RU2546715C1 - Device to measure density of current in local volumes of solid media - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: sensor of the device represents a thick-walled tube-dowel 1, made of a dielectric plastic material, on the outer cylindrical surface of which there are current electrodes 2 and 3, split by plastic dielectric gaskets 4 and on the reverse side. The tube-dowel with electrodes is inserted into a hole 6, previously drilled in the solid medium, and is spaced in it with a screw 10 screwed into it. Gaskets 4 and 5 are installed perpendicularly in a current line 8, flowing in the solid medium. A current recorder 9 is connected between current electrodes 2 and 3. A rotary lever-indicator 11 is fixed on the upper part of the dielectric tube perpendicular to the split of the tube-dowel.

EFFECT: increased accuracy of current density measurement in local volumes of solid media.

2 cl, 1 dwg

Description

The invention relates to measuring technique, in particular to devices for measuring current density in local volumes of solid conductive media with any current direction.

A device is known for measuring the electrical conductivity of liquid media and measuring the current density in local volumes of liquid solutions and melts, containing a sensor in the form of a dielectric tube, two disk-shaped current electrodes installed inside it, separated by an insulating gasket, three ring-shaped measuring electrodes, one inside the tube and two outside opposite the inner ring-shaped and the current electrode closest to it, a current recorder and a variable resistor connected in series between the current and electrodes, two of the same type of voltage recorder, included respectively between the outer electrodes and between opposite located internal electrodes [1]. The device can be used both for measuring the electrical conductivity of liquid media and the current density in them. When measuring the current density, the tube is installed in the local volume of the liquid medium under study and rotated so that its longitudinal axis is located in the direction of the current lines, while the voltage recorder connected between the external electrodes shows the maximum value. In the found position of the sensor, by changing the resistance of the variable resistor, the readings of the voltage recorders are set to the same. According to the testimony of the current recorder, taking into account the known section of the tube, the current density is found.

To use this device in solving the problem of measuring the current density in local volumes of solid conductive media, placing the device’s sensor inside a solid, turning and orienting the tube along the current lines is impossible, which excludes the use of the device when measuring in solid media. The increased dimensions of the sensor additionally burden the solution of the problem of measuring current in local volumes of solids.

The closest in technical essence to the proposed device is a device for measuring current density in liquid media, containing three identical measuring elements [2]. Each of them contains a dielectric tube with two current disk-shaped electrodes located in it, separated by a dielectric spacer, and a current recorder connected between the current electrodes. In this case, the dielectric spacers are made lamellar, and the current electrodes are flush with the ends of the tubes. The dielectric tubes of the measuring elements of the device are rigidly fixed one relative to another in three mutually perpendicular planes using a non-conductive frame.

The technical result of the invention of a device for measuring current density in local volumes of solid media provides enhanced technological capabilities, improved accuracy and simplifies the procedure for measuring current density in local volumes of solid media: simplification of preparatory operations by using a simple and reliable drilling operation not only on drilling machines, but and a variety of portable drills: this allows measurements not only in laboratory conditions, but for most operating facilities at the place of installation; use as a tube of a typical, easy to manufacture, cheap, widespread building element - dowels; simplicity and manufacturability of the manufacturing device sensor; simple preparation and measurement technique, high measurement accuracy by reducing the size of the sensor placed in the measuring space of a solid conductive medium.

The essence of the invention lies in the fact that the device for measuring the current density in local volumes of solid media, containing a current recorder connected between two current electrodes separated by a dielectric spacer, located flush with the dielectric tube rigidly fixed by the frame, further comprises a second dielectric spacer, current the electrodes are a thin shell attached externally to the lower cylindrical part of a thick-walled plastic tube-dowel and cut together f with the bottom of the dowel-tube along the axis of symmetry, flush with the tube on both sides flush with the tube, and plastic dielectric spacers inserted flush with the formed current electrodes on the outside, the tube-dowel with a sliding fit is placed in the hole previously drilled in solid medium through the measurement site perpendicular to the current vector, and the function of the frame is performed by a screw pre-screwed into the dielectric tube and bursting it in the hole, in addition, the device can add It is necessary to contain a rotary lever-pointer mounted on the upper part of the dielectric tube-dowel perpendicular to its section.

The current density is found by the formula:

$$J=I/S,\mathrm{BUT}/m^2,\text{(one)}$$

where I is the current strength according to the current recorder, A;

S = h × d is the central sectional area of the hole, m ² ;

h is the distance from the lower edge of the current electrodes:

- to the upper edge of the current electrodes when the electrodes are completely immersed in a current-carrying solid medium, m;

- to the upper edge of the current-carrying solid medium with partial immersion of the electrodes in the current-carrying solid medium, m;

d - hole diameter, m

Therefore, the sensor of the device consists of a rotary bay, which is a plastic thick-walled tube-dowel, on top of the outer cylindrical surface of which thin-sheet current electrodes bent by a half-shell are fixed. The lower part of the dowel tube is cut along the axis of symmetry, and dielectric gaskets are inserted flush with the inner cylindrical surface of the dowel tube and the outer surface of the current electrodes. The electrodes are interconnected by a current recorder and are made of foil or of plates 0.05-1 mm thick. The plate gasket has a thickness of 0.5-1 mm.

In a local place of a conductive solid medium (for example, in a steel plate over which electric current flows), in which it is necessary to measure the current density, a hole with a diameter of 5-10 mm is drilled. The hole provides free sliding and rotation of the bay. For example, in a plate, a hole is drilled from the surface nearest to the local volume under investigation and passes through it so that the center of the local volume is located on the symmetry axis of the dowel tube at a distance h / 2 from the lower edge of the current electrode. The bay is installed in the hole. An electric current passes through each current electrode and is recorded by a current recorder.

When the bay of the device is rotated, the strength of the current flowing from one current electrode through the current recorder to the second current electrode changes. All the current will pass through the current recorder, provided that the dielectric spacers are perpendicular to the current vector. Otherwise, only part of the current will flow between the current electrodes. The measurement error increases and reaches a maximum with the direction of the dielectric spacers along the current vector flowing through the measured local volume. In this case, no current will flow between the current electrodes at all.

Unlike the prototype, where the shape of the current electrodes is flat, in the proposed device, the electrodes are made in the form of half shells. This is due to the technological features of obtaining holes in the measured solid medium. Obtaining holes in the form of a recessed disk, grooves with rounding at the edges, a parallelepiped is technologically many times more difficult than a cylindrical one. They are created by milling, chiselling on specialized rarely used and expensive machines, while cylindrical ones are easily created by conventional widespread drilling. To do this, you can use not only drilling machines, but also a variety of portable drills. This allows measurements to be made not only in laboratory conditions, but also for most operating objects at the place of their installation.

The use of current electrodes in the form of half-shells instead of flat ones does not significantly change the measurement process. The area of the active cross section of the current involved in the measurement process is S = h × d, m ² , where h is the height of the central section involved in passing current between the electrodes, m, d is the diameter of the hole, m. This rectangle is essentially equal to the width of the hole captured by the current electrode during the measurement, multiplied by the height of this hole.

In the preparatory period of measurement by turning the bays maximize the recorded current. Then a screw is screwed into the dowel tube, bursting it and tightly pressing the current electrodes to the walls of the hole. The contact area of the current electrodes with the wall of the hole increases to the maximum at a given pressure of the electrode against the wall of the hole.

The current recorder, like current electrodes, has high electrical conductivity. Given the small diameter of the hole, the small thickness and diameter of each current electrode, the device has a negligible effect on the conductivity, and hence the current density in the process unit. This allows for high measurement accuracy.

The thickness of the dielectric strip and electrodes is reduced to a minimum, due to which the dimensions of the bay and the holes, the degree of their influence on the electrical conductivity and current density in the technological unit are minimized. A screw serves as a frame. He bursts the dowel tube in the hole, maximizing the contact of the current electrodes with the wall of the hole.

The use of the second dielectric strip allows you to separate the current electrodes on one side of the cylindrical outer surface of the dowel tube, while the first dielectric strip separates the current electrodes on the other side.

The implementation of the current electrodes in the form of a thin shell allows, firstly, to reduce the size of the bay - the moving part of the sensor, consisting of a dowel tube, two current electrodes, two dielectric spacers and a screw. Secondly, lowering the stiffness of the current electrodes while reducing the thickness of the curved plates contributes to a better fit of the current electrodes to the hole in the measured solid medium.

Failure to use a flat sensor-shaped sensor circuit consisting of a set of flat current electrodes separated by a flat dielectric gasket allows using a simpler, more technological and economical way to create a hole for placing the device’s sensor in the local measurement area — drilling through the studied area of a solid medium. This made it possible for the movable part of the sensor — to give the bay a cylindrical shape, which made it possible to use the hole as efficiently as possible to accommodate current electrodes. The bay with a small gap occupies almost the entire space of the drilled cylindrical hole.

The mounting of the current electrodes from the outside to the lower cylindrical part of the thick-walled plastic tube-dowel makes it possible to arrange them in the immediate vicinity of the hole wall. This allows, firstly, in the preparatory period of the measurement to search for the direction of the current vector and to install dielectric spacers perpendicular to the current vector. Secondly, due to a small expansion of the bay when screwing in the screw, to achieve a good fit of the current electrodes to the wall of the hole.

A hole pre-drilled in a solid medium through the measurement site perpendicular to the current vector allows you to place and use a cylindrical sensor in the measured local volume.

The manufacture of the device’s sensor in a cylindrical form provides, firstly, the possibility of its rotation and, as a result of adjusting the position of the sensor in a solid local area according to the criterion of maximizing the current flowing through the current electrodes, setting it so that the dielectric spacers are located perpendicular to the current vector in the measured local area. Secondly, it makes it possible to minimize the violation of the initial shape of the measurement object itself - an electrically conductive solid medium: under the conditions of the possibility of adjusting the device to a current vector in a local volume with rotation of the sensor in space, the technological hole made in this case is of a minimum volume.

The placement of the dowel tube with a sliding fit in the hole of the electrically conductive solid medium allows the sensor to rotate in the solid during the preparatory period of measurement. This makes it possible to correctly position the current electrodes in the measured local area and to maximize the current strength during the preparation of the device. As a result of such positioning, the width of the cross section of the electrodes involved in conducting the current turns out to be known and equal to the diameter of the hole. Ultimately, this fact allows us to find the current density.

A screw acting as a frame provides access to the measured local volume for bursting of the dielectric tube in the hole and establishing reliable contacts of the current electrodes with the surface of the hole by screwing it into the dielectric tube (for example, with a screwdriver or screwdriver).

The use of a rotary lever-pointer facilitates not only the rotation of the bay in the hole, but also its installation. Strengthening the pointer lever on the upper part of the dielectric dowel tube allows its functions to be carried out even with a significant immersion of the dowel tube in the hole. The direction of the pointer arm perpendicular to the section of the tube-dowel combines the direction of the current vector with the direction of the pointer arm.

The lever-pointer performs a special role in the subsequent refinement of the current density by taking into account the current component in the plane passing through the pointer-lever and the axis of symmetry of the dowel. In this case, the second measurement is carried out with a new drilling of the solid under study. The direction of drilling is determined, inter alia, by the pointer of the lever-pointer.

Thus, a comparison of the claimed solution with other technical solutions shows that the newly introduced elements are known. However, their introduction in this connection with other elements of the device, as well as their mutual arrangement, leads to the appearance of the new properties mentioned above, which make it possible to measure the current density in any local volume of a solid conductive medium in any direction of the current vector.

The drawing shows a General view of a device for measuring current density in local volumes of solid media.

The device comprises a thick-walled tube-dowel 1 made of a dielectric plastic material, on the outer cylindrical surface of which are mounted current electrodes 2 and 3, separated by plastic dielectric spacers 4 and from the reverse side 5 (gasket 5 is not shown). The tube-dowel with electrodes is inserted into the hole 6, previously drilled in a solid medium, and raspred in it by a screw 10 wrapped in it. Gaskets 4 and 5 are installed perpendicular to the streamline 8 flowing in the solid medium. Between current electrodes 2 and 3, a current recorder 9 is turned on. On the upper part of the dielectric tube perpendicular to the section of the tube-dowel, a rotary pointer arm 11 is mounted.

The dielectric tube-dowel 1, the dielectric gaskets 4, 5 are made of plastic material with a low dielectric constant in the operating temperature range of the studied liquid medium. For example, at a low temperature of a solid conductive medium, dielectric tubes and gaskets can be made of polyvinyl chloride, polystyrene, polyethylene, and at high temperature - from fluoroplastic.

The shape of the tube-dowel is practically no different from the plastic dowels used in construction for fixing screws in drilled wall openings. The lower end of the tube-dowel is cut into 2 or more parts along the axis of symmetry of the tube. This is done to facilitate the bursting of the tube in the formed hole. For the purpose of measuring the current density in a low-temperature electrically conductive solid medium, building dowels, the outer surface of which is strictly cylindrical, without bumps and diameter fluctuations, can be used as a dowel tube. On both sides, dielectric spacers are inserted into the through slot in the lower part of the dowel tube, separating the current electrodes from each other, including during plastic settlement during the expansion of the dowel tube in the hole. Current electrodes are fixed on the two remaining external semi-cylindrical surfaces of the lower part of the tube-dowel.

The electrodes 2 and 3 are rolled from a thin plate in the form of half shells with a diameter of d with negative tolerances that provide free sliding of the bay, consisting of a dowel tube with current electrodes fixed to it, in a drilled hole. The electrodes are made of a plastic material with high electrical conductivity and low tendency to surface polarization. At low temperature - from a foil of copper, silver. At an elevated temperature of the medium under study, the electrode material should have sufficient heat resistance, and at an increased aggressiveness of the medium, corrosion resistance or heat resistance. For example, from steel foil 12X18H9T, platinum.

At the boundary "current electrode - measured solid medium" at each given temperature, a potential difference is formed, which is determined by the difference in electrochemical potentials of the electrode material and the material of the studied solid medium at the measurement site. For example, copper M1 and steel St3ps. However, in a measuring circuit made up of the “test medium - first current electrode” contact — the first current electrode — the first current lead wire — the current recorder — the second current lead wire — the second current electrode — the second current electrode – test medium contact, they are included in the circuit two identical potential differences. They are mutually destroyed and practically do not affect the measurement results.

Electrodes with current leads on the dowel tube are fixed, for example, by gluing, soldering. Welding-soldering proved itself when the current electrode warms up with a soldering iron, bringing the plastic of the tube-dowel to soften (for polyethylene - 200-215 ° C). When this occurs, welding-soldering of the electrode to the surface of the tube-dowel. Before welding, the surface of each electrode should be degreased in known solutions.

A hole in a solid conductive medium is obtained by drilling. To reduce the surface roughness of the hole and improve the fit of the current electrodes to the surface of the hole, reaming, honing, grinding and other methods are used.

A steel screw is used as a frame. The diameter and pitch of the screw cutting depends on the diameter of the dowel tube. So, with a tube-plug diameter of 6 mm, a screw with a diameter of 3 mm is used. A screw is screwed into the bay during the preparatory period of measurement. After taking the testimony of the registrar, he turns out and frees the bay from a close connection with the hole. The bay is freely removable. The possibility of re-applying the same bay for the next measurement should be determined by the results of verification of its integrity.

In the case of the manufacture of a pointer arm of plastic (polyvinyl chloride, polyethylene, etc.), it is attached to the dowel tube by welding. When it is made of metal, fastening to the dowel tube can be performed using a tie on the base of the dowel tube.

The type of current recorder is determined by technological factors and depends on the type, polarity, expected range of current strength, the required measurement accuracy, etc.

The device operates as follows.

The location of the local volume of the solid medium in which measurements are required is determined. The approximate direction of the current vector in this local volume is preliminary estimated. The possibility (including safety of consequences) of drilling into this local volume is evaluated. If possible, a hole is drilled through the indicated local volume perpendicular to the expected direction of the current vector in this volume. Whenever possible, drilling is carried out from the surface of a solid body closest to it. The local volume should be drilled through with an overrun of l ₂ = 3S and by cutting in l ₁ = d / 2 · tg φ, where S is the drill feed, mm / rev., Φ is half the angle at the top of the drill, φ≈59 °.

The dimensions and position of the current electrodes on the dowel tube are measured. The sensor is inserted into the hole - the investigated local volume of the solid medium. Under the conditions of electric current flowing through the local volume by rotation of the dowel-tube, the current strength recorded by the current recorder is maximized. A screw is wrapped, bursting the tube-dowel. The final readings of the current recorders are counted. In case of incomplete immersion of the sensor in the measured solid medium, the dimensions of the unloaded part of the current electrodes are measured. Calculated h is the distance from the lower edge of the current electrodes:

- to the upper edge of the current electrodes when the electrodes are completely immersed in a current-carrying solid medium, m;

- to the upper edge of the current-carrying solid medium with partial immersion of the electrodes in the current-carrying solid medium, m

The current density is found by the formula (1).

After the required number of measurement repetitions and providing the required measurement statistics, a decision is made on the appropriateness of the further use of the hole. If necessary, it is eliminated. For this, various known recovery methods are used: welding, soldering, caulking, etc.

To clarify the unmeasured part of the current flowing in the plane A, which coincides with the direction of the pointer arm and passing through the axis of symmetry of the dowel, another similar measurement is carried out in plane A. For this, another hole perpendicular to plane A is drilled near the studied local volume , and the measurement is repeated.

In this case, the total current density in the studied local area is found by the formula:

$$J=\sqrt{J_{\mathrm{one}}^2+J_2^2},A/m^2,(2)$$

where J ₁ is the current density in the first dimension, A / m ² ;

J ₂ - current density in the second dimension, A / m ² .

It should be noted that when measuring in two planes, the measurement accuracy is practically independent of the accuracy of setting the direction of the first drilling. If there is no guarantee that the direction of the current vector in the local volume is correct before drilling the first hole, it is advisable to conduct a study in two planes. In this case, it is advisable to arrange both holes on both sides of the center of the measured local volume at a distance of 1-2 d, where d is the diameter of each hole. Moreover, lower values correspond to a smaller recommended hole diameter.

The metrological characteristics of the device were checked by multiple measurements of the current density flowing along a steel plate 100 mm long, 25 mm wide and 25 mm high from the VDU-1201 welding rectifier complete with RB-301 ballast resistors. Current electrodes were made of copper foil 0.3 mm thick, which ensured stable operation of the device at a constant current in the recorder up to 1 A.

Using 30 measurements, the average actual current density in the plate J _{p was} calculated, which was compared with the results of 30 current density measurements with the device J _y .

The absolute measurement error was Δ _u = J _y -J _p = 0.220 · 10 ^-6 -0.217 · 10 ^-6 = 0.007 · 10 ^-6 A / m ² . The relative error was δ = (Δ / J _y ) · 100% = (0.007 · 10 ^-6 / 0.220 · 10 ^-6 ) · 100% = 3.18%.

As an example, the use of a device for assessing the degree and nature of the spreading of stray currents in the hull of a vessel was considered. The results of using the device confirm its operability in a wide range of current density changes. In this case, it is recommended to use a multi-range ammeter as a current recorder.

The proposed device allows the measurement of current density in solid conductive media for any direction of current in a wide range of products.

Information sources

1. RF patent No. 2007707, priority dated 01/27/1992, cl. G01N 27/07, G01R 27/22 IPC ⁶ "Device for measuring electrical conductivity and current density."

2. RF patent No. 2055353, priority of 02/01/1993, cl. G01N 27/07 IPC ⁶ "Device for measuring current density in liquid media" is a prototype.

Claims (2)

1. A device for measuring current density in local volumes of solid media, comprising a current recorder connected between two current electrodes separated by a dielectric spacer, flush with a dielectric tube rigidly fixed by a frame, characterized in that it further comprises a second dielectric spacer, current the electrodes are a thin shell attached externally to the lower cylindrical part of the thick-walled plastic dowel tube and cut together with the lower part of the pipe the dowel-plugs along the axis of symmetry, flush with the tube on both sides flush with the tube, and plastic dielectric spacers inserted flush with the formed current electrodes on the outside, the tube-plug with a sliding fit is placed in a hole previously drilled in a solid medium through measurements perpendicular to the current vector, and the function of the frame is performed by a screw previously screwed into the dielectric tube and bursting it in the hole. 2. The device according to claim 1, characterized in that it further comprises a rotary pointer arm mounted on the upper part of the dielectric tube-dowel perpendicular to its section,
current density is found by the formula:
J = I / S, A / m ² ,
where I is the current strength according to the current recorder, A;
S = h × d is the central sectional area of the hole, m ² ;
h is the distance from the lower edge of the current electrodes:
- to the upper edge of the current electrodes when the electrodes are completely immersed in a current-carrying solid medium, m;
- to the upper edge of the current-carrying solid medium with partial immersion of the electrodes in the current-carrying solid medium, m;
d - hole diameter, m