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paths are used, the more infor- mation on the ow pro le pre- vailing at the measurement spot can be gained. The total ow rate in this case is the total of the individual ow rates. Using multiple measure- ment paths hence will increase the accuracy of the ow rate determination. Arranging the sensors of a multi-path meas- urement crosswise reduces the e ects of disturbing ows crossing the main ow direc- tion. Cross ows may cause measurement errors. Using a multi-path measurement setup may also reduce the length of intake and discharge sections required to calm down the ow pro le at the measurement point. Thanks to CFD models (Computational Fluid Dynam- ics) and comprehensive testing at renowned institutes, in u- ence and behaviour of ow pro les downstream of stand- ard disturbances could be examined. Based on the results it is now possible to integrate ow pro les downstream of elbows and other disturbances into calculation models 14 WET NEWS NOVEMBER 2017 Transit time method – made for flow detection T he relevance of water vol- ume measurement is growing particularly in large diameters. The challenge with cooling water or fresh water processes is to generate accurate ow recordings or to document and to control indi- vidual consumptions and with- drawal quantities. Flow detection, however, particularly in large diameters is highly demanding. In such cases, the transit time meas- urement is literally made for permanent ow metering because of its high exibility as a reliable and cost-eƒ cient measurement system. The requirement for many processes is to feed as little fresh water as possible. Water feed and withdrawal volumes need to be monitored con- stantly as well. All these tasks require to permanently investi- gate and to verify ow rates. Integration into higher sys- tems (such as SCADA systems) is indispensable since the sys- tems are generally used within large areas. To ensure constant ow recording it is necessary to use measure such as electrical conductivity, uctuating tem- peratures or viscosity. This measurement principle is based on directly measuring the transit time of an acoustic signal between two ultrasonic sensors. Two sonic impulses are transmitted successively a† er each other and the di er- ent transit times between transmitter and receiver are measured. The impulse head- ing downstream reaches the receiver sooner than the impulse heading upstream. The required times are measured by utilising highly accurate time measurements as well as a signal correlation. This signal correlation com- pares the transmitted signal with the signal received by the opposite converter. The com- parison therefore enables to determine the accurate moments of transmission and reception of the measurement signal. The di erence between both determined times is pro- portional to the average ow velocity within the measure- ment path.‡ The more measurement directly in the transmitter of the measurement system. Only the type of disturbance and the distance to the measurement spot need to be speci ed. From these speci cations the meas- urement automatically deter- mines the correction factors to use. The result of the ow measurement is therefore highly accurate and can be even used together with shorter calming and intake sections.‡ The new NIVUS device types allow using the transit time method both as invasive measurement and clamp-on system. Here the type of sensor used must be selected depend- ing on the situation on the measurement place. Highest measurement accuracy can be achieved by using a multi-path system with wetted sensors in a de ned arrangement. If it is not possible to insert the sen- sor into the process (abrasive, corrosive or other problematic media), the sensors can be installed on the pipeline from the outside without process interruption (clamp-on meas- urement system). The schematic highlights the multiple measurement paths a measurement system capable of determining the medium velocity covering the entire wetted area. This is important particularly with uctuating ow conditions. Many meas- urement systems commonly used either feature spot veloc- ity measurement only or do provide the required penetra- tion depth. Quite simply, some meas- urement systems cause too much costs or require too many employees when it comes to installation. Measur- ing high medium velocities to many systems is an impossible task, too. A cost-eƒ cient method to obtain reliable information on the prevailing discharge / ow is the meas- urement using the ultrasonic transit time di erence princi- ple. Such systems stand out for low maintenance expenses and high operational safety. They can be exibly used with all needed sizes and media. Compared to other methods, the measurement system more- over has the advantage to be largely independent of the properties of the media to " INSIGHT Filtration & screening The high flexibility of transit time measurement, says NIVUS's Ralf Bruening, makes it perfect for permanent flow metering.

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