Typical Ranges and Their Interrelationship**
Electromagnetic flowmeters rely on three key electrical characteristics—coil resistance, electrode-to-ground insulation resistance, and electrode-to-electrode resistance—to ensure stable measurement and reliable performance.
Although values vary among manufacturers and models, industry consensus ranges and their significance are well established.
1. Typical Resistance Ranges
1.1 Coil Resistance
The excitation coil generates an alternating magnetic field. Coil resistance depends on wire gauge, number of turns, and construction.
Typical values
Low-frequency square-wave excitation: 5–50 Ω (commonly 10–30 Ω for DN50–DN300)
Large diameter meters: 1–5 Ω (fewer turns, thicker wire)
Small diameter meters: up to 50–100 Ω (more turns, thinner wire)
📌 Two excitation coils should have symmetrical resistance; deviation >5% may indicate winding shorting or manufacturing faults.
1.2 Electrode-to-Ground Resistance
The measuring electrodes must be electrically insulated from the flowmeter body.
Measured using 500 V insulation megger:
Normal condition: ≥100 MΩ
Acceptable field level: ≥50 MΩ
Fault indication: <10 MΩ (potential liner damage, moisture ingress, or contamination)
Note: For rare conductive liners, electrode-to-ground resistance may be very low by design.
1.3 Electrode-to-Electrode Resistance
This value is not inherent to the instrument—
📌 It is determined by the conductivity of the process fluid.
Conductive media (water, acids, electrolytes): hundreds to thousands of ohms
Non-conductive or empty pipe: ≥100 MΩ (or open circuit)
2. Functional Relationship Between Resistances
2.1 Coil Resistance → Magnetic Field Strength
At fixed excitation voltage:
Lower resistance → higher current → stronger magnetic field
Higher resistance → lower magnetic field
Abnormal cases:
Very low resistance (short-circuit): risk of amplifier damage
Open circuit: no magnetic field → no signal
2.2 Electrode-to-Ground Resistance → Signal Integrity
The induced electromotive force (millivolt level) must remain isolated from chassis ground.
If insulation deteriorates:
Signal leaks to housing → measurement drops, fluctuates or collapses
Moisture ingress often causes temporary reduction in insulation
Severely low resistance may short electrodes to body → zero flow reading
2.3 Electrode-to-Electrode Resistance → Process Conditions
This resistance confirms whether:
The pipe is full (conductive path established)
The fluid is sufficiently conductive
The sensor should trigger empty pipe detection
Low-conductivity applications (e.g., deionized water) may require:
Highly sensitive low-frequency drive
Specialized flowmeter models
3. Measurement Guidelines
Measure coil resistance only when the meter is powered off
Use a megger (not a standard multimeter) for insulation testing
Confirm resistance compliance after installation, maintenance, or repairs
Investigate immediately if readings fall outside expected ranges
4. Practical Summary
| Parameter | Typical Range | Meaning |
|---|---|---|
| Coil resistance | 5–50 Ω (common) | Produces magnetic field |
| Electrode–ground | ≥50–100 MΩ | Ensures signal isolation |
| Electrode–electrode | Depends on fluid | Confirms full pipe & conductivity |
📌 All three must be normal to achieve accurate measurement
If any parameter diverges significantly, the measurement loop fails—
no magnetic field, no usable signal, or uncontrolled leakage.
Conclusion
The coil, electrode insulation, and electrode-to-fluid path form a complete chain enabling magnetic field generation, EMF induction, and signal transmission.
Understanding the resistance ranges and their interdependence allows engineers to diagnose installation faults, process conditions, or instrument failure long before they escalate into downtime.