Ultrasonic flow meters are highly convenient instruments, particularly useful for large-diameter water supply pipelines. The portable ultrasonic flow meters can be easily installed on the exterior of pipelines, enabling flow measurement without interrupting the flow or damaging existing pipelines. This ease of use has made them widely popular.
To evaluate the quality of a flow meter, a flow standard device is typically used for calibration. Upon reviewing ultrasonic flow meter calibration results over the years, we find that the performance of ultrasonic flow meters is comparable to other velocity-based flow meters. Notably, when the flow speed is sufficiently high, ultrasonic flow meters exhibit excellent linearity.
However, why do many users experience inaccuracies when using ultrasonic flow meters? Through research and analysis, we’ve identified several critical factors that are often overlooked during operation:
1. Failure to Properly Calibrate or Verify the Ultrasonic Flow Meter
Before using any flow meter, it is essential to calibrate or verify it. This is especially important for portable ultrasonic flow meters. These meters typically come with three sets of transducers (small, medium, and large), each suited to different pipe diameters. Each transducer set is essentially a separate flow meter system.
For instance, if a portable ultrasonic flow meter is calibrated using a small-diameter transducer on a flow standard device designed for smaller pipes, using a large-diameter transducer on a larger pipeline without recalibration would lead to inaccurate readings. This is because, due to current technological limitations, transducer interchangeability cannot be guaranteed. The differences between small and large transducers represent a significant variable that should not be ignored.
The proper approach is to calibrate or verify the ultrasonic flow meter on a flow standard device that closely matches the actual pipe diameter where it will be used. Ideally, all sets of transducers should be verified for accurate performance. Calibration certificates typically specify the device and transducer details, ensuring that users are aware of the limits and specifications for each setup. This avoids any misunderstanding, as treating portable ultrasonic flow meters as if they are fixed-diameter meters can lead to errors.
Each calibration certificate will include the necessary correction factors for the device. Different flow meters, due to their operating principles and manufacturing differences, will have a correction factor specified during calibration. This factor varies depending on the transducer set used. Therefore, it is crucial to ensure that the correct correction factor is applied during actual measurements. It is a good habit to verify the correction factor before starting measurements to avoid incorrect calibration and inaccurate results.
2. Ignoring Installation Conditions and Environmental Requirements
All velocity-based flow meters, including ultrasonic flow meters, have certain requirements for the flow profile within the pipeline. Ultrasonic flow meters are no exception. If the installation site does not meet the necessary straight pipe length before and after the flow meter, the flow profile may become unstable, leading to significant measurement errors.
For example, ultrasonic flow meters using time-difference technology are highly sensitive to gas bubbles in the water. These bubbles can cause fluctuating readings. If gas bubbles accumulate in certain areas of the pipeline and align with the transducer, it could render the flow meter inoperative.
To mitigate these issues, the installation of ultrasonic flow meters should avoid locations susceptible to gas accumulation, such as near pump outlets or high points in the pipeline. Ideally, the transducers should be installed at a 45° angle to the horizontal diameter of the pipe, and care should be taken to avoid installation near welds or pipe defects. Additionally, the installation environment should be free from strong electromagnetic interference and vibration. In practice, we have observed that high-voltage power lines, heavy traffic areas, and the use of mobile phones or radios near the device can cause measurable interference and inaccuracies.
3. Inaccurate Pipeline Parameters Lead to Measurement Errors
Portable ultrasonic flow meters measure the flow velocity of the fluid inside the pipe by placing transducers on the outside of the pipe. The flow rate is then calculated by multiplying the flow velocity by the cross-sectional area of the pipe. The accuracy of this calculation depends heavily on the correct input of pipeline parameters such as pipe diameter and sound path length.
Even if the flow velocity is measured accurately, incorrect pipeline parameters lead to erroneous flow rate calculations. This is because small errors in the measurement of pipe dimensions—especially pipe diameter—have a disproportionately large impact on the final result, particularly for smaller pipes. For example, a measurement error of just 1mm in the diameter of a DN100 pipe (100mm) results in a relative error of 1%. However, the same error on a DN1000 pipe (1000mm) results in only 0.1% error. The flow rate is proportional to the square of the pipe’s diameter, meaning small errors in diameter have a larger effect on the flow rate in smaller pipes.
When using portable ultrasonic flow meters on small pipes (e.g., DN100), it is crucial to measure the pipe’s dimensions as accurately as possible. Even minor deviations can lead to significant errors in flow measurement.
Moreover, the installation distance between transducers is another crucial parameter that must be accurately maintained. Often, users adjust the transducer positions to optimize signal strength, but moving a single transducer without adjusting both can distort the measurements. The correct procedure is to move both transducers together while ensuring the required installation distance is maintained.
Pipe linings, such as cement mortar or other coatings, can also significantly affect measurements. These linings may have irregular thickness, surface roughness, or debris buildup, which can distort the ultrasonic signals. If the existence of a lining is not accounted for during the calibration or measurement process, it could introduce large errors.
Conclusion
Ultrasonic flow meters are valuable tools, but their accuracy is heavily influenced by proper calibration, installation, and parameter measurement. To achieve reliable results, it is essential to ensure that the device is properly calibrated, that installation conditions meet the required specifications, and that accurate pipeline parameters are used. Paying attention to these details will ensure that ultrasonic flow meters provide accurate, reliable measurements, especially in challenging conditions such as large or small diameter pipes, or when working in complex environmental settings.
By following the proper procedures and addressing these common pitfalls, users can optimize the performance of ultrasonic flow meters and avoid errors that could affect flow measurement accuracy.