What does the manufacturer distinguish?
NI's Relationship Between NPLC and Reading Rate in DMM Specification, updated 7 February 2026, distinguishes aperture duration from the reading process, which also involves settling. Its example specifications concern the PXI-4065 and are not a universal timing table for other meters.
Commissioning Docs is independent and is not affiliated with NI. This guide interprets documentation; it does not prescribe a test connection, setting, accuracy claim or noise-rejection guarantee.
How is a cycle count converted into time?
Illustrative example. For a fictional acquisition using 1 PLC and a confirmed 60 Hz line-frequency basis, the integration time is 1 / 60 second, approximately 16.67 ms. The same 1 PLC on a 50 Hz basis is 1 / 50 second, or 20 ms. The frequency basis belongs beside the cycle count.
These calculations describe the integration window only. Taking the reciprocal and labelling it the achieved reading rate ignores other acquisition stages. The actual instrument specification, mode and evidence control the timing interpretation.
Which fields make the acquisition record reviewable?
Keep values with their units and their source. A setting displayed in cycles and an observed interval displayed in milliseconds are different fields.
| Field | Evidence to retain |
|---|---|
| Instrument and mode | Exact model, firmware and measurement function |
| Integration basis | NPLC and confirmed frequency basis |
| Time basis | Aperture in seconds or milliseconds |
| Other stages | Applicable settling and instrument processing |
| Observed timing | Original data and actual reading timestamps |
| Decision | Approved method and qualified technical review |
What should happen if the timing is unclear?
Preserve the original trace, settings and instrument reference. Ask the responsible technical reviewer whether the acquisition chain supports the claimed observation. Do not replace missing timing evidence with the inverse of the NPLC conversion or a display refresh rate.
This is narrower than a logger saving interval: the question is how one measurement is acquired. The responsible commissioning professional controls the approved method. Engineering sign-off is performed only by the registered engineer on the job.
Common questions
What else should the bid team know?
Does 1 PLC always mean 20 ms?
No. The time depends on the confirmed line-frequency basis.
Does aperture alone prove achieved readings per second?
No. Retain the complete instrument and mode-specific timing evidence.
Can this example approve a meter setting?
No. Follow the approved procedure and responsible technical review.