Electromagnetic Flow Meter (EMF) is commonly installed for measuring conductive liquids, operating based on the well-established principle of Faraday’s Law of Induction. Inside every EMF sensor, energised excitation coils generate the magnetic field to produce a flow signal.
Monitoring excitation coil resistance is a simple yet powerful method for verifying the health of an electromagnetic flow meter. By including this check in routine maintenance schedules and understanding the impact of temperature, plant engineers can predict failures before they happen, ensuring process reliability and measurement accuracy.
What is Excitation Coil Resistance?
Coil resistance of electromagnetic flow meter is a critical parameter for ensuring accuracy and diagnosing long-term health. It is the DC resistance of the copper wire wound inside the flow meter sensor. This value is determined by the length and thickness of the wire used. For our flow meters, this resistance typically falls within a range of 60Ω to 120Ω. The resistance can vary based on the meter’s size and manufacturer specifications.
Why Does Excitation Coil Resistance Change?
Excitation coil resistance is not a static value. They are subject to a positive temperature coefficient, because the windings are made of copper. This means resistance increases as temperature rises.
Copper resistance increases by approximately 0.393% per °C relative to its value at 20°C, exhibiting a linear increase as temperature rises. For every 10°C rise in temperature, the resistance increases by roughly 4%, causing higher power loss and lower conductivity.
Therefore, if the process medium temperature fluctuates or the ambient environment changes, the coil resistance will naturally be affected. A shift in resistance is not always a sign of failure. It often indicates a change in temperature.
Simple Steps to Measure EMF Coil Resistance
- Turn Off the Power: Make sure the electromagnetic flow meter is powered off and the sensor is disconnected from the transmitter or power supply.
- Find the Coil Terminals: Locate the excitation coil terminals using the sensor wiring diagram (usually labeled X/Y or 7/8).
- Set the Multimeter: Set the multimeter to resistance (Ω) mode.
- Measure the Resistance: Place the multimeter probes on the two coil terminals.
- Read the Value: Check the resistance reading shown on the multimeter.
- Compare the Result: Compare the reading with the manufacturer’s specified coil resistance range to confirm if the coil is in good condition.
Flow Meter Health Diagnostic
Checking excitation coil resistance is therefore a useful diagnostic and preventive maintenance practice, especially during installation, troubleshooting, and periodic servicing. By comparing measured values with the manufacturer’s reference data, technicians can identify potential issues early and avoid unexpected process downtime.
Abnormal Resistance Causes
- Short-Circuited Windings: Insulation breakdown between coil turns causes lower-than-normal resistance.
- Open Circuit: Broken coil wire or disconnected internal connection leads to very high or infinite resistance.
- Overheating: Excessive temperature damages coil insulation and alters resistance characteristics.
- Moisture Ingress: Water or humidity entering the sensor can cause corrosion, insulation failure, and unstable resistance.
- Aging and Insulation Deterioration: Long-term exposure to heat, chemicals, or vibration gradually degrades coil insulation.
- Mechanical Vibration or Stress: Continuous vibration may loosen internal connections or damage coil windings.
- Improper Installation or Wiring Damage: Incorrect wiring or damaged terminals can affect the coil’s electrical integrity.
- Manufacturing Defects: Rare cases of coil winding or material defects may cause abnormal resistance early in operation.
Beyond Basic Resistance
While coil continuity is vital, it should be considered alongside insulation resistance. Insulation resistance tests, measured in Mega-Ohms (MΩ) check whether moisture or damage has compromised the isolation between the coil and the process fluid. Low insulation resistance can cause signal noise and instability, even if the coil itself appears electrically continuous.
