1. Description of the problem and scope
This guide covers unstable, floating or erratic measurement signals encountered on process transmitters (pressure, temperature, flow, level). These symptoms are classified as critical in aerospace and energy environments, where data integrity conditions system security and compliance with NF/EN standards.
This document is intended for maintenance technicians and reliability engineers seeking to isolate the root cause between electromagnetic disturbances (EMC), grounding failures, wiring insulation degradation, or an internal transmitter fault.
2. Safety Precautions
WARNING: Any intervention on live equipment or in ATEX zones presents a fatal risk.
1. Mandatory lockout and tagout (Lockout/Tagout) according to site procedures.
2. Check the absence of voltage (VAT) with a EN 61243-1 compliant device before any handling.
3. Wear appropriate PPE: class 00 insulating gloves, protective glasses, flame-retardant clothing (if ATEX zone).
4. Pay attention to stored energy (capacitors, valve springs) and residual pressures.
3. Required diagnostic tools
| Tool | Minimum specification | Usage |
|---|---|---|
| RMS multimeter | Category IV 600V, accuracy 0.05% | 4-20mA loop measurement, voltage, continuity |
| Megohmmeter | 500V DC, range 0-1000 MΩ | Wiring insulation test |
| Handheld oscilloscope | 100 MHz bandwidth, galvanically isolated | EMI/RFI noise analysis |
| Process Calibrator | Source/Measurement 4-20mA, HART | Signal simulation, transmitter test |
| Current clamp | Resolution 1mA | Leakage current detection on shielding |
4. Initial Checklist
| Verification | Observation |
|---|---|
| Alarm history | Correlation with starts of motors, variators or switches? |
| Physical state | Cables damaged, crushed, or exposed to corrosive fluids? |
| Packaging | Separation distance (minimum 300mm) respected with power cables? |
| Process parameters | Stability of other measurements on the same I/O card? |
5. Diagnostic logic
- Step 1: Isolate the Signal (Local Loop vs. System)
- Measure the series current (mA) at the transmitter.
- If the signal is stable locally but unstable at the PLC (PLC): Wiring or ground problem.
- If the signal is unstable locally: Transmitter problem or massive disturbances.
- Step 2: CEM analysis (EMI/RFI)
- Observe the signal with the oscilloscope: presence of HF signals (>1kHz)?
- Check continuity and shield connection (must be grounded on one side only, usually on the PLC side, to avoid ground loops).
- Step 3: Insulation and Ground Test
- Measure the AC voltage between the shield and ground: should be < 0.5V AC.
- Test insulation resistance: should be > 50 MΩ at 500V DC.
6. Cause-symptom matrix
| Symptom | Probable causes | Diagnostic test | Expected result |
|---|---|---|---|
| Custom 50Hz noise | Ground loop | AC voltage on shield | < 0.5V AC |
| Random drift (EMI) | Shielding poorly connected / Power proximity | Oscilloscope | Clear signal (4-20mA) |
| Unstable/Erratic Signal | Degradation of cable insulation | Megohmmeter | > 50 MΩ |
| Frozen or saturated signal | Transmitter electronic fault | 4-20mA simulation | Signal follows the simulator |
7. Root cause analysis
Ground loops
Occurs when a cable's shield is grounded at both ends, creating a potential difference. Current flowing through the shield induces interference on the internal conductors. Confirmation: AC voltage > 1V between disconnected shield and earth point.
Degradation of insulation
Due to mechanical abrasion, chemical exposure or excessive heat (polymer aging). This reduces insulation resistance, causing current leakage to ground or between conductors.
8. Resolution procedures
- EMC correction: Connect the shield to earth only on the API bay side, on a clean equipotential bonding strip. Tightening torque: 1.2 Nm.
- Cabling repair: If resistance < 50 MΩ, replace the cable with a twisted shielded cable conforming to EN 50288-7. Avoid splices.
- Transmitter replacement: If the signal is stable at 12mA with the calibrator but unstable with the transmitter, replace it according to the OEM procedure. Check the terminal block seal.
9. Preventive measures
| Cause | Strategy | Frequency |
|---|---|---|
| EMI/RFI | Visual inspection of power/instrumentation separation | Annual |
| Ground loop | Checking AC voltage on shield | Bi-annual |
| Aging | Megohmmeter test (power off) | Tri-annual |
10. Spare parts
| Description | Specification | UNITEC category |
|---|---|---|
| Instrument cable | Shielded, twisted, 2x2x0.75mm² | Wiring |
| Ground terminal | DIN rail, high conductivity | Connection |
| Replacement transmitter | See nameplate | Sensors |
Consult our technical catalog for exact references: https://www.unitecd.com/e-catalog/
11. References
- NF C 15-100: Low voltage electrical installations.
- EN 61000-6-2: Electromagnetic compatibility (EMC) - Generic standards.
- Aerospace maintenance procedures (manufacturers' guides).
