As-found and as-left are the paired before-and-after readings that turn a calibration from a private adjustment into a documented, defensible record. As-found captures how the instrument was reading when the technician first arrived; as-left captures how it read after any correction. Together they prove whether the instrument had drifted and by how much - the exact evidence an auditor, a custody-transfer review, or a regulator wants to see. This guide explains what each reading is, why the pair matters more than the adjustment itself, and how the record feeds oil and gas measurement compliance.
As-Found / As-Left Calibration in one line: As-found is the set of readings taken from an instrument exactly as it was discovered, before any adjustment, and as-left is the set of readings taken after the technician has finished adjusting it; the paired records document whether the device had drifted out of tolerance and confirm it was returned to specification, forming the auditable proof of calibration that measurement and quality reviews require.
When a technician calibrates a transmitter, the temptation is to just fix it and move on. As-found/as-left discipline says: record what it read first, before touching anything. The as-found data - the instrument's output at several input points compared against a reference standard - is a snapshot of its actual condition in service. Only after that snapshot is captured does the technician adjust the zero and span or trim the digital values, and then take the as-left readings to confirm the device is now within tolerance. The adjustment is the same either way; the value is in bracketing it with a documented before and after.
The pair matters because it answers two different questions. As-left proves the instrument is good going forward - it will measure correctly until the next calibration. As-found proves how good the instrument was during the period just ended - the interval since its last calibration. That second question is the one people forget and the one that carries the most weight. If a flow transmitter is found reading two percent high, every measurement it produced since its last good calibration is suspect, and only the as-found record makes that error visible and quantifiable.
The gap between as-found and as-left is the physical evidence of drift. An instrument that reads the same as-found as as-left had not drifted and needed no correction - a valuable finding in itself, because it justifies stretching the calibration interval. An instrument whose as-found readings sit outside tolerance and whose as-left readings are back inside it has documented, in numbers, both that a problem existed and that it was fixed. Over many cycles, the history of as-found deviations reveals how fast a given instrument or instrument type drifts, which is the rational basis for setting calibration frequency rather than guessing.
This is also why as-found must be recorded before adjustment, without exception. If a technician adjusts first and documents afterward, the as-found is lost forever and there is no way to reconstruct how the instrument was behaving in service - the calibration proves the device is good now but says nothing about the measurements it produced before. In regulated measurement, that missing history can invalidate an entire period of data. The rule is simple and rigid: capture as-found untouched, then adjust, then capture as-left.
In oil and gas, as-found/as-left records are central to the audit trail that custody transfer, allocation, and regulatory measurement demand. When money changes hands based on a meter reading, both parties and any reviewing authority need proof that the measuring instruments were within tolerance during the accounting period - and if one was not, the as-found deviation drives the correction or reallocation of the disputed volumes. Measurement audits and regulatory reviews routinely ask to see the calibration certificates with their as-found and as-left columns; a certificate showing only the final good reading is incomplete because it hides whether an error existed.
A cloud SCADA platform strengthens this record without replacing the calibration itself. The calibration is performed at the instrument with a reference standard and documented on a certificate, but Merobix provides the surrounding evidence: the continuous trend of the tag before and after the calibration visit, which shows the operational impact of a discovered error, and an event log noting when the instrument was serviced. If an as-found record reveals a transmitter was reading high, Merobix's history lets an engineer see exactly what that tag reported over the affected period and support any volume correction from a browser. The paired readings live on the certificate; the platform ties them to the live measurement history that gives them meaning.
As-found readings are taken from the instrument exactly as discovered, before any adjustment, and show its actual condition in service. As-left readings are taken after the technician has adjusted it and confirm it is back within tolerance. As-found documents how the device was behaving in the interval just ended; as-left documents that it is good going forward.
Because once you adjust, the original in-service behavior is gone and cannot be recovered. If a technician adjusts first, there is no proof of how the instrument was reading during the period it was in service, so any measurements it produced cannot be validated or corrected. In regulated measurement, a missing as-found record can invalidate an entire accounting period of data.
Because they prove both that an instrument was within tolerance and, if it was not, by how much it had drifted. In custody transfer and allocation, money depends on meter accuracy, so reviewers need evidence the instruments were good during the accounting period. An as-found deviation drives any volume correction, so a certificate showing only the final good reading is considered incomplete.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.