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(Image: Principle of the network measurement [VDIVDE2640]) A further measuring method that manages without cross sectional restrictions is network measurement. In this method, two sizes for determining the volume flow are also necessary: one is the flow depth over which the surface of flow is to be calculated and the other is the median flow velocity in the open channel. To find the velocity profile the individual velocities are measured within a … |
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure Without Cross-Sectional Restriction All the methods mentioned below have in common that two variables must be found for calculating the volume flow, one is the cross section A and the other is the median flow velocity vm. For known geometrical dimensions of the open channel, the cross section is easily found with all methods by means of flow depth measurement. For determining the median flow velocity, however, a different measurement and long calculation method must be used that is … |
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure Without Cross-Sectional Restriction The calculation of the flow with Dilution or Tracer Methods is based on a measurement of the concentration of a marker substance. A tracer (marker substance) is added to the sewage upstream of the measuring point at a known concentration and at a constant dosing rate. After traversing a mixing section, its thinning is measured in the measuring cross section. The continuity equation: QTracer · CTracer = Q · aTracer where QTracer = Constant dosing quantity … |
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Nov 19, 2004 The following methods belong to the flow measurement procedures without cross-sectional restrictions:
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure with Cross-Sectional Restrictions
In hydraulic measuring methods with cross sectional reductions, the installation of measuring weirs or venturi sections forces a change of flow from streaming to jetting. The flow depth upstream of the measuring cross section, because of the change of flow, has a direct relationship to the flow quantity [Preis92] [Hager95]. … |
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure with Cross-Sectional Restrictions
If hydraulic measuring methods that are other than those described in the above section are to be utilised, then, besides the depth of flow h, also the median flow velocity in the cross section must be determined either by means of approximations or with considerable measuring and calculation effort … |
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Nov 19, 2004 The measuring methods described in the following differ from the methods in the previous section in two aspects. For one, structures are always required in the open channel profile, which sometimes substantially reduce the hydraulic capability of the section of the sewer and can thus create backwaters. Thus, these methods cannot be used to solve all the problems. Besides this rather practical point of view, there also occurs a common feature: The … |
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Nov 19, 2004 Flow measurements in the sewage field are only seldom direct measurements. The volume flow through a particular cross section is much easier deduced indirectly from a metrologically easier acquired size that is proportional to the volume flow. For measurements in a gravity open channel with known dimensions, two parameters are normally necessary to calculate the volume flow: the depth of flow h and the median flow velocity vm. From these measurement … |
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Nov 19, 2004 The planning of a flow measurement requires a knowledge of the subject and experience in the field of hydrometry and must always be individually adapted to the respective case requirements as well as the structural and hydraulic conditions at the measuring site. The catalogues of numerous portable flow measurement devices promise ease of handling and universal application possibilities for every hydraulic problem. In fact, portable measuring devices … |
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Nov 19, 2004 For flow measurements in the sewage sector, direct or indirect acting contact transducers with pressure sensors are mainly used. In the direct pressure measurement, a pressure box is installed in the open channel invert, which measures the hydrostatic water pressure as a size proportional to the flow depth. Sensors that operate according to the bubbling-through method, measure the hydrostatic water pressure in that, in a hosepipe that extends to … |
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Feb 16, 2011 The standard transducer for contact-less flow depth measurement is the ultrasonic sonar. The principle of the measurement is based on the reflexion of pulsed ultrasonic signals from the surface of the water (Bild 4.4.2.1.2). Depending on the depth of flow, the signals require different time periods until they are again received by the sonar. This period is inversely proportional to the depth of flow in the open channel. Each sonar device must be positioned … |
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Nov 19, 2004 For most of the measuring methods in open channels, one of the measurements, or the only measurement necessary, is the flow depth. The transducers required for this purpose are divided into two large groups: sensors that are positioned in the medium and transducers that acquire the flow depth without contact. In some borderline cases (extraneous water measurement), contact sensors cannot be used due to the fact that the sensor with its large size … |
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Feb 16, 2011 (Image: Schematic of a magnetic inductive throughflow sensor [Hager95]) MID transducers are based on the electrical conductivity of the sewage. The basis of the principle of measurement is Faraday's law of induction: a voltage is induced in a conductor that is positioned at right angle to the lines of force of a magnetic alternating field. The same effect occurs when the conductor (in this case the flow of sewage) is moved through a static magnetic … |
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Nov 19, 2004 |
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Feb 16, 2011 Mechanical measuring propellers (Bild 4.4.2.2.2.1) (Bild 4.4.2.2.2.1) are precision instruments with friction resistant bearing mounted propeller blades whose speeds of revolution are directly linearly proportional to the median flow velocity in the pipe to be measured. The propeller blades trigger a measuring impulse with each complete rotation that is acquired by a mechanical or electrical counter and that is included with the results of a time … |
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Feb 16, 2011 (Image: MID velocity sensor for measuring local flow velocities) Magnetic inductive velocity sensors, when correspondingly designed, can also be used to measure quasi-point velocities in a flow cross section (Bild 4.4.2.2.2.2), i.e. they represent a kind of "MID measuring propeller". The magnetic field is created only at the face of the magnetic coil of the open measuring transducer in a tightly limited space in the inflowing water. The voltage induced … |
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Nov 19, 2004 For the measurement of local velocities the application of
has predominantly proved itself. |
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Nov 19, 2004 (Image: Principle of measurement of an ultrasonic Doppler measuring sensor) Sensors and measuring instruments that are meant to detect the maximum velocity in a flow profile usually operate in sewage according to the ultrasonic Doppler principle. Doppler measurements are based on the reflexion from solid particles in the flow that are always present in the sewage. The sensor sends an ultrasonic signal at an angle β (Bild 4.4.2.2.3.1) upwards. By … |
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Nov 19, 2004 The measurement of maximum velocity in a flow profile presents a special case of the measurement of local velocities. By means of correct installation, calibration and setting, the sensors and measuring instruments designed for this take over from the user the task of looking for the region of the maximum velocity in open channels (Abschnitt 4.4.1.5). |
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Nov 19, 2004 The flow velocity measurement may relates to
velocity of flow. |
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Instruments with Combined Detectors for Measuring Combined Flow Depth and Velocity Either magnetic inductive (MID) or ultrasonic Doppler sensors are built into the detectors for determining the flow velocity. Neither of them directly measure the flow velocity required for the volume flow calculations but rather a local point velocity. The MID sensor measures a local velocity in a limited flow tube above the combined detector. The ultrasonic Doppler sensor provides the connected converter with a frequency signal from the sonic lobe … |
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Feb 16, 2011 Rehabilitation and Maintenance of Drains and Sewers Instruments with Combined Detectors for Measuring Combined Flow Depth and Velocity (Image: Determining the flow profile with the Pulse Doppler method) The measuring instruments described in the previous section with their velocity sensors can only detect a local velocity in the profile. Based on assumptions on the structure of the velocity profile, the median velocity is calculated on partly empirical relationships. New measuring instruments on the market that work in a similar manner to the ultrasonic Doppler measurements, also … |
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Nov 19, 2004 (Image: Streamline shaped detector with combined flow depth and flow velocity probe) It is becoming increasingly common for measurements of volume flow to utilise instruments with combined flow depth and velocity sensors in a single detector housing. This trend reflects the wishes of the user for compact and easy-to-install measuring systems that will work without any installations that have restricting effects on the cross section. The measuring … |
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Nov 19, 2004 Various transducers or measuring instruments are necessary in order to be able to acquire the required measuring information from the methods described above. These, according to DIN 19559-1 [DIN19559-1:1983] should be "robust and require little maintenance" and should measure "without additional impairment of the flow cross section with a high degree of flow constancy, uninfluenced by extreme environmental conditions, with a good degree of accuracy … |
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The solving of hydraulic problems requires that all the relevant information for determining the situation is collected and evaluated. A calculated determination of the dry and storm weather flows, with the addition of an inventory of bad connections and leaks, is insufficient in many cases for comprehensively describing the hydraulic loads on the sewers. The actual flow in a drain and sewer network, or part network, can deviate substantially from … |
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