Calibration, verification, and fitness for purpose
Separate adjustment, evidence against a reference, and the decision that an instrument is suitable for a particular measurement.
Field diagramA quality-check decision
Estimated reading
8 min
Estimated practice
10 min
Equipment
A supplied instrument record or manufacturer certificate; optional water-only balance or pipette example under an approved procedure.
Progress stays in this page session. Allow lesson storage in Cookie settings to remember it; browser settings may also prevent saving.
Objective
By the end, you can:
Explain the different roles of calibration, adjustment, verification, and validation.
Choose a check that matches the measurement range and intended decision.
Record evidence, acceptance basis, and disposition without inventing a tolerance.
Four questions that sound alike
Calibration compares an instrument indication with a reference and documents the relationship and uncertainty. Adjustment changes the instrument so it indicates differently; an adjustment may be part of a calibration service, but it is not the same evidence. Verification asks whether the instrument or method meets a stated requirement at the time and conditions of use. Validation asks whether the complete method is fit for its intended purpose. A manufacturer’s calibration certificate can support a claim, but it does not automatically prove that today’s method is suitable for every volume, temperature, or sample type.
Start with the decision. If the instrument will distinguish 9.8 from 10.2 units, a check designed only for a broad range may be irrelevant. Define the measurand, range, reference, conditions, acceptance criterion, and response to failure before collecting a check result. The criterion must come from a method, manufacturer specification, contract, regulation, or documented scientific rationale. Never copy a tolerance from an unrelated instrument.
Evidence is a chain
A useful record links instrument ID and software or configuration to reference identity, reference value and uncertainty, date, operator, conditions, raw indication, calculation, criterion, and decision. ISO/IEC 17025 frames competence and consistent laboratory operation; the practical lesson is to make the evidence auditable even when you are working on a small project. A failed verification does not prove every earlier result is wrong, but it does create a question about the affected interval and requires a documented investigation.
Calibration: what relationship did the reference establish?
Verification: did the current result meet the pre-stated criterion?
Fitness: is the uncertainty small enough for this decision?
Failure: what data and actions are affected?
Worked example
A bounded decision
A method needs a balance indication near 10 g and defines an allowable difference of 0.010 g. A check reference is 10.000 g with a documented uncertainty; the balance reads 10.006 g.
01Difference: +0.006 g.
02Decision: passes this stated check if the method’s comparison rule says the difference is within 0.010 g and uncertainty treatment is satisfied.
03Limit: this result does not establish performance at every load or for every sample.
Materials
Set out what you need.
Instrument record template
Manufacturer instructions
Reference certificate or supplied check data
Decision and deviation log
Safety & stop conditions
Pause if the work no longer fits the plan.
Use a supervised, approved procedure for any physical check; this lesson does not authorize opening or servicing instruments.
Use only water in practical demonstrations and stop if a balance, pipette, or container is damaged or contaminated.
Do not label an instrument calibrated or fit for purpose from a single informal observation.
Method
Work through the steps.
01
Define the intended use
Write the measurand, range, units, environment, and decision the measurement will support.
02
Find the basis
Locate the applicable manufacturer specification, method, certificate, regulation, or documented project criterion. Record its version or date.
03
Select the check
Choose a reference and check point that exercise the relevant range and uncertainty. State why the check represents the intended use.
04
Compare and decide
Record the raw indication, correction or difference, uncertainty information, criterion, and accept/investigate decision. Do not back-fit a criterion after seeing the result.
05
Control the consequence
If the check fails, identify the affected measurements, place the instrument in the method’s defined status, and document review or repeat actions.
Checkpoints
Observe, record, investigate.
The criterion existed before the result was interpreted.
Reference identity and uncertainty are recorded.
The check covers the range used rather than a convenient unrelated point.
The disposition states what happens to data if the check fails.
Common mistakes
Correct the process, preserve the record.
A certificate is treated as proof of current fitness.
Use it as part of the evidence, then verify current performance under the intended conditions when the method requires it.
Adjustment and calibration are used as synonyms.
Describe whether the instrument was compared, changed, or judged against a criterion.
A pass/fail limit is invented from display resolution.
Find a justified criterion or state that the check cannot support a pass/fail conclusion.
A failed check is fixed and the original result deleted.
Preserve the failed record, document the action, and assess affected data before repeating.
Practice
Read a verification record
Use this supplied record: a 10.000 g reference gives 10.006 g and a 50.000 g reference gives 50.021 g. The predeclared comparison limits are ±0.010 g and ±0.050 g respectively. Decide whether the record supports the intended 10 g water-weighing exercise, and identify missing evidence.
