1. Introduction
Brief role of pressure transmitters in industrial automation.
Importance of fault diagnosis and preventive maintenance.
2. No Output from the Transmitter
Inspection & Testing (check power polarity, 24VDC supply, display head, loop continuity).
Solutions (correct wiring, ensure ≥12V supply, replace faulty display, check loop impedance ≤250Ω).
3. Transmitter Output ≥20 mA
Inspection & Testing (over-range pressure, sensor damage, wiring issues).
Solutions (select proper range, repair/replace damaged sensor, tighten wiring).
Table 1 — Common Causes of Over-Range Output in Pressure Transmitters
| Cause | Description | Recommended Solution |
|---|---|---|
| Actual Process Pressure Exceeds Range | The applied process pressure is higher than the transmitter’s selected span. | Select a transmitter with a higher range or adjust span setting. |
| Sensor Overload / Diaphragm Damage | Severe overpressure may damage the sensing diaphragm permanently. | Return to manufacturer for repair or replace damaged sensor. |
| Power Supply Abnormality | Supply voltage too low or unstable, causing incorrect loop current. | Ensure supply ≥12 VDC at the transmitter terminals; check loop impedance. |
| Loose or Incorrect Wiring | Loose terminal screws or wrong polarity connections. | Reconnect wires securely with correct polarity. |
| Electrical Load Too High | Loop resistance beyond transmitter’s capability. | Reduce loop load or increase supply voltage (must remain <36 VDC). |
| External Interference / Noise | EMI or transient spikes may push output to maximum. | Apply shielding, grounding, and separate signal cables from power cables. |
4. Transmitter Output ≤4 mA
Inspection & Testing (low power supply, sensor damage, wrong range selection).
Solutions (adjust load, re-select transmitter, factory repair if sensor damaged).
5. Incorrect Pressure Indication
Inspection & Testing (reference instrument accuracy, mismatched ranges, wiring correctness, grounding, cable routing, sensor health, process conditions).
Solutions (use accurate reference, adjust load resistance, add cooling/condensate pot, clean impurities).
6. Installation Issues
Cases with saturated steam, superheated steam, and external steam supply.
Problems due to improper condensate pot height, short impulse lines, static pressure differences.
7. Impulse Line Blockage
Causes: impurities, corrosion, dust deposition.
Symptoms: pressure reading unchanged, only little water drains from vent.
Solutions: regular flushing, filtration at pressure ports.
8. Transmitter Hardware Failure
Example: lube oil pressure transmitter triggering shutdown due to sudden internal failure.
Long-term drift and sensitivity degradation.
Solutions: regular calibration, replacement of aged sensors.
9. Electromagnetic Interference (EMI)
Cases: compressed air discharge pressure signals fluctuating near high-voltage rooms.
Solutions: cable shielding, grounding, separating signal and power cables.
10. Preventive Measures & Maintenance Guidelines
Use filters to avoid impulse line blockage.
Install condensate pots correctly for steam measurement.
Separate signal and power cabling to minimize EMI.
Conduct periodic calibration and verification.
Replace or repair sensors after long-term operation.
11. Conclusion
Pressure transmitters are robust but prone to multiple fault categories.
Proper diagnosis, structured troubleshooting, and preventive maintenance significantly extend their lifecycle.