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Operating characteristics and measurement errors of target flowmeters Author: Le Yi, engineer at Xiamen Hongkong. The operating characteristics of the HKB series of target flowmeters are primarily determined through fluid calibration experiments. While ensuring the reproduction of experimental conditions, calibration can also be carried out by using weights; that is, the gravity generated by the weights is utilized to replace the force exerted by the fluid on the target during operation, in order to calibrate the flow meter – this method is known as dry calibration. Additionally, dry calibration can also be used to periodically check the instruments. Operating characteristics of the HKB target flowmeter 1. Flow coefficient The flow coefficient C1 of the HKB target flowmeter is related to the Reynolds number Red. It is also influenced by factors such as the pipe diameter D, the ratio of the target diameter to the pipe diameter B, the shape of the target, the structure of the target chamber, the precision of the manufacturing of the target and the target chamber, the sharpness of the edges at the inlet of the target, as well as the degree of concentricity and surface finish between the target and the pipe. Generally, the relationship curve between the flow coefficient C1 and Re, D, and B is determined through experiments, as shown in Figure 4-29. As can be seen from the curve in the figure, the critical Reynolds number is lower than that of the differential pressure flow meter mentioned earlier; therefore, this measurement method exhibits its advantages more significantly for fluids with high viscosity and low Reynolds numbers. However, at lower Reynolds numbers, the viscous friction force acting on the target has a significant impact; only when the C1 value remains essentially constant within the actual flow measurement range can measurement accuracy be ensured. 2 Pressure loss: When a real fluid passes through the target, viscous friction is generated, and vortices form behind the target; all of these factors consume part of the fluid’s energy, resulting in pressure loss. The pressure loss is related to the diameter of the flowmeter, the target-diameter ratio, the flow rate, and the properties of the fluid. Under the same fluid, flow rate, and diameter, the smaller the target diameter ratio, the smaller the pressure loss B as well. The pressure loss curves for different diameters and target diameter ratios are shown in Figure 4-0. The pressure loss of target flowmeters is generally lower than that of throttle differential pressure flowmeters. When testing and comparing flowmeters using orifice plates with A=0.8 and B=0.5, the pressure loss of target flowmeters is only 40% to 50% of that of the orifice plates. 4.5.3.3 Measurement Error There are two main reasons for the measurement errors associated with target flowmeters: (l) When calculating the flow rate using a target flowmeter, we ignore the effect of the viscous friction exerted by the fluid on the perimeter of the target. However, when measuring fluids with high viscosity, this viscous friction increases the actual force acting on the target, resulting in a higher flow rate as indicated by the instrument. (2) When the measured flow rate is low, due to the effect of viscous force, the force exerted by the fluid on the target is not proportional to the square of the fluid velocity. Features of HKB target flowmeters The main features of target flowmeters are: (l) No moving parts, robust and simple structure; no need for pressure transduction tubes or other auxiliary fittings, making installation and maintenance easy, and they are less prone to clogging. (2) It has low pressure loss, making it suitable for small diameters (0.00–0.5 to 0.2 nl, 11) and low Reynolds numbers Re. =The fluid with (l--5)x 103 compensates for situations where standard throttling devices are difficult to use. (3) The lower measurement limit is low, the child-to-adult ratio is 3:1, and the basic error is ±1%. (4) It has a wide range of applicable measurement objects. l) It is mainly used to measure the flow rate of fluids with low Reynolds numbers and high viscosity in pipelines; 2) It can measure the flow rate of slurries containing an appropriate amount of solid particles, such as mud, pulp, mortar, and ore slurry; 3) It can measure dirty fluids such as sewage, crude oil, and high-temperature oils; 4) It can also be used to measure the flow rate of ordinary liquids, gases, and vapors; 5) When the target placed in the pipeline is made of corrosion-resistant materials, the instrument can also measure the flow rate of various corrosive media. (5) The instruments have not yet been standardized, and individual actual flow calibrations are required to ensure their accuracy. (6) When high-flow velocity impacts the target plate, vortices are generated behind it, causing the output signal to oscillate and affecting its stability; therefore, high-flow velocity measurement applications should be used with caution.