DE202017007116U1 - Measuring unit, pipe section and flow meter for determining the flow rate - Google Patents

Abstract

A measuring unit (12) for determining a total flow (Q _G ) of a fluid flowing through a pipe section (14), comprising:
an inlet (18), an outlet (22) and a measuring channel (20) extending therebetween; and
a first ultrasonic transducer (26) for emitting an ultrasonic signal into the measuring channel and a second ultrasonic transducer (28),
for receiving the ultrasonic signal after passing through the measuring channel,
characterized in that
the inlet and the outlet for detachable connection to the pipe section are formed so that the total flow is divided into a main flow (Q _H ) in the pipe section and a tributary (Q _N ), which is passed through the measuring channel and then reunited with the main flow becomes.

Description

The invention relates to a measuring unit for determining an overall flow of a fluid which flows through a line section, with an inlet, an outlet and a measuring channel extending therebetween and with a first ultrasonic transducer for emitting an ultrasonic signal into the measuring channel and a second ultrasonic transducer for receiving the ultrasonic signal Going through the measuring channel.

Such measuring units are used for example in flow meters for heat meters in the field of district heating. Using such heat meters, it is possible to determine the amount of heat that is supplied to consumers via hot water pipes. For this purpose, the volume of warm water flowing through the measuring channel is measured. Temperature sensors additionally determine the temperature difference between two measuring points in the flow and return, ie before and after delivery of the amount of heat stored in the water. Based on these two measurements, the heat meter can determine the amount of heat consumed. Further fields of application include water consumption measurement in distribution centers and in residential, commercial or industrial buildings (so-called ultrasonic large water meters).

An example of an ultrasonic measuring unit can be found in DE 10 2013 012 139 A1 , Disclosed is an ultrasonic counter for detecting the flow rate of a fluid, with a preferably tubular housing which is connectable to a fluid network and having a flow channel, and arranged within the flow channel ultrasonic measuring section with measuring channel for detecting the flow rate of the fluid. The ultrasonic measuring section is designed as a bypass channel, which is arranged within the flow channel, so that a flow fraction of the total flow can be branched off and this flow fraction serves as a representative component for the detection of the total flow rate.

In order to ensure and also prove the accuracy of the measurement, it is helpful and often required that the measuring unit is regularly checked or calibrated before and during use. For this purpose, the entire function group is examined in a suitably equipped test stand. Especially with larger pipe diameters, such as distribution points or in large residential or industrial plants, this calibration process is therefore relatively expensive. This also applies to a one-time calibration to be performed before the first use.

In view of this problem, it is an object of the present invention to accurately measure larger and larger flow rates, while keeping the cost of calibration, verification and calibration low.

According to a first aspect of the present invention, this object is achieved in a measuring unit according to the preamble of claim 1, characterized in that the inlet and the outlet for releasably connecting to the pipe section are formed such that the total flow is divided into a main flow in the pipe section and a Creek, which is passed through the measuring channel and then rejoined with the main river.

Another aspect of the invention relates to a conduit for determining an overall flow of a fluid, having connections disposed at the beginning and end for connection to subsequent fluid conduits; a first connection port and a second downstream communication port for releasably connecting a measuring unit as described above; and a flow resistance for generating a pressure drop between the first port and the second port. The object is achieved according to the invention in that the first connection opening for connection to the inlet of the measuring unit and the second connection opening for connection to the outlet of the above-described measuring unit are formed.

A third aspect of the invention relates to a flow meter. In this case, the stated object is achieved by a measuring unit and a line piece, as described above.

Preferred embodiments of the invention are described in the dependent claims. It should be understood that the metering unit, tubing, flow meter and method may be configured in accordance with the embodiments described for the meter unit, tubing, flow meter and method in the dependent claims.

By determining or measuring a (fluid) flow is meant in particular the determination of a flow rate, or the determination of a volume flow. The fluid may in particular be water, but it may also be another liquid such as oil or, in principle, also a gas.

According to the invention, the measurement is not performed inside, but outside of the pipe section. For this purpose, the inlet and the outlet of the measuring unit are designed for detachable connection to the line piece. The measuring unit can in particular from the outside to an outer wall of the Line piece to be coupled. Connection is understood as a detachable fastening from the outside, in particular screwing, coupling, plugging or making a magnetic connection. In the connected state, the measuring unit is completely outside the line piece and is easily solvable or disassembled or decoupled.

Through the outlet and the inlet of the measuring unit, a tributary is branched from the total flow, which is determined in the measuring channel by means of two ultrasonic transducers. The measuring unit forms a kind of bypass channel to the line section.

The line piece according to the invention has two connection openings which correspond to the inlet and the outlet. Between the two connection openings of the line piece, a flow resistance is arranged, through which a pressure difference or a backflow between the two connection openings is generated. As a result of this backflow, fluid passes through the first connection opening or through the inlet into the measuring unit, passes through the measuring channel and is then guided back into the line piece through the outlet or the second connection opening.

The line piece according to the invention replaces a previous line piece in a line system and comprises a connection for connecting to adjoining or adjacent line sections. This connection is preferably designed as a flange connection, by means of which a direct connection to adjacent line sections or an integration into a line network can take place. The installation effort is minimal.

Measuring unit and line piece together form the flow meter. The inventive arrangement of the measuring unit is achieved that the inflow is proportional to the main flow. Therefore, it can be concluded from the determination of the tributary flow on the skin flow or the total flow. In other words, the measurement is independent of the line segment.

For a given pipe diameter of the pipe section, the proportionality is independent of the total flow, so that a single determination of a corresponding proportionality factor is sufficient. Therefore, the measuring unit can be calibrated separately. The calibration of the entire flow meter is carried out by the calibration of the measuring unit.

In previous ultrasound-based flowmeters, it is necessary that even in systems with large diameters, the line piece is removed together with internal measuring unit for the calibration. This is avoided by the invention, since the measuring unit is connected in the manner according to the invention to the line piece connected or detachable. The ratio between the main flow in the pipe section and the secondary flow in the measuring unit is constant, so that only the measuring unit and not the entire flow meter has to be used for the calibration. Only the external measuring unit is decoupled and calibrated, the line piece can remain in the pipe system.

The present invention is therefore particularly advantageous for large pipe diameters and overall flows, in which the calibration was previously very complex. In addition, in the case of a defect or a required maintenance, a simple and quick replacement of the measuring unit can be made. The time required for maintenance is significantly reduced.

In a preferred embodiment of the measuring unit, the inlet comprises a first connection section, which runs perpendicular to the measuring channel; and the outlet comprises a second connection portion which is perpendicular to the measurement channel. Inlet and outlet sections are designed as connecting pieces running orthogonally to the measuring channel, which correspond to the connection openings on the line section and flow back through the fluid into the measuring channel or from the measuring channel into the line section. In the case of a round pipe section, the connection sections are radially aligned.

In a further advantageous embodiment, the ultrasonic transducers face each other and the measuring channel is arranged between the ultrasonic transducers, so that the ultrasonic signal passes through the measuring channel parallel to the flow direction of the fluid. Such an arrangement of the ultrasonic transducers would not be possible due to the inflowing and outflowing fluid at within the line piece arranged measuring unit. According to the invention, it is not necessary that the ultrasonic signal is deflected once or several times when passing through the measuring channel. This results in improved accuracy and a higher dynamics.

In a further advantageous embodiment, the measuring channel has a constant cross section. A constant cross section ensures that the flow in the interior of the channel is uniform. A uniform flow, which is traversed along the direction of flow of the ultrasonic signal, allows a particularly accurate flow determination. Preferably, the measuring channel is round in cross-section, whereby edge effects are minimized. The accuracy of the measurement or the dynamics are increased.

In a further preferred embodiment of the measuring unit, the inlet at the connection to the measuring channel comprises a first Conditioning section with a cross-sectional constriction in the flow direction of the fluid; and / or the outlet at the connection to the flow conditioning measurement channel comprises a second conditioning section having a cross-sectional widening in the flow direction of the fluid. Such a cross-sectional constriction at the inlet of the measuring channel and / or a cross-sectional widening at the outlet of the measuring channel causes a more uniform flow in the measuring channel. The flow is conditioned. Such a uniform flow with a nearly constant profile causes an increased accuracy of the ultrasonic flow measurement. The conditioning sections are preferably funnel-shaped.

Preferably, the measuring channel runs parallel to the line piece when the measuring unit is coupled to the line piece. An arrangement of the measuring channel parallel to the line piece causes a constant flow can be generated with a nearly constant flow profile, whereby the measurement accuracy is further improved.

Advantageously, the inner wall of the measuring channel made of plastic. The plastic causes the ultrasound in the measuring channel to be less reflected, thereby increasing the measuring accuracy. Compared with a metal embodiment, as variously proposed in the prior art, in particular when other measuring principles are used, results in improved accuracy and higher dynamics.

In a preferred embodiment of the line piece, the flow resistance is designed as a nozzle which has a first nozzle section with a cross-sectional constriction in the direction of flow of the main flow and a subsequent second nozzle section with a smaller constant cross section. Through the nozzle, a pressure difference between the first connection opening, which cooperates with the inlet of the measuring unit, and the second connection opening, which cooperates with the outlet, generated. Due to the pressure difference, a sufficiently large constant fluid flow flows through the measuring unit. The nozzle may in particular be a Venturi nozzle.

Preferably, the line piece comprises an inlet chamber, which is arranged in the flow direction of the main flow in front of the nozzle and at which the first connection opening is arranged; and an outlet chamber disposed downstream of the nozzle in the flow direction of the main flow and on which the second connection opening is arranged. Between the chambers created by the nozzle a pressure difference.

Advantageously, the second nozzle section opens into the outlet chamber and the outlet chamber comprises a section which coaxially surrounds the second nozzle section, wherein the second connection opening is arranged in the region of the second nozzle section. By this arrangement the least possible impairment of the flow is achieved.

More preferably, the connection chamber in the region of the second connection opening comprises a continuously curved radial wall. Through this wall, the flowing back from the measuring unit in the line piece inflow is deflected. A dead water zone is avoided; the flow is conditioned. As a result, both the flow resistance is minimized and the flow in the measuring unit or in the measuring channel kept as constant as possible, so that a high measuring accuracy can be achieved.

In an advantageous embodiment of the flowmeter, the latter comprises a processor which is designed to determine the tributary flow based on the received ultrasound signal; and by multiplying the tributary flow by a proportionality factor to calculate the total flow, the proportionality factor indicating a ratio of the tributary flow to the main flow. The constancy of the proportionality between main flow and tributary flow over the entire dynamic range is used to deduce the total flow from the measurement of the tributary flow.

As described above, the measuring unit is calibrated alone and without a line piece. An expansion and recurring calibration of only the measuring unit is much easier to perform than a calibration of the entire, much larger, flow meter due to the constant proportionality between the main flow and the tributary. The running costs are minimized.

• 1 einen Durchflussmesser in einem vereinfachten Schnittbild;
• 2 ein Ersatzschaltbild des Durchflussmessers von 1;
• 3 den Verlauf des Strömungswiderstands in der Messeinheit in Abhängigkeit vom Gesamtfluss;
• 4 den Verlauf des Strömungswiderstands im Leitungsstück in Abhängigkeit vom Gesamtfluss;
• 5 den Verlauf des Proportionalitätsfaktors zwischen Nebenfluss und Gesamtfluss;
• 6 die Messeinheit des Durchflussmessers der 1 in einer Schnittansicht;
• 7 das Leitungsstück des Durchflussmessers der 1 in einer Schnittansicht; und
• 8 ein alternatives Leistungsstück.

Embodiments of the invention are described below with reference to the accompanying drawings and explained in more detail. Show it:

• 1 a flow meter in a simplified sectional view;
• 2 an equivalent circuit diagram of the flow meter of 1 ;
• 3 the course of the flow resistance in the measuring unit as a function of the total flow;
• 4 the course of the flow resistance in the pipe section as a function of the total flow;
• 5 the course of the proportionality factor between the tributary flow and the total flow;
• 6 the measuring unit of the flow meter of 1 in a sectional view;
• 7 the pipe section of the flow meter of 1 in a sectional view; and
• 8th an alternative service.

According to 1 includes the flow meter 10 a measurement unit twelve connected to a pipe section 14 can be connected or coupled. When connected, it becomes a total flow to be measured Q _G (Volume flow in the flow meter or total volume flow) into a main flow Q _H (Volume flow in the pipe section or main volume flow) through the pipe section 14 and a tributary Q _N (Volume flow in the measuring unit twelve or secondary volume flow).

The tributary Q _N flows out of the pipe section 14 through a first connection opening 16 of the pipe section 14 into an inlet 18 the measuring unit twelve , traverses the measuring channel 20 the measuring unit twelve and passes through an outlet 22 or a second connection opening 24 back into the pipe section 14 , Downstream of the second connection opening 24 pool tributary Q _N and main river Q _H back to the total flow Q _G ,

inlet 18 and outlet 22 can from the outside to the pipe section 14 to be shot.

In 2 is shown schematically an equivalent circuit of the flow in the flow meter. Starting from a total pressure difference Ap becomes the total flow O _G into a main river Q _H and a tributary Q _N divided up. The main flow is the flow resistance R _H suspended in the pipe section, the tributary Q _N is the flow resistance R _N suspended in the measuring unit. After the tributary Q _N has passed through the measurement unit unit tributary Q _N and main river Q _H back to the total flow Q _G ,

The flow resistance R _i is the ratio between pressure difference and flow (volume flow): $${\text{R}}_i=\frac{\Delta \mathrm{<figure-callout\; id="2"\; label="p\; Q\; i"\; filenames="DE202017007116U1\_0006.png"\; state="\{\{state\}\}">p\; </figure-callout>}}{Q_i^{}}$$

Furthermore, the total flow results Q _G as the sum of tributary Q _N and main river Q _H : $$Q_G=Q_N+Q_N$$

ergibt sich ein im Wesentlichen konstantes Verhältnis zwischen Hauptfluss Q_H und Nebenfluss Q_N unabhängig vom Gesamtfluss Q_G .Based on a calculation of the flow resistance R _I based on the pressure loss coefficient ζ, the pipe friction coefficient λ, the density of the fluid ρ, and the length I and the diameter d of the pipe section and its cross-sectional area A to $$R_i={\zeta }_i\cdot \frac{\mathrm{<figure-callout\; id="2"\; label="\rho "\; filenames="DE202017007116U1\_0006.png"\; state="\{\{state\}\}">\rho </figure-callout>}}{2}\cdot \frac{1}{{\mathrm{<figure-callout\; id="2"\; label="A\; i"\; filenames="DE202017007116U1\_0006.png"\; state="\{\{state\}\}">A\; </figure-callout>}}_i^{}}+{\lambda }_i\cdot \frac{\mathrm{<figure-callout\; id="2"\; label="\rho "\; filenames="DE202017007116U1\_0006.png"\; state="\{\{state\}\}">\rho </figure-callout>}}{2}\cdot \frac{{\text{l}}_i}{d_i}\cdot \frac{1}{{\mathrm{<figure-callout\; id="2"\; label="A\; i"\; filenames="DE202017007116U1\_0006.png"\; state="\{\{state\}\}">A\; </figure-callout>}}_i^{}}$$

results in a substantially constant ratio between the main flow Q _H and tributary Q _N independent of the total flow Q _G ,

In 3 is an exemplary course of the flow resistance in the measuring unit R _N depending on the total flow Q _G shown. Once a minimum flux level is reached, the flow resistance is R _N essentially constant and independent of the total flow Q _G ,

In 4 is an exemplary course of the dependence of the flow resistance in the pipe section R _H from the total flow Q _G shown. Again, the flow resistance R _H in principle, constant and independent of the total flow when reaching a minimum flow rate Q _G ,

In 5 is the course of the proportionality factor K that determines the relationship between total flow Q _G and tributary Q _N indicates, depending on the tributary Q _N shown. The proportionality factor K remains essentially constant.

Consequently, tributary are Q _N and main river Q _H independent of the total flow Q _G essentially directly proportional to each other. This proportionality is utilized in the flowmeter according to the invention. The actual measurement takes place in the measuring unit, whereby only the tributary flow is determined. Due to the proportionality between the tributary and the main river, it is possible to deduce the main flow or the total flow with very high accuracy and safety.

In 6 is the measurement unit twelve enlarged and shown alone. The fluid flows through a first connection section 30 of the inlet 18 in the measuring unit twelve , passes through the measuring channel there 20 and leaves the measurement unit twelve again through a second connection section 32 the outlet 22 ,

inlet 18 and outlet 22 the measuring unit twelve are designed to be detachably connected to the pipe section. By detachable connection, it is understood that a connection can be made which can also be disconnected again. The connection sections 30 . 32 are orthogonal to the pipe section and can engage in corresponding receptacles on the pipe section, whereby the detachable connection or the connection between the connection openings of the pipe section and the inlet 18 and outlet 22 the measuring unit twelve is possible. The connection sections 30 . 32 So they are a kind of neck. The attachment takes place for example via appropriate fastening means, such as screws, magnets, adhesive, etc., or via a clamping mechanism.

In contrast to previous measuring units, which are flowed through by the total flow, therefore, the measuring unit twelve from the pipe section 14 be solved, that is decoupled or removed. As the flow conditions within the pipe section 14 Do not change is a calibration of the pipe section 14 not mandatory. Only the measuring unit twelve must be calibrated. The calibration process is thus significantly simplified, since the required in previous systems removal of the pipe section 14 is no longer necessary for the calibration. The pipe section 14 rather remains within the management system. Only the much smaller measuring unit twelve is solved to be calibrated in a calibration device.

In the measuring unit twelve are two ultrasonic transducers 26 . 28 (Transducer) in the area of the inlet 18 or the outlet 22 arranged. By the ultrasonic transducers 26 . 28 On the one hand, an ultrasonic signal is sent to the measuring channel 20 coupled, which is then received on the other side. Both ultrasonic transducers 26 . 28 can be both transmitter and receiver.

The ultrasound penetrates the tributary before it passes from the second ultrasound transducer 28 is received again. Depending on the flow rate of the water, the running time or the frequency of the received ultrasonic signal changes.

The ultrasonic signal can be the measuring channel 20 basically run in any direction, wherein at least a portion or a signal component must also be in the flow direction or against the flow direction to allow a measurement of the flow rate.

In the illustrated embodiment, the ultrasound passes through the measurement channel 20 parallel to the tributary Q _N , The measuring channel 20 is going through in length. It is possible that the ultrasonic signal of the measuring channel 20 runs counter to the flow direction or in the direction of flow of the fluid.

Because it is due to the inventive arrangement of the measuring unit twelve outside the pipe section 14 allows the measuring channel 20 is traversed parallel to the flow direction of the fluid, no deflection of the ultrasonic signal to located within the measuring tube mirrors is necessary. As a result, increased measurement accuracy and increased dynamics are achieved.

As in 6 to see is in the inlet 18 a first conditioning section 34 provided with a cross-sectional constriction, in which for flow conditioning of the tributary Q _N in the direction of the measuring channel 20 is narrowed. This creates a uniform flow profile within the measurement channel 20 reached. A uniform flow profile is understood to mean one over the entire diameter of the measuring channel 20 uniform profile.

Such a profile makes it possible to make a precise statement about the flow rate by means of the ultrasonic measurement method. Mirror image of the cross-sectional constriction in the first conditioning section 34 in the inlet 18 is in the outlet 22 a second conditioning section 36 provided with a cross-sectional widening. In this second conditioning section 36 this will be out of the measuring channel 20 outflowing fluid is intercepted such that no backflow forms, whereby the flow in the measuring channel 20 is also conditioned to a uniform flow profile over the entire measuring channel 20 sure.

The evaluation of the received ultrasonic signal and the control of the ultrasonic transducer 26 . 28 takes place in a processor (not shown). From the received signal, the flow rate is determined. In particular, due to the predefined proportionality factor, it is possible to extrapolate from the measured tributary flow to the main flow, and from this the total flux can be calculated.

In the area of the measuring channel 20 is the measurement unit twelve preferably formed of plastic or has at least one inner wall 38 made of plastic. Plastic reflects ultrasound poorly, leaving reflections on the walls of the measuring channel 20 , which could falsify the measurement result, can be avoided.

In 7 is the line piece according to the invention 14 shown alone and enlarged. Between an inlet chamber 40 that with the first connection opening 16 communicates, and an outlet chamber 42 connected to the second connection opening 24 is, is a flow resistance 44 arranged. The connection openings 16 . 24 are radial to the pipe section 14 aligned.

The flow resistance 44 is designed as a Venturi nozzle and has a first nozzle portion 46 with cross-sectional constriction and an adjoining second nozzle section 48 with a smaller, constant cross section. The nozzle sections 46 . 48 are coaxial in the pipe section 14 arranged. By the flow resistance 44 is a pressure difference between the first port 16 and the second connection opening 24 generated. Due to the pressure difference, fluid flows out of the pipe section 14 through the first connection opening 16 in the measuring unit twelve ,

The flow resistance 44 can also be designed in other ways. For example, the flow resistance 44 as an extension, in the line piece 14 protrudes, be trained. It is understood that other possibilities for generating a pressure difference are conceivable. For example, a bend in the pipe section may be provided.

The pipe section 14 has here for example a round cross-section. The pipe section 14 has connections at its ends 50 For connection to subsequent fluid lines or for installation in a pipe. The connections 50 are formed in the example shown as a flange.

The second nozzle section 48 flows into the outlet chamber 42 that have a surrounding section 52 comprising the second nozzle portion 48 radially surrounds. That through the second connection opening 24 inflowing fluid becomes in this surrounding section 52 reunited with the main river.

8th shows the line piece 14 alone in an alternative embodiment. In the area of the second connection opening 24 is in the outlet chamber 42 in the area of the second connection opening 24 a continuously curved radial wall 54 intended. The wall is rounded so that the tributary flow out of the measuring unit is directed towards the total flow. Turbulences or backwater are avoided. The continuously curved radial wall 54 thus causes a conditioning of the flow. A dead water area is avoided and the flow in the measuring channel and in the line section 14 will be further optimized.

LIST OF REFERENCE NUMBERS

10

Flow meters

12

measuring unit

14

line section

16

first connection opening

18

inlet

20

measuring channel

22

outlet

24

second connection opening

26

first ultrasonic transducer

28

second ultrasonic transducer

30

first connection section

32

second connection section

34

first conditioning section

36

second conditioning section

38

inner wall

40

inlet chamber

42

outlet

44

flow resistance

46

first nozzle section

48

second nozzle section

50

connection

52

environment section

54

continuously curved radial wall

Ap

pressure difference

Q _G

total flow

Q _N

tributary

Q _H

main river

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102013012139 A1 [0003]

Claims (14)

A measuring unit (12) for determining a total flow (Q _G ) of a fluid passing through a conduit (14), comprising: an inlet (18), an outlet (22), and a measurement channel (20) extending therebetween; and a first ultrasonic transducer (26) for emitting an ultrasonic signal into the measuring channel and a second ultrasonic transducer (28), for receiving the ultrasonic signal after passing through the measuring channel, characterized in that the inlet and the outlet are designed for releasable connection to the line piece such that the total flow is divided into a main flow (Q _H ) in the pipe section and a tributary (Q _N ), which is passed through the measuring channel and then rejoined with the main flow. Measuring unit (12) according to Claim 1 characterized in that the inlet comprises a first connection portion (30) which is perpendicular to the measurement channel (20); and the outlet comprises a second connection portion (32) which is perpendicular to the measurement channel. Measuring unit (12) according to any one of the preceding claims, characterized in that the ultrasonic transducers (26, 28) facing each other and the measuring channel (20) is arranged between the ultrasonic transducers, so that the ultrasonic signal passes through the measuring channel parallel to the flow direction of the fluid. Measuring unit (12) according to one of the preceding claims, characterized in that the measuring channel (20) has a constant cross-section. Measuring unit (12) according to one of the preceding claims, characterized in that the inlet (18) at the connection to the flow conditioning measuring channel (20) comprises a first conditioning section (34) with a cross-sectional constriction in the direction of flow of the fluid; and or the outlet (22) at the connection to the flow conditioning measuring channel (20) comprises a second conditioning section (36) with a cross-sectional widening in the flow direction of the fluid. Measuring unit (12) according to one of the preceding claims, characterized in that the measuring channel (20) runs parallel to the line piece (14) when the measuring unit is connected to the line piece. Measuring unit (12) according to one of the preceding claims, characterized in that an inner wall (38) of the measuring channel (20) consists of plastic. Conduit (14) for determining a total flow (Q _G ) of a fluid, comprising: initial and terminal connections (50) for connection to subsequent fluid conduits; a first connection opening (16) and a second connection opening (24) arranged at a distance downstream for releasably connecting a measuring unit (12) to one of the Claims 1 to 7 ; and a flow resistance (44) for generating a pressure drop between the first port and the second port, wherein the first port for connection to the inlet (18) of the measuring unit and the second port for connection to the outlet (22) of the measuring unit are formed. Line piece (14) to Claim 8 characterized in that the flow resistance (44) is formed as a nozzle having a first nozzle portion (46) with a cross-sectional constriction in the direction of flow of the main flow (Q _H ) and a subsequent second nozzle portion (48) with a smaller, constant cross section. Line piece (14) to Claim 9 characterized by an inlet chamber (40) disposed in the flow direction of the main flow in front of the nozzle and at which the first connection opening (16) is arranged; and an outlet chamber (42) disposed downstream of the nozzle in the flow direction of the main flow and to which the second connection opening (24) is arranged. Line piece (14) to Claim 10 characterized in that the second nozzle portion (48) opens into the discharge chamber and the discharge chamber comprises an ambient portion (52) coaxially surrounding the second nozzle portion, the second connection opening being located in the region of the second nozzle portion. Line piece (14) to Claim 11 , characterized in that the connection chamber in the region of the second connection opening comprises a continuously curved radial wall (54). Flow meter (10) for determining a total flow (Q _G ) of a fluid, comprising a measuring unit (12) according to one of Claims 1 to 8th and a pipe section (14) according to one of Claims 8 to twelve to which the measuring unit is connected. Flow meter (10) after Claim 13 with a processor designed to determine the tributary flow (Q _N ) based on the received ultrasound signal; and by multiplying the tributary flow by a proportionality factor (K) to calculate the total flow (Q _G ), the proportionality factor indicating a ratio of tributary flow to main flow (Q _H ).

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