Troubleshooting Erratic Sensor Values: EMI/RFI, Grounding, Cable Degradation and Transmitter Diagnostics

1. Problem Description & Scope

Erratic sensor readings manifest as unpredictable, fluctuating, or incorrect readings that do not reflect actual process conditions. This can lead to unreliable process control, false alarms and, in critical cases, unsafe situations or production interruptions. This guide focuses on diagnosing and solving these problems in various industrial sensors and transmitters, including pressure, temperature, level and flow sensors, as well as proximity switches and encoders.

The severity of the problem is classified as follows:

• Critical: Immediate process interruption, acute safety risk, significant quality deviations. Requires immediate action.
• Major: Reduction in productivity, unreliable process monitoring, increased risk of failures. Requires prompt correction.
• Minor: Small, inconsistent deviations that do not yet have an immediate impact, but indicate potential future problems. Requires planned correction.

2. Safety measures

WARNING: Work on electrical installations and process instrumentation involves risks. Always follow your organization's applicable safety procedures and relevant standards.

• Lock/Tag out (LOTO): Always perform a Lockout/Tagout procedure in accordance with NEN 3140 and EN 1037 before inspecting, de-energizing, or replacing electrical circuits. Check for absence of voltage with a suitable voltage detector.
• Personal Protective Equipment (PPE): Always wear suitable PPE, such as safety glasses (EN 166), insulating gloves (according to EN 60903) when working on electrical systems, and anti-static footwear (according to EN ISO 20345) in potentially explosive environments.
• Residual energy: Always check for residual energy in capacitors and mechanical systems (e.g. spring-loaded components) before working on the system. Discharge this energy safely.
• Earth leakage protection: Ensure that adequate earth leakage protection (according to NEN 1010) is present and functions correctly. Switch off the earth leakage protection if this is required for the diagnostics, but restore it immediately afterwards.
• Danger zones (ATEX): Work in potentially explosive environments (ATEX zones) only with intrinsically safe and ATEX-certified tools and equipment (according to ATEX 2014/34/EU). A work permit must be drawn up and validated before each intervention.

3. Required Diagnostic Tools

For effective troubleshooting, specific and calibrated tools are essential. Below is an overview of the recommended equipment:

4. Initial Assessment Checklist

Before beginning in-depth diagnostics, a systematic initial assessment is essential. This helps gather crucial information and can already reveal the root cause, preventing unnecessary disassembly.

5. Systematic Diagnosis Flowchart

This flowchart guides you through a logical sequence of steps to isolate the root cause of erratic sensor readings.

1. Symptom: Erratic Sensor Values
   • Initial Assessment (according to checklist):
   • Is the visual inspection (step 1.1) OK?
   • YES: Proceed to step 2.
   • NO: Repair visible damage (e.g. broken cable, corrosion) and retest the sensor. If problem resolved, complete procedure.
   • Does the alarm history indicate intermittency or external triggers?
   • YES: Focus on EMI/RFI or loose connections as the primary suspect. Go to step 2.
   • NO: Proceed to step 2.
2. Sensor/Transmitter Power Supply Check:
   • Measure the supply voltage directly at the sensor or transmitter with a DMM (DC or AC, depending on the type).
   • Expected: 24VDC ±10% (i.e. 21.6V to 26.4V) for most industrial sensors, or 100-240VAC ±10% for AC powered equipment.
   • Result:
   • Voltage WITHIN the specified range and stable: Proceed to step 3 (Signal Integrity & Noise).
   • Voltage OUT of range or fluctuates (more than 5% deviation):
   • Probable Cause: Power supply problems (defective power supply unit, overload, voltage drop due to long/undersized cables, loose connections in power supply circuit).
   • Diagnosis: Check the power supply, wiring to the sensor, and the total load on the power supply. Measure voltage at the power source.
   • Action: Restore power supply by repairing/replacing power supply, checking/optimizing wiring, or reducing load. Test again.
3. Signal Integrity & Noise (EMI/RFI):
   • 3.1. Visual inspection of cable shield:
   • Check that the cable shield is correctly connected on one side, usually on the PLC/DCS side, via a low-impedance connection.
   • YES: Go to 3.2.
   • NO:
   • Probable Cause: Missing, incorrectly connected, or shield grounded on both sides (creating a ground loop).
   • Action: Connect the shielding correctly in accordance with the standards (e.g. ground on one side). Test again.
   • 3.2. Measurement with Oscilloscope:
   • Connect the oscilloscope to the sensor signal (preferably measure directly at the sensor output and at the PLC/DCS input).
   • Setting: AC coupling, time base 100 µs/div, vertical sensitivity 100 mV/div.
   • Result:
   • Clean signal (< 50mV peak-to-peak noise on a 0-10V signal, or < 0.2mA peak-to-peak on a 4-20mA signal): Proceed to step 4 (Ground Integrity).
   • Significant noise or irregularities present (> 50mV peak-to-peak noise or > 0.2mA peak-to-peak noise):
   • Probable Cause: EMI/RFI interference.
   • Diagnosis:
   • Measure the RF field strength with an EMI/RFI detector near the cable and sensor. Values ​​> 3 V/m (in accordance with NEN-EN-IEC 61000) indicate significant interference.
   • Turn off potential sources of interference (VFDs, large motors, switching power supplies, wireless transmitters) one at a time and observe the sensor output.
   • Action:
   • Optimize cable routing: maintain a minimum distance of 30 cm between signal cables and power cables. Cross power cables at a 90 degree angle if necessary.
   • Use proper shielded cables (e.g. EN 50289, with foil and/or braid) and ensure proper grounding practices (see step 4).
   • Install ferrite cores on signal cables to attenuate high-frequency noise.
   • Consider optical isolators for signals with large potential differences.
   • Test again.
4. Ground integrity:
   • 4.1. Control Ground Loop & Resistance:
   • Measure the resistance between instrument ground and process ground (or general equipotential bonding) with a DMM.
   • Use an earthing tester for a comprehensive check of the earthing quality of the panel and the installation (in accordance with NEN 1010).
   • Result:
   • Resistance between ground points < 1 Ω (ideally): Grounding is probably sufficient. Proceed to step 5 (Cable Degradation).
   • Resistance > 1 Ω or open circuit:
   • Probable Cause: Poor ground connection, corrosion, defective ground terminal, or ground loop.
   • Diagnosis: Inspect all grounding points, bonding terminals, and grounding strips. Measure the resistance of individual ground connections.
   • Action: Repair or replace defective ground connections. Eliminate ground loops by consistently grounding shields on one side. Ensure solid equipotential bonding. Test again.
5. Cable degradation:
   • 5.1. Insulation test:
   • WARNING: Completely disconnect the cable from both the sensor and the PLC/DCS before performing an insulation test.
   • Perform an insulation test with a Megger on all individual cores against earth and each other (e.g. 500V test voltage for 24VDC systems).
   • Result:
   • Insulation resistance > 100 MΩ (for new installations, minimum 1 MΩ is required according to NEN 1010): Cable insulation is acceptable. Proceed to step 6 (Transmitter Diagnostics).
   • Insulation resistance < 100 MΩ:
   • Probable Cause: Cable degradation, moisture ingress, damaged insulation (physical, chemical, thermal).
   • Diagnosis: Perform a thorough visual inspection of the entire cable route for kinks, chafing, pinching, water penetration, or signs of overheating.
   • Action: Replace the entire defective cable. Test again.
   • 5.2. Continuity/Short Circuit (DMM & TDR):
   • Use a DMM to perform a continuity test of each core of the cable. Also measure for short circuits between cores and between core and earth.
   • Use a TDR to confirm exact cable length and accurately locate opens or shorts.
   • Result:
   • Continuity OK, no short circuit, cable length in accordance with documentation: Cable is electrically intact. Continue to step 6.
   • Opening, short circuit or cable length discrepancy:
   • Probable Cause: Cable break, short circuit, connector problem or terminal problem.
   • Action: Locate and repair the break/short circuit, or replace the affected cable/connector. Test again.
6. Transmitter Diagnostics (Sensor/Transmitter):
   • 6.1. Local Output Test & Calibration:
   • If safe and possible, disconnect the transmitter from the process and the receiving PLC/DCS.
   • Use a process calibrator to simulate a known input (e.g. pressure, temperature) to the sensor/transmitter or to force a known output (e.g. 4 mA or 20 mA on a 4-20mA transmitter).
   • Measure the output directly at the transmitter with a calibrated DMM.
   • Result:
   • Output correct and stable over the entire measuring range and corresponds to the simulated input: Transmitter/sensor is probably functioning correctly. Proceed to step 7 (PLC/DCS input).
   • Output incorrect, fluctuates, or differs from the simulated input:
   • Probable Cause: Faulty transmitter/sensor, calibration problem, sensor contamination/wear.
   • Diagnosis: Check the calibration with reference equipment and clean the sensor element.
   • Action: Recalibrate the transmitter/sensor, clean the sensor element, or replace the transmitter/sensor if defective. Test again.
   • 6.2. Status indicator Transmitter:
   • Check any status LEDs or displays on the transmitter for error codes or diagnostic indications.
   • Action: Consult the manufacturer's manual for the meaning of any error codes and take the recommended corrective actions.
7. PLC/DCS Input Diagnostics:
   • 7.1. PLC/DCS Input Test:
   • Inject a known, stable signal (e.g. 4mA, 12mA, 20mA for a 4-20mA input) directly onto the input terminals of the PLC/DCS module, with the sensor disconnected.
   • Check the value that the PLC/DCS reads internally via the software (HMI, SCADA).
   • Result:
   • The PLC/DCS reads the injected value correctly and stably: The PLC/DCS input is OK. The fault probably lies outside the PLC/DCS (sensor, cabling, EMI/RFI).
   • The PLC/DCS reads an incorrect or fluctuating value:
   • Probable Cause: Defective PLC/DCS input module, incorrect configuration of the input (scaling, type).
   • Diagnosis: Check the configuration of the PLC/DCS input in the programming software. Swap the input module with a known good module if possible.
   • Action: Correct the configuration, or replace the defective PLC/DCS input module. Test again.

6. Error Cause Matrix

This matrix presents an overview of common symptoms, their likely causes (ranked by likelihood), the diagnostic test and the expected results if the cause is confirmed.

7. Root Cause Analysis for Each Error

A deep understanding of the root causes is crucial for sustainable solutions.

Electromagnetic Interference (EMI/RFI)

• Why it happens: Electromagnetic interference occurs when unwanted electromagnetic fields (e.g. from large motors, frequency converters, switching power supplies, radio transmitters, high-frequency welding equipment) induce voltages or currents in nearby signal cables. These induced signals are then interpreted as legitimate measurements by the PLC/DCS, resulting in erratic values. This problem is even more critical with unbalanced or inadequately shielded signal circuits. The NEN-EN-IEC 61000-4-x series of standards specifies immunity tests for equipment in industrial environments.
• How to confirm: An oscilloscope will show a distinct noise pattern on the sensor signal, often at a frequency that correlates with the source of interference. An RF detector will measure an increased electromagnetic field strength in the vicinity of the signal cables. Turning off potential sources of interference (e.g. a VFD) and observing the disappearance of the noise confirms the cause.
• Damage if unresolved: False alarms, incorrect process control that can lead to quality deviations, production loss, and even accelerated wear of mechanical components due to incorrect controls.

Grounding problems

• Why it happens: Improper grounding does not create a stable, common reference point for all electrical and instrumentation circuits. This results in potential differences between different ground points, leading to circulating currents (ground loops) in signal cables. These currents are then detected as signal noise. Poor ground connections (corrosion, loose terminals) increase the impedance of the ground path, negating the effectiveness of shielding. NEN 1010 sets the requirements for low-voltage installations and the earthing systems therein.
• How to confirm: Measuring significant potential differences (> 0.5V) between metal parts that should be grounded, or between the sensor ground and the PLC/DCS ground, with a DMM. A high ground resistance (>1 Ω for instrumentation) measured with a ground tester on a panel's ground bar confirms poor ground quality.
• Damage if left unresolved: Constant equipment failure, corrosion due to electrochemical reactions (galvanic corrosion), and a significant safety risk due to insufficient equipotential bonding in the event of a fault.

Cable degradation

• Why it happens: Industrial cables are subject to harsh conditions. Physical damage (kinking, abrasion, impact), moisture penetration into the cable sheath or connectors, chemical attack on the insulation (oils, acids), or thermal overload and aging can drastically reduce the electrical properties of cables. This leads to reduced insulation resistance, which causes leakage currents and short circuits, or to interruptions in wires that disrupt or completely block the signal.
• How to confirm: An insulation tester (Megger) will demonstrate low insulation resistance (< 100 MΩ, or < 1 MΩ is critical) between cores and/or between core and earth. A DMM will indicate an open circuit (infinite resistance) or a short circuit (very low resistance). A TDR can pinpoint the exact location of opens or shorts in the cable. Visual inspection can confirm physical damage such as kinks, cracks, or traces of moisture.
• Damage if unresolved: Unreliable signals, total sensor failure, short circuits leading to tripping of fuses or damage to power supplies, and in extreme cases fire hazard due to overheating at high leakage currents.

Transmitter/Sensor Defects

• Why it happens: The sensor or transmitter itself may be the cause of erratic readings. This may be due to internal component defects (e.g. failing electronics due to thermal stress or aging), wear of the measuring element (e.g. drift in a Pt100 element, mechanical play in an encoder), contamination of the sensor element (e.g. scaling on a pressure sensor, particles on a flow sensor), or thermal drift (temperature-dependent deviations that are not compensated).
• How to confirm: During calibration with a calibrated reference, deviations from the expected values ​​outside the specified tolerances will occur. An unstable or incorrect output when the transmitter is tested with a simulated input (via a process calibrator) indicates an internal defect. Error codes on the transmitter display, if present, provide direct indications.
• Damage if unresolved: Inaccurate measurements lead to inefficient process control, resulting in higher energy costs, incorrect product quality, and potential process downtime due to incorrect control.

8. Step-by-Step Troubleshooting Procedures

The following procedures describe corrective actions for each root cause, including specific values and verification steps.

With EMI/RFI Interference

1. Isolation and Identification: Identify all potential sources of interference in the vicinity of the sensor cable. If possible, turn these off sequentially and observe the sensor output on an oscilloscope to pinpoint the specific source.
2. Cable Routing Optimization: Reroute signal cables to maintain a minimum distance of 30 cm from power cables (according to EN 50174). If intersections are unavoidable, ensure they are at a 90 degree angle to minimize inductive coupling.
3. Shield Correction: Check that shielded cables (according to EN 50289, with a shield of at least 85% coverage) are connected correctly; the shielding must be grounded on one side (usually the PLC/DCS side) via a low-impedance connection (< 0.1 Ω). Avoid double-sided grounding of the shield, as this can create a ground loop.
4. Ferrite cores Installation: Place split ferrite cores (e.g. nickel-zinc ferrite, suitable size for cable diameter) around signal cables at both the transmitter and receiver to attenuate high-frequency noise. Place 2-3 turns through the core for maximum effectiveness.
5. Signal filtering: Consider installing RC filters, passive or active signal conditioners at the PLC/DCS input to suppress specific noise frequencies. Consult the specifications of the sensor and PLC module.
6. Verification: Monitor the sensor signal continuously with an oscilloscope. The peak-to-peak noise after correction should not exceed 1-2% of the full scale range of the signal (e.g. < 100mV for a 0-10V signal).

For grounding problems

1. LOTO: Always perform a Lockout/Tagout procedure before working on grounding systems.
2. Checking and Cleaning Connections: Thoroughly inspect all ground points, equipotential bonding connections, clamps, and ground straps for corrosion, loose connections, or physical damage. Clean contact points with sandpaper and use a conductive copper paste on the connections for optimal contact resistance. Tighten clamps to the specified torque (refer to the clamp manufacturer's manual, typically 1.5-3 Nm).
3. Earth Resistance Measurement: Measure the earth resistance of the earthing system and individual earth connections with a earth ground tester. This must be less than 1 Ω for instrumentation and less than 10 Ω for general electrical installations in accordance with NEN 1010. Improve or add ground electrodes if necessary.
4. Eliminate Ground Loops: Identify and eliminate all ground loops. Make sure that the shielding of signal cables is grounded on only one side (usually the PLC/DCS side). Use optical isolators or galvanic signal isolators for signals that inherently connect multiple ground points or must communicate between zones with potential differences.
5. Potential bonding Optimization: Ensure adequate potential bonding between all metal parts of the process, the instrumentation, and the electrical installation. Use earthing conductors of sufficient cross-section (e.g. min. 6mm² copper for instrumentation panels).
6. Verification: Re-measure the resistance between all relevant ground points with a DMM; this must be < 1 Ω. Check the stability of the sensor signal after restoring the ground.

In case of cable degradation

1. LOTO: Always perform a Lockout/Tagout procedure before working on cables.
2. Locating Fault: Use a TDR to determine the exact location of opens, shorts, or other impedance changes in the cable. This minimizes the need for disassembly.
3. Cable replacement: Replace the entire defective cable. Repairs to damaged instrumentation cables are not recommended due to the impact on signal integrity. Use cables specifically designed for the environmental conditions (temperature range, chemical resistance, UV resistance, mechanical stress, according to EN 50288).
4. New Cable Routing: Route the new cable in a manner that minimizes physical damage, extreme temperatures, and exposure to harsh chemicals. Use cable ducts, protective conduits and sufficient strain relief.
5. Testing after Installation: After installing the new cable, perform an insulation test (with Megger, > 100 MΩ) and a continuity test (with DMM) on the new cable (in accordance with NEN 3140).
6. Verification: Reconnect the sensor and PLC/DCS and check the stability of the sensor signal during operation.

For Transmitter/Sensor Defects

1. LOTO: Always perform a Lockout/Tagout procedure before disconnecting or replacing sensors or transmitters.
2. Cleaning and Inspection: Clean the measuring element of the sensor thoroughly. Inspect for physical wear, corrosion, or damage. Check the condition of internal connectors in the transmitter housing.
3. Calibration: Perform a full calibration of the sensor/transmitter with calibrated reference equipment (traceable to national standards, according to ISO 9001). Adjust the calibration according to the specific manufacturer's manual. For a 4-20mA pressure sensor, set the 0% range accurately to 4.00 mA and the 100% range to 20.00 mA. The acceptable deviation should be within ±0.25% of full scale. Also check linearity over the entire range.
4. Replacement: If calibration is not possible, the transmitter gives a hard error code, or the sensor is physically damaged, replace the transmitter/sensor with an identical model or a certified alternative (CE, TUV, ATEX if applicable).
5. Verification: After calibration or replacement, test the sensor in operation. Compare the reported values ​​with other process references or manual measurements. Monitor stability and accuracy over time to confirm the problem has been resolved.

9. Preventive Measures

Preventive measures are essential to prevent recurrence of erratic sensor values and to ensure the reliability of the instrumentation.

10. Spare Parts & Components

Inventory management of critical spare parts is essential for fast response and minimal downtime.

Visit our e-catalog for a complete and up-to-date overview of spare parts and components, including detailed specifications and stock information: https://www.unitecd.com/e-catalog/

11. References

• NEN 1010: Safety provisions for low-voltage installations in the Netherlands.
• NEN 3140: Operation of electrical installations - Additional Dutch provisions for low-voltage installations.
• EN 1037: Safety of machinery - Prevention of unexpected start-up.
• EN 50288: Multi-element metallic cables used in analog and digital communication and control.
• EN 50174: Information technology - Cabling installation.
• NEN-EN-IEC 61000-4-x: Series of standards relating to electromagnetic compatibility (EMC) - Part 4: Testing and measurement techniques for immunity.
• ATEX 2014/34/EU: Directive on equipment and protective systems intended for use in potentially explosive atmospheres.
• ISO 9001: Quality Management Systems - Requirements.
• OEM manuals: Manufacturer specific documentation for the installed sensors and transmitters.