At present, the most widely used ultrasonic flowmeter in the water supply industry is the liquid portable ultrasonic flowmeter. In actual use, many users do not have a good grasp of the use points of the ultrasonic flowmeter and the measurement effect is not ideal, so they have a problem with the ultrasonic flowmeter. All kinds of doubts: "Is this flow meter accurate?" Questions such as these have become a hot topic in the industry.
The best platform for evaluating the quality of a flowmeter is the flow standard device. Through the statistics of the ultrasonic flowmeter verification conclusions over the years, we found that the indicators of the ultrasonic flowmeter are comparable to other velocity flowmeters, and when The linearity is particularly good when the flow rate is large enough. So why do you feel that the measurement of the ultrasonic flowmeter is inaccurate in actual use? Through research and analysis, we found that the following problems are often ignored in use.
1. The ultrasonic flowmeter is not properly calibrated or calibrated
Verification or calibration is required before any flowmeter can be used, and portable ultrasonic flowmeters are especially important in this regard. As we all know, the portable ultrasonic flowmeter has three sets of probes to choose from (large sensor, medium sensor, small sensor), which are suitable for different pipe diameter ranges. The combination of each set of probes and the host is an independent set in a sense flowmeter.
If the portable ultrasonic flowmeter is only calibrated or calibrated with a small probe on a flow standard device with a small diameter, then if you use a large probe to measure the flow of a large pipeline, you are using an uncalibrated or calibrated flowmeter. When measuring the flow rate of a single flowmeter, its measurement accuracy cannot be guaranteed, because the current technical level cannot guarantee the interchangeability of probes, not to mention the difference between large and small probes is a variable that cannot be ignored.
Therefore, the correct method should be: based on the user's own usage as a reference, as far as possible, the portable ultrasonic flowmeter should be verified or calibrated for multiple pipelines on a flow standard device with the same or close diameter as the pipeline used. At least ensure that each set of probes configured with the flowmeter is calibrated.
The purpose of recording the experimental caliber and probe number on the certificate when the verification or calibration agency issues the verification or calibration certificate is to prevent misunderstandings. Those who equate portable ultrasonic flowmeters with other fixed-caliber flowmeters are wrong. It is also one of the reasons why some users do not use portable ultrasonic flowmeters well.
The meter correction factor is given on the portable ultrasonic flowmeter verification or calibration certificate. All kinds of flowmeters will give a meter coefficient when calibrating due to principles, manufacturing and other reasons, but the names and expressions are different. The portable ultrasonic flowmeter is equipped with multiple sets of probes and may have several meter correction coefficients for different calibers. When using the flow meter to measure the flow, it is necessary to ensure the correct use of the meter correction factor. Do not forget to use it and pay attention not to mix it. It is a good habit to confirm whether the meter correction factor set in the host is correct before the formal measurement.
2. Neglecting the requirements for the use conditions and environment of the flowmeter
Any velocity type flowmeter has certain requirements on the flow field of the fluid in the measured pipeline, and the ultrasonic flowmeter is no exception. When the installation position of the flowmeter cannot guarantee the required length of the straight pipe sections before and after it, the measurement error caused by the unstable flow field cannot be ignored. Many users are limited by the measuring well of the instrument and measure in the position that cannot meet the installation requirements, which causes the increase of the measurement error.
The time difference ultrasonic flowmeter is particularly sensitive to the bubbles mixed in the water. The bubbles flowing along with it will cause the instability of the flowmeter's indication value. If the accumulated gas coincides with the installation position of the probe, the flowmeter will not work. Therefore, the installation of the ultrasonic flowmeter should try to avoid the pump outlet, the highest point of the pipeline and other places that are easily affected by gas. The installation point of the probe should also try to avoid the upper and bottom of the pipeline, and install it within a range of 45° from the horizontal diameter. , but also pay attention to avoid pipeline defects such as welds.
The installation and use environment of the ultrasonic flowmeter should pay attention to avoiding strong electromagnetic interference and vibration. In use, we found that the use of mobile phones or walkie-talkies near the host will have a more or less impact on the measurement.
3. Inaccurate measurement due to inaccurate measurement of pipeline parameters
The portable ultrasonic flowmeter probe is installed outside the pipe. It directly measures the flow rate of the fluid in the pipe. The flow rate is the product of the flow rate and the flow area of the pipe, and the pipe area and channel length are the pipe parameters manually input by the user from the host. Calculated, the accuracy of these parameters directly affects the measurement results. That is to say: even if the flow meter measures the flow rate accurately, if you enter an inaccurate set of pipe parameters, the measurement result will be inaccurate.
The best way to obtain the pipeline parameters is to use the actual measurement method. Errors may occur in the way of checking the design drawings and inquiring about the personnel familiar with the situation, because the actual construction situation often differs from the design parameters, and the manufacturing deviation of the pipeline is sometimes It is also not negligible, and parameters such as the wall thickness of the pipe used for a period of time will also change a lot with time.
In the actual measurement of pipeline parameters, attention should also be paid to the rationality of the methods and methods, and the measuring tools and instruments used for measurement should be calibrated. When measuring the outer diameter of the pipe, pay attention to the possible influence of the outer protective layer of the pipe and the rust and dirt on the outer layer on the measurement.
When using a portable ultrasonic flowmeter for flow measurement on a small pipe diameter, the error caused by the inaccurate input of the pipe inner diameter cannot be ignored. For DN100 pipeline, the relative error of inner diameter is 1.0%. The flow rate is proportional to the square of the inner diameter (the area of the inner diameter of the pipe). The measurement error of the inner diameter of 1 mm is only about 0.3% for the flow measurement error caused by the DN1000 pipeline. The error is about 3%. It can be seen that the larger the diameter of the pipeline, the easier and more accurate the measurement is. The smaller the diameter of the pipeline, the more difficult it is to measure. Therefore, it is reasonable to recommend that the portable ultrasonic flowmeter should be used in pipelines above DN300.
The installation distance of the probe is an important measurement parameter that must be accurately guaranteed. In order to ensure the best signal strength, some users move a probe to find the signal. Often, the signal has the wrong installation distance of the probe, and they ignore the other. The correct method is to move the two probes in parallel, and find the best installation point on the premise that the installation distance of the probes meets the requirements. After the signal is found, be sure to use the actual measurement method to confirm that the installation distance of the probes is correct.
The influence of the pipeline lining on the measurement is also great. Some old pipelines are lined with cement mortar to prevent corrosion. If the existence of it is ignored during the measurement, a large error will occur, because the thickness of the cement mortar lining is generally not negligible. The number, coupled with its rough surface, easy to adhere to mud and sand, and easy to fall off in pieces, may have a great impact on the measurement. When measuring, if the lining parameters are not input, and the signal cannot be found according to the normal operation, it should be considered that the pipeline may be lined.