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Author:
Talip BIYIK
How to Set Calibration Intervals and Intermediate Checks
A calibration interval is the time between two calibrations of a measuring instrument. No standard says "this instrument must be calibrated once a year"; the owner sets the interval and justifies it. Too long an interval risks months of measurements with a faulty instrument; too short an interval means unnecessary cost and frequent downtime. An intermediate check is a simpler, more frequent check that confirms the instrument is still reliable between calibrations.
This guide explains how to set calibration intervals, the methods in the ILAC-G24 / OIML D 10 guideline, when to extend or shorten an interval, and how to plan and record intermediate checks, with examples.
Who sets the calibration interval?
The owner and user of the instrument. ISO/IEC 17025 requires calibration laboratories not to put a recommended interval on the certificate unless agreed with the customer, because you, not the laboratory, know how the instrument is used. ISO 9001 also requires measuring resources to be verified at specified intervals or before use, and leaves setting those intervals to the organisation.
How to choose the initial interval
A new instrument has no history yet. ILAC-G24 / OIML D 10 recommends considering the following when choosing the initial interval:
- The manufacturer's recommendation
- Frequency and severity of use
- Influence of the environment (temperature, humidity, vibration, dust)
- The required uncertainty and the margin to the acceptance limit
- The instrument's tendency to drift over time or with use
- Whether it has been adjusted, and the risk of overload or misuse
- Historical calibration data of similar instruments
- The frequency and effectiveness of intermediate checks
In practice the initial interval often starts at 12 months and is reviewed as data accumulates. What matters is that the reason for the chosen interval is recorded.
Interval review methods in ILAC-G24
| Method | How it works | When it suits |
|---|---|---|
| Automatic adjustment or "staircase" (calendar time) | If the instrument is within limits at calibration, the next interval is extended; if not, it is shortened. | Few instruments, simple to apply. Use with care, as it relies on a single result. |
| Control chart (calendar time) | The same calibration points are charted; drift and scatter are calculated to find the optimum interval. | Instruments with regular data and measurable drift. |
| In-use time | The interval is tied to actual hours or number of uses rather than the calendar. | Instruments with very variable usage, where a counter or usage log exists. |
| In-service checking ("black-box" testing) | Critical parameters are checked frequently with a check device; the instrument goes for full calibration when a problem appears. | Complex instruments where a check set-up is available. |
| Other statistical approaches | The interval is calculated against a reliability target using data from groups of similar instruments. | Organisations managing many similar instruments. |
The ILAC-G24 guideline describes these methods in detail and when to choose each.
When to extend and when to shorten
Signs to shorten: an out-of-limit or near-limit result, a deviation found in an intermediate check, drift in the same direction over successive calibrations, a drop or overload, repair, increased usage.
Evidence needed to extend: several successive calibrations passing with a wide margin, a stable trend and clean intermediate checks. A single good result is not enough evidence to extend.
Example: staircase interval adjustment
The rules below are for illustration only; define your own rules in your procedure.
| Calibration | Result (% of acceptance limit) | Decision | Next interval |
|---|---|---|---|
| 1st | 35% | Pass, wide margin | 12 months (initial) |
| 2nd | 40% | Pass, wide margin; two good results in a row | Extended to 15 months |
| 3rd | 85% | Pass but close to the limit | Reduced to 12 months |
| 4th | 110% | Out of limit; retrospective impact assessment done | Shortened to 9 months |
Out-of-limit instrument: retrospective impact assessment
When an instrument is found out of limits, adjusting it is not enough. You must assess whether measurements made with it since the last good calibration, and the products or results relying on them, were affected. This also shows why the interval matters: the longer the interval, the more work there is to review.
What is an intermediate check?
An intermediate check is a check performed between calibrations to maintain confidence in the instrument's performance. ISO/IEC 17025 requires that, where intermediate checks are needed, they are carried out according to a procedure. An intermediate check does not replace calibration; the instrument is not fully calibrated, only quickly confirmed to work as expected.
Intermediate check methods
- Check standard: a part, weight or reference material of known value is measured regularly.
- Cross-check: the same quantity is measured with two instruments and the results compared.
- Zero and single-point check: the zero and a critical point in the working range are checked.
- Comparison with a reference at a few points: the laboratory's own reference is used at a limited number of points.
How often?
Frequency is risk-based: the more critical, heavily used and easily affected the instrument, the more frequent the checks. A precision balance may be checked at the start of each working day, a reference thermometer monthly and a hand tool every few months. An extra check after transport, a drop or repair is good practice.
Example intermediate check form
| Field | Description |
|---|---|
| Instrument | Inventory no., serial no., location |
| Check standard | Part, weight or reference instrument used and its identification |
| Date and performed by | Who and when |
| Environmental conditions | Temperature, humidity if they affect the result |
| Measured values | Readings and mean |
| Acceptance limit | Limit defined in the procedure or control chart limits |
| Result | Pass / fail |
| Action if failed | Take out of use, send for calibration, impact assessment |
Using a control chart for intermediate checks
Charting intermediate check results shows much more than a list of "pass" entries: slow drift is noticed before a limit is exceeded. For example, a balance is checked every morning with a 200 g check weight. If the first 20 results have a mean of 200.0003 g and a standard deviation of 0.0002 g:
- Warning limits: mean ± 2s = 199.9999 g to 200.0007 g
- Action limits: mean ± 3s = 199.9997 g to 200.0009 g
A single value outside the action limits, or several consecutive values in the warning zone, signals that the instrument should be taken out of use and investigated. The numbers are illustrative; calculate limits from your own data. Control chart limits must also be consistent with the instrument's permissible error.
Tracking intervals and intermediate checks with software
As the number of instruments grows, tracking dates in Excel gets harder, and an overdue calibration is often noticed only at an audit. For companies that outsource calibration, KalDataV2 proposes an annual plan from instrument dates, records in-house intermediate checks and verifications, opens a task for the department when an intermediate check fails, and performs chained impact analysis. For organisations that calibrate in their own laboratory, departments open calibration and intermediate check requests in the In-House Calibration Software; on return the next date is calculated and reminder e-mails are sent. We cover reference instruments in reference device tracking.
Frequently Asked Questions
How is a calibration interval determined?
The owner sets it, considering the manufacturer's recommendation, frequency of use, environment, required uncertainty and calibration history. The ILAC-G24 / OIML D 10 guideline provides methods for this.
Must calibration be done once a year?
No. A yearly interval is a common starting point but not a requirement. It can be extended or shortened according to use and history, with the reason recorded.
Can the laboratory set the calibration interval?
ISO/IEC 17025 requires the laboratory not to put a recommended interval on the certificate unless agreed with the customer. The owner sets the interval.
What is an intermediate check?
A simpler and more frequent check than calibration, performed between calibrations to confirm the instrument still works as expected.
Does an intermediate check replace calibration?
No. It maintains confidence in the instrument but does not provide the traceable result and uncertainty evaluation of a calibration.
How often should intermediate checks be done?
According to the instrument's criticality, intensity of use and risk of being affected. An extra check after transport, a drop or repair is recommended.
What should an intermediate check form contain?
Instrument identification, check standard used, date and person, measured values, acceptance limit, result and the action taken if it failed.
What if an instrument fails calibration?
Take it out of use, have it adjusted or repaired and recalibrated, and assess whether measurements since the last good calibration were affected. Shorten the interval if needed.