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Guide

How to Read a Calibration Certificate

A calibration certificate looks like paperwork and is actually the only proof an instrument reads what it claims. Four sections carry the meaning: the as-found results, the stated uncertainty, the reference standards used, and the traceability chain behind them.

By the Akermin Editorial Team · 3 min read

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Read a calibration certificate for four things. The as-found results: what the instrument read at each test point when it arrived, compared with the reference, and whether that was within tolerance. The uncertainty: the ± figure with a coverage factor (usually k=2, about 95 percent confidence) that qualifies every result. The reference standards: what the instrument was compared against and their own calibration status. Traceability and accreditation: the chain to a national standard and whether an accredited laboratory (ISO/IEC 17025) issued it. As-left results, conditions, dates and the recommended interval complete the picture.

Sections and what they mean

Section Contains Why it matters
Identification Instrument model, serial number, owner, certificate number, date Confirms the certificate belongs to this unit
Procedure and conditions Method, temperature, humidity during calibration Results are valid under those conditions
Reference standards Instruments used as references, their certificate numbers and due dates The chain of trust
As-found results Reference value, instrument reading, error, tolerance, pass/fail at each point How the instrument performed before adjustment; validates past data
As-left results Same after any adjustment How it performs now
Uncertainty Expanded uncertainty with k factor per point Qualifies every number
Traceability statement Link to NIST, NPL, PTB or another national institute Makes results comparable worldwide
Accreditation UKAS, A2LA, DAkkS, NATA or similar logo and scope Independent assurance of the laboratory’s competence
Recommended interval Suggested time to next calibration Planning; not a guarantee

As-found versus as-left

As-found tells you whether measurements made since the last calibration were trustworthy. If an instrument arrives out of tolerance, data from the previous period may need correcting or discounting. As-left tells you how it performs now. A certificate showing only as-left values has hidden the more important half. See calibration vs adjustment.

Uncertainty

Each result carries an expanded uncertainty, for example ±0.15 °C (k=2). The instrument’s error at that point is known only to within that range. The uncertainty must be smaller than the tolerance being tested for the pass/fail to mean anything; a common rule is a ratio of at least 4:1. See what measurement uncertainty is.

Traceability and accreditation

Traceability means the reference standards were calibrated against higher standards in an unbroken chain to a national institute, each step with stated uncertainty. Accreditation to ISO/IEC 17025 means an independent body has assessed the laboratory’s competence for the listed scope. An accredited certificate carries the accreditation body’s mark and a scope reference; a non-accredited “traceable” certificate relies on the laboratory’s own claim.

Reading a result line

Example: Reference 50.0% RH; Instrument 51.3% RH; Error +1.3%; Tolerance ±2.0%; Uncertainty ±0.8% (k=2); Result: Pass. Interpretation: at 50 percent RH the instrument reads about 1.3 points high, known to within 0.8 points, and that is inside its ±2 percent specification. A reading of 60 percent on this instrument probably corresponds to about 58.7 percent true.

What to do with it

  1. File it with the instrument’s serial number.
  2. Apply corrections if the errors are significant for your use.
  3. Note the calibration date and set a reminder for the interval.
  4. If as-found was out of tolerance, review the data taken since the previous calibration.

Instruments that ship with certificates, such as professional loggers and multimeters, are covered across the environmental monitoring and electrical testing categories; the comparison of specifications is in how to compare sensor accuracy, resolution and repeatability.

Data Logger, Temperature and Humidity, USB

Data Logger, Temperature and Humidity, USB

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Fluke 117 Digital Multimeter, Non-Contact AC Voltage Detection, Measures Resistance/Continuity/Frequency/Capacitance/Min Max Average, Automatic AC/DC Selection, Low Impedance Mode

Fluke 117 Digital Multimeter, Non-Contact AC Voltage Detection, Measures Resistance/Continuity/Frequency/Capacitance/Min

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

What is the difference between a calibration certificate and a certificate of conformance?

A calibration certificate reports measured values against a reference with uncertainties. A certificate of conformance states the product meets its specification without reporting measurements. Only the former tells you how the instrument actually performs.

What does traceable to NIST mean?

The reference used to calibrate the instrument was itself calibrated against a standard that links, through an unbroken chain of comparisons with stated uncertainties, to a national metrology institute.

Does a calibration certificate expire?

It reports performance on the date of calibration. The recommended interval printed on it is the period after which performance should be re-checked; the certificate does not guarantee anything beyond the calibration date.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.