AT410595B - Fluid flow rate and/or speed measurement unit, comprises mechanical expansion unit, at least one expansion strip and tubular body - Google Patents

Abstract

An arrangement for measuring the flow rate and/or speed of a fluid stream, comprises at least one mechanical expansion unit (12,13) whose expansion depends upon the pressure drop of the medium passing through a flow restriction. At least one expansion strip is mounted on the expansion body. The body is tubular and the medium flows through it axially.

Description

AT 410 595 B

The invention relates to a device for measuring the flow speed and / or rate of a fluid medium, with at least one mechanical expansion body, the expansion of which is subject to the pressure drop of the medium via a flow restriction, and at least one strain gauge element arranged on the expansion body.

Such devices are used, for example, in volume measuring devices of water measuring devices, e.g. as heat meter. According to the state of the art, ultrasonic transit time measuring devices and sensors based on the magnetoinductive principle are primarily used for this application. When using ultrasonic transit time measuring devices, the particularly short transit times of the ultrasonic pulse and the deposits that occur over time on the reflector surfaces or sound sensors, which lead to a significant weakening of the reflected sound pulses and measuring sensitivity, are a problem. Air bubbles and air pockets also lead to incorrect measurements. The processing electronics can also cause problems with regard to their electromagnetic compatibility. In addition, the sensors used are mechanically sensitive. In the event of operational interventions in the piping system, the individual devices often have to be vented, which causes costs. Another problem is the cleaning of deposits after some operating time.

In the case of sensors based on the magnetoinductive principle, changes in the ion concentration and type of ions in turn create problems, and readjustments have to be carried out. The occurrence of air and vapor bubbles in the medium also leads to incorrect measurements.

A mass flow meter of the type mentioned in the introduction is known from CH-PS 604 132. In this device, the pressure before and after the flow constriction is conducted via channels to membrane-shaped expansion bodies. This structure is complex and the sensitivity is limited.

The aim of the invention is to provide a particularly simple device for measuring the flow rate and / or rate of a fluid medium, which device also has a high sensitivity in the lowest flow area.

This goal is achieved with a device of the type mentioned at the outset, which is distinguished according to the invention in that the expansion body is tubular, the medium flows axially and is arranged downstream of the flow restriction, the expansion body being surrounded by an annular sealing body with formation of an annular space, and wherein the annular space is fed via a feed line from the medium upstream of the flow restriction.

With the use of a tubular expansion body according to the invention, a particularly streamlined design with the simplest construction can be achieved. In addition, the axial flow allows a particularly large contact area with the medium, so that the sensitivity is maximized. The guiding of the medium also to the outside of the expansion body enables a differential measurement, which compensates for the temperature dependencies of the medium.

A particularly advantageous embodiment of the invention is characterized in that the feed line opens into the annular space at one end and from the other end a discharge line emerges, which opens into the medium downstream of the expansion body and has a smaller passage cross section than the feed line. As a result, an at least slight flow through the annular space is achieved, which also prevents deposits in the annular space which could impair the measuring accuracy.

It is particularly favorable if the passage cross section of the exhaust line is adjustable. As a result, the measuring sensitivity and measuring curve of the measuring device can be set.

The flow restriction is preferably formed by a nozzle body to which the expansion body is connected, the unit comprising the expansion body and nozzle body being held at both ends essentially coaxially in the sealing body by means of a flow-permeable centering element, and the passage cross section of the inlet-side centering element being the feed line and the passage cross section the outlet-side centering element forms the exhaust line. This results in a particularly streamlined construction, which largely prevents deposits.

A particularly simple, yet precise type of setting of the exhaust line is achieved if an annular disk faces the outlet-side centering element and is axially adjustable in the direction of the centering element.

In any case, it is particularly favorable if, according to a further feature of the invention, the expansion body is inserted into a flange of the nozzle body, which is a stable construction 2

AT 410 595 B results.

It is particularly advantageous if the expansion body has at least one longitudinal slot that is spanned by an expandable bellows. This can significantly increase the sensitivity in the lower flow range. Alternatively, the expansion body can have at least one longitudinal slot, which is spanned by a highly elastic material, preferably 2-component silicone rubber.

A particular problem is the sealing of the bellows in the axial direction against the fluid flowing through. To solve this problem, the invention proposes a preferred shape of the bellows, which is characterized in that the bellows is composed of a first V-shaped plate part and at least a second V-shaped plate part, the end edges of the V-legs of the first slat part are connected to the edges of the slot and the V-apex of the first slat part points into the interior of the expansion body, each of which has a side edge of the V-leg of the second slat part connected to the one side edge of the V-leg of the first slat part and the V-apex of the second lamellar part points into the interior of the first lamellar part, and the respective other side edges of the V-leg of the second lamellar part are connected to one another. Seen in the flow direction of the tubular expansion body, the parts of the bellows pointing into the interior of the tubular expansion body therefore do not form their own permeable channels, so that the entire expansion body can be sealed particularly easily by an envelope.

Another preferred embodiment of the invention is characterized in that additional, curved expansion plates are welded in the circumferential direction on the outside of the expansion body facing away from the slot. This increases the resistance of the expansion body to high pressure differences, while maintaining its sensitivity with small forces.

The expansion body is preferably surrounded by a tubular, pressure-resistant sealing body to form an annular space, the same medium also flowing through this annular space.

The risk of temperature differences is therefore extremely small. The medium flowing into the device is focused in terms of flow into the interior of the expansion body by means of a rotating nozzle body, so that a high flow rate can occur. This creates a vacuum zone in this area. This rotary nozzle body is screwed to the tubular sealing body at three or more locations. What is important for the mode of operation is the fact that the rotationally shaped nozzle body does not close off the tubular sealing body (holding tube). This is ensured by radial recesses on the circumference of the rotary nozzle body. This means that part of the medium flow can shift into the annular space, the flow speed being significantly lower.

Accordingly, a pressure difference arises in the annular space to the flow inside the expansion body, which acts on the expansion body with negative expansion (compression). The mechanical deformation that occurs represents a function of the flow velocity. At the end of the annular space there is also a star-shaped holder with recesses in the radial direction and at the same time serves as a holder for the expansion body.

By means of a steel disc with a fine thread that can be adjusted in the axial direction, it is possible according to this invention to set the flowing medium quantities. This has a direct consequence on the pressure distribution. It is thus possible to adjust the characteristics of the measuring device.

A particularly noteworthy advantage of this device is that the system is immediately ready for operation after installation of the device. In addition, impurities are rinsed out immediately. This also affects air and steam bubbles. It is therefore not necessary to vent the device.

Since the expansion body is in the medium water, moisture-proof strain gauge elements already available on the market are used, which are additionally enclosed with polyurethane. It is particularly advantageous if, according to a further feature of the invention, the expansion body has at least one longitudinal slot that is spanned by an expandable bellows. As a result, the measurement sensitivity in the lower flow range can be increased significantly. 3

AT 410 595 B

A particular problem is the sealing of the bellows in the axial direction against the fluid flowing through. To solve this problem, the invention proposes a preferred form of bellows according to claim 9. The bellows is preferably composed of a first and two further lamella parts, the second lamella parts with their interconnected V-leg side edges facing each other, which ensures a symmetrical, also symmetrically expanding structure.

For heat and corrosion resistance, the lamella parts are preferably made of laser-welded individual parts made of high-quality spring steel.

For cost reasons in series production, it is possible to press the appropriate shape of the spring steel, preferably 1.4301 steel, to press the complete shape and at the same time to roll the tubular expansion body (automatic production) and to create connections by laser welding only at two points. In any case, it is particularly advantageous if, according to a further feature of the invention, a plurality of strain gauge elements are arranged on the outer surface of the expansion body, which are connected to at least one Wheatstone measuring bridge, which increases the measuring accuracy. In order to increase the sensitivity even further, several Wheatstone bridges are preferably provided, which measure the material expansion in the tangential and / or axial direction.

A particularly simple assembly results if the Wheatstone measuring bridges are preferably connected in thin-film technology and manufactured on a carrier film which is applied to the expansion body. Furthermore, by applying semiconductor strain sensors, including a piezoelectric version, the measurement signal level can be significantly increased.

In order to compensate for the transducer curve of the strain gauges or measuring bridges, the wall thickness of the expansion body can preferably increase linearly in the flow direction. It is particularly advantageous if the wall thickness of the expansion body increases in a non-linear manner in the throughflow direction in such a way that the output signal of the strain gauge element or of the measuring bridge assumes a linear profile with the flow velocity.

Due to the quadratic relationship between differential pressure and measurement signal, on the other hand, in order to obtain good results in the lowest flow range, the way could be taken to provide expansion plates according to claim 10. These preferably have a non-constant wall thickness, so that they have a very subordinate effect in the area of the smallest expansions, that is to say with the smallest medium throughputs. If there is a greater mechanical expansion, they bring about a steady reduction in the actual mechanical expansion of the tubular expansion body and the system becomes less sensitive, so that the sensitivity to measurement in the lowest flow range is greatly increased. In addition, this measure is a protection against sudden pressure surges often occurring in line systems.

Since the average power loss of the strain gauge elements is limited due to the necessary heat dissipation, but according to the relationship AR / R0 = ks (k factor is the strain sensitivity), the measuring bridge signal increases linearly with the magnitude of the bridge supply voltage, and in order to make full use of the measuring sensitivity, it is It is particularly advantageous to apply short needle pulses with a higher voltage level to the bridge supply voltage. This brings not only a high measurement signal, but also a low energy consumption, which makes possible battery operation of the device possible. The measuring bridge diagonal signals are stored in analog capacitors on charging capacitors and then processed further. If several strain gauge bridges are used, a cascade can be built up via buffer capacitors. An extremely sensitive measuring system is thus achieved.

Further objects, features and advantages of the invention will become apparent from the following description of a preferred embodiment in conjunction with the accompanying drawings.

Fig. 1 shows an embodiment of the device according to the invention in longitudinal section, Fig. 2 shows the section "A" of Fig. 1, Fig. 3 shows the section "B" of Fig. 1,

4 illustrates the expansion body in a schematic perspective view, FIG. 5 shows a front view of the expansion body from FIG. 4,

6 shows the measure according to the invention for raising the smallest and weakening large measured values in a likewise schematic view (perspective view), and

7 shows a block diagram of the wiring of the strain gauge elements of the strain gauge 4

AT 410 595 B body.

According to FIG. 1, the device for measuring flow velocities and / or rates of a fluid medium has a sealing body 1, which is closed at its end faces with end plates 2, 4. The end plates 2, 4 are penetrated by central openings and provided on the outside with end flanges 3, 5 in order to integrate the entire device into a pipe (not shown) which transports the fluid to be measured. Each end plate 2, 4 is equipped with an inner flange 6, 7 on its inside. The sealing body 1 contains the cable connection screw connection 8.

In the interior on the right of the sealing body 1, a rotationally shaped nozzle body 9 (see FIG. 2) with radial recesses 10 is connected to the sealing body 1 via fastening screws 11. The webs remaining between the cutouts 10 form centering elements for the central nozzle part of the sealing body. A sealing sleeve 12 is glued in the nozzle body 9. This carries an expansion body 13 with strain gauge elements. The tight connection to the expansion body 13 is established via further centering elements in the form of a holding device 14 (see FIG. 3) and a rubber sleeve 15.

The holding device 14 also has radial recesses 16 and is connected to the sealing body 1 by means of connecting screws 17. Accordingly, there is an annular space 18. The annular space 18 is covered in the drawing on the right by an annular disk 19 which can be adjusted in the axial direction by means of adjusting screws 20 depending on the desired flow distribution. Most of the fluid flow of the medium reaches the interior of the expansion body 13 through the central opening 21 of the nozzle body 9, which creates a vacuum zone in the interior of the expansion body 13. The fluid then re-enters the flow path via the circular opening 22 of the ring disk. Part of the fluid flow enters the annular space 18 and is braked due to the larger passage area in the annular space 18 and via the annular disc 19.

The speed reduction of the fluid medium in the annular space 18 can be adjusted in the axial direction due to the adjustable distance between the annular disc 19 and the holding device 14. This also changes the flow distribution between the expansion body 13 and the annular space 18. The static flow component comes into play in the annular space 18. Therefore, there is always an overpressure zone in the annular space 18 in the case of fluid flow and the expansion body 13 experiences a negative expansion (compression).

Fig. 2 shows the centering elements of the nozzle body 9. With 23 fastening threads are shown. Fig. 3 illustrates the bracket 14 of the expansion body 13 via the rubber sleeve 15, which is also glued. With 24 fastening threads for the screws 17 are shown. The structure of the expansion body 13 is explained with reference to FIGS. 4 and 5. The expansion body 13 consists essentially of a thin-walled tube 25 made of highly elastic, high-quality spring steel, which has a longitudinal slot 26 which is spanned by an expandable bellows 27. The bellows 27 could be a simple bellows, the fold lines of which run in the axial direction.

As illustrated, the bellows 27 is preferably composed of a first V-shaped lamellar part 28 and two second V-shaped lamellar parts 29,30 in the following manner:

All end edges of the V-legs are provided with folds 31, 32. The end edges 33 of the V-legs of the first lamellar part 28 are initially connected to the edges of the slot 26, the V-apex 34 of the first lamellar part 28 pointing into the interior of the expansion body 13. Furthermore, one side edge 35 of the V-leg of a second lamella part 29, 30 is connected to one side edge 36 (congruent in the drawing) of the V-leg of the first lamella part 28. The V-legs 37 of the second lamella parts 29, 30 point into the interior of the first lamella part 28. The respective other side edges 38 of the V-legs of a second lamella part 29, 30 are connected. The sealing sleeves 12, 15 are glued inside on the edge of the expansion body and thus connected in a sealing manner. The V-shaped gap remaining between the lamella parts 29, 30 is filled with high-quality 2-component silicone rubber on its top. The small gap remaining between the lamella parts 29, 30 is likewise sealed at the lamella ends with silicone rubber, and the silicone material for covering the V-shaped gaps in the lamella parts 29, 30 is in each case preferred and glued to the top with the sealing sleeves 12, 15.

The expansion body 13 is made of high quality spring steel, preferably 1.4301 steel. It 5

Claims (10)

AT 410 595 B makes it possible to press a suitable material cut of the spring steel into the complete shape in a single operation and at the same time to roll the tubular expansion body. Accordingly, laser welding or soldering is only necessary at two connection points. One or more strain gauge elements or complete Wheatstone measuring bridges with strain gauge elements 39-42 are arranged on the outer surface of the expansion body 13. These can also be realized using thin film technology. A design with semiconductor strain sensors or piezoelectric elements is also possible. The wall thickness of the expansion body 13 can be designed to be unequal in order to compensate or change the transducer curve of the expansion sensors or measuring bridges. In particular, it can increase linearly in the direction of flow of the fluid, etc. so that the output signal of the strain gauge elements or measuring bridges assumes a linear course to the flow velocity. The physical disadvantage that in a device that operates according to Bernoulli's law, according to which small measurement signals occur in the lower fluid flow area after the quadratic relationship, can largely be compensated for with a device according to FIG. 6. On the basis of a design of the device for the highest measuring sensitivity, for example by appropriate adjustment of the fluid throughput in the annular space 18 in relation to the medium speed in the expansion body 13, two preferably curved expansion plates 43, 44 are welded to the outside of the slot 26 by laser. The strain gauge elements 41, 42 are preferably applied in between. In accordance with the mode of operation of the device, negative expansions (compressions) occur in the Sigma-1 stress field when the medium flows. The expansion plates are made with a running thickness, with a very low material thickness starting from the laser weld seams. The largest material thickness of the expansion plates 43, 44 is in the middle. The consequence of this measure is a given strain distortion in the sigma-1 stress field. This is recognized in the Wheatstone bridge by sensors 41, 42. At low elongation values in the apex of the tubular expansion body 13, the expansion plates are largely ineffective. With increasing fluid throughput, the expansion of the expansion plates 43, 44 shifts in the direction of greater material thickness, and thus the negative expansion course is weakened; the system becomes less sensitive to measurement. This measure has the advantage that the sensitive measuring system is largely mechanically protected against pressure surges in the line network. This is a great advantage over the highly sensitive sensors of known ultrasonic transit time volume measuring devices. 7, each measuring bridge 45-47 is controlled by suitable semiconductor switches 46-50 by a microprocessor 52 and supplied with needle pulses by a power supply unit 52 in order to keep the power loss low on average. On the one hand this suppresses temperature errors caused by self-heating, on the other hand it is possible to design the device for battery operation. The third aspect is the possibility to work with higher bridge feed levels. Since the output signal increases linearly with the bridge supply voltage, the highest output signals can be achieved. The measuring bridges 45-47 are polled in particular cyclically at intervals. The output signals of the measuring bridges 45-47 are applied via suitable semiconductor switches 48-50 to buffer capacitors 51-53, which in turn are connected in series. The output signal of the series circuit is available at the input 54 of the microprocessor 51 for further processing. In particular, the flow velocity and flow rate of the fluid passing through the device can be determined therefrom in a manner well known to the person skilled in the art. The invention is of course not limited to the illustrated embodiments, but includes all possible variants that fall within the scope of the attached claims. PATENT CLAIMS: 1. Device for measuring the flow velocity and / or rate of a fluid medium, with at least one mechanical expansion body, the expansion of which is subject to the pressure drop of the medium via a flow restriction, and at least one strain gauge element arranged on 6 AT 410 595 B of the expansion body, characterized in that the expansion body (13) has a tubular construction, the medium flows axially through it and is arranged downstream of the flow restriction (9), the expansion body (13) being surrounded by an annular sealing body (1) to form an annular space (18), and wherein the annular space (18) is fed via a feed line (10) from the medium upstream of the flow restriction (9). 2. Device according to claim 1, characterized in that the feed line (10) opens into the annular space (18) at one end and from the other end a discharge line (16) emanates, which opens into the medium downstream of the expansion body (13) and has a smaller passage cross section than the feed line (10). 3. Apparatus according to claim 2, characterized in that the passage cross section of the exhaust line (16) is adjustable. 4. Apparatus according to claim 2 or 3, characterized in that the flow constriction (9) is formed by a nozzle body (9) to which the expansion body (13) connects, the unit consisting of expansion body (13) and nozzle body (9) both ends via a flow-permeable centering element (9, 14) is held essentially coaxially in the sealing body (1), and wherein the passage cross-section (10) of the inlet-side centering element (9) the feed line (10) and the passage cross-section (16) of the outlet-side centering element (14) forms the exhaust line (16). 5. The device according to claim 4, characterized in that the outlet-side centering element (14) faces an annular disc (19) which is axially adjustable in the direction of the centering element (14). 6. Device according to one of claims 1 to 5, characterized in that the expansion body (13) is inserted into a flange of the nozzle body (9). 7. Device according to one of claims 1 to 6, characterized in that the expansion body (13) has at least one longitudinal slot, which is spanned by a highly elastic material, preferably 2-component silicone rubber. 8. Device according to one of claims 1 to 6, characterized in that the expansion body (13) has at least one longitudinal slot (26) which is spanned by an expandable bellows (27). 9. The device according to claim 8, characterized in that the bellows from a first V-shaped lamellar part (28) and at least a second V-shaped lamellar part (29, 30) is composed, the end edges (33) of the V-leg of first lamellar parts (28) are connected to the edges of the slot (26) and the V-apex (34) of the first lamellar part (28) points into the interior of the expansion body (13), each having a side edge (35) of the V-leg of the second slat part (29, 30) are each connected to one side edge (36) of the V-leg of the first slat part (28) and the V-vertex (37) of the second slat part (29, 30) into the interior of the first slat part (28), and the respective other side edges (38) of the V-legs of the second lamella part (29, 30) are interconnected. 10. Device according to one of claims 7 to 9, characterized in that additional, bent expansion plates (43, 44) are welded in the circumferential direction on the outside of the expansion body facing away from the slot. THEREFORE 4 SHEET DRAWINGS 7