ultrasonic flow meter advantages
The Advantages of Using Ultrasonic Fluid Measurement
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.
Transit-Time Ultrasonic Fluid Measurement
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. Additionally, it will determine the direction of the flow with the aid of an ultrasonic beam.
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.
Traditionally flow meters that use the transit time method were used only in conditions of clean fluids, with barely any gas bubbles or suspended solids. However, advances in transit time technologies mitigates some of these fluid conditions. The diagram below describes the transit time technique.
Doppler Ultrasonic Fluid Measurement
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. However Doppler techniques are used less and less as the technology of Transit-time is constantly being improved to measure dirty fluids with better accuracies than Doppler techniques. The diagram below describes the theory of operation for Doppler techniques.
Transit Time versus Doppler Ultrasonic Fluid Measurements
Transit-time ultrasonic flow meters are usually considerably more accurate than Doppler measurement. However, Transit-time fails to measure fluid flow accurately 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 reach between 1-2% concentrations. These products are often time low cost and for relatively clean and stable fluids gives users a smart solution in terms of a cost benefit ratio. Very few manufacturers’ employee ultrasonic technologies that can measure less than 5-10%, while only one manufacturer, Smartmeasurement Inc. from USA has a patented technique that can measure accurately fluid flow of up to 30% bubbles/solids in 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.
SmartMeasurement’ 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 AlsonicDSP product line are used in both fixed and portable application as well as more complicated Area Velocity open channel techniques for sewers, small channels and rivers.
The AlsonicDSP product line 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, the AlsonicDSP ultrasonic product line flow meters can be used to measure fluids that contain up to 30 percent gas bubbles or solids.
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. Most transit time ultrasonic flow meters 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. However, to alleviate some of its inaccuracies, transit-time ultrasonic meters can also use in multiple path configurations to achieve higher accuracies.
Historically the market is moving away from Doppler to transit time technologies because transit-time techniques are continuing to improve while Doppler technologies remain stagnant. Currently, over 90% of ultrasonic fluid measurement uses transit-time ultrasonic technologies of various scan frequencies, unique algorithms, noise cancellation techniques. Transit-time measurements are used from low cost hot/cold water meters to large channels and rivers in more complicated open channel measurement using multipath techniques. The wide variety application usages of transit time measurement mean they are mass produced making them much cheaper than Doppler as only a few manufactures remain. Most ultrasonic flow measurement applications are liquids that are relatively clean with less than 1-2% concentration of bubbles or solids in a flow stream. However, users often don’t know if they have excess bubbles or particles in their flow stream and often times user higher scan technologies with higher accuracies. Smartmeasurement is the only manufacturer that uses both low frequency transit-time technologies and high frequency technologies as well as Doppler techniques.
Benefits of Ultrasonic technologies
- Obstruction less flow – clamp on outside the pipe
- Pressure drops equal to an equivalent length of straight pipe
- Bi-directional flow capability
- Low flow cutoff
- Accuracy about 1% of flow rate standard and 0.5% with dual path
- Relative low power consumption
Benefits of Transit time Ultrasonic technologies
- Lower costs
- Widely used in majority ultrasonic applications.
- Ability to use multi-path techniques for flow profiling in large pipes or open channel applications.
- Many vendors
- Higher accuracies than Doppler
- Technology is constantly being improved
- Variety of scan rates, noise cancellation, Digital Signal Processing (DSP), etc. gives user many choices
Benefits of Doppler Ultrasonic technologies
- Excessive particles or bubbles in the flow stream up to 40% or more.
- Excellent for sewers and small channels for Area Velocity open channel measurement.
- Doppler open channel measurement can use one transducer to measure velocity, level and fluid conductivity
Read more about the different types of Ultrasonic Flow meters: