Ultrasonic Flowmeter

Prinsip kerja meter aliran ultrasonik

Fluktuasi frekuensi gelombang suara karena pantulan cairan mengalir dan diskontinuitas adalah prinsip utama yang ultrasonik flow meter bekerja. Ketika cairan dilewatkan melalui pipa, gelombang suara ultrasonik juga ditransmisikan bersama dengan itu dan perbedaan dalam frekuensi gelombang ultrasonik menunjukkan diskontinuitas cairan. Jadi jelas, bahwa frekuensi gelombang ultrasonik berbanding lurus dengan tingkat mengalir cairan melalui pipa

Ultrasonik flow meter adalah teknologi canggih dan inovatif yang bermanfaat untuk aplikasi air limbah dan industri. Ini adalah salah satu teknologi yang banyak digunakan digunakan untuk mengukur aliran air kotor yang banyak teknologi aliran lain tidak mampu melakukan. Teknologi ini volumetrik yang bermanfaat di daerah-daerah tertentu di mana ada penurunan tekanan dan bekerja dengan kompatibilitas kimia. Flow meter tersedia dalam rentang yang berbeda seperti pusaran flow meter, portabel ultrasonik flow meter.
Portabel Ultrasonic Flow Meters
Portabel meter aliran ultrasonik digunakan terutama dalam keperluan industri. Manfaat yang diberikan di bawah ini:
• Hal ini tidak invasif di alam
  • Faktor Kompatibilitas, kontaminasi, material dan risiko korosi dieliminasi
  • Biaya Mudah instalasi dan mengurangi
  • hardware Fundamental sama dengan yang tetap
  • Sepasang sensor yang terhubung ke pipa pengolahan
  • Mudah perawatan
Membedakan fitur portabel meter aliran ultrasonik:
• Kekuatan untuk sensor dan pemancar diberikan oleh baterai isi ulang
  • Dibangun pada akuisisi data untuk merekam kekuatan sinyal, kecepatan aliran, kecepatan suara dll
    • Analog input untuk menerima dan merekam data
  • Dilengkapi dengan panduan langkah
  • Memiliki 2 sensor.
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 The Transit-Time Flowmeter
Design Overview: Like its Doppler cousin, transit-time meters utilize an ultrasonic pulse that is projected into and across the pipe. The design works on a slightly different principle, however. The basic premise of the transit-time meter is to measure the time difference (or frequency shift) between the time of flight down-stream and the time of flight up-stream. This frequency shift can then be correlated into a fluid flowrate through the pipe. To help explain one type of transit-time design, Figure 4a shows two transducers attached to a pipe.

Figure 4a This diagram of a transit-time flowmeter shows the downstream signal being projected between the two transit-time sensors.

In this figure, V is the average fluid velocity, Z is the distance from the upstream transducer to the downstream transducer, and q is the angle between the ultrasonic-beam line and the horizontal fluid flow. The time it takes for the ultrasonic signal to go from the upstream transducer to the downstream transducer can be written as
t_down = Z/(V_s + V cosθ)
where V_s is the velocity of sound through the liquid. The upstream time can be written as (Figure 4b):
t_up = Z/(V_s - V cosθ)
Because the upstream and downstream frequencies can be generated in proportion to their respective transit-times, we can say the following:
f_down = 1/t_down
and
f_up = 1/t_up
where f_down and f_up represent the downstream and upstream frequencies respectively. The change in frequency can then be given as
Δf = f_down - f_up = 1/t_down - 1/t_up
By substitution, one obtains
Δf = (V_s + V cosθ)/Z - (V_s - V cosθ)/Z = (2 cosθ/Z)V
Since (2 cosθ/Z) is just a constant, one can write the final equation as
Δf = kV
with
k = 2 cosθ/Z

Figure 4b This diagram shows the upstream signal projection. The frequency difference between the upstream and downstream times is proportional to the flow velocity.

This, then, is the basic relationship used to determine flow velocity from the measured frequency shift. The flow rate can then be calculated using a Reynolds-number correction for velocity profile and by programming in the internal pipe diameter. The Reynolds-number correction takes into account the behavior of the fluid as being laminar, transitional or turbulent. These calculations are made electronically and the flowrate or flow total can then be displayed in the engineering units of choice. Interestingly enough in this instrument, the frequency shift is measured independently of V_s. This is an advantage, since corrections will not have to be made for the variance of V_s because of line-pressure and temperature fluctuations. Most transit-time applications involve liquids, but designs are available to handle gases, as well.
In light of the single path design discussed above, note that a single ultrasonic pulse will average the velocity profile across the transit path, and not across the pipe cross-section, where better accuracy would be obtained. Some flowmeters on the market send several ultrasonic pulses on separate paths in order to average this velocity profile; these meters tend to have better accuracy than their single-pulse counterparts. Transit-time flowmeters generally exhibit accuracies of around ±1% of the measured velocity. Pipe-material recommendations are the same as those given for Doppler flowmeters.
Advantages: As pointed out, the main advantage of the transit-time meter is that it works non-invasively with ultrapure fluids. This allows the user to maintain the integrity of the fluid while still measuring the flow. Some of the other advantages are listed below.

• Easy installation—transducer set clamps onto pipe
• No moving parts to wear out
• Zero pressure drop
• Can detect zero flow
• No process contamination
• Works well with clean and ultrapure fluids
• Works with pipe sizes ranging from 1" to 200"
• No leakage potential
• Meters available that work with laminar, turbulent, or transitional flow characteristics
• Battery powered units available for remote or field applications
• Sensors available for pulsating flows
• Advanced software and datalogging features available
• Insensitive to liquid temperature, viscosity, density or pressure variations

Disadvantages: Transit-time flowmeter performance can suffer from pipe-wall interference, and accuracy and repeatability problems can result if there are any air spaces between the fluid and the pipe wall. Concrete, fiberglass and pipes lined with plastic can attenuate the signal enough to make the flowmeter unusable. Because these factors can vary from one design to the next, it is advisable to check with the manufacturer to ensure that the pipe material is appropriate.
As mentioned before, the transit-time meters will not operate on dirty, bubbly, or particulate-laden fluids. Sometimes, the purity of a fluid may fluctuate so as to affect the accuracy of the flow measurement. For such cases, there are hybrid meters on the market that will access the fluid conditions within the pipe and automatically chose Doppler or transit-time operations where appropriate. These units are especially useful if the unit is to be used in a wide variety of different applications which may range from dirty to clean fluids.
Applications: Transit-time meters have wide applicability for flow measurement of clean or ultrapure streams. Some of these applications are listed below.

• Clean water flowrate in water treatment plants
• Hot or cold water in power plants, airports, universities, shopping malls, hospitals and other commercial buildings
• Pure and ultra-pure fluids in semiconductor, pharmaceutical, and the food & beverage industries
• Acids and liquefied gases in the chemical industry
• Light to medium crude oils in the petroleum refining industry
• Water distribution systems used in agriculture and irrigation
• Cryogenic liquids
• Gas-stack flow measurement in power plant scrubbers