The short answer
Calibration Checklist
A calibration event is a procedure with a beginning and an end that are as important as the middle: before the instrument is touched, operations is told, the loop is placed in manual or the controller is set to hold, the alarms on the tag are inhibited with a record, the reference standard is confirmed in date and adequate for the tolerance, and the instrument is isolated safely; the as-found test is recorded before any adjustment, the adjustment is made only if the error exceeds the tolerance, the as-left test is recorded, and a loop check confirms the value reaches the display correctly; then the isolation is removed, the loop is returned to automatic, the alarm inhibits are cleared, the operator confirms the reading, the record is completed with the standard used and the next due date, and the instrument carries a label that says when it was done and when it is due.
Key points
- Tell operations, put the loop in manual, inhibit the alarms with a record: the process must not act on a calibrator.
- The reference standard is in date and at least four times better than the tolerance, and its identity is on the record.
- As-found before any adjustment; adjust only if out of tolerance; as-left after.
- The loop check to the display is part of the calibration; a correct transmitter on a wrong channel has not been calibrated.
- Restore everything: isolation, mode, alarm inhibits, and confirm with the operator.
- Record, label, and set the next due date; a calibration without a record did not happen.
Before
| Step | Check | Note |
|---|---|---|
| Work order | The instrument tag, the procedure, the tolerance, and the range, from the instrument list and the calibration schedule | The tolerance comes from the loop requirement, not the transmitter specification |
| Notify operations | The operator knows which tag, when, and what the display will do | The reading will be wrong for the duration |
| Loop in manual | Any controller using the tag placed in manual or hold; any interlock on the tag reviewed and, if it must be bypassed, bypassed by procedure with a record | A calibrator pressure on a transmitter that trips a pump is an outage |
| Alarms inhibited | The alarms on the tag shelved or placed out of service with the reason and the time | They are restored at the end; the record proves it |
| Reference standard | In calibration, certificate on file, accuracy at least four times better than the tolerance, suitable for the range | Serial number on the record |
| Safety | The isolation valves, the bleed, the process pressure and temperature, the chemical, the electrical hazard, the PPE, and the permit if one applies | A pressure transmitter on hypochlorite is a chemical hazard |
| Isolate | Close the root valve; vent or drain; connect the calibrator; confirm the instrument sees the calibrator and not the process | For an analyzer: sample flow stopped, the sensor accessible |
The test
- 1
As-found
Apply the test points across the range, rising and falling, and record the instrument output at each. Do not adjust anything yet.
- 2
Compute the errors
Each point against the reference, as a percent of span. Compare with the tolerance.
- 3
Decide
In tolerance at every point: no adjustment; as-left equals as-found. Out of tolerance: adjust.
- 4
Adjust
Per the instrument procedure: zero trim, span trim, sensor trim, or the analyzer calibration sequence. Record what was adjusted.
- 5
As-left
Repeat the test points and record them. They are within tolerance or the instrument is replaced.
- 6
Loop check
With the calibrator at one or two points, read the value on the HMI and in the controller. It matches the instrument within the loop tolerance, in the right units, on the right tag.
After
| Step | Check |
|---|---|
| Restore the instrument | Calibrator removed; the bleed closed; the root valve opened; the sample flow restored; the transmitter reading the process |
| Compare with the process | The reading is plausible against the process and any local gauge or reference |
| Restore the loop | The controller returned to automatic, bumpless, with the operator watching; any bypass removed with its record closed |
| Restore the alarms | Inhibits cleared; the alarm list shows the tag active and normal |
| Confirm with operations | The operator agrees the reading is right and the loop is behaving |
| Record | As-found and as-left values, errors, adjustments, the standard used with its serial and due date, the loop check result, the technician, the date, pass or fail |
| Label | The calibration label on the instrument with the date, the technician, and the next due date |
| Schedule | The next due date entered in the calibration schedule, adjusted if the as-found drift warrants a shorter or longer interval |
Special cases
- Analyzers: the calibration is against a standard solution or a reference method, the sample system is part of the test, and the as-found is taken before the sensor is cleaned as well as after.
- Level: the reference is a tape or a sight glass at two levels; the transmitter reference point is re-measured.
- Flow: verification against a clamp-on meter or a drawdown, since a flowmeter is rarely calibrated in place; electronic verification for magnetic meters.
- Chemical feed pumps: the drawdown test with the calibration column is the calibration.
- Compliance instruments: the procedure and the record format the regulator expects, and the frequency the rule requires.
Frequently asked questions
- Can the operator just watch the HMI instead of inhibiting alarms?
- The operator will be distracted, the notification system will page the on-call person, and the journal will fill with alarms that were not real. Inhibit with a record, restore with a record. It takes a minute each way.
- What if the reference standard is out of date?
- Do not calibrate with it. A calibration against an uncertified standard proves nothing and can make a good instrument worse. Get the standard calibrated or use another, and put the standard calibration on a schedule so this does not recur.
- Do we adjust an instrument that is barely within tolerance?
- No. Adjusting a passing instrument loses the drift record and adds its own uncertainty. Note the trend, and if it is approaching the limit, shorten the interval so it is checked before it fails.
- Who signs the record?
- The technician who did the work, and where the plant requires it, a reviewer. The record is the evidence for the regulator and the investigation, and an unsigned record is a claim.
Related topics
- Calibration ProceduresHow a utility calibrates its instruments consistently: the as-found and as-left discipline, tolerance from the loop rather than the data sheet, the reference standard and its traceability, the five-point check with hysteresis, adjusting only when needed, intervals set from history, and the procedure written once per instrument type.
- Calibration DocumentationThe records a calibration program produces and what each is for: the calibration certificate per event, the instrument history, the reference standard file, the schedule, the tolerance and interval register, and how the records are organized so a regulator, an auditor, or the next technician can find what they need.
- Loop ChecksProving every I/O point end to end before a system goes live: what a loop check is and is not, the three-point analog check, discrete and output checks, the two-person method with the sheet that records it, what to do with a point that fails, and why a loop check is not a calibration.
- Pressure CalibrationWhat calibrating a pressure transmitter means: as-found and as-left, the five-point test and hysteresis, tolerance and the reference standard, sensor trim versus output trim versus reranging on a smart transmitter, gauge versus absolute, and intervals.
- How to Calibrate a Pressure TransmitterA five-point calibration with a pressure source and a reference: isolate and vent, record as-found, decide whether to adjust, trim the sensor and the output separately, verify as-left, and record it so the next calibration means something.
- Alarm ShelvingThe operator tool for temporarily silencing a nuisance alarm: what shelving is and is not, the rules a philosophy sets, who may shelve what and for how long, automatic unshelving, the shelved list, the record, and how to implement it on any platform.
Direct contact
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