Electromagnetic flow meters are widely used in wastewater treatment due to their accuracy and non-intrusive design. However, various environmental and operational factors can introduce measurement errors. In this article, we’ll explore the common causes behind these inaccuracies and how to prevent or troubleshoot them effectively.

1. Partially Filled Pipes

One of the most common reasons for inaccurate readings is the pipe not being completely filled with liquid. This can result from improper installation or insufficient backpressure.

  • If small amounts of gas cause stratified or wavy flow, the flow reading may be higher or lower than actual.
  • In cases of bubble or slug flow, measurements may fluctuate significantly as bubbles pass the electrodes.
  • If the electrodes are not submerged due to extreme underfilling, the flow meter may register maximum (overflow) output errors.

Tip: Always install the electromagnetic flow meter in a location where full pipe conditions are ensured — typically in vertical upward flow or a properly pressurized horizontal section.

2. Presence of Solid Particles in the Liquid

Wastewater often contains particles such as fibers, sand, or sludge. These can cause:

  • Signal noise (slurry noise)
  • Fouling or coating of electrode surfaces
  • Conductive or insulating deposits reducing internal diameter
  • Liner wear or buildup altering flow characteristics

Regular maintenance and choosing a flow meter with appropriate lining and self-cleaning electrode designs can help minimize such issues.

3. Crystallization of Liquid

Some wastewater streams may carry crystallizable chemicals that solidify as temperature drops.

  • Pipes may be insulated or heated, but the sensor body often lacks heat tracing, making it prone to internal crystallization.
  • This can lead to blockages, reduced accuracy, or sensor failure.

Recommendation: Use compact, short-body electromagnetic flow sensors and ensure upstream piping is well-insulated.

4. Incorrect Material Selection for Electrodes and Grounding Rings

If the electrode material isn’t compatible with the fluid, several issues may occur:

  • Chemical reactions forming passivation layers
  • Polarization and stray voltages
  • Catalytic effects forming gas bubbles on the electrode surface

Materials must be carefully selected based on the chemical composition of the wastewater. For aggressive media, use electrodes made of platinum or Hastelloy.

5. Low Electrical Conductivity of the Fluid

Electromagnetic flow meters require a minimum conductivity level to function. For most meters, this is typically ≥5 μS/cm.

  • If conductivity is too low (e.g., in distilled or deionized water), the signal may become unstable or fluctuate.
  • Actual field conditions often require the conductivity to be 10–100 times higher than the published minimum.

Tip: Always confirm the conductivity of your process fluid before selecting or installing a flow meter.

Conclusion

Inaccurate readings from a wastewater electromagnetic flow meter are often caused by avoidable factors such as improper installation, low conductivity, solid contaminants, or material mismatches. Understanding these causes helps ensure accurate and reliable measurements.

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