A pH meter’s service label may be current while today’s check result is unacceptable. Before measuring the next sample, the analyst needs to know whether the instrument can be used and what must happen if it cannot.
Use the laboratory’s readiness procedure for that instrument and measurement method.
Choose checks for the measurement
A pH procedure might cover the electrode, buffers, temperature handling, and response stability. A balance procedure might cover cleanliness, levelling, operating conditions, and a specified performance check. Set the checks and frequency using equipment instructions, experience, and the consequences of an unreliable result.
For example, the Fisher pH/Ion 510 manual discusses buffers around the expected sample range and warns against reusing calibration solutions. Those instructions concern that equipment; check the manual for the instrument actually on your bench.
Distinguish calibration, verification, adjustment, and maintenance
The international metrology vocabulary distinguishes four activities that can appear in an equipment history:
- Calibration: establishing the relationship between reference values, indications, and measurement results, including uncertainty under specified conditions.
- Verification: establishing that defined requirements have been met.
- Adjustment: changing the measuring system to give prescribed indications; subsequent recalibration is usually needed.
- Maintenance: care or servicing, such as cleaning an electrode or replacing a component.
Record what the instrument’s “calibrate” operation does and the results it produces. Include any changed settings. NIST’s traceability discussion also explains why a list of logged activities alone does not establish metrological traceability.
Keep the readings with the acceptance criteria
| Part of the check | Information to keep |
|---|---|
| Equipment | Asset, probe or accessory, and configuration |
| Method | Procedure and revision |
| Reference material | Identity, lot, assigned value and relevant status |
| Conditions | Temperature and other relevant observations |
| Result | Actual readings, units and original output where applicable |
| Assessment | Criteria, operator, time and required review |
| Outcome | Ready, restricted or unavailable, with follow-up |
A pass box alone hides how close a result was to its limit. Keep the criteria that applied at the time, even if a later procedure changes them.
What to do with a failed check
In a fictional example, a pH check fails before a new sample set. The analyst records the result and restricts use under the local procedure. Investigation considers the buffer, electrode, temperature, and execution of the check.
If the fault might affect earlier work, identify the measurements made since the relevant satisfactory evidence. Assess the likely effect on those results. A failure does not automatically invalidate all earlier measurements, and a later pass does not explain the original failure.
Record the investigation, any justified repeat work, and the basis for return to use. Keep the instrument’s current status visible to the next analyst. Longer-term repairs belong in its maintenance history.
Sources: BIPM-hosted metrology definitions for calibration, verification and adjustment; Fisher pH/Ion 510 manual; NIST measuring-system traceability discussion.
