The non-contact ultrasonic flow meter is a non-invasive flow meter available in two formats: transit time and doppler. Both technologies feature clamp on designs with transducer arrangements that can detect in-situ flow rates from outside the pipework, without any interruption to flow or process. Transit time flow meters, or also known as time of flight meters are mainly consigned to the measurement of clean liquids, although many flow measurement devices have a maximum tolerance of 5% solids. The principle behind this type of non-intrusive clamp-on (strap on) flow meter is that acoustic waves with a frequency of >20kHz are emitted from one transducer to the other side of the pipe back to the opposite transducer requires less time than when travelling in the opposite direction. The differential transit time of the synchronised signals is proportional to the flow rate of the fluid。
Ultrasonic meters have no moving parts, they suffer no pressure loss and theyprovide maintenance-free operation - important advantages over conventionalmechanical meters such as positive displacement meters (PDs), turbines, orificeplates and vortex meters, and also in many cases coriolis mass meters.
Moreover, ultrasonic flowmeters are invariably more accurate and reliablethan many competing systems. Price has been a primary stumbling block,but the case for ultrasonics is now stronger than ever with the emergenceof 3-beam ultrasonic meters developed to replace the mass, vortex, positive displacement (PD) and turbine flowmeters that are used to measure non-conductive fluids.
Ultrasonic flow measurement uses the transit time principle, whereby opposite sending and receiving transducers are used to transmit signals through the flow.
The signal travels faster when moving with the flow stream rather than against the flow stream. The difference between the two transit times is used to calculate
the flow rate.
The measured average flow velocity between acoustic sensors does notalways represent the required average flow velocity. This is because flow velocity across the pipe is not uniform and can differ in character. Thereare two main areas of flow velocity distribution (i.e. flow profile): laminar and turbulent flow and these have posed potential problems in flow measurement.
The Reynolds number reflects the behaviour of fluids flowing through a pipe. The number indicates the ratio between the inertial forces and viscous forcesin a flowing stream. In theory, for Re >2300 there will be a turbulent flow profile (flattened shape), whereas for Re < 2300 the laminar profile (parabolic shape) will have been established. Also, a flow profile called the transition area exists in a Reynolds range from 1500 to 4000.
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