The short answer
Pressure Calibration
Calibrating a pressure transmitter means comparing its output with a reference standard at several points across its range, recording the as-found error, adjusting if the error exceeds the tolerance, and recording the as-left error. On a smart transmitter the adjustment is a sensor trim that corrects the digital measurement against the reference, or an output trim that corrects the 4-20 mA against a reference ammeter; reranging, which changes the pressure at 4 and 20 mA, is configuration and calibrates nothing. A five-point test rising and falling reveals zero, span, linearity, and hysteresis errors, the reference should be at least four times more accurate than the tolerance, and the interval between calibrations is set from the drift history and any regulatory requirement.
Key points
- As-found before touching anything; as-left after. The as-found record is what tells you the drift and the interval.
- Five points up and five down: zero and span errors, linearity, and hysteresis each have their own signature.
- Sensor trim corrects the measurement; output trim corrects the milliamps; reranging corrects nothing.
- The reference standard is four times better than the tolerance, and it has its own certificate.
- Tolerance comes from the loop requirement, not from the transmitter specification.
- Elevation and static pressure effects are installation offsets, corrected by a zero at the installed condition.
What calibration is
Calibration is a comparison. A known pressure from a reference standard is applied to the transmitter, and the transmitter output is read and compared with what it should be. The difference is the error, expressed as a percentage of span. If the error at every point is within the tolerance, the transmitter passes and nothing is adjusted. If it is not, the transmitter is adjusted and tested again. The records of both tests are the calibration. A transmitter that was adjusted without an as-found record has lost the information that mattered most: how far it had drifted and therefore how often it needs to be checked.
Error (% of span) = (Reading − Reference) / Span × 100
- Reading = the transmitter output converted to pressure units, or the mA reading converted through the range
- Reference = the pressure applied by the standard
- Span = the upper range value minus the lower range value
The five-point test
Apply 0, 25, 50, 75, and 100 percent of span rising, then 75, 50, 25, and 0 falling, and record the output at each. The pattern of errors identifies the fault. Every point off by the same amount is a zero error. Errors that grow in proportion to the pressure are a span error. Errors that are largest in the middle and small at the ends are nonlinearity, which trimming cannot remove and which usually means the sensor is damaged. Different readings at the same pressure rising and falling are hysteresis, also a sensor condition. A single-point check at zero, which is what most field zeroing amounts to, finds only the first of these.
| Pattern | Meaning | Action |
|---|---|---|
| Same error at every point | Zero shift | Zero trim, or check the installation for an elevation or static pressure effect |
| Error proportional to pressure | Span shift | Span trim against the reference at the upper point |
| Error largest mid-range | Nonlinearity | Sensor damage or overrange history; replace if beyond tolerance |
| Rising and falling differ | Hysteresis | Sensor condition; replace if beyond tolerance |
| Output correct in the display, wrong in mA | Output stage error | Output trim against a reference ammeter |
| Random, unrepeatable | Noise, a leak in the test setup, an unstable reference | Fix the test before judging the transmitter |
Trims on a smart transmitter
- Sensor trim
- Corrects the digital pressure value the transmitter computes from its sensor, against the reference pressure. A zero trim at zero pressure, and a full trim at two points. This is the calibration of the measurement.
- Output trim
- Corrects the 4-20 mA output against a reference ammeter, independent of pressure. The transmitter is set to output exactly 4 and exactly 20 mA and the digital-to-analog converter is adjusted. This is the calibration of the signal.
- Rerange
- Sets the pressures that correspond to 4 and 20 mA, the lower and upper range values. It changes what the transmitter reports, not how accurately it measures. Reranging a transmitter that is out of calibration produces a wrongly calibrated transmitter with a new range.
- Zero at installed condition
- A zero trim performed with the transmitter installed, the isolation valve closed, and the line at its normal fill, removing the elevation and static offsets. It is an installation adjustment, distinct from the bench calibration, and recorded separately.
Reference standards
The reference has to be better than the transmitter it checks. The usual rule is a test accuracy ratio of four to one: the reference uncertainty is no more than a quarter of the tolerance being applied. A digital pressure calibrator or a pressure module with a hand pump satisfies that for most process transmitters; a deadweight tester is the laboratory standard. The reference has its own calibration certificate traceable to a national standard, and its due date is checked before it is used. A reference gauge of the same grade as the transmitter proves nothing.
Gauge, absolute, and vacuum
A gauge transmitter measures relative to atmospheric pressure and is zeroed vented to atmosphere. An absolute transmitter measures relative to a vacuum, reads atmospheric pressure when vented, and is zeroed against a reference that knows the barometric pressure or against a vacuum. Confusing the two puts an error equal to atmospheric pressure, about 14.7 psi at sea level, into the calibration. Vacuum ranges are checked with a vacuum pump and a reference that reads negative gauge or absolute correctly.
Tolerance and interval
The tolerance is set by what the loop needs, not by the transmitter specification. A transmitter capable of 0.1 percent used for a discharge pressure alarm with a tolerance of 1 percent passes easily and needs checking rarely; the same transmitter used for a compliance measurement with a 0.25 percent requirement needs a better reference and a shorter interval. The interval starts at a year for most process transmitters and moves with the evidence: as-found errors well inside tolerance for several cycles justify a longer interval, and any as-found failure shortens it. Regulatory requirements override the evidence where they apply.
The record
- Tag, model, serial, range, and the tolerance applied.
- Reference standard used, its serial, and its certificate due date.
- As-found readings at each point, rising and falling, and the computed errors.
- Adjustments made, and which trim.
- As-left readings and errors.
- Pass or fail, the technician, the date, and the next due date.
Frequently asked questions
- The transmitter reads correctly on the display but the controller shows a different value. Is it out of calibration?
- Not the sensor. The difference is in the output stage, the loop, or the controller scaling. Check the mA output against the reference ammeter, then the controller input against the mA, then the scaling. Output trim fixes the first; the others are wiring or configuration.
- Can I calibrate with the transmitter installed?
- Yes, with the root valve closed and a pump connected at the bleed or a calibration port, provided the reference and the process are isolated from each other. Many transmitters are calibrated in place this way. Zero at atmosphere first, then apply the test points. Remember the elevation offset if the test pressure enters at a different height from the tap.
- The as-found error is within tolerance. Should I trim it anyway?
- No. Adjusting a transmitter that passes adds nothing and loses the drift record, and each adjustment has its own small uncertainty. Record the as-found values as the as-left values and move on. Adjust when the error is out of tolerance or approaching it.
- How is a calibration different from a loop check?
- Calibration proves the transmitter measures pressure correctly against a standard. A loop check proves the signal from the transmitter reaches the controller and the HMI correctly and the alarms and controls respond. Both are needed; a calibrated transmitter wired to the wrong input has passed one and failed the other.
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.
- 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 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.
- Pressure Transmitter InstallationWhere and how to install a pressure transmitter so it reads the process and not its own plumbing: tap location, mounting above or below the tap, isolation and bleed valves, line slope, pulsation, elevation, freeze protection, seals, vibration, and sun.
Direct contact
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