Electromagnetic flow measurement

With the magnetic-inductive flow measurement, it is possible to measure conductive liquids with the highest accuracy. A measurement method based on Faraday's law of electromagnetic induction is used. An electrical signal is generated from the flow. The magnetic-inductive flow measurement is versatile and measures precisely even under extreme measuring conditions.

For which applications is this flow measurement intended?

Magnetic-inductive flow measurement is intended for conductive liquids, including water, pulp, sludge, acids, alkalis, juices and emulsions. This offers the possibility for many applications:

• Chemistry
• Pharmaceuticals
• Hygienic and sterile applications
• Filling and dosing applications
• Water, waste water
• Paper, pulp

Magneto-inductive flow measurement can also be used under special conditions. A very important factor is that the measurement can be carried out in potentially explosive atmospheres. Furthermore, a wide range of communication is required (HART, PROFIBUS DP/PA, FOUNDATION fieldbus, Modbus, EtherNet/IP, Bluetooth).

Function and structure

The basic principle of electromagnetic flow measurement is as follows: If a conductive liquid flows through a pipe filled with it through a magnetic field, an electrical voltage is generated whose magnetic field strength is proportional to the flow movement.

A magneto-inductive flowmeter has two field coils that generate a constant magnetic field over the entire cross-section of the pipe. Two electrodes are installed orthogonally, which pick up electrical voltage.

If there is no flow, no electrical voltage can be generated and no measurement takes place. The electrically charged particles of the medium are evenly distributed (on the drawing these particles are shown as red and blue dots). As soon as the medium begins to flow, the magnetic field exerts a force on the particles. Positive and negative particles are separated in the flowmeter area and accumulate at the electrodes. This creates an electrical voltage that is detected and measured by the electrodes. Stress is proportional to flow motion (velocity).

With the known measuring tube cross-section, the flow volume can be calculated.

How is interference voltage prevented?

Interference voltages detected by the electrodes must be isolated. This happens by building up the magnetic field using a clocked current field. For this purpose, the magnetic field is constantly reversed. The voltage tapped off at the electrodes also changes its polarity. This procedure eliminates all constant sources of interference, such as electrochemical effects in the medium and electromagnetic interference fields from the outside.

How are other disruptive factors excluded?

The prerequisite for electromagnetic flow measurement is that a magnetic field builds up inside the pipe and the voltage can be used without interference. Two excitation coils generate the magnetic field. It is important that austenitic steel is used as the tube material, as this is not magnetisable and therefore does not deflect the magnetic field of the exciter coils.

In order to avoid the possibility of further voltage discharge, the measuring tube has an insulating inner lining. The inner lining consists mostly of plastic (PP, PTFE, PEEK,...) or technical ceramics (alumina or zirconium oxide). It prevents short circuits between the conductive liquid and the metallic measuring tube.

The medium itself must be an electrical conductor. A minimum electrical conductivity between 20 µS/cm and 0.5 µS/cm must be (depending on the device).

The right electrode material

Selecting the right material for the electrodes is crucial for a correct measurement.

Galvanic signal aspect

Since the electrodes are in direct contact with the medium, they must be corrosion-resistant and ensure a well-functioning transition to the medium. The most common materials for this are:

• Stainless steels
• CrNi alloys
• Platinum
• tantalum
• Titanium
• Zirconium

If the sensors are equipped with ceramic measuring tubes, sintered electrodes are used.

Non-contact capacitive signal aspect

Non-contact capacitive signal termination is used for media with extremely low electrical conductivity and for media that form insulating deposits on the pipe wall that interrupt contact with the electrode.

Capacitor plates are attached to the pipe from the outside over a large area. If ceramic measuring tubes are used, the capacitor plates are sintered onto the aluminum oxide measuring tube.

Advantages and disadvantages of electromagnetic flow measurement