What should I know when choosing a flow meter? (2) Posted by : admin / Posted on : Mar 14， 2020

Flowmeter performance requirements
(3) Repeatability
Repeatability is determined by the principle of the flow meter itself and manufacturing quality. It is an important technical indicator in the use of the flow meter and is closely related to the accuracy of the flow meter. Generally, the metering performance requirements in the verification regulations not only specify the accuracy level of the flowmeter, but also specify the repeatability. Generally, the repeatability of the flowmeter must not exceed the maximum allowable error specified by the corresponding accuracy level. 1/3 ～ 1/5.
Repeatability is generally defined as the consistency of multiple measurements of a certain flow value in the same direction for a short period of time under the same environmental conditions and media parameters. However, in practical applications, the repeatability of the flowmeter is often affected by changes in the viscosity and density parameters of the fluid. Sometimes these parameter changes have not reached the level that requires special correction, which may be mistaken for the repeatability of the flowmeter. . In view of this situation, a flow meter that is not sensitive to this parameter change should be selected. For example, float flowmeters are easily affected by fluid density. Small-caliber flowmeters are not only affected by fluid density, but may also be affected by fluid viscosity. Turbine flowmeters are affected by viscosity when used in high viscosity ranges; some are not modified The processed ultrasonic flowmeter is affected by the temperature of the fluid and so on. This effect may be more pronounced if the output of the flowmeter is non-linear.
(4) Linearity
The output of the flowmeter mainly has two kinds of linear and non-linear square root. Generally speaking, the non-linear errors of the flow meter are not listed separately, but are included in the error of the flow meter. For a generally wide flow range, where the output signal is pulsed and used as a total flow meter, linearity is an important technical indicator. If a single meter coefficient is used within its flow range, the linearity difference will be Will reduce the accuracy of the flow meter. For example, the turbine flow meter uses an instrument coefficient in the flow range of 10: 1, and its accuracy will be lower when the linearity is poor. With the development of computer technology, its flow range can be segmented and fitted by the least square method. The output flow-meter coefficient curve corrects the flow meter, thereby improving the accuracy of the flow meter and extending the flow range.
(5) Upper limit flow and flow range
The upper flow is also called the full flow or maximum flow of the flowmeter. When we choose the caliber of the flowmeter, we should configure it according to the flow range used by the pipeline to be tested and the upper and lower flow rates of the selected flowmeter. It cannot be simply matched according to the pipeline diameter.
Generally speaking, the maximum flow rate of the designed pipeline fluid is determined by the economic flow rate. If it is too low, the pipe diameter is large, and the investment will be large; if it is too high, the transmission power is large, which increases the operating cost. For example, low-viscosity liquids such as water have an economic flow rate of 1.5 to 3 m / s and high-viscosity liquids of 0.2 to 1 m / s. The upper limit flow rate of most flow meters is close to or higher than the economic flow rate of the pipeline. Therefore, when the flowmeter is selected, its caliber is the same as that of the pipeline, which is convenient for installation. If they are not the same, they will not differ too much. Generally, the specifications of the next and next gear can be connected by reducers.
In the selection of flowmeters, different types of flowmeters should be noted. The upper limit flow rate or upper limit flow rate is greatly different due to the limitation of the measurement principle and structure of the respective flowmeter. Taking a liquid flow meter as an example, the upper limit flow rate is the lowest with a glass float flow meter, generally between 0.5 and 1.5 m / s, a volumetric flow meter between 2.5 and 3.5 m / s, and a vortex flowmeter with a higher flow rate. Between 5.5 and 7.5 m / s, the electromagnetic flowmeter is between 1 and 7 m / s, and even between 0.5 and 10 m / s.
The upper limit flow rate of the liquid also needs to consider that cavitation cannot occur because the flow velocity is too high. The location of the cavitation phenomenon is generally at the position with the highest flow rate and the lowest static pressure. To prevent the formation of cavitation, it is often necessary to control the minimum flowmeter. Back pressure (max flow).
It should also be noted that the upper limit of the flow meter cannot be changed after ordering, such as a positive displacement flow meter or a float flow meter. Once the differential pressure flow meter, such as the orifice plate of the throttling device, is designed and determined, its lower limit flow cannot be changed. The upper limit flow can be changed by adjusting the differential pressure transmitter or replacing the differential pressure transmitter. For example, some models of electromagnetic flowmeter or ultrasonic flowmeter, some users can reset the upper limit of flow rate by themselves.