A transmitter returns from the lab with a neat report, and most people only check the stamp and due date. The real value lies in the numbers inside, which tell you whether past readings could be trusted and how well the instrument performs now.
A calibration report is a technical record, not just a compliance sticker. This guide walks through every important field, explains pass and fail decisions and shows how to calculate error as a percent of span.
What Is a Calibration Certificate?
A calibration certificate is the formal record issued by a calibration laboratory that states the measured values of an instrument against traceable reference standards, the errors found, the measurement uncertainty and, where requested, a statement of conformity to a specification. For accredited work the content follows ISO 17025 laboratory requirements.
Quality Magazine lists the required contents of a calibration certificate, including a title, the lab name and address, a unique identifier, the customer, the method, the item, the dates, results with units, uncertainty, conditions and the authorising signature. Anything missing weakens the record during an audit.
Before reading a calibration certificate, it helps to know the vocabulary of error, accuracy and tolerance explained in instrument calibration common terms.
8 Vital Fields on a Calibration Certificate
Quality Magazine stresses that results before and after any adjustment must be reported where available. This baseline on the calibration certificate tells you whether the instrument was in tolerance during the whole previous period.
As Found Versus As Left Data
| Aspect | As Found | As Left |
|---|---|---|
| When recorded | Before any adjustment | After adjustment or repair |
| Main purpose | Judge past measurement quality | Prove present performance |
| If out of tolerance | Triggers an impact review | Must be within tolerance to return |
| Used for | Drift analysis and interval review | Starting point for next period |
Micro Precision notes that leaving out as found data prevents meaningful drift analysis and interval decisions. That drift history feeds directly into calibration interval determination.
If the as found data on a calibration certificate fails, the product or process measured since the last calibration may need review. Quality teams must record that impact assessment.
Error, Tolerance and Percent of Span
Error is the measured reading minus the applied reference value at each test point. Many a calibration certificate also lists a correction, which is the same value with opposite sign, as described in correction factor in calibration.
Error % of span = Error ÷ (URV minus LRV) × 100
Example, pressure transmitter 0 to 10 bar:
Applied 5.000 bar, measured 5.030 bar
Error = 0.030 bar
Error % span = 0.030 ÷ 10 × 100 = 0.30%
Tolerance 0.50% of span, result PASS
Five points across the range reveal zero, span and linearity faults, as used in level transmitter 5 point calibration. A single midpoint check hides most of them.
Percent of Span Error Calculator
Run the calculator for each test point on the calibration certificate, both upscale and downscale. The worst point decides the overall result.
Uncertainty and Traceability Explained
UKAS LAB 5 explains that the reported expanded uncertainty is a standard uncertainty multiplied by a coverage factor k of 2, giving a coverage probability of about 95%. The method behind the number is covered in measurement uncertainty in calibration.
Quality Magazine adds that the result and its uncertainty must be in the same or relatable units. An accredited lab also cannot report an uncertainty smaller than its calibration and measurement capability.
Traceability should be stated to the SI units through an unbroken chain, not simply as traceable to one national institute. Primary pressure references such as a dead weight tester sit near the top of that chain.
Decision Rules and Pass or Fail
Pass if the reading is within tolerance, ignoring uncertainty.
Acceptance limit is tolerance minus uncertainty.
Reading within tolerance but uncertainty crosses the limit.
UKAS LAB 5 requires the decision rule to be described in the calibration certificate unless it is already part of the cited specification. Agree the rule with the lab before sending instruments.
- Proves past readings were valid.
- Supports audits and ISO 9001 records.
- Reveals drift trends early.
- Helps set smarter intervals.
- Ignoring as found failures.
- Mixing units of uncertainty and error.
- Unstated decision rules.
- Expired reference standards.
Field teams often verify transmitters with a multifunction process calibrator between lab visits. The procedure in how to calibrate a pressure sensor shows the same as found and as left logic.
Troubleshooting related drift problems is covered in the calibration and troubleshooting guide.
Reporting Calibration Results PDF
Understanding Certificates Video
Calibration Certificate FAQ
It is the laboratory record of measured values, errors and uncertainty for an instrument checked against traceable standards. It may also state pass or fail against a tolerance that the customer specified on the order.
Accredited certificates follow the content rules in ISO 17025. Each one carries a unique number, the calibration date and an authorised signature.
As found data are the readings taken before any adjustment or repair is carried out. They show how the instrument really performed during the previous calibration period.
If these readings fail, past measurements may need an impact review. That review should be documented by the quality team together with any corrective action taken for affected products.
As left data are the readings recorded after the technician adjusts the instrument. They prove that the instrument is now within tolerance and fit to return to service.
These readings become the starting point for the next calibration period. Comparing them with the next as found results shows how much the instrument drifts over one full interval.
No measurement is perfect, so the laboratory states how much doubt surrounds each result. It is usually an expanded value calculated with a coverage factor k equal to 2.
That gives roughly 95% coverage for a normal distribution of errors. The uncertainty must be stated in the same units as the measured result, or in clearly relatable units.
It is the method used to decide pass or fail when measurement uncertainty is taken into account. Simple acceptance and guarded acceptance are the two most common rules in practice.
UKAS requires the rule to be stated clearly in the report. Agree the rule with the laboratory before sending instruments for calibration, ideally on the purchase order itself.
Accredited laboratories normally do not set a due date unless the customer asks for one. The user decides the interval from drift history, usage and measurement risk.
Some certificates show a recommended date when requested on the order. Treat that date as advice from the laboratory, not as a fixed rule for your own quality system and procedures.
Confirm the instrument identity, dates, reference standards and environmental conditions first. Then compare every as found and as left point with your own process tolerance for that tag.
Check that the uncertainty and the decision rule are clearly stated. File your review with the instrument history record so auditors and future engineers can trace every decision later.
Related Articles
- ISO 17025 Calibration Laboratory Requirements
- Measurement Uncertainty in Calibration
- Instrument Calibration Common Terms
- Calibration Interval Determination
- Correction Factor in Calibration
External References
- LAB 5 Reporting Calibration Results, UKAS
- How to Read and Interpret ISO/IEC 17025 Calibration Certificates, Quality Magazine
- Calibration, Wikipedia
What We Learn Today
- A calibration certificate records errors, uncertainty and traceability for each test point, and every field matters when auditors review your measurement history.
- As found data judge the previous period while as left data prove present performance, and together they reveal drift for smarter interval decisions.
- Pass or fail depends on tolerance, expanded uncertainty and a stated decision rule, so always agree that rule with the laboratory in advance.