability (method 5): R(t) = s / n, where n is the number of instruments calibrated and s the number found in tolerance. The model R(t) = e^(−λt) is fitted and the interval for a target reliability R* is t = −ln(R*) / λ.
Examples from the CENAM paper
70 MPa dead-weight tester, calendar time: tolerance ±8.1×10⁻⁵ and drift −2.75×10⁻⁶ per month → 29-month interval.
Same tester, in-use time: drift −0.26×10⁻⁶ per hour → 310 hours of use.
Group of hygrometers, S2 method: 32 calibrations grouped by weeks; with a target reliability of 85 % the interval was 23.6 weeks.
You can load all three examples in the calculator with the "Load CENAM example" button.
Frequently asked questions
Is the 12-month interval mandatory? No. It is the most common one because many manufacturers recommend it, but the right interval depends on actual use and on the stability of the instrument. Only regulated instruments (in Mexico, those set by the Ministry of Economy, such as commercial scales or fuel dispensers) have legal intervals.
What should I do if the instrument arrives out of tolerance? Reduce the interval and assess the impact on measurements made since the previous calibration; under ISO/IEC 17025 it is handled as nonconforming work.
Which method suits a small laboratory? The staircase method is the easiest to apply. As soon as you have three or more calibrations of an instrument, the control chart (drift) gives a better-supported result.
Sources: ILAC-G24:2007 / OIML D 10:2007, Guidelines for the determination of calibration intervals of measuring instruments; B. Soriano, V. Aranda and N. Gutiérrez, "Determinación de intervalos de calibración", 2004 Metrology Symposium, CENAM; NCSL RP-1, Establishment and Adjustment of Calibration Intervals.