I. N. Kraev, V. N. Titov

Induction Flow Meter

(Recommended by the joint scientific seminar of the departments)

The ever-increasing use of measuring instruments in the chemical industry and in related branches makes it necessary to create more advanced means for measuring the flow rate of liquids. Work associated with high rates of chemical reactions often requires the use of liquid-flow instruments that have low inertia, a simple measurement scheme, and make it possible to ensure reliable technological control.

Measurement of the flow rate and quantity of substances participating in the technological processes of chemical production plays an important role not only as a means of monitoring the results of the production process, but also as one of the principal factors in controlling the course of production.

The specific conditions of chemical production, caused by the use of aggressive media, high pressures, etc., often require the use of special instruments. An induction flowmeter can measure the volumetric flow rate of liquids possessing electrical conductivity, in particular caustic acids, viscose, wastewater, liquid acidic ores, sandy and aqueous slurries, paper stock, resin glue, detergents, and food products.

Fig. 1. Diagram of an induction flowmeter:
1 — pipeline; 2 — magnet; 3 — measuring electrodes; 4 — recording instrument.

The creation of an induction flowmeter is a new method for solving the problem of measuring the flow rate of liquids, and moreover a non-hydraulic method. The principle on which it is based is widely known and consists in the following (Fig. 1): when a conductor moves in a magnetic field, in a direction perpendicular to this conductor and to the magnetic lines of force, an emf is induced in it, the magni-

is directly proportional to the number of lines of force crossed by the conductor per unit time.

The induction flow meter has a number of substantial advantages over flow meters based on measuring a pressure drop and over flow meters with a variable-area flow section.

In this flow meter there is no throttling device, no head loss, and no pressure tap; as a result, the flow rate of the above-mentioned liquids is measured with constant accuracy.

The flow meter, manufactured in the laboratory of the institute, consists of a transducer, in which an e.m.f. is induced due to the motion of the liquid in a magnetic field, and a secondary instrument.

A photograph of the transducer is shown in Fig. 2. The magnetic system of the transducer consists of an armored-type magnet having two frameless saddle-shaped windings, intended to create a uniform magnetic field between the electrodes of the transducer. The transducer tube is made of nonmagnetic stainless steel. The corrosion resistance of the instrument is ensured by the fact that the inner surface of the transducer tube is covered with an insulating material, for which hard rubber was used. Depending on the operating conditions of the flow meter, the insulating layer is applied from hard rubber, fluoroplastic (of various grades), fiberglass, epoxy resin, etc.

The electric potential arising during the motion of the liquid is taken from two small-diameter electrodes located at diametrically opposite points of the tube cross section.

Fig. 2. Photograph of the transducer: 1 — magnet core; 2 — excitation winding; 3 — measuring tube.

The end surfaces of the electrodes are sealed so that they do not cause disturbances in the liquid flow. The transducer tube is rigidly fastened in the air gap of the electromagnet and is placed in a ferromagnetic housing consisting of two halves, which also performs the function of a shield. The overall dimensions of the instrument transducer, with an outside pipeline diameter of 60 mm, do not exceed \(100 \times 100 \times 200\) mm.

The electrode leads are made with shielded wires. The supply lead-in to the electromagnet windings and the removal of the e.m.f. induced at the transducer electrodes are carried out by means of feedthrough insulators (Fig. 3).

Measurements carried out of the distribution of magnetic induction in the air gap of the transducer showed that the field nonuniformity in this electromagnet design, both in the axial and in the radial directions—

is 1–1.5%. Therefore the dependence of the instrument readings on the velocity profile is insignificant.

The schematic diagram of the flowmeter is shown in Fig. 4. The emf induced in the transducer and proportional to the measured flow rate of the liquid is fed through a shielded communication-line cable to the input of the amplifier.

Fig. 3. Structural design of the transducer:
1 — measuring tube;
2 — magnet excitation winding;
3 — magnet core;
4 — electrode;
5 — insulating gasket;
6 — insulating layer;
7 — shield;
8 — insulator.

Fig. 4. Schematic diagram of the flowmeter.

In order that practically the entire signal induced at the transducer electrodes be applied to the amplifier input, the input resistance of the amplifier must be large. For this purpose a cathode follower, assembled on tube \(L_1\), is placed at the amplifier input. On tubes \(L_2\) and \(L_3\) a voltage amplifier with resistance-capacitance coupling is assembled. The amplified signal is fed to a phase-splitting cascade.

assembled on tube \(L_4\). The presence of the phase shifter is explained by the need to match the phase of the amplifier output voltage with the phase of the voltage that ignites the neon lamps.

Compensation of the transformer emf is carried out by means of a special phase-selective element. In the circuit used, this element is a bridge multiplication circuit. An increase in the sensitivity of the phase-selective element is achieved by connecting four photoresistors with identical parameters in a bridge circuit. The photoresistors are illuminated by means of MN-8 neon lamps.

For a sinusoidal magnetic field, the signal taken from the sensor electrodes consists of two components, namely:

\[ e_0 = (e_1 \cos \omega t + e_2 \sin \omega t), \tag{1} \]

where \(e_1 \cos \omega t\) is the voltage in phase with the magnetic field, proportional to the flow velocity;

\[ e_2 \sin \omega t \]

is the interference voltage, independent of the flow and shifted in phase by \(90^\circ\).

In order to obtain a voltage proportional to the liquid flow, it is necessary to isolate the first term in equation (1) and suppress the second term. This can be done if the voltage taken from the electrodes is multiplied by a term of constant amplitude, coinciding in phase with the magnetic field, \(e_3 \cos \omega t\).

In our case, the neon lamps are supplied by the voltage \(e_3 \cos \omega t\). The voltage \(e_0\), taken from the electrodes, is applied to the supply diagonal of the bridge.

The multiplication circuit operates as follows: in the absence of illumination, all photoresistors are under identical conditions and the bridge is balanced.

During the first half-period the lamps \(L_6, L_7\) are lit, introducing an unbalance into the operation of the bridge; the lamps \(L_5, L_8\) are extinguished. The values of the resistances \(FS_2\) and \(FS_3\) during the first half-period vary from \(R_{\mathrm{d}}\) to \(R_{\mathrm{c}}\) and to \(R_{\mathrm{t}}\), i.e., the frequency of the change in resistance is equal to the frequency of the change of \(e_3 \cos \omega t\).

The maximum unbalance of the bridge occurs when the phase of the voltage supplying the lamps coincides with the phase of the voltage applied to the supply diagonal of the bridge. At the end of the first half-period all the lamps are extinguished and the bridge is again balanced.

During the second half-period \(e_3 \cos \omega t\) ignites the lamps \(L_5, L_8\); the pattern of resistance variation is repeated.

Analytically, the result of multiplication can be represented in the form

\[ e'_0 = (e_1 \cos \omega t + e_2 \sin \omega t)e_3 \cos \omega t = \]

\[ = \frac{1}{2}\,(e_1 e_3 + e_1 e_3 \cos 2\omega t + e_2 e_3\, 2\omega t). \tag{2} \]

By placing a filter at the output of the multiplication circuit, one can isolate the pure signal

\[ e = \frac{1}{2} e_1 e_3. \tag{3} \]

The result obtained characterizes the signal from the sensor electrodes. A indicating instrument and a recorder are provided as the recording devices.

Figure 5 presents the experimental dependence of the sensor output signal on the magnitude of the flow rate. Repeated measurements showed that the scatter of the results is insignificant.

Fig. 5. Dependence of the readings of the recording instrument on the flow rate: 1 — calculated; 2 — experimental.

Fig. 6. Diagram of the test installation.

The study of the experimental model of the electromagnetic flow meter was carried out on a water-metering test installation (Fig. 6). Water is drawn by pump 1 from supply tank 8 and delivered to pressure tank 3, located at a height of 10 m. The liquid from pressure tank 3 flows through pipeline 4 into measuring tank 6. The sensor is mounted on a section of pipeline 4. Valve \(B_2\) serves to regulate the flow rate of the liquid through the sensor. The measuring tank of the installation had previously been calibrated by means of a measuring vessel.

To maintain a relatively constant level in pressure tank 3, pipeline 9 is used, through which the liquid, when a certain level is exceeded, flows into storage tank 8. Pipeline 7 and valve \(B_3\) serve to drain water from measuring tank 6 into storage tank 8.

The principal error of the sensor does not exceed \(\pm 2\%\).