Symptom
The pressure gauge, sight glass, level tape, flow indicator, or portable meter reads differently from the value shown on the operator screen; the difference is constant, or proportional to the reading, or appears only during changes; operators have begun to distrust one or the other.
The short answer
Analog Does Not Match Field Indicator
When the field indicator and the SCADA value disagree, one of them is wrong, both may be, and the disagreement has a location: in the transmitter, in the loop, in the controller scaling, in the display, or in the physical difference between what the two instruments measure. The first step is to establish a reference with a third instrument that is calibrated, a test gauge or a portable meter, applied at the same point. Then the difference is walked: the transmitter output current against what the transmitter should send for that value, the controller raw counts against the current, the scaled value against the counts, and the display against the scaled value. A difference that is constant across the range is a zero offset or a reference elevation; one that grows with the value is a span or scaling error; one that appears only when the process moves is damping or filtering.
Key points
- Establish a calibrated reference at the same measurement point before deciding which instrument is wrong.
- A constant difference is a zero or an elevation offset; a proportional difference is a span or scaling error; a difference during changes is damping.
- Walk the signal: transmitter value, output current, raw counts, scaled value, display.
- Field gauges drift and get damaged too; the screen is not automatically wrong.
- Units, reference points, and range changes account for most large constant differences.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Scaling mismatch in the controller | The controller range differs from the transmitter range; the difference is proportional across the range. Compare both against the design. |
| Transmitter zero or span drift | The transmitter differs from a calibrated reference at the transmitter itself; a calibration record that is old. Calibrate. |
| Field gauge drifted or damaged | The gauge differs from the reference and the transmitter agrees with it; a gauge that was overpressured, frozen, or has a bent pointer. Replace the gauge. |
| Different elevation or reference point | A pressure gauge at a different height from the transmitter, or a level transmitter zeroed at a different datum than the sight glass. A constant offset equal to the head difference. |
| Different units or a units change | Pounds per square inch against feet of water, gallons per minute against million gallons per day, gauge against absolute pressure. Convert and compare. |
| Damping or filtering | The screen lags the gauge during changes and matches at steady state; transmitter damping and controller filtering. Reduce or accept. |
| Loop voltage drop or a resistance error | The controller raw counts do not match the loop current; a wrong shunt resistor, a second receiver in series, or a poor connection. Measure the current and the voltage across the input. |
| Range changed in one place only | A transmitter re-ranged during a replacement without the controller, or the reverse; the design record settles it. |
| Display scaled or clamped differently | The controller value and the screen differ; the display tag has its own scaling, a decimal place, or a clamp. |
| Impulse line, tap, or sensor problem | A blocked or partly filled impulse line, a tap at a different location, a level probe fouled; the transmitter sees a different process than the gauge does. |
Locate the difference
- 1
Reference
A calibrated test gauge or portable meter at the same tap, the same elevation, and the same time as the two readings. Record all three.
- 2
Characterize
Compare at two or three points in the range: is the difference constant, proportional, or only present while the process moves.
- 3
Transmitter
Its own display or a communicator: the measured value and the output current it is sending. If the transmitter differs from the reference, calibrate it and stop; if it agrees, the difference is downstream.
- 4
Loop
The current at the controller terminals with a meter; it should be what the transmitter says it is sending.
- 5
Controller
Raw counts against the current, then the scaled value against the counts and the configured range. Wrong range values live here.
- 6
Display
The screen tag against the controller value: its own scaling, units, and formatting.
- 7
Physical differences
Elevation, datum, tap location, and units, if the difference is constant and every stage agrees with the one before.
| Difference behaves | Usual cause | Where |
|---|---|---|
| Constant across the range | Zero offset, elevation head, datum, or a units offset such as gauge versus absolute | Transmitter zero, or physical |
| Grows with the reading | Span error or a range mismatch | Transmitter span or controller scaling |
| Only while the process changes | Damping, filtering, or a slow impulse line | Transmitter damping, controller filter |
| Random and changing | Noise, a poor connection, a fouled sensor | Loop wiring or the sensor |
| Present at the screen but not in the controller | Display tag scaling or formatting | The screen |
Head offset (psi) = Δh × 0.433 × SG
- Δh = elevation difference between the gauge and the transmitter, feet
- 0.433 = pounds per square inch per foot of water
- SG = specific gravity of the liquid
Deciding which to trust
The transmitter is the instrument that controls the process and feeds the record, so it is the one that is calibrated against a traceable reference and documented. A field gauge is a convenience for the operator at the equipment, and it is replaced when it disagrees with a calibrated transmitter rather than used to adjust one. When operators trust the gauge more than the screen, the real problem is a transmitter that has not been calibrated on schedule; the calibration program is the fix.
After the fix
- Record the as-found and as-left values for whichever instrument was adjusted.
- Update the instrument list if a range, a unit, or a datum changed.
- Check every consumer of the value: alarms, setpoints, interlocks, reports, and the historian scaling.
- Put the gauge and the transmitter on the same calibration schedule, or take the gauge off the pipe if nobody needs it.
Frequently asked questions
- The difference is exactly 2.3 psi, always.
- A constant difference that size is an elevation difference of about five feet of water, a gauge that was never zeroed, or a transmitter zero offset. Compare the mounting heights first; then zero whichever instrument is off against the reference.
- The screen reads 10 percent high everywhere.
- A proportional error is a span or a range problem: the transmitter upper range value against the controller upper scaling value. One of them was changed without the other, or the transmitter span has drifted.
- The gauge responds instantly and the screen takes thirty seconds.
- Damping in the transmitter, a filter in the controller, a slow scan, or a partly blocked impulse line. Each is found by watching the transmitter display against the gauge during a step: if the transmitter itself lags, it is damping or the impulse line.
- Which one goes on the compliance report?
- The calibrated, documented instrument, which should be the transmitter. If it is not calibrated, calibrate it before the report, and note the as-found error.
Related topics
- Transmitter Reads Wrong ValueThe reading is steady, plausible, and wrong. How to prove it against a reference, then separate a scaling mismatch, a range change, a zero shift, an installation effect, and a genuine transmitter fault, with the order of checks that finds it fastest.
- 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.
- Analog Scaling: Raw Counts to Engineering UnitsThe arithmetic that turns an analog card reading into feet, psi, or gallons per minute — plus the range mismatches that silently corrupt it.
- 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.
- Impulse LinesThe tubing between the process tap and the pressure or DP transmitter, and everything that goes wrong in it: slope and routing for liquid and gas, elevation head and how to correct for it, air and condensate, blocking and manifold valves, freezing, plugging on dirty service, and the diaphragm seal alternative.
- 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.
Direct contact
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