SU1597580A1 - Potentiometric level gauge-conductivity apparatus - Google Patents

Abstract

The invention relates to the field of instrumentation and makes it possible to combine the measurement of the level of a liquid with the measurement of its resistivity. The AC stabilizer 6 is connected to the lower end of the high-resistance electrode 1 and to the low-resistance electrode 2. From the device 7, a voltage is removed that is proportional to the level of the liquid and is independent of its electrical conductivity. To extend the functionality, a second voltage meter 8 is inserted, connected between the low-resistance electrodes 2 and 3. The product of the voltages measured by the devices 7 and 8 is proportional to the resistivity of the liquid. 2 Il.

Description

I Fig.}

This invention relates to instrumentation engineering and can be used in measuring the level and resistivity of electrically conductive liquids, including liquids containing suspended particles.

The purpose of the invention is to enhance the functionality of the conductivity level gauge by providing simultaneous measurement of the level and resistivity of the fluid.

Figure 1 shows schematically a level gauge-conductometer; figure 2 - the same, with an illustration of the output of the basic ratios of isic quantities.

The device contains the first 1, second 2 and third 3 electrodes, partially immersed in an electrically conductive liquid 4, located in a tank with a horizontal dielectric bottom 5. Between the electrode 2 and the lower end of the electrode 1, an AC stabilizer 6 is turned on, and between the upper and lower ends of the electrode 1 is a voltage meter 7 with a high input resistance. A second voltage meter 8 is connected between the second third electrodes, which is represented by a combination of a differential amplifier 9 and a secondary device 10. The common point of a two-pole power source (not shown) of a differential amplifier 9, denoted by 1, is connected to the lower end of electrode 1. Performance of meter 8 stress may be different. The electrodes are connected to external devices through wires.

11-16.

Electrode 1 must be sufficiently thin and may be a tensioned string of metal with high resistivity, low temperature coefficient and linear expansion coefficient, for example, nichrome, manganin or constantan.

Electrodes 2 and 3 can be made of a material suitable for conductometric measurements, such as stainless steel. The cross section of the electrodes 2 and 3 can be significantly larger than that of the electrode 1. In this case, the electrode 3 can be positioned in a very different way with respect to the remaining electrodes. Wires 12, 13, and 17 can pass through the measured fluid, but in this case they must be hermetically sealed. Wires 14-16 may be connected to any point of the respective electrodes.

The level gauge conductometer works as follows.

Through the liquid 4 between the electrodes 2 and 1 flows the current stabilizer 6 current. A part of the current lines in the fluid also passes through the electrode 3. The current 1o flows into the electrode 1 through its side surface and passes through the immersed portion of the electrode 1, which practically does not branch into the voltage meter circuit 7.

Due to the relatively high electrical conductivity of electrodes 2 and 3, their surfaces are equipotential, and in electrode 1 the vertical component of the current density vector prevails. It can be considered that it is distributed evenly over any cross section of electrode 1, if the frequency of the alternating current is small, but the value of this component varies along the submerged part of the electrode. The capacitance between the electrodes and the inductance of the sensor in the first approximation can be neglected.

An infinitesimal region of the dx electrode 1 between the points a and i is selected. b (figure 2). Let I (x) be the current through the upper section of the element dx. then the current i (x + dx), flowing through the lower section of the element dx, has the form

(x + dx) i (x + A

dx.

(one)

thirty

A current di6oK, equal to

di6oK gUdx, (2)

where g is the linear interelectrode electrical conductivity;

and is the voltage between the dx elements of electrode 1 and electrode 2.

According to the first law of Kirchhoff

i (x) -l (x + dx) + di6oK 0, (3)

40, where the + and - signs are taken with regard to the conditional positive directions shown in Fig. 2, the electrode portion 1 with the length dx being considered as a node.

45 By substituting expressions (1) and (2) in (3), one obtains

6 dx

-gU -0.

(four)

On the other hand, in the closed loop a-b-c-d between the points and there is a voltage U (x), and the voltage U (x + dx) between the points b and c is equal to

U (x + dx) U (x) +. (five)

The voltage dUs between the ends of the dx element of the electrode 1 is equal to

dUg ridx, (6)

where g is the linear resistance of the electrode 1.

The voltage between points c and d is zero, since the surface of electrode 2, as noted, is equipotential.

According to the second Kirchhoff law, for the a-b-c-d contour, taking into account the conditional positive directions, we can write dUg - U (x + dx-) + U (x) 0.

Substituting relations (5) and (6) here, we obtain

f-

(7)

or

sh (ah)

(eleven)

0

The voltage drop Uh at electrode 1 is obtained by integrating relation (6) over the entire submerged part of the electrode

h Uh / ridx.

about

Substituting the right side of the expression (11) instead of i, we get

Differential equations (4) and (7) are a system to solve which differentiate equation (4)

dx and substitute the value -dx

from equation (7). Then get

dl

dx

 - rgi o

(eight)

A common solution to this equation is

I, (9)

where a vVgi and the constants AI and A2 are determined from the boundary conditions at the ends of the immersed part of electrode 1. A current of the stabilizer 6 flows through the bottom base of the electrode, and the current through the cross section of the electrode at the height of the liquid level is zero. Therefore, when X h 1 1о, (10)

and at X o, where h is the value of the liquid level.

Substituting (10) in (9), get

A2-Ai;

1o Ai (e - e), i.e.

Ai

1s

ah

A2

..ah -ah

her

By substituting the values of AI and A2 in (9), we obtain a relation expressing the dependence of the current flowing through one or another section of electrode 1 on the x coordinate:

r1

)

h1o ch (ah) a- sh (ah)

(12)

This is the output signal of the level gauge, recorded by the voltage meter 7.

Expression (12) is a monotonically increasing function of S, with the minimum signal value approaching zero at h. However, the dependence of the output signal on h is not linear, the output signal also depends on the conductive properties of the fluid. By decomposing (12) in the Maclaurin series in powers of ah, it can be shown that with ah "1

(13)

those. the signal is proportional to h and does not depend

from electric wire 1

) WATER

 Since a Vrg, has the form

11X fluid properties, linearity condition

Vrg h "1. and even more so if

 1o «1, (14)

where 1o is the maximum possible measured level value equal to the length of electrode 1.

The ratio (14) can be written in vid.

vrlo vglo "1 or

"RU

(15)

where Ri2 is the interelectrode resistance of the sensor at the maximum liquid level;

RE - the resistance of the electrode 1 between its ends.

The voltage between electrodes 2 and 3 depends on both the height and the resistivity of the fluid and has the form

U2,

(sixteen)

where U2.3 is the voltage between the electrodes 2 and 3, fixed by the device 10;

Ef - specific resistivity of the fluid;

A is a dimensionless constant depending on the mutual arrangement of all three electrodes and is determined experimentally.

To obtain information on the resistivity of the fluid p, multiply the measurement of the magnitude of UH and 1) 2,3. Then, as follows from formulas (13) and (16), the result of multiplication turns out to be proportional to RJ. It is also possible to use a straining device, applying signals Uh and 1) 2,3 to its inputs. In the stationary state of a fluid, the effect of its heating with a current of 1 ° takes place, but it is insignificant in the case of a flow-through reservoir.

The small value of the signal Uh taken from the low-resistance electrode 1 does not present significant difficulties in measuring, since pickups from third-party sources of EMF sit on the relatively low resistance of the electrode 1.

The effect of the particles settled to the bottom on the measurement result is relatively small. It decreases with increasing level, since all the fluid layers are involved in the formation of the DM and 1) 2.3 signals.

Claims (1)

The invention of the Potentiometric level gauge-conductometer containing two vertically arranged metal electrodes with a cylindrical side surface, partially immersed in a controlled electrically conducting fluid, as well as a stabilized source power supply and voltage meter, each of which is connected by one of the poles to the lower end of the first electrode, characterized in that, in order to enhance the functionality by providing a simultaneous measurement of the resistivity of the fluid, a third electrode with a cylindrical side surface is inserted into it, placed similarly to the first two, and a second meter voltage connected between the second and third electrodes, while the stabilizer was used as an alternating current stabilizer, the second pole of which is connected to the second electrode, and the second pole of the first voltage meter connected to the upper end of the first electrode. FIG. 2