In this article8 sections
An instrument calibration certificate is the record of one calibration. It names the instrument that was tested and its serial number, the procedure used, the dates, the result at each test point with its unit, the uncertainty of those results, the conditions during the work, how the measurements trace back to national standards, and the people who performed the calibration and authorized the certificate. When the instrument was adjusted or repaired, it also gives the results from before and after. For a field instrument those two sets of readings are the as-found and as-left record, and deliverable lists name the calibration reports that carry them among a plant's commissioning and maintenance documents.
The certificate records what was measured. The procedure, the interval between calibrations and the tolerance an instrument has to meet come from the plant's calibration program, the manufacturer's specification, and the quality system and regulations that apply to the site.
In short
- A calibration certificate covers one calibration of one identified item, with the method, the dates, the results and their units, the uncertainty, the conditions, the traceability and the names of the people responsible.
- As-found is what the instrument read before anyone adjusted it. As-left is what it read when the work was finished. When nothing was adjusted, the two are the same.
- One calibration's as-left readings, set beside the next calibration's as-found readings, show how far the instrument drifted in between.
- A laboratory's certificate does not set the next due date. The interval and the pass mark belong to the plant's calibration program.
- The record joins the instrument index, the data sheet, the I/O list, the loop folder and the maintenance system through the tag, the serial number and the calibrated range.
Three documents that get called a calibration certificate
| Document | Who issues it | What it records |
|---|---|---|
| Certificate of test | The manufacturer, at production | That the instrument passed its stated accuracy in production, against its serial number, without every measured result |
| Calibration certificate | A calibration laboratory, or the manufacturer's factory calibration | Each test point and its result against tolerances and uncertainties, with the conditions and the traceability, for one serial number |
| Calibration report | The plant's instrument technicians or a calibration contractor | The as-found and as-left readings of an installed instrument, the date, the technician, the test equipment used and any deviations |
Deliverable lists keep the vendor's papers and the plant's reports apart. One list of 82 instrumentation documents names the instrument test certificates supplied by the vendor, with batch numbers, traceability, tolerance and test range, and separately the instrument calibration reports, with the as-found and as-left data, the date, the technician's name, the test equipment and any deviations.
A calibration report on an installed instrument leans on another certificate. The report names the test equipment, and each calibrator carries its own laboratory certificate with its own traceability statement. That chain, from the field instrument to the calibrator, to the laboratory's standards, to a national metrology institute, is what makes a field reading traceable.
What a calibration certificate carries
NIST's recommended procedure for preparing calibration certificates (SOP 1, 2019) lists their contents against section 7.8 of ISO/IEC 17025:2017, the standard for the competence of testing and calibration laboratories, and publishes a review checklist built on the same items.
| Field | What it records |
|---|---|
| Title | A title such as "Calibration Certificate", so the document states what it is |
| Laboratory | Its name and address, and any other location where the work was done, such as a temporary or mobile facility |
| Certificate number | A unique number, each page marked as part of the whole (page 1 of 2), and the end of the certificate clearly marked |
| Customer | The customer's name and contact details |
| Method | The procedure used, with its full title and version date, and any addition, deviation or exclusion the customer approved |
| Item | An unambiguous description and identification of the instrument, such as its serial number, and its condition when that matters |
| Dates | Receipt of the item, the calibration itself and the issue of the certificate |
| Results | The result at each test point with its unit, set out in a table |
| Before and after | The results before and after, when the item was adjusted or repaired |
| Uncertainty | The measurement uncertainty in the same unit as the result, with its coverage factor and level of confidence |
| Conditions | The environmental or other conditions that influence the results |
| Traceability | A statement of how the measurements are metrologically traceable, for example to the International System of Units through a national metrology institute |
| Conformity | Where the certificate states pass or fail, the results it covers, the specification they were judged against and the decision rule applied |
| People | The name, title and signature of who performed the calibration and who authorized the certificate |
| Scope | A statement that the results relate only to the item calibrated |
The certificate leaves two things out on purpose. The first is the next due date. Under section 7.8.4.3 of ISO/IEC 17025, a calibration certificate or label carries no recommended calibration interval, except where a regulation requires one or the customer asked for it. The second is a tolerance of its own. The uncertainty describes the measurement, and NIST's example certificate says its expanded uncertainty "is not to be confused with a tolerance limit for the user during application". A pass or fail on a certificate is a conformity statement against a named specification, with the decision rule stated beside it.
The as-found and as-left record
The as-found test is the first test on an instrument, before any adjustment. If its results fall outside the instrument's specification, the instrument is adjusted, and a second test, the as-left test, shows it within specification across its range. When nothing was adjusted, the as-left readings are recorded as the same as found.
Keeping both is what makes the record useful later. The calibration guide of the HART Communication Foundation puts it in one sentence, "By comparing the As-Left data from one calibration to the As-Found of the next calibration, you can determine the instrument's drift." A record that keeps only one of the two loses that comparison, and excessive drift is often a sign that an instrument is heading for failure.
The as-found readings also answer a question about the past. A practitioner exercise on the subject sets it in a pharmaceutical plant. A recorder logging the temperature of a critical batch vessel is calibrated every month, and months later patients report complications from a batch processed in that vessel. Whether the recorder read correctly while that batch was made can be answered only from the as-found readings of the calibrations around it. Regulation writes the same logic down where it applies. In US drug manufacturing, 21 CFR 211.68 has automatic, mechanical and electronic equipment "routinely calibrated, inspected, or checked according to a written program designed to assure proper performance", with written records of those checks kept. Among its laboratory controls, 21 CFR 211.160 calls for a written calibration program with limits for accuracy and precision and "provisions for remedial action in the event accuracy and/or precision limits are not met".
The usual layout is a table of test points with the ideal response, the actual response and the error at each point. The error is the instrument's reading minus the standard's, so its sign matters. An instrument that reads -79 °C against a standard at -78.112 °C has an error of -0.888 °C and reads low. As a percentage of span, the error is the deviation divided by the span, times 100. From the same readings, the HART Communication Foundation's guide derives the maximum error, the zero error (the error at the lower range value), the span error, the linearity and the hysteresis, and hysteresis needs points taken in both directions.
One transmitter, found and left
In this invented example, PT-101 in Unit 34 is a pressure transmitter calibrated 0 to 10 bar g with a 4 to 20 mA output, so its span is 16 mA. The technician applies five pressures rising through the range from a pressure calibrator, reads the output with a milliamp calibrator, and records the as-found readings before touching the transmitter.
| Point | Applied (bar g) | Ideal (mA) | As-found (mA) | Error (% of span) | As-left (mA) | Error (% of span) |
|---|---|---|---|---|---|---|
| 0% | 0.00 | 4.000 | 4.040 | +0.25 | 4.000 | 0.00 |
| 25% | 2.50 | 8.000 | 8.056 | +0.35 | 8.008 | +0.05 |
| 50% | 5.00 | 12.000 | 12.072 | +0.45 | 12.000 | 0.00 |
| 75% | 7.50 | 16.000 | 16.088 | +0.55 | 15.992 | -0.05 |
| 100% | 10.00 | 20.000 | 20.104 | +0.65 | 20.000 | 0.00 |
As found, PT-101 reads high at each point, and the error climbs from 0.25 to 0.65 percent of span, a zero error of 0.25 percent with a span error of 0.40 percent on top. Whether that put PT-101 out of tolerance is decided by the tolerance the plant set for it, and the as-found readings are kept either way. After the adjustment, the as-left readings sit within 0.05 percent of span. At the next calibration, the as-found readings set beside this as-left column show how far PT-101 moved in between.
The table travels with the facts that make it usable later. The record names the tag and the serial number, the calibrated range, the date, the technician, the test equipment, so that each calibrator's own certificate can be found, and any deviation from the procedure.
Smart transmitters carry two readings
A HART transmitter turns its sensor's measurement into a digital process variable, then converts that value to its 4 to 20 mA output, and the two steps are tested and trimmed separately. A sensor trim corrects the digital reading, read with a communicator against the applied input. A 4 to 20 mA trim corrects the analog output against a milliamp calibrator. When the digital value is used for monitoring or control, the input section is tested and adjusted on its own, and when the milliamp output is used, so is the output section.
The same guide draws a line the record has to keep. A range change made with a communicator references no calibration standard, so the guide counts it as a configuration change. Pressing the zero and span buttons to correct a zero shift moves the range instead of correcting the sensor reading, and the guide's example ends with a transmitter ranged 1 to 101 inches of water whose digital reading still shows 1 inch with both ports vented. A re-range is recorded as a change of range, because a control system still scaled to the old range reads the signal wrongly.
Where the record meets the other documents
| Document | What it shares with the calibration record | The mismatch to look for |
|---|---|---|
| Instrument index | The tag, its service and its calibrated range | A record for a tag the index does not list, or a range it does not show |
| Instrument data sheet | The calibrated range, the output signal and the manufacturer's accuracy | A transmitter calibrated to a range other than its data sheet's |
| I/O list and control system scaling | The range and units the control system scales the signal to | A transmitter re-ranged in the field while the I/O list and the control system keep the old range |
| Loop folder | The loop's test records from commissioning on | A loop folder without the as-left record of its transmitter |
| Test equipment records | Each calibrator used, with its own certificate | A calibrator whose certificate had lapsed on the day it was used |
| Maintenance system and asset register | The serial number, the calibration history and the next due date | A serial number on the record that is not the one installed |
Keeping the record
A certificate that has to change after issue is not edited in place. The change goes out as a further document marked "Amendment to Calibration Certificate", or as a new certificate with its own number that refers to the one it replaces, and the laboratory keeps legible copies of each certificate it issued or amended. On the plant side, calibration reports feed maintenance planning and audits, and the history they build, one as-found and as-left pair at a time, is the evidence of how each instrument holds its calibration.