Discrete liquid level and tank filling rate control sensor

Abstract

FIELD: instrument making.SUBSTANCE: sensor comprises a body with a printed circuit board in the form of a narrow thin plate, on which a substrate with two thermistors is installed, made in a “point” form with dimensions (0.15…0.5) mm × (0.15…0.5) mm and located symmetrically at a distance of no more than 1.5 mm from each other. The short side of the printed circuit board is rigidly attached to the base of the body, and the opposite unattached side comprises an opening above which a substrate with two thermistors is placed, wherein the substrate is made of a heat-insulating material, and in the centre of the substrate between the thermistors there is an opening. The distance from the thermistors to the edge of the hole is at least 0.2 mm, and the width of the hole is at least 5 times the size of the thermistors. The aspect ratio of the printed circuit board is selected in the range of 1:3…1:5, and the hole in the printed circuit board is a cutout made to the edge of one long side of the board, the width of which exceeds the width of the substrate, and the length exceeds the width of the hole in the substrate by at least 1.5 times. The substrate with thermistors is shifted as far as possible to the edge of the cutout.EFFECT: increase in the accuracy of measuring the rate of change of the liquid level in the tank.1 cl, 3 dwg

Description

[0001] The invention relates to the field of instrument making and can be used in various industries, including space, for discrete measurement of the level and rate of filling of a tank.

[0002] A liquid level sensor is known (RU Patent No. 2295115, published March 10, 2007, IPC G01F 23 / 00, G01F 1 / 68). The known sensor comprises a housing with a printed circuit board and a sensing element (thermistor) on a substrate mounted on the printed circuit board. The printed circuit board is designed as a narrow (with an aspect ratio of 1:3...1:5) thin plate. The short side of the plate is rigidly attached to the base of the housing, and at the edge of the unattached side there is a hole with a diameter 2...4 times larger than the width of the substrate with the thermistor, placed above the hole and made of heat-insulating material. The thermistor is a point-shaped resistor with dimensions of (0.15...0.5) mm × (0.15...0.5) mm and a thickness of no more than 0.005 mm. A thin film serves as the substrate, and the thermistor is located at the bottom edge of the film.

[0003] A disadvantage of this device is the inability to measure the rate of change of the liquid level at the location of the sensing element. Calculating the rate of change of the liquid level in the tank in this case is only possible by calculating the ratio of the distance between individual sensors to the time between drops in their electrical resistance. This approach does not provide high calculation accuracy and does not allow for estimating the local rate of change of the liquid level.

[0004] Based on the combination of its essential features, the closest invention to the claimed invention (the prototype) is a sensing element with two thermistors on a single thermally insulated polyimide substrate, described in the patent for the invention "Liquid Level Monitoring Sensor" (RU Patent No. 2310173, published on November 10, 2007, IPC G01F 23 / 00, G01F 1 / 68). The liquid level monitoring sensor comprises a housing with a printed circuit board, a sensing element (thermistor) made in a "point form" with dimensions of (0.15 ... 0.5) mm × (0.15 ... 0.5) mm and a thickness of no more than 0.005 mm and placed on a substrate no more than 50 μm thick, fixed above an opening in the printed circuit board with a diameter 2 ... 4 times greater than the width of the substrate with the thermistor. Additionally, a second thermistor is used in the sensor, and the thermistors are located symmetrically at a distance of no more than 0.2-0.5 mm from the upper and lower edges of the substrate, and the distance between the thermistors is no more than 1.5 mm.

[0005] The use of two thermistors in a single sensing element allows for higher accuracy in determining the rate of change of liquid level compared to a sensing element containing a single thermistor. However, despite polyimide's low thermal conductivity, it is not zero, and heat dissipation from the upper thermistor begins before it contacts the liquid surface. Therefore, a significant drawback of this invention is the steeper drop in the electrical resistance of the upper thermistor compared to the lower one when the sensing element is immersed in liquid, reducing the accuracy of measuring the rate of change of liquid level.

[0006] The problem that the claimed invention is aimed at solving is the creation of a device for high-precision discrete control of the liquid level and the filling speed of the tank.

[0007] The technical result of the invention is to increase the accuracy of measuring the rate of change of the liquid level in the tank.

[0008] This result is achieved due to the fact that in the discrete liquid level and tank filling rate monitoring sensor, comprising a housing with a printed circuit board in the form of a narrow thin plate, on which a substrate with two thermistors is mounted, made in a "point" form with dimensions of (0.15 ... 0.5) mm × (0.15 ... 0.5) mm and located symmetrically at a distance of no more than 1.5 mm from each other, wherein the short side of the printed circuit board is rigidly attached to the base of the housing, and the opposite unattached side contains an opening above which a substrate with two thermistors is placed, wherein the substrate is made of heat-insulating material, in the center of the substrate between the thermistors there is an opening. This opening is made in such a way that the distance from the thermistors to the edge of the opening is not less than 0.2 mm, and the width of the opening is not less than 5 times the size of the thermistors.The PCB aspect ratio is selected in the range 1:3...1:5, and the PCB opening is a cutout extending to the edge of one long side of the board. The width of the cutout exceeds the substrate width, and the length exceeds the substrate opening width by at least 1.5 times. The substrate containing the thermistors is offset as far as possible toward the edge of the cutout.

[0009] As the tank fills, the printed circuit board containing the sensor's sensitive element is immersed in the liquid. When the sensor's sensitive element contacts the liquid surface, a sequential drop in the electrical resistance of both thermistors is observed: the lower thermistor's resistance drops first, followed by the upper one. The thermal conductivity of the substrate affects the steepness of the upper thermistor's resistance drop, reducing the accuracy of determining when the thermistor crosses the liquid surface. However, a hole in the substrate, interrupting the heat flow between the thermistors, mitigates this effect, thereby improving the accuracy of liquid level detection.

[0010] The essence of the invention is explained by drawings.

[0011] Fig. 1 shows the printed circuit board of the discrete liquid level and tank filling rate control sensor.

[0012] Fig. 2 shows a sensitive element consisting of two independent thermoelectric resistances formed in the form of thin-film thermistors on the surface of a substrate with an opening.

[0013] Fig. 3 shows the oscillogram of the characteristics of the discrete liquid level and tank filling rate control sensor.

[0014] The discrete liquid level and tank filling rate monitoring sensor (Fig. 1) comprises a housing (not shown) with a printed circuit board 1 in the form of a narrow thin plate, the aspect ratio of which is selected in the range of 1:3...1:5. The short side of the printed circuit board 1 is rigidly attached to the base of the housing (not shown), and the opposite unattached side contains a cutout 2 made to the edge of the long side of the board 1.

[0015] On printed circuit board 1, substrate 3 with two thermistors 4 is installed above cutout 2 as close to its edge as possible (see Fig. 2). Thermistors 4 are implemented as "point" types with dimensions of (0.15...0.5) mm × (0.15...0.5) mm. Thermistors 4 are located on substrate 3 symmetrically at a distance of no more than 1.5 mm from each other. The substrate 3 is made of a heat-insulating material, and in the center of the substrate 3 between the thermistors 4 an opening 5 is made. This opening is made in such a way that the distance from the thermistors 4 to the edge of the opening 5 is not less than 0.2 mm, and the width of the opening 5 is not less than 5 times the size of the thermistors 4. The width of the cutout 2 in this case exceeds the width of the substrate 3, and its length exceeds the width of the opening 5 of the substrate 3 by at least 1.5 times.

[0016] The device operates as follows.

[0017] A discrete liquid level and tank filling rate sensor with two thin-film thermistors 4 on a substrate 3 with an opening 5 is mounted in the monitored tank within a special structure that ensures the liquid surface remains steady as the tank fills. As the tank fills, the liquid surface sequentially crosses the sensors installed at the control points. When the surface crosses the sensor's sensitive element, the electrical resistance of the two thermistors 4 of the sensitive element drops sequentially: the lower thermistor drops first, followed by the upper one.The data on the resistance drops are converted by the electronic circuit into electrical signals, the registration of which indicates the filling of the reservoir to the control point and, taking into account the calculation of the ratio of the distance between the thermistors 4, specified by the method of precision photolithography, to the time delay between the drop in resistance of both thermistors 4, the local filling rate of the reservoir at the control point is determined with high accuracy.

[0018] Fig. 3 shows an oscillogram of the characteristics of a discrete liquid level and tank filling rate sensor, demonstrating a sharp drop in the resistance characteristics of the lower and upper thermistors, respectively (as they are immersed in liquid), which indicates high accuracy in tracking the moment the thermistors cross the liquid surface.

[0019] The claimed invention can be implemented using commercially available components and materials. Polyimide is preferably used as the heat-insulating substrate material, as this material is characterized by low thermal conductivity. Titanium film, which has high adhesive properties, can be used as the resistive material of the sensing element (thermistor). Thin-film thermistors are formed using magnetron sputtering of film layers and optical photolithography.

[0020] Thus, the claimed invention enables highly accurate control of the filling process (measuring the rate of change in liquid level) of a tank. This solution will find wide application in various industries, including the space industry.

Claims

A sensor for discrete monitoring of liquid level and tank filling rate, comprising a housing with a printed circuit board in the form of a narrow thin plate, on which a substrate with two thermistors is mounted, made in a "point" form with dimensions of (0.15 ... 0.5) mm × (0.15 ... 0.5) mm and located symmetrically at a distance of no more than 1.5 mm from each other, wherein the short side of the printed circuit board is rigidly attached to the base of the housing, and the opposite unattached side contains an opening above which a substrate with two thermistors is placed, wherein the substrate is made of a heat-insulating material, characterized in that in the center of the substrate between the thermistors an opening is made in such a way that the distance from the thermistors to the edge of the opening is not less than 0.2 mm, and the width of the opening is not less than 5 sizes of the thermistors, wherein the ratio of the sides of the printed circuit board is selected in the range of 1:3 ... 1:5,and the hole in the printed circuit board is a cutout made to the edge of one long side of the board, the width of which exceeds the width of the substrate, and the length exceeds the width of the hole in the substrate by at least 1.5 times, and the substrate with the thermistors is shifted as much as possible to the edge of the cutout.

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