In addition to the various types of ultrasonic flowmeters mentioned above, recent years have also seen the emergence of digital ultrasonic flowmeters using digital circuits, integrated ultrasonic flowmeters that combine a transducer and converter, and more. Faced with this multitude of ultrasonic flowmeter types, users must select the appropriate type based on their actual needs and measurement requirements.
1. Doppler Ultrasonic Flowmeter
This type of flowmeter can only measure fluids containing moderate amounts of particles or bubbles that reflect ultrasonic signals, such as factory effluent, untreated sewage, and factory process fluids with stable impurity content. It is important to note that the measured medium has stringent requirements: it cannot be clean water, and the impurity content must be relatively stable for proper measurement. Furthermore, the performance and requirements of the instrument vary from manufacturer to manufacturer. Choosing this type of ultrasonic flowmeter requires a thorough understanding of the measured medium and the performance, accuracy, and requirements of the selected ultrasonic flowmeter.
2. Portable Ultrasonic Flowmeter
This is suitable for temporary measurements, primarily used to calibrate the operating status of other flow meters installed on a pipeline, conduct fluid balance tests within a region, and check current flow conditions in a pipeline. For these applications, portable ultrasonic flowmeters are convenient and economical.
3. Transit-Time Ultrasonic Flowmeter
Currently, the most widely produced and applied type is the transit-time ultrasonic flowmeter. It is primarily used to measure the flow rate of clean fluids and is widely used by water companies and industrial water companies. It can also measure the flow rate of uniform fluids with low impurity content (impurity content less than 10g/L, particle size less than 1mm), such as sewage, with an accuracy of up to ±1.5%. Practical applications have shown that transit-time ultrasonic flowmeters can achieve satisfactory measurement results for various fluids.
4. Pipeline Ultrasonic Flowmeter
This type offers the highest accuracy, reaching ±0.5%, and is not limited by pipeline material or lining, making it suitable for applications requiring high flow measurement accuracy. However, as pipe diameter increases, costs also increase. Generally, it's more economical to choose a small or medium-sized pipe segment ultrasonic flowmeter.
5. Fixed Ultrasonic Flowmeter
If there's sufficient installation space, using an insertable transducer instead of an external transducer completely eliminates the effects of pipe lining, scaling, and pipe wall on ultrasonic signal attenuation, resulting in higher measurement stability and significantly reduced maintenance. Furthermore, because insertable transducers can be installed continuously, their use is growing.
Some manufacturers have introduced ultrasonic flowmeters with internal digital circuitry. These feature digital signal processing, enhanced error correction capabilities, timely sampling, and improved accuracy (analog circuits have an accuracy of ±1.5%, while digital circuits can achieve ±1.0%). Furthermore, they offer increased integration, significantly reduced instrument size, and a variety of signal output modes. These flowmeters have demonstrated excellent performance in practice. Users can compare them with analog ultrasonic flowmeters.
The choice of ultrasonic flowmeter functionality can be determined based on actual needs. If you're measuring bidirectional fluids, you should definitely choose an ultrasonic flowmeter with positive and negative metering capabilities. If you need to regularly monitor the flow rate over a specific time period, choose an ultrasonic flowmeter with a printer. In short, the functions of the ultrasonic flowmeter you choose should meet your needs, but don't overdo it, leaving many features unused and increasing the purchase cost.
Ultrasonic Flowmeter Applications
When using ultrasonic flowmeters, the following aspects require careful consideration:
1. Correct Selection
This is essential for the proper functioning of ultrasonic flowmeters. Improper selection can result in flow measurement failure or user inconvenience. Specific selection principles have been detailed previously.
2. Proper Installation
Improper transducer installation is a major cause of ultrasonic flowmeter failure. Transducer installation requires consideration of both location and installation method. When determining the location, in addition to ensuring sufficient upstream and downstream straight pipe runs, it is particularly important to avoid locations with contaminating power sources such as variable frequency drive units and welding machines. There are three main installation methods: face-to-face mounting, V-mount mounting, and Z-mount mounting, as shown in Figure 3. Doppler ultrasonic flowmeters use face-to-face mounting, while transit-time ultrasonic flowmeters use V-mount mounting and Z-mount mounting. Generally, the V-mount mounting method is used for pipes with a diameter less than 300 mm, and the Z-mount mounting method is used for pipes with a diameter greater than 200 mm. For transducers that can be mounted in either a V or Z-mount mounting method, the Z-mount mounting method is preferred. Practice has shown that transducers installed in a Z-position have high ultrasonic signal strength and good measurement stability.
3. Timely Calibration
For users with a large number of fixed ultrasonic flowmeters installed on-site, a portable ultrasonic flowmeter of the same type can be used to calibrate the on-site instruments. First, adhere to the principle of "one installation, one calibration"—that is, calibrate each newly installed ultrasonic flowmeter during installation and commissioning to ensure proper positioning, installation, and measurement accuracy. Second, if a sudden flow rate change occurs during online operation, a portable ultrasonic flowmeter should be used for timely calibration to determine the cause of the sudden change and determine whether it is a malfunction or a genuine change in flow rate.
4. Regular Maintenance
Compared to other flowmeters, ultrasonic flowmeters require relatively little maintenance. For ultrasonic flowmeters with external transducers, there is no water pressure loss or potential leakage after installation. Regular inspections are sufficient to ensure the transducer is not loose and the adhesive between the transducer and the pipe is in good condition. For insertable ultrasonic flowmeters, the probe should be regularly cleaned of deposited impurities, scale, and other leaks. For integrated ultrasonic flowmeters, the flange connection between the flowmeter and the pipe should be checked for proper function, and the effects of on-site temperature and humidity on its electronic components should be considered. Regular maintenance ensures the long-term stable operation of the ultrasonic flowmeter.