Ultrasonic flowmeters can not be ignored in use

The ultrasonic flowmeter's specifications are comparable to other speed flowmeters, and their linearity is particularly good when the flow rate is large enough. But why do you feel that the ultrasonic flowmeter is not accurate when it is actually used? Through investigation and analysis, we find that users often ignore the following problems during use.

1. There is no correct verification or calibration of the ultrasonic flowmeter.

Any flow meter needs to be calibrated or calibrated before use. Portable ultrasonic flow meters are especially important at this point. As we all know, the portable ultrasonic flowmeter has three sets of probes to choose from (large, medium and small), which are suitable for different pipe diameter ranges. The combination of each set of probes and the host is in a sense a set of independent flowmeters. .

If the portable ultrasonic flowmeter is calibrated or calibrated with a small probe on a small-diameter flow standard device, if you use a large probe to measure the flow of a large pipe during use, it means that you are using uncalibrated or calibrated. The flowmeter is measuring the flow rate, and the measurement accuracy is not guaranteed, because the current technical level can not guarantee the interchangeability of the probe, and the difference between the size probe is a variable that cannot be ignored.

Therefore, the correct method should be: based on the user's own use, as much as possible to verify or calibrate multiple portable flowmeters on a flow standard device that is the same or close to the pipe diameter. At a minimum, ensure that each set of probes configured for the flowmeter is calibrated.

The purpose of the verification or calibration mechanism to record the test caliber and probe number on the certificate when issuing the calibration or calibration certificate is to prevent misunderstanding. The understanding of the flowmeter that equates the portable ultrasonic flowmeter with other fixed calibers is wrong. It is also a reason why some users do not use portable ultrasonic flowmeters.

The instrument correction factor is given on the portable ultrasonic flowmeter calibration or calibration certificate. Various flowmeters will give a meter factor at the calibration time due to principles, manufacturing, etc., but the names and expressions are different. The portable ultrasonic flowmeter is equipped with multiple sets of probes, which can be used for different calibers and may have several instrument correction factors. When using the flowmeter to measure the flow rate, it is necessary to ensure the correct use of the meter correction factor. Don't forget to use it and be careful not to use it. It should be a good habit to confirm whether the instrument correction factor set in the host is correct before the formal measurement.

Second, neglect the requirements of the flowmeter use conditions and the use of the environment

Any speed flowmeter has certain requirements on the flow field of the fluid in the pipeline to be tested, and the ultrasonic flowmeter is no exception. When the installation position of the flowmeter cannot guarantee the length of the straight pipe section before and after, the measurement error due to the instability of the flow field cannot be ignored. Many users are limited by the metering wells and are measured at locations that do not meet the installation requirements, resulting in increased measurement errors.

The time difference type ultrasonic flowmeter is particularly sensitive to the air bubbles mixed in the water, and the air bubbles flowing therethrough will cause the flowmeter to be unstable. 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 avoid the position of the pump outlet, the highest point of the pipeline, etc., which is susceptible to gas. The installation point of the probe should also avoid the upper part and the bottom of the pipeline as much as possible, and install it within a range of 45° from the horizontal diameter. Also pay attention to avoiding pipeline defects such as welds.

Ultrasonic flowmeter installation and use environment should pay attention to avoid strong electromagnetic interference and vibration. In use, we found that under the high-voltage line, the dense side of the road, the use of mobile phones or walkie-talkies near the host will have more or less impact on the measurement.

Third, the inaccurate measurement of pipeline parameters can result in inaccurate measurement

The portable ultrasonic flowmeter probe is installed outside the pipeline. It directly measures the flow velocity of the fluid in the pipeline. The flow rate is the product of the flow velocity and the flow area of the pipeline. The pipeline area and the channel length are the pipeline parameters manually input by the host by the host. Calculated, the accuracy of these parameters directly affects the measurement results. That is to say: even if the flow rate is measured accurately, if you input a set of inaccurate pipe parameters, the measurement results are not accurate.

It is best to use the actual measurement method to obtain the parameters of the pipeline. It is possible to use the method of checking the design drawings and inquiring about the personnel who are familiar with the situation. The actual construction conditions often differ from the design parameters, and the manufacturing deviation of the pipeline sometimes occurs. It is also not negligible. Parameters such as the wall thickness of the pipe used for a period of time will also change over time.

The actual measurement of the pipeline parameters should also pay attention to the rationality of the method, and the gauges and instruments used for the measurement should be calibrated. To measure the outer diameter of the pipe, attention should be paid to the possible influence of the outer protective layer of the pipe and the rust and dirt on the outer layer.

When using a portable ultrasonic flowmeter for small-diameter flow measurement, the error caused by the inaccurate input of the inner diameter of the pipe can not be ignored. For example, the absolute error of the inner diameter measurement is also 1 mm, then the inner diameter of the DN1000 pipeline is relatively The error is 0.1%; for the DN100 line, the relative error of the inner diameter is 1.0%. The flow rate is proportional to the square of the inner diameter (the inner diameter of the pipe). The same measurement error of the inner diameter of 1 mm is only about 0.3% for the flow measurement error caused by the DN1000 pipeline, and the flow measurement for the DN100 pipeline. The error is about 3%. It can be seen that the larger the diameter of the portable ultrasonic flowmeter is, the easier it is to measure accurately. The smaller the diameter of the pipeline, the harder it is to measure.

Therefore, it is reasonable to recommend that portable ultrasonic flowmeters 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 that the best signal strength is found, some users use a probe to find the signal. It is often the signal that the installation distance of the probe is incorrect. The correct method is to move the two probes in parallel, and find the best installation point on the premise of ensuring that the installation distance of the probe meets the requirements. After the signal is found, the actual installation method must be used to confirm that the installation distance of the probe is accurate.

Pipeline lining has a great influence on the measurement. Some old pipelines are lining with cement mortar. If they are neglected during the measurement, great errors will occur because the thickness of cement mortar lining is generally not negligible. The number, together with its rough surface and easy to adhere to the mud sand, easy to fall off, may have a great impact on the measurement. When measuring, if the lining parameters are not input, if the signal is not found in the normal operation, it should be considered that the pipeline may be lined.

Ultrasonic flowmeter is an excellent flow meter. It gives us the convenience and economy of measuring flow. It is unmatched by other flowmeters. We don’t misunderstand it because of our carelessness. Practice has proved that we are using ultrasonic waves. Most of the problems that occur when the flowmeter measures flow are caused by human factors, so the above problems can no longer be ignored.