Why Some Electromagnetic Flowmeters Become Unstable Over Time?
Many engineers focus on lining material, electrode type, or brand when selecting an electromagnetic flowmeter.
However, in real industrial applications, one hidden factor often determines long-term stability and accuracy:
๐ Excitation Mode
A wrong excitation type can lead to:
- Zero drift
- Unstable readings
- Noise interference
- Frequent maintenance
In this guide, weโll break down the three main excitation types and show you how to choose the right one for your application.
Why Do Electromagnetic Flowmeters Need Excitation?
Electromagnetic flowmeters work based on Faradayโs Law of Electromagnetic Induction.
When a conductive liquid flows through a magnetic field, it generates a voltage proportional to flow velocity.
๐ The key question is:
Where does the magnetic field come from?
It is generated by excitation coils powered by an electrical signal.
Different types of excitation signals create different magnetic field behaviors โ which directly affect:
- Measurement stability
- Noise resistance
- Power consumption
- Application suitability
The 3 Main Excitation Types Explained
1. AC Excitation (Alternating Current)
How it works:
- Uses standard 50Hz or 60Hz sine wave
- Magnetic field changes continuously
Advantages:
- Fast response (~20 ms)
Major problems:
- โ Severe zero drift (caused by eddy currents)
- โ Strong interference from power frequency (50Hz noise)
- โ High energy consumption
Application:
- Old systems (before 1990s)
- Low-accuracy indication only
โ ๏ธ Recommendation:
AC excitation is outdated and NOT recommended for new projects.
2. Low Frequency Pulse Excitation (Mainstream Solution)
How it works:
- Uses low-frequency square wave (typically 2.5โ25 Hz)
- Alternates between positive and negative excitation
- Measures signal only during stable periods
Advantages:
- โ Extremely stable zero (almost no drift)
- โ Strong resistance to 50Hz interference
- โ Low power consumption (supports 2-wire systems)
- โ Mature and reliable technology
Limitations:
- Slower response (100โ500 ms)
- Not suitable for very low conductivity (<5 ฮผS/cm)
- Limited performance in slurry with bubbles
๐ Best choice for:
- Water / wastewater
- Chemical liquids
- Food & beverage
- Most industrial applications
โ Covers over 90% of use cases
3. Dual Frequency Excitation (Advanced Solution)
How it works:
- Combines low frequency + high frequency signals
- Low frequency โ stability
- High frequency โ noise suppression
Advantages:
- โ Works with very low conductivity liquids (down to ~0.05 ฮผS/cm)
- โ Excellent performance in slurry and bubble conditions
- โ Suppresses polarization effects on electrodes
- โ Faster response than low-frequency (~50โ100 ms)
Limitations:
- Higher cost (1.5โ3ร standard models)
๐ Best choice for:
- Slurry (mining, pulp, desulfurization)
- Liquids with bubbles
- Low conductivity fluids
- Unstable process conditions
โ ๏ธ Important:
If your application involves slurry or unstable conductivity โ dual frequency is NOT optional.
Quick Comparison of Excitation Types
| Feature | AC | Low Frequency | Dual Frequency |
|---|---|---|---|
| Response Speed | โญโญโญโญ | โญโญ | โญโญโญ |
| Zero Stability | โ Poor | โ Excellent | โ Excellent |
| Noise Resistance | โ Weak | โ Strong | โ Very Strong |
| Low Conductivity | โ No | โ ๏ธ Limited | โ Excellent |
| Slurry Handling | โ Poor | โ ๏ธ Medium | โ Best |
| Power Consumption | โ High | โ Low | โ ๏ธ Medium |
| Cost | โ Low | โ Medium | โ High |
How to Choose the Right Excitation Type (Simple Rule)
๐ Just follow this:
- โ Clean liquids (water, chemicals) โ Low Frequency
- โ Slurry / bubbles / unstable fluids โ Dual Frequency
- โ AC excitation โ Avoid for new installations
๐ Another key threshold:
- Conductivity < 5 ฮผS/cm โ MUST choose dual frequency
Real Case: Wrong Excitation Selection Causes Measurement Failure
Case: Paper Mill Application
A paper plant used a standard low-frequency electromagnetic flowmeter for pulp measurement.
Problem:
- Unstable readings
- Sudden spikes and negative values
- Control system malfunction
Root cause:
- Low conductivity (~3 ฮผS/cm)
- Air bubbles in slurry
Solution:
- Replaced with dual-frequency excitation flowmeter
๐ Result:
- Stable readings
- Accurate control
- No more false alarms
Industry Recommendations
| Industry | Recommended Type |
|---|---|
| Water / Wastewater | Low Frequency |
| Chemical | Low Frequency |
| Food & Beverage | Low Frequency |
| Mining Slurry | Dual Frequency |
| Paper & Pulp | Dual Frequency |
| Desulfurization | Dual Frequency |
Final Thoughts
Excitation mode is not just a technical detail โ
๐ it is the core factor that determines performance and reliability.
- AC โ outdated
- Low frequency โ standard solution
- Dual frequency โ advanced solution for difficult conditions
Choosing the right one can save:
- Maintenance cost
- Downtime
- Measurement errors
Need Help Selecting the Right Flowmeter?
๐ฉ Not sure which excitation type fits your application?
Our engineers can help you select the most suitable model based on your:
- Medium type
- Conductivity
- Flow conditions
- Installation environment
๐ Contact us now for technical support + quotation