For several decades, Electromagnetic Flowmeters (EMFs) have been the primary choice for measuring volumetric flow of conductive liquids through pipes. This flow technology is especially effective for water-based media or fluids with a minimum 5-20 micro Siemens of electrical conductance. Primary industries for EMFs include food and beverage, chemical, water/wastewater, pharmaceutical, and water or utility applications.
The measuring principle is based off of Faraday's Law of Induction, where the voltage that is induced at the electrodes is directly proportional to the velocity of the liquid moving within the magnetic field. By measuring the velocity and having fixed sensor geometry, a volumetric flow measurement is output from the converter. The wall of the sensor tube itself is electrically isolated with only the electrodes contacting the fluid. With KROHNE flow meters, the liners are typically Hard Rubber, different elastomers, or ceramic liners for extremely corrosive applications. The liners and electrodes must be suitable and chemically compatible with the application.
Importance of Grounding the EMF
Proper grounding of the EMF is critical for ensuring a fixed reference potential for the small millivolt signal voltage in order to provide accurate measurement.
The traditional grounding of EMFs has been done through :
Grounding to electrically conductive pipes - The liquid is in direct contact with the metal of the pipe and therefore it has the same potential as the grounded pipe.
Utilization of grounding rings between the pipe and the meter before and after the meter - The rings will be in contact with the liquid, along with the meter electrodes, ensuring that they have the same ground reference point for the measurement. However, this can be problematic for wetted material compatibility and the need for exotic metals for the grounding rings, which can be cost prohibitive.
Grounding with grounding electrodes - having exotic materials at the grounding electrode means lower cost on the material, but also means less surface for the ground to be in contact with the fluid.
The latest alternative to traditional grounding: Virtual Reference
In many applications, traditional grounding methods for EMFs pose problems. In processes with cathodic corrosion protection or galvanization, a voltage is present across the electrodes and earth ground. Also, applications with corrosive or chemically aggressive fluids require expensive grounding rings to be chemically compatible that may fail over time, resulting in more maintenance and replacement of the measurement point.
KROHNE solves these problematic grounding methods with Virtual Reference, sometimes referred to as "virtual grounding." With Virtual Reference, the flow sensor can be installed in any pipe or process without the need for additional grounding.
The new patented technology works by measuring the fluid's electrical potential at the electrodes and creating an offsetting voltage that eliminates the induced electrical noise. The voltage is then used as the reference potential at the flow converter for signal processing.
Key Takeaways and Advantages
No additional fluid grounding method is necessary - resolving one of the most common causes for error with EMFS
No risk of electrolytic destruction where there are potential differences in the system - Ungrounded use is also possible where voltage and current are applied to the pipes for electrolytic or galvanic treatment
Lower cost of install for chemically aggressive fluids - save costs on exotic materials for grounding rings.
For more information, please see the article below for practical examples where this technology has solved challenges in volumetric flow using electromagnetic flowmeters.
Reference: "Desalination Plants Benefit from Mag Meters with Unique Virtual Reference Feature" Joe Incontri, Director of Marketing KROHNE USA; 2013