Magnetic flow meter

Magnetic-inductive flow meter, briefly MID, use a measuring method, which is based on Faraday's law of electromagnetic induction. The sensor generates an electrical signal from the usable flow.

  • 4.1 advantages
  • 4.2 disadvantages

History

The first basis for magnetic-inductive measuring the flow velocity was recorded in 1832 in a paper by Michael Faraday.

The modern electronic circuit design of the 70s of the last century in connection with alternating magnetic fields enabled the filtering of interfering signals, which are due to electrochemical processes in generating the magnetic field. Thus, the widespread industrial use magnetic-inductive flowmeter ( MID), nothing stood in the way.

Aufbau principle

A magneto-inductive flowmeter is the basic principle of a measuring tube made of metal, which is traversed by a minimum electric -conductivity material to be measured, which is permeated by a magnetic field generated by coils. In the magnetic field there are ( at least) two of the measuring tube opposite transverse to the magnetic field are arranged measuring electrodes which are provided for detecting the measurement voltage generated inductively. The metallic measuring tube is provided with an electrically insulating inner lining or coating.

The complete measuring point of an electromagnetic flowmeter consists of a sensor and an associated transmitter.

Principle of measurement

The measurement principle of this flow meter utilizes the separation charges moving in a magnetic field. Through a pipe of non-magnetic material, the inner liner has an electrically insulating, to be measured flows through the conductive liquid.

From the outside, a vertically oriented to the flow direction magnetic field is applied by means of coils. Present in the conductive liquid carriers, ions or charged particles are deflected by the magnetic field, the positive charge carrier, for example to the left, the negative to the right. To the perpendicular to the magnetic field are arranged measuring electrodes formed by the charge separation, a voltage is sensed with a measuring device ( analyzer ). The amount of the measured voltage is proportional to the flow velocity of the carriers, ie to the flow velocity.

Magnetic field

The magnetic field is generated in modern constructions by a pulsed DC current of alternating polarity. This ensures a stable zero point and makes the measurement insensitive to influences of multiphase materials and inhomogeneities in the liquid. Even with low conductivity a useful measuring signal is reached.

When operated with pure AC magnetic fields, it comes to induction of interference voltages at the electrodes, which can be largely suppressed by elaborate and appropriate filters, however.

Useful voltage

Now, a measuring fluid moves through the pipe, is according to the law of induction, the two measuring electrodes which are arranged on the measuring tube perpendicular to the flow direction and the magnetic field B, a voltage U in. This voltage is directly proportional to the mean flow velocity in a symmetrical flow profile and a homogeneous magnetic field v. The inductive flow measuring method is able to generate directly from the flow an electrically usable signal for further processing. From this, the relationship calculated as follows:

With U = voltage, k = proportionality factor, B = magnetic field, D = pipe diameter, v = flow velocity

By a downstream evaluation unit, the voltage is converted into a corresponding useful signal.

Electrodes

Choosing the correct electrode material is crucial for a reliable function and accuracy of a magnetic-inductive flow measurement.

Galvanic signal tap

The measuring electrodes are in direct contact with the medium and thus must be sufficiently resistant to corrosion and ensure good electrical contact with the material to be measured. Electrode materials are usually stainless steel, stainless alloy, platinum, tantalum, titanium, zirconium. For flowmeters with ceramic measuring tubes sintered- electrodes are used.

Capacitive signal pickup

For fluids with extremely low electrical conductivity and for media that can form insulating deposits on the pipe wall and would thus break the contact between the medium and the electrode, today transducers come with non-contact capacitive signal to the application.

Here, the electrodes are replaced by large-area capacitor plates and attached to the outside of the lining of the non-conducting measuring tube. In MID with capacitive signal using Kermik measuring tubes, the condenser surface is sintered onto the alumina flow tube.

FLOW

The value of the flow rate Q can therefore be derived from the pipe diameter D and the average flow velocity V:

For both laminar and for turbulent flow results in a linear dependence of the useful voltage U of the flow velocity v. The flow rate depends on the flow rate and the nominal size of the flow meter.

Applications

Magnetic-inductive flow meter for conductive liquids: water, slurries, pastes, slurries, acids, alkalis, juices and emulsions, including liquids with a minimum conductivity of 0.5 ĩS / cm. A variety of product features and technical characteristics guarantee its suitability for almost all applications such as:

  • Hygienic and Sanitary applications
  • Filling and Dosing applications
  • Chemistry
  • Pharma
  • Water networks
  • Sewage
  • Pulp and Paper

Electromagnetic Flowmeter for special applications:

  • Use by special construction Nutzsignalabgriffe in partially filled pipe and liquid level detection.
  • Ex - Design

Benefits

  • Measuring principle practically independent of pressure, density, temperature and viscosity
  • Also, solids-laden liquids (eg ore slurry, pulps )
  • High dynamic range up to 1000:1
  • No moving parts, therefore no wear
  • No pressure losses
  • No obstructions, measuring route as pipeline
  • CIP-/SIP-reinigbar, piggable as free pipe cross-section
  • Linear output signal
  • Even for aggressive and corrosive products
  • No influence of the conductivity when greater than 5 ĩS / cm
  • High accuracy even in solids loading and gas inclusions
  • High reproducibility and long term stability
  • Minimal maintenance and care maintenance

Disadvantages

  • Requirement of a minimum conductivity
  • Maximum fluid temperature at about 200 ° C
  • Minimum flow velocity (pick ) ca.0, 5 m / s
  • Tends in the raw water area to inaccuracies, since iron deposits reduce nominal cross section
Highway Addressable Remote Transducer Protocol EtherNet/IP
539840
de