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The ultrasonic flow meter provides numerous advantages over other types of flow measuring meters. The main advantage provided is ultrasonic measurements are non-invasive. The flow of fluid is measured by clamping a set of transducers on a pipe. Because in most cases the transducers are placed on the outside, they do not come in contact with the fluid.
There are multiple industrial applications that call for the accurate flow measurement. However, traditional methods aren't always an option, hence the need for non-destructive, non-invasive technologies and methods. Ultrasonic flow meters are the equipment of choice whenever the contact with the measured fluid isn't possible. Ultrasonic measurements are made by attaching two transducers onto the outside of a pipe.
This installation method is perfect for all situations in which neither cutting the pipes nor the shutting down of the process aren't possible. These externally applied flow meters don't suffer any mechanical wear and tear. They don't cause the pressure to drop, and they can be used for bidirectional measurements.
This method is different than other flow meters. It is perfect when an application cannot be shut down or a pipe cannot be cut during installation. No pressure drops are experienced with clamp-on flow meters. The flow meter will not suffer normal wear and tear. Bidirectional measurements can be done with clamp-on flow meters.  There are two types of ultrasonic technologies, Transit-time and Doppler techniques.
Transit time ultrasonic flow meters measure the time difference between the ultrasonic pulse going upstream and the one propagating downstream. These sound waves are generated by a pair of transducers. The difference between the two intervals of time is a function of flow direction and velocity, and it represents an accurate measure for the average velocity of the liquid flowing through a pipe.
The velocity of the fluid is indirectly measured by beaming an ultrasonic pulse from one transducer to the second one, and then back again. The time needed to arrive is measured in both situations. The difference between these two amounts of time is directly proportional to the mean velocity of the liquid. This is the parameter being used by ultrasonic flowmeters to calculate the liquid flow rate. These calculations take into consideration some user-entered data, as well.
These piezo-type transducers can be mounted either on the exterior or inside the pipes. In most situations, the exterior installation works very well. However, in case of uneven wall thickness or corrosion problems, the transducers have to be inserted through the pipe, in order to be able to do their job properly. Advanced technologies of digital signal processing have made it possible to attach digital coding to the beamed sound waves. This is a good method to eliminate lots of problems such as noise and density variations. Flow meters that use the transit time method can be used only in conditions of clean fluids, with barely any gas bubbles or suspended solids. Besides, the fluid has to be inside a full and closed pipes system.
Doppler ultrasonic flowmeters are based on the Doppler-shift or the phenomenon in which the wavelength of an approaching sound source is shorter than the wavelength of that same source as it is moving away. The transducer emits a sonic beam into the process and entrained particles or bubbles reflect the beam back to the transducer. The measured difference in the wavelengths (transmitted verses reflected) is proportional to the process' velocity. These ultrasonic flow meters are used on closed full pipes and the transducer may be mounted either on the outside of the pipe or inserted (hot tapped) into the pipe.  SmartmeasurementTM can provide this type of flowmeter, however Doppler techniques are used less and less as the technology of Transit-time is constantly being improved with accuracies and flexibility to use in any fluid flow application.
Transit-time techniques uses an ultrasonic pulse sent through the fluid via the upstream and downstream transducers to determine the fluid velocity. The difference in time it takes for a pulse to traverse the pipe determine the velocity of the fluid. The transit time in flow meters is used to measure the difference between the time it takes to go with and against the flow of the fluid with the help of ultrasonic pulses. The piezo type sensing transducers produce sound waves. One wave will travel against the flow while the other one will travel with the flow. The difference in the amount of time it takes to travel in each direction helps to determine the velocity of the flow. Additionally, it will determine the direction of the flow with the aid of an ultrasonic beam. 
However where Transit-time fails to measure fluid flow accurately is when there are bubbles and particles in the fluid stream which blocks the sound wave reaching across the pipe to measure its transit time.  Most ultrasonic manufactures use low frequency sound pulses to measure the transit time there by severely limits their accuracy when bubbles/particles reaches between 1-2% concentrations.  Very few manufacturers’ employee ultrasonic technologies that can measure less than 5-10%, while only one manufacturer, SmartmeasurementTM has a patented technique that can measure accurately fluid flow of up to 30% bubbles/solids in stream.  Usually the lower the cost of transit-time ultrasonic meters, the less ability to accurately measure fluids with bubbles/solids in the flow stream. 
Transit time ultrasonic flow meters are used in the vast majority of flow application due to its superior technology in obtaining excellent accuracy versus Doppler technology as well as its ability to measure liquids with up to 30% solids and bubbles in the flow stream.  Doppler technologies are only used where the fluid which has a high concentration of solids and bubbles in the flow stream and often times cannot measure fluid flow accurately, due to its unique technique in using particle or bubble velocities in the flow stream to measure the overall flow rate of the fluid.  Since particles or bubbles in a stream flow at different speeds, it is impossible for this technology to measure all the particles/bubble velocities to calculate the true average flow in a flow stream.   Doppler technologies are only used where Transit-times techniques fails which is where there is a very high concentration of particles/bubbles.
Both Doppler and Transit-time techniques have transducers that can either be inserted into a pipe or on the outside of a pipe. Typically, the transducer is inserted into a pipe when the pipe is not uniform in size or the pipe is too thick due to corrosion or pipe insulation. Digital signal processing has made it possible to use a transmitted signal via the use of digital signal coding. Digital signal processing alleviates many of the problems caused by noisy environments. The transit times of an ultrasonic flow meter require that the process either be clean, meaning there are few solids or gas bubbles in a liquid, or has no trapped liquid in a gas, and the piping must be closed. Ultrasonic meters can also use multiple paths to achieve results in a number of custody transfer applications.
The vast majority of flow measurement applications are liquids with various concentration of bubbles or solids in a flow stream.  Low cost employing low frequency transit-time pulses can successfully be used when the liquid is very clean with less than 1-2% solids/bubbles and fail when these concentrations exceed their limits.  However in most application this technology works well.  SmartmeasurementTM is the only manufacturer that uses both low frequency transit-time technologies and high frequency technologies.  There is no reason to use more expensive high frequency transit-time techniques where lower low frequency techniques can effectively measure fluid flow.
SmartMeasurementTM’s AlsonicDSP family of ultrasonic flow meters that use Digital Signal Processing techniques and their patented "fine time measurement technologies" that involve sound waves to be beamed at picoseconds to ensure accurate measurements of liquids that contain solids or gas bubbles. The SmartMeasurementTM’s AlsonicDSP flow transmitters use Cross Correlation to erase noise and make 3-d cross sections of the flow profile's travel speed through a pipe. It also allows users to use a Fast Fourier Transform technology, which allows for two signals to use the same frequency. This increases the accuracy of the flow rate measurement. Finally, SmartMeasurementTM’s AlsonicDSP ultrasonic flow meters can be used to measure fluids that contain up to 30 percent gas bubbles or solids.