Electromagnetic flowmeters (magnetic meters) there since about 1939. The Swiss clergyman and inventor Thürlemann Father Bonaventura (1909-1997) was a pioneer in the industrial use of this measurement principle.
The physical phenomenon in which this technique is known, however, much earlier. The English physicist Michael Faraday (1791-1867) realized that moving a conductive metal rod length (L) with speed (v) within a magnetic field (B), induces an electric current is generated between the two ends of the bar voltage (Ue) of a few millivolts (Fig. below). Faraday also discovered that the magnitude of the voltage induced in this way is directly proportional to the velocity (v) of movement and intensity (B) the magnetic field.
Ue: Stress-induced
B: Magnetic field
L: length of electrical wire (corresponding to the distance between electrodes in the measuring tube)
v: speed of movement of the driver (corresponding to the flow velocity in the measuring tube)
Figure: Principle of electromagnetic induction as formulated by M. Faraday
In a magnetic flowmeter (Figure below), the conductive fluid flowing through the measuring tube inside the metal bar corresponds to the experiment of Faraday. Two coils on both sides of the measuring tube generate a magnetic field of constant intensity. Two electrodes on the inside of the pipe wall detect the voltage induced by the fluid in circular motion within the magnetic field. The measuring tube is electrically isolated from the fluid and the electrode by a non-conductive coating (eg, rubber, Teflon, etc.)..
Given a constant intensity magnetic field (B), the above equation shows that the induced measuring voltage (Ue) is directly proportional to fluid velocity (v). On the other hand, the cross section of the pipe (A) is a known parameter, so that the volume flow (Qv) is calculated directly from the following expression:
Figure: Principle of electromagnetic flow measurement.
Figure: Design and construction of an electromagnetic sensor
Measuring tube (a)
In physical terms, it is important that the measuring tube does not dampen or distort the magnetic field. Evidently, then, no electromagnetic pipe going to choose. The most common materials used are stainless steel and plastic.
Coating (b)
The insulation coating is required between the electrodes and the measuring tube, which prevents the induced voltage is discharged through the pipe. It is also important to consider the properties of fl-sicasy chemical resistance of polyurethane coating to fluido.El, gum Duray PFA / PTFE (eg Teflon) are among the most commonly used materials.
Coil system (c)
The magnetic field is generated by two coils of copper wire magnetic core mounted outside the measuring tube. Excitation systems can be either of CCY CA.
Electrodes (d1-d3)
Besides the usual measurement electrodes, some manufacturers offer special types of electrodes:
- Measuring electrodes (d1) for detection of induced voltage. The conditions imposed by the process of manufacturing the electrode material, which can be stainless steel, hastelloy, tantalum o-platino/rhodio. Capacitive electrodes.
- Reference or ground electrode (d2) for equipotential bonding between the flowmeter and the fluid. For the same purpose can also install disks (rings) of independent ground.
- Probe empty pipe detection (d3) for detecting whether the measuring tube is empty or partially filled. The transmitter sounds an alarm if the electrode is exposed.
In practice, the signal detected at the electrodes from various sources of tension that interfere with the induced voltage and filtration is required before processing. The typical sources of interference include the Earth's magnetic field, electrical charges on the inner wall of the pipe and the fluid and surface galvanic effects between the electrodes and the fluid.
The main advantage of this measuring principle is that it is insensitive to pressure, temperature and viscosity. The flow velocity profile has a minimal effect on the measurement result. These properties make magnetic flowmeters extremely attractive to a wide range of industrial applications for flow measurement. In fact, this principle of measurement is used worldwide.
Figure: Magnetic flowmeters are manufactured in various nominal bores from DN2 (1 / 12 ") to DN2.000 (80").
