Ultrasonic flow measurement technique, Advantages and disadvantages of ultrasonic flowmeters

Field Instrumentation Flow Measurement
1

Ultrasonic flowmeters pass high-frequency sound wave to fluid path to measure the fluid velocity. The ultrasonic method utilises the change in the propagation time in the flowing fluid. There are two major types of ultrasonic flowmeters: Doppler and transit time type

Doppler flowmeter:

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The Doppler flowmeter works based on the principle of Doppler’s effect, which says that the frequency of the sound waves shifts when reflected by a moving object.

This type of flowmeter requires objects in the flow stream. The incident wave hit the object or bubbles and reflects back as the reflected wave, the difference in the frequencies of the incident and reflected waves are measured which is proportional to the function of fluid flow in the stream.

If the reflected wave returns from a bubble advancing toward the ultrasonic transducer, the reflected frequency will be greater than the incident frequency. If the flow reverses direction and the reflected wave returns from a bubble travelling away from the transducer, the reflected frequency will be less than the incident frequency.

The relation between fluid flow and frequency shit:

^f = frequency shift f = incident frequency v = velocity of fluid c = speed of wave in process fluid

Transit-time:

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Also known as counter propagation flowmeter. Transit time flow meter uses a pair of transducers both are capable of sending and sensing the sound signals. Usually used transducers are piezoelectric transducers.

The transducers are mounted, one opposite to another with an elevation which forms an upstream and downstream. The incident wave is received at the other end with a time span. The time lag in receiving the signal is proportional to the flow of the fluid. The signal sends in both directions upstream and downstream.

The relationship between flow speed and transit time:

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Q = Calculated volumetric flow K = constant of proportionality t up = Time for sound pulse to travel from downstream location to upstream location t down = Time for sound pulse to travel from upstream location to downstream location

Advantages of ultrasonic flowmeter:

  • No moving parts and allow unabstracted flow

  • Can operate in both direction

  • Unaffected by temperature, density and concentration

  • Low maintenance

  • No additional pressure drop

Disadvantages of ultrasonic flowmeter:

  • Transit time required clean liquid and doppler meter need objects in the liquid.

  • Sound beam must transverse a representative cross section, therefore flow profile dependent

  • Long inlet and outlet section required.

2

Ultrasonic flow meters measure fluid velocity by analyzing ultrasonic signals passing through the flow. There are two main techniques:

  1. Transit-Time (Time-of-Flight) Method Two transducers send ultrasonic pulses alternately upstream and downstream. The time difference (ΔT) between pulses is proportional to flow velocity. Best for clean liquids (water, oils, chemicals).
  2. Doppler Effect Method A single transducer emits ultrasonic waves that reflect off particles/bubbles in the fluid. The frequency shift (Doppler effect) is used to calculate flow velocity. Best for slurries or dirty liquids (wastewater, mining slurries).
  3. Hybrid Ultrasonic Meters Combine both transit-time and Doppler for wider fluid compatibility. :white_check_mark: Advantages of Ultrasonic Flow Meters
  4. Non-Intrusive (Clamp-On Option Available) No pipe cutting required (external sensors possible). No pressure drop or flow obstruction.
  5. No Moving Parts → Low Maintenance Unlike turbine or mechanical meters, no wear and tear.
  6. Bidirectional Flow Measurement Can measure flow in both directions.
  7. Wide Range of Applications Works with liquids, gases (special models), and steam. Suitable for large pipe diameters (no size limitation).
  8. High Accuracy (Transit-Time: ±0.5–1%, Doppler: ±1–5%) Transit-time is highly accurate for clean fluids. Doppler is less precise but works with dirty flows.
  9. No Conductivity Requirement (Unlike Magmeters) Can measure non-conductive fluids (oils, hydrocarbons).
  10. Minimal Installation Downtime Clamp-on meters can be installed without stopping flow. :x: Disadvantages of Ultrasonic Flow Meters
  11. Sensitive to Flow Profile & Turbulence Requires straight pipe runs (10D upstream, 5D downstream) for accuracy.
  12. Doppler Needs Particulates/Bubbles Fails with ultra-clean liquids (requires reflectors).
  13. Limited for High-Temperature/Pressure Applications Clamp-on meters may struggle with very hot pipes (>200°C).
  14. Higher Cost Than Basic Flow Meters More expensive than mechanical meters (but cheaper than Coriolis).
  15. Signal Attenuation in Certain Fluids Aerated liquids or viscous fluids can weaken ultrasonic signals.
  16. Calibration Challenges Pipe material & lining can affect readings (needs proper configuration).