The short answer
Calibration Procedures
A calibration procedure compares an instrument against a reference of known accuracy at several points across its range, records what it read before any adjustment, decides from a tolerance whether adjustment is needed, adjusts if so, records what it reads after, and files the result. The tolerance comes from what the loop needs, the reference is traceable and at least four times more accurate than the tolerance, the points include both directions to show hysteresis, and the interval is set from how the instrument has drifted in the past. Written once per instrument type and followed every time, the procedure is what makes a calibration record mean something.
Key points
- As-found before touching anything. The as-found record is the reason to calibrate.
- Tolerance is what the loop needs, decided before the test, not the accuracy on the data sheet.
- The reference is traceable and at least four times better than the tolerance.
- Five points, up and down. Adjust only when a point is out of tolerance.
- Set the interval from the as-found history: extend it when the instrument passes, shorten it when it drifts.
Why a procedure
Calibration done differently by each technician produces records that cannot be compared: one adjusts every instrument to zero error, one checks two points, one uses a gauge that was last checked in the previous decade. The value of a calibration program is the history it builds, and the history is only useful if every entry was produced the same way. A procedure per instrument type, a page or two, followed by everyone, is the whole difference.
The elements
- Tolerance
- The maximum error acceptable for the loop the instrument serves, expressed as a percent of span or in engineering units. A wet well level for pump control may tolerate 1 percent; a compliance flow meter or a chlorine residual analyzer tolerates what the permit and the method allow. The tolerance is decided by the engineer and written on the instrument list before the first calibration. It is not the accuracy printed on the transmitter data sheet, which is what the instrument can do, not what the loop needs.
- Reference standard
- The calibrator, test gauge, or standard solution the instrument is compared against. It is traceable, meaning its own calibration is documented back to a national standard through an unbroken chain, and it is at least four times more accurate than the tolerance, ten times where practical. Its calibration due date is checked before use and recorded on the sheet.
- Test points
- Five points across the range, 0, 25, 50, 75, and 100 percent, applied increasing and then decreasing, so that hysteresis and linearity show. Three points for a simple loop where the procedure says so; more for a compliance instrument. Analyzers are calibrated at the points their method prescribes, usually one or two standards.
- As-found
- The readings at every point before any adjustment. This is the record of how the instrument had drifted since the last calibration, and it is what sets the interval and what a regulator reads.
- The decision
- If every as-found point is within tolerance, the instrument passes and is not adjusted. If any point is out, it is adjusted per the instrument procedure, zero and span or the sensor trim, and the points are repeated.
- As-left
- The readings after adjustment, or a copy of the as-found readings where no adjustment was made, with every point within tolerance.
- The record
- Tag, date, technician, reference standard and its due date, tolerance, as-found, as-left, adjustment made, pass or fail, and the next due date.
A procedure by type
| Instrument | Reference | Points | Notes |
|---|---|---|---|
| Pressure transmitter | Pressure calibrator or deadweight tester; milliammeter for the output | Five, up and down | Isolate and vent; sensor trim and output trim separately; the pressure transmitter page |
| Level transmitter, hydrostatic | Pressure calibrator or a known head of water | Five, up and down | Specific gravity in the calculation; vent tube on submersibles |
| Level transmitter, radar or ultrasonic | Tape measure or a target at known distances | Two or three levels | Configuration verification more than calibration; the radar page |
| Flow meter, magnetic | Manufacturer verification tool for the electronics; a volumetric or a comparison meter for the whole meter | Verification of coil and electrode values; two or three flows where a comparison is possible | Most mag meters are verified electronically and compared against a reference flow periodically |
| Flow, differential pressure | Pressure calibrator on the DP transmitter | Five points on DP, checked through the square root | The element is inspected, not calibrated; the DP flow page |
| Temperature | A dry block or a bath with a reference thermometer | Three points across the range | Immersion depth and stabilization time matter |
| pH | Buffers, two points bracketing the process | Two | Slope and offset recorded; the pH page |
| Chlorine residual | DPD grab sample on a calibrated bench instrument | One point at the process residual, zero on chlorine-free water | Compare before adjusting; the chlorine page |
| Turbidity | Primary formazin standard for calibration; secondary sealed standard for verification | Per the instrument method | Compliance records; the turbidity page |
| Analog input channel | Milliamp source at the panel terminals | Three or five | A card channel is calibrated like an instrument when the loop tolerance is tight |
Setting the interval
An instrument that has passed as-found at every calibration for two years is not drifting, and its interval can be extended. One that fails every six months is drifting and its interval is shortened, or it is replaced. The starting interval is set from the manufacturer guidance and the criticality, commonly six months for compliance and control-critical instruments and a year for the rest, and the as-found history moves it. Regulatory instruments have minimum frequencies the permit sets, and those are the floor.
The written procedure
- Scope: the instrument type and the tags it applies to.
- Safety: isolation, process notification, lockout, chemical handling.
- Equipment: the reference standard by type and the accuracy required.
- Preparation: how to isolate, vent, connect, and stabilize.
- Test points and the order.
- Tolerance and the decision rule.
- Adjustment method, by instrument model where it differs.
- Return to service and the live check.
- Record: the form and where it is filed.
- Revision and approval, so the procedure itself is under control.
Frequently asked questions
- What accuracy ratio between the reference and the instrument is required?
- Four to one is the widely used minimum: the reference uncertainty is no more than a quarter of the tolerance being checked. Ten to one is comfortable. Below four to one, the reference error is a significant part of what is being measured, and the result cannot be trusted at the tolerance.
- Do we need to send our calibrators out for calibration?
- Yes, on a schedule, to a laboratory that provides a traceable certificate, typically annually. A calibrator with no current certificate is not a reference, and every calibration done with it is undermined. The certificate and the due date are part of the calibration record.
- Can a calibration be done in place without isolating the instrument?
- A comparison against a reference at the operating point can be done in place and is valuable as a check between calibrations. It does not exercise the range and it is not a calibration for the record unless the procedure for that instrument says a single-point comparison is acceptable, which is the case for some analyzers.
- What do we do with an instrument that fails as-found badly?
- Record the failure, adjust or replace it, and ask what the loop did with the wrong value between the last calibration and this one. A compliance instrument that was out of tolerance for months may require reporting; a control instrument may explain a process problem. The as-found record is what makes that question answerable.
Related topics
- 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.
- 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.
- Chlorine Residual AnalyzersAmperometric and colorimetric residual analyzers: what each measures, free versus total chlorine, pH and flow dependence, sample line design, calibration against a DPD grab sample, and using the signal for feed control.
- Turbidity MeasurementNephelometric turbidimeters for filter effluent and raw water: how 90-degree scattering is measured, the methods and the NTU, low-range versus high-range instruments, bubbles and the debubbler, formazin calibration and verification standards, and the compliance record the instrument exists to produce.
- Pressure TransmittersGauge, absolute, and differential pressure transmitters: how they sense, how range, turndown, and accuracy specifications work, the process connection and its accessories, and the installation errors that show up as calibration problems.
- Instrument ListsThe index of every measuring and final control device on a project: what the instrument list records, how it grows from the P&ID into specification sheets and calibration records, the columns that matter at each stage, and how it becomes the asset register the utility maintains.
Direct contact
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