Electromagnetic consumption meters
The principle of operation of electromagnetic (induction) flow meters is based on the measurement of the EDW generated in the current-carrying fluid under the influence of an external magnetic field. The scheme of the induction flow meter is shown in Figure 4.7.
Between the N and S poles of the magnet, the liquid pipe 1 passes perpendicular to the direction of the magnetic field lines of force. The part of the pipe passing through the magnetic field is made of non-magnetic material (fluoroplast, ebonite, etc.). Diametrically opposite measuring electrodes 2 are installed on the pipe walls. Under the influence of the magnetic field, the ions in the liquid move and give their charges to the measuring electrodes, forming an electric field in them. Proportional to the flow rate, the value of EYUK, when the magnetic field is constant, is determined by the basic equation of electromagnetic induction:
E=B·D·vort (4.16)
here, V is the electric magnetic induction, Tl, formed between the magnetic poles; D is the inner diameter of the pipe (distance between the electrodes), m; Vort — the average speed of the stream, m/s.
Let's express the speed by volume consumption Q
(4.17)
From this expression, it follows that the value of EYUK in a constant magnetic field is directly proportional to the consumption. Induction flowmeters have an electrical conductivity of 10-3…
It is used in liquids not less than 10-5cm/m.
The main drawback of induction flowmeters with a constant magnetic field is the occurrence of polarization and galvanic EDUC in the magnetic electrodes. These drawbacks prevent or make it difficult to accurately measure magnetic field-induced EYUK in a moving fluid. Therefore, flowmeters with a constant magnetic field are used for measuring the flow of liquid metals, pulsating flow of liquid, and for short-time measurements where polarization has no effect. Most of the current induction flowmeters use an alternating magnetic field. If the magnetic field changes with frequency f at time t, EYUK is determined by the following equation:
(4.18)
here,
—amplitude value of induction.
In a changing magnetic field, electrochemical processes are less affected than in a constant field. The principle scheme of an induction flowmeter with a variable magnetic field is shown in Figure 4.8. The following symbols are used in the drawing: SBEO' is the primary electromagnetic transducer of the variable magnetic field consumption meter; The magnetic field is created using an electromagnet 4: OK—transmitter with a measuring amplifier with a 0…5 mA DC output signal; O'A—measuring instrument, integrator, etc.; R is resistance.
In the non-magnetic part of the tube 1, it is evenly divided by means of an electromagnet 4
a magnetic field is created. EYUK generated in the liquid under the influence of the magnetic field is directly proportional to the liquid capfi and is transmitted through electrodes 2 and 3 to the intermediate measuring amplifier, where an amplified signal proportional to the consumption n is output. The amplified signal comes to the measuring instrument calibrated in the unit of consumption. The presence of a unified electrical output signal (O...5mA) allows the use of secondary control devices.
Induction flow meters have a number of advantages.
These are not inertial, but rapidly changing costs
when measuring and using them in automatic adjustment systems
very important. Measurement results are not affected by particles and gas bubbles in the liquid. The readings of the flow meter do not depend on the properties of the measured liquid (viscosity, density) and the nature of the flow (laminar, turbulent).
The disadvantages of electromagnetic consumption meters include the requirement for the minimal value of the electrical conductivity of the medium being measured, which limits their scope of application. The complexity of the measurement scheme.
Induction flow meters can provide flow measurements in the range of 1...2500 m3/hour and more in pipes with a diameter of 3...1000 mm and more, when the linear velocity of the liquid is up to 0,6...10 m/s. Accuracy class of instruments is 0,6; 1; 1,5; 2; 2,5.
