Field work needs a valid permit, isolation and the site's procedures. Tell operations before you touch a loop that feeds control or a safety system, and put the loop in manual or bypass only with their agreement. Device menus, limits and tolerances vary by make and model: always check the manufacturer's manual.
The method: signal, power, process
Split the loop in three and prove each part before you move on. Most time is wasted by replacing a good transmitter when the fault was a plugged impulse line.
- Ask what changed. When did it start? After a shutdown, a rainstorm, a maintenance job, a process change?
- Compare. Is there a local gauge, a redundant transmitter or a related reading (flow, level) that agrees or disagrees?
- Read the mA at the transmitter (clamp meter, or the test terminals) and at the control system. Do they agree with each other and with the DCS value?
- Check the power: voltage at the transmitter terminals while it draws current.
- Check the device: HART status, alarms, configured range and damping.
- Check the process connection: isolation valves, manifold, impulse lines, drains and vents.
- Prove it with a known pressure (handpump or calibrator) if still in doubt.
What the mA value tells you
| Reading | Likely meaning | Next check |
|---|---|---|
| 0 mA | No loop current: open circuit, blown fuse, no supply | Supply voltage, fuse, wiring, terminal screws |
| < 3.6 mA | Transmitter driving a low fault signal (NAMUR NE 43) or very low supply | HART diagnostics, supply voltage |
| 3.8–4.0 mA | Process at or below the lower range value (saturated low) | Is the range right? Is the process really that low? |
| 4–20 mA, but wrong value | Range mismatch, zero shift or process connection problem | Compare range in transmitter and DCS; check impulse lines |
| 20.5 mA | Saturated high: process above the upper range value | Range, blocked low side on a DP |
| > 21 mA | High fault signal (NE 43) or wiring short | HART status, wiring |
| Flat, never moves | Isolated, plugged line, frozen line, or damping set very high | Valves, tracing, damping |
| Noisy / jumping | Pulsation, entrained gas, loose wiring, poor screen earthing | Damping, wiring, earth at one end only |
Power and wiring checks
A two-wire transmitter needs a minimum terminal voltage at 20 mA. If the supply is marginal the reading collapses only at high values, which looks like a process problem.
- Measure terminal voltage with the loop at a high reading. Compare with the minimum in the data sheet (often 10–12 V, more with a display or HART).
- HART needs at least about 230–250 Ω in the loop. Without it, a handheld cannot talk to the transmitter.
- Water in the housing or cable gland causes leakage currents and drifting readings. Check the covers and glands are tight.
- Earth the cable screen at one end only (normally the control-room end).
The Loop Load Check app does this budget for you.
Impulse lines and manifolds
Most "transmitter faults" on DP service are impulse line faults. Learn the patterns:
| Symptom | Likely cause | Fix |
|---|---|---|
| DP reads low or zero, flow looks low | Equalising valve on the 3- or 5-valve manifold left open or passing | Close the equaliser; check it holds |
| DP reads high | Low-side impulse line plugged or isolated | Check low-side valve; blow down if allowed |
| Slow, lazy response | Partly plugged line or high damping | Blow down; check damping |
| Liquid DP reads wrong and wanders | Gas trapped in liquid-filled legs | Vent the transmitter body at the high points |
| Gas DP reads wrong | Liquid (condensate) collected in gas legs | Drain at low points; lines should slope back to the pipe |
| Steam DP drifts after start-up | Condensate legs not filled or at different heights | Fill both legs to equal height; wait for condensate pots to settle |
| Fails only in winter | Frozen line or wax | Check heat tracing and insulation |
Taking a DP transmitter out of service on a 3-valve manifold
- Tell operations; put the loop in manual.
- Open the equalising valve.
- Close the high-side isolation valve.
- Close the low-side isolation valve.
- Now the transmitter is isolated and equalised: it should read zero DP (4 mA on a 0-based range).
To return it to service, reverse the order: open high side, close the equaliser, open low side. Never have both isolation valves open with the equaliser open: process can blow through from high to low side.
Zero shifts and range mistakes
- Mounting position: a transmitter rotated or moved after calibration can shift zero slightly. Do a zero trim with the manifold equalised (DP) or vented to atmosphere (gauge).
- Wet legs and seals: on level and remote-seal DP, the fill-fluid head creates a fixed offset. That offset is expected and is part of the calibrated range; don't "trim it away".
- Range mismatch: the transmitter range (for example 0–250 mbar) must match the DCS range. A mismatch gives a reading that is right at one end and wrong at the other.
- Square root in two places: on DP flow, take the square root either in the transmitter or in the DCS, never both. Doubled square root reads too high at low flow.
A DP level transmitter with a wet leg read 8 % after maintenance. The wet leg had been drained and not refilled, so the reference head was missing. Refilling the leg to the marked height brought the reading back to 0 %. No re-ranging was needed.
Using HART diagnostics
- Read the device status first: sensor failure, electronics failure, configuration changed, analog output fixed or saturated.
- Check the PV, % range and mA on the handheld. If PV is right but mA is wrong, the output stage or a loop test left the output fixed. If PV is wrong too, look at the sensor and process.
- Check damping and range against the instrument datasheet.
- After any work, make sure the output is not left in fixed-current (loop test) mode.
More in HART Communication.
Proving it with a known pressure
If all checks pass and the reading is still in doubt, isolate the transmitter, connect a handpump and reference gauge or calibrator, and apply 0, 50 and 100 % of range. Compare the mA with the expected value:
If it is out of tolerance, follow the full five-point calibration.
Q&A quiz
Related: Pressure Calibration · DP Transmitters · Loop Checking