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The last edit to this post was made by 955559 on 2018-9-9 at 12:06. Vortex flowmeter 1. Instantaneous flow calibration method: Start the frequency pump, open electric gate valve 1 and electric gate valve 2 using the electric gate valve controller; adjust the manual valve while observing the reading on the glass rotor flowmeter, and adjust the manual valve until the flow rate reaches the specified calibration value. Once the flow rate stabilizes, observe the readings of the glass rotor flow meter and the vortex flow meter to be calibrated. Using the glass rotor flow meter as the standard, calculate the relative error between the two instruments; then enter this error into the error table of the vortex flow meter to be calibrated via the keyboard. 2. Cumulative flow calibration method: For cumulative flow calibration, first determine a calibration flow rate following the steps used for instantaneous flow calibration. Once the flow rate stabilizes, use the electric gate valve controller to close electric gate valve 1 and open electric gate valve 2, thereby draining the water in the cumulative flow calibration cylinder. Then, use the electric gate valve controller to open electric gate valve 1 and close electric gate valve 2 to start the cumulative flow calibration process. At this point, immediately record the cumulative flow value displayed by the vortex flow meter to be calibrated. When the volume of water in the cumulative flow calibration cylinder reaches a certain level, a water column will appear on the scale rod. At that time, use the electric gate valve controller to close electric gate valve 1, and simultaneously record both the scale value (representing the volume of water) on the scale rod of the calibration cylinder and the cumulative flow value displayed by the vortex flow meter to be calibrated. The relative error, obtained by comparing the difference between the two cumulative flow values with the scale value, is entered into the error table of the vortex flow meter to be calibrated via the keyboard. Cumulative flow calibration generally takes a few minutes. The above only completes the calibration of one flow calibration site. As stated in the measurement principle described in section 2.1.1, to perform nonlinear correction on a vortex flowmeter, it is necessary to calibrate at 7 different flow calibration points. The manual calibration system controls the flow rate via manual valves, then reads the value from the glass rotameter (the standard gauge), compares it with the reading of the gauge to be calibrated in order to determine the relative error, and adjusts the corresponding error parameters through the keyboard to carry out correction for nonlinear errors. Parallax can be introduced into the readings of glass rotameters, thereby affecting the calibration accuracy. When calibrating at low flow rates, since the reading error of glass rotameters is greater at this time, the instantaneous flow calibration method cannot be used; instead, the cumulative flow calibration method must be employed. It takes a few minutes to calibrate one flow point using the cumulative flow calibration method, and only one flow point of a single flow meter can be calibrated at a time, resulting in very low calibration efficiency. In summary, the manual calibration system has issues such as low calibration accuracy and low calibration efficiency. Given the problems existing in manual calibration systems, it is necessary to develop an automated calibration system to address these issues. 2. Requirements analysis and overall design of the automated calibration system 1. System requirements analysis The automated calibration system represents an automated version of the existing manual calibration systems, designed to overcome problems such as low calibration accuracy and low calibration efficiency. An automated calibration system involves establishing a computer-controlled system to achieve precise calibration of flow rates, as well as the ability to calibrate multiple vortex flow meters simultaneously in order to improve calibration efficiency. Based on the structure of manual calibration systems and using the standard gauge method for calibration, an automated calibration system has the following requirements. (1) Computer control platform: The computer control platform is used to carry out flow regulation control, access high-precision flow meter standards and the vortex flow meters to be calibrated, facilitate human-computer interaction, and schedule the calibration steps. (2) Flow control device: As a device for setting the flow rate, the flow control device needs to be able to communicate with the computing control platform in order to receive flow control commands from it. (3) High-precision flow meter standard gauge: As a standard gauge for flow calibration, a high-precision flow meter needs to be able to communicate with a computer control platform and receive flow-related commands from it. (4) Simultaneous calibration of multiple meters: The automated calibration system needs to be capable of calibrating multiple vortex flowmeters that are to be calibrated at the same time. 5) Communication protocol: Since the flow rate setting devices and flow rate detection equipment are purchased from external suppliers, an industry-standard communication protocol should be used. 2. Overall system design: The overall system design is carried out based on the requirements analysis of the automated calibration system; the schematic diagram of the system’s hardware platform is shown in Figure 2-6. The system mainly consists of a τ controller, a 0.1-class high-precision vortex flow meter, the vortex flow meter to be calibrated, an inverter, an inverter-driven pump, an RS-232 to RS-485 converter, a water storage tank, and water delivery pipes. Among them, the high-precision flow meters, the flow meters to be calibrated, and the frequency converters all have RS-485 communication interfaces with Modbus protocol. The computer communicates with these devices via the RS-485 bus, using the Modbus protocol, which is widely used in the industrial sector. (1) Selection of flow regulation scheme. There are generally two ways for flow regulation: one is to use an electric control valve and a power-frequency pump to form a flow regulation unit, with the flow being adjusted by changing the opening degree of the control valve; the other is to use an inverter and an inverter-driven pump to form a flow regulation unit, with the flow being adjusted by changing the set frequency of the inverter. When adjusting the flow rate using a control valve, the power-frequency pump operates at full speed; that is, the flow rate of water pumped out by the pump is the maximum possible flow rate. The control valve regulates the flow rate by adjusting its opening degree. The problem with this method of regulation is that, regardless of the desired flow rate, the power pump operates at full speed throughout, resulting in high power consumption, which goes against the principles of energy saving. When the control valve adjusts its opening degree, the water pressure in the pipeline changes, causing fluctuations in the flow rate within the pipeline – a situation that is not allowed in a calibrated system. Therefore, this control method is not adopted; by using an inverter to regulate the flow rate, the problems associated with using control valves for flow regulation can be effectively solved. An inverter can regulate the flow rate of a variable-frequency pump by adjusting its speed via frequency variation. The lower the flow rate, the lower the frequency output by the inverter, which in turn results in less power consumption by the variable-frequency pump. Moreover, since this method of flow rate regulation does not involve the use of control valves, the water pressure in the pipeline remains relatively stable, and as a result, the flow rate of the water is also stable. This makes it suitable for calibrating the flow rate in systems. In summary, using a frequency converter and a variable-frequency pump to form a flow control component for regulating flow offers advantages such as energy savings and stable output flow; therefore, this approach is employed in automated calibration systems to regulate flow. The computer accesses the frequency converter via the RS-485 bus and sets the frequency parameters to enable the frequency converter to change the frequency, thereby altering the flow rate of water pumped by the variable-frequency pump, and this achieves the purpose of flow rate regulation. To avoid system instability caused by frequent communication and complex closed-loop control algorithms, the frequency converter, variable-frequency pump, and standard meter do not form a closed loop but operate in an open-loop manner. This requires us to conduct experiments on the frequency converter in advance to obtain the relationship between the frequency parameters and the instantaneous flow rate of water pumped by the variable-frequency pump; based on this relationship, the computer can roughly control the flow rate set value. The flow rate values at the calibration sites do not need to be exact; they can vary. A standard gauge is used to determine the current instantaneous flow rate, and the other gauges under test use this value as their own calibration point. (2) Selection of calibration methods and standard tables: The common calibration methods for liquid flow rate include the standard volume method and the standard table method. The standard volume method offers high calibration accuracy, but it takes a long time to calibrate; it usually takes several minutes to calibrate one flow rate point. Moreover, the standard measuring containers need to be replaced frequently depending on the different flow range, which is very inconvenient. Therefore, this calibration method is not used. The standard gauge method involves using a high-precision flow meter as a standard instrument to calibrate other gauges that need to be adjusted. This method is simpler, more convenient, and cost-effective, so it is adopted as the calibration method for this system. The selection principle for standard instruments is that the allowable error of the standard instrument should be less than 1/3 to 1/5 of the allowable error of the instrument being calibrated, so that the error of the standard instrument can be ignored. The accuracy required for the reference meter is grade 0.5; therefore, a high-precision vortex flow meter of grade 0.1 was chosen as the reference meter. This high-precision vortex flow meter replaced the original glass rotameter, enabling real-time measurement of the current instantaneous flow rate. In the calibration system, it serves as the standard meter for calibrating the vortex flow meters to be calibrated. The computer can access this reference meter via the RS-485 bus to obtain the current instantaneous flow rate, thereby completing the flow measurement using the reference meter. 3) Simultaneous calibration of multiple meters: The number of flowmeters to be calibrated simultaneously is temporarily set at 8. These 8 vortex flowmeters to be calibrated are connected in series with standard meters; the piping connection is in an S-shaped configuration and all are parallel to the ground. An S-shaped connection is used to save space occupied by the system components – if the connections were made in a straight line, too much space would be required, as the straight sections before and after the vortex flowmeters need to meet certain length requirements and cannot be too short. (4) Human-machine interaction: The human-machine interface is used to set the traffic calibration parameters and initiate the calibration process. 5) Calibration schedule: For the nonlinear correction of vortex flowmeters, a total of 7 calibration points are required. At each calibration flow point, the 8 vortex flowmeters to be calibrated must be calibrated in a specific order.