Electromagnetic flow meter
Electromagnetic flow meters , or MID for short , use a measuring method based on Faraday's law of electromagnetic induction . The sensor generates an electrically usable signal from the flow .
history
The first basis for magnetic-inductive measurement of flow velocity was recorded in a publication by Michael Faraday in 1832.
The modern electronic circuit technology of the 1970s in connection with alternating magnetic fields made it possible to filter interference signals that can be traced back to electrochemical processes during the generation of the magnetic field. Thus, nothing stood in the way of the widespread industrial use of electromagnetic flow meters (MID).
Construction principle
A magnetic-inductive flow meter basically consists of a measuring tube made of metal through which a material to be measured flows through which has a minimum electrical conductivity and is penetrated by a magnetic field generated by coils. In the magnetic field there are (at least) two measuring electrodes arranged opposite one another on the measuring tube, transversely to the magnetic field, which are provided for detecting the inductively generated measuring voltage. The metallic measuring tube is provided with an electrically insulating inner coating or lining.
The complete measuring point of an electromagnetic flowmeter consists of a measuring sensor and an associated measuring transducer.
Measuring principle
The measuring principle of this flow meter uses the separation of moving charges in a magnetic field ( Hall effect ). The conductive liquid to be measured flows through a tube made of non-magnetic material, which has an electrically insulating lining on the inside.
A magnetic field oriented perpendicular to the direction of flow is applied from outside by means of coils. The charge carriers, ions or charged particles present in the conductive liquid are deflected by the magnetic field: the positive charge carriers to the left, for example, the negative to the right. The charge separation creates a voltage on the measuring electrodes, which are arranged perpendicular to the magnetic field, which is recorded by a measuring device (evaluation device). The level of the measured voltage is proportional to the flow velocity of the charge carriers, i.e. H. their flow rate.
Magnetic field
In modern designs, the magnetic field is generated by a clocked direct current of alternating polarity. This ensures a stable zero point and makes the measurement insensitive to the effects of multiphase substances and inhomogeneities in the liquid. A usable measurement signal can be achieved even with low conductivity.
In the case of magnetic fields that are operated with pure alternating voltage, interference voltages are induced on the electrodes, which, however, can be largely suppressed using complex and suitable filters.
Useful voltage
If a measuring liquid is now moved through the tube, a voltage U is applied to the two measuring electrodes, which are arranged on the measuring tube perpendicular to the direction of flow and the magnetic field B, according to the law of induction. With a symmetrical flow profile and a homogeneous magnetic field, this voltage is directly proportional to the mean flow velocity v. The inductive flow measurement method is able to generate an electrically usable signal for further processing directly from the flow. From this the relationship can be calculated as follows:
with U = voltage, k = proportionality factor, B = magnetic field, D = pipe diameter, v = flow velocity
A downstream evaluation unit converts the voltage into a corresponding useful signal.
Electrodes
The selection of the right electrode material is a decisive factor for a reliable function and measuring accuracy of a magnetic-inductive flow measurement.
Galvanic signal tap
The measuring electrodes are in direct contact with the medium and must therefore be sufficiently corrosion-resistant and guarantee a good electrical transition to the material to be measured. Electrode materials are mostly stainless steels, CrNi alloys, platinum, tantalum, titanium, zirconium. Sintered electrodes are used for transducers with ceramic measuring tubes.
Capacitive signal tap
For media with extremely low electrical conductivity and for media that can form insulating deposits on the pipe wall and thus interrupt the contact between the media and the electrode, sensors with non-contact capacitive signal pick-up are used today.
The electrodes are replaced by large capacitor plates and attached to the outside of the lining of the non-conductive measuring tube. In the case of EMFs with capacitive signal tapping using ceramic measuring tubes, the capacitor surface is sintered onto the aluminum oxide measuring tube.
Flow
The value of the flow rate Q can be derived from the pipe diameter D and the mean flow velocity v :
For both laminar and turbulent flows , there is a linear dependence of the useful voltage U on the flow velocity v . The volume flow depends on the flow velocity and the nominal diameter of the flowmeter.
Applications
Electromagnetic flow meters for conductive liquids: water, pulps, pastes, slurries, acids, bases, juices and emulsions, including liquids with a minimum conductivity of 0.5 µS / cm. Diverse product functions and technical properties guarantee suitability for almost all applications such as:
- Hygienic and sterile applications
- Filling and dosing applications
- chemistry
- Pharma
- Water, networks
- sewage
- Paper and pulp
Electromagnetic flow meters for special applications:
- Use due to special construction of useful signal taps with partially filled pipelines and level detection .
- Communication via HART , PROFIBUS DP / PA , FOUNDATION fieldbus, Modbus, EtherNet / IP , Bluetooth
- can be used in potentially explosive areas
advantages
- Measuring principle practically independent of pressure, density, temperature and viscosity
- Also liquids containing solids (e.g. ore sludge, pulp pulp)
- No moving parts, therefore no wear
- No pressure loss
- No annoying fixtures and fittings, measuring path such as pipeline
- CIP - / SIP -reinigbar Piggable as Free pipe cross section
- Linear output signal
- Also for aggressive and corrosive products
- No influence of conductivity if it is greater than 5 µS / cm
- High measurement accuracy even under solid load and with gas inclusions
- High reproducibility and long-term stability
- Minimal maintenance and upkeep
disadvantage
- Requirement for a minimum conductivity
- Maximum medium temperature around 200 ° C
- Minimum flow velocity (response range) approx. 0.5 m / s
- Tends to inaccuracies in the raw water area, as iron deposits reduce the nominal cross-section
See also
literature
- Heinz Bernard, Frank Grunert, Frank Dornauf, Armin Brucker, Friedrich Hofmann: Flow measurement technology . (= Atp practical knowledge compact. Volume 5). Oldenbourg, Munich 2008, ISBN 978-3-8356-3074-1 .
- Fritz L. Reuther, Adalbert F. Orlicek: On the technology of volume and flow measurement of liquids. R. Oldenbourg, Munich 1971, ISBN 3-486-39111-9 .
- Urs Endress et al: Flow Primer. Endress + Hauser Flowtec AG, Reinach 1990, ISBN 3-905615-03-7 .
Web links
- 3D animation of the magnetic-inductive flow measurement principle
- Example of a particularly compact sensor based on the magnetic-inductive measuring principle (company ifm)
- PDF document - Flow meter from page 7 ( Memento from January 25, 2011 in the Internet Archive ) (813 kB)
- Electromagnetic flowmeters for the water sector PDF file (107 kB)
- ABB flow measurement
Individual evidence
- ↑ a b Inductive measurement methods ( Memento from January 25, 2011 in the Internet Archive ) (PDF; 832 kB)