Design of automatic flow calibration system for heat meters

introduction

The household metering method of centralized heating is an energy-saving and environmentally friendly heating metering system. According to the "Interim Measures for Urban Heating Price Management" issued by the National Development and Reform Commission of China in August 2008, my country's heating metering method will gradually transition from area-based billing to household-based billing. The heat meter is an important means to achieve household-based heating billing. It measures the actual heat supply to users by measuring the flow rate of the fluid and the temperature difference between the inlet and return water. The calculation formula is as follows:

In the formula, Q represents the heat released or absorbed by the heat meter, qm represents the mass flow rate of water flowing through the heat meter, qv represents the volume flow rate of water flowing through the heat meter, ρ represents the density of water, △h represents the enthalpy difference of water, and the enthalpy value is equal to the product of temperature and specific heat capacity at constant pressure, and t represents time. It can be seen from formula (1) that the flow measurement accuracy of the heat meter is an important indicator, which directly affects the accuracy of heat supply metering. Therefore, each heat meter needs to be calibrated according to regulations before leaving the factory. At present, the flow rate adjustment device of the heat meter calibration device of most domestic heat meter manufacturers adopts manual mode, which is complicated to operate. The writing of the flow correction coefficient requires manual operation, which is prone to human errors. The heat meter standard calibration device of the Metrology Bureau adopts the overall calibration method, which has high accuracy, but high detection cost and low efficiency. It is not suitable as a production equipment and is mostly used for the final calibration of heat meters. In order to improve the automation and efficiency of heat meter calibration, an automatic flow calibration system for heat meters was designed. The MSP430F149 single-chip microcomputer was used to realize automatic control of the system calibration process. The high-precision and low-cost weighing method was used to obtain the standard flow rate of the fluid in the system. Up to 12 heat meters can be automatically calibrated at the same time.

System Design

The entire calibration system is a closed-loop test system, and the calibration fluid can be recycled to save costs. As shown in Figure 1, the system consists of a computer, a control unit with MSP430F149 as the main control chip, an electronic scale, a flow control valve, a constant temperature water tank, a pressure regulating tank, a water storage tank, a solenoid valve, a water pump, etc. The heat meters to be calibrated are connected in series in the same straight pipeline, and data is exchanged with the control unit through the optoelectronic transceiver interface to realize automatic detection of batch heat meters and automatic writing of correction coefficients. The programmable flow control valve is used for automatic flow control. The electronic scale and the water storage tank are used to weigh and calculate the standard flow, and the constant temperature water tank, the water pump and the pressure regulating tank are used to provide the calibration fluid. In order to be close to the on-site working conditions of the heat meter, the constant temperature water tank controls the temperature of the calibration fluid at about 50°C. At the same time, corresponding insulation measures are added to the pipeline to reduce the heat dissipation of the circulating pipeline.

The flow calibration process of the heat meter is fully automatically controlled by the host computer through the control unit. Since bubbles in the pipeline will cause errors in flow measurement, the control unit starts the solenoid valve to start the exhaust process at the beginning of the test. After the exhaust is completed, the flow calibration begins. For each flow point, the control unit automatically reads the data from the electronic scale after the electronic scale reading stabilizes, and then converts the flow Q0. At the same time, the control unit collects the data Q1 of the heat meter, and calculates the correction coefficient C=Q0/Q1 of each heat meter at this flow point. The five flow points specified by the National Metrology Verification Regulations of the People's Republic of China are tested in turn to complete a round of calibration. After the entire calibration process is completed, the control unit automatically writes the correction coefficient into the corresponding heat meter and uploads the data to the host computer, which determines whether the heat meter is qualified or not.

The standard flow rate of the calibration system is determined by weighing with an electronic scale. Therefore, the range of the electronic scale must meet both the weighing at the minimum flow rate and the weighing at the maximum flow rate. Its measurement accuracy directly affects the accuracy of the heat meter. Because it is a circulation system, the volume of the container is twice the volume of the fluid required at the maximum flow point, and the volume of the constant temperature water tank is three times the volume of the fluid required at the maximum flow point to meet the requirements. In order to ensure the stability of the fluid flow through the heat meter, the straight pipe section of the upstream section of the heat meter to be calibrated should meet more than 5 times the pipe diameter, and the straight pipe section of the downstream section should meet more than 3 times the pipe diameter. The number of heat meters in series should not be too many. Too many will not only occupy a large area, but also cause excessive pressure difference in the entire circulation pipeline to affect the calibration results. In this system, a maximum of 12 heat meters are connected in series. During the operation of the water pump, the circulating pipeline fluid often pulsates, which will introduce errors in the flow measurement of the heat meter. When designing the system, a pressure regulating tank is installed at the rear end of the water pump to avoid pulsation shock. In addition, a transparent pipe is configured during the design of the circulation pipeline to facilitate the operator to observe whether there are bubbles in the fluid. If no bubbles are observed during the exhaust process, the exhaust can be stopped manually in advance; if bubbles are found during the calibration process, the stop button can be used to end the test and recalibrate. The automatic calibration process of the heat meter flow is completed by the control unit and the host computer software.

System control unit

The control unit block diagram of this system is shown in Figure 2, which is mainly composed of keyboard circuit, temperature control circuit, solenoid valve control circuit, photoelectric transceiver interface circuit, communication interface circuit, and sound and light indication circuit. The MSP430F149 microcontroller is used as the control core, and an external 6MHz crystal oscillator is connected.

The power supply of the microcontroller inside the control unit is 3.3V, and the power supply of other chips and analog devices is between 3V and 10V. The power input of the control unit is provided by an external switching power supply. At the production site, the switching power supply and the water pump motor share one AC power. The water pump will cause the output fluctuation of the switching power supply during operation. If no anti-surge measures are taken, it will inevitably affect the normal operation of the microcontroller. The system power supply module designed according to the above analysis is shown in Figure 3. Inductor L1 plays a role in anti-surge protection; the input voltage outputs 12V voltage after passing through the switching power supply chip MC34063, and then generates two 3.3V power supplies through two NCP1117ST33 voltage regulator chips to power the microcontroller and the photoelectric receiving circuit respectively.

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In order to realize the temperature control of the constant temperature water tank, the temperature of the constant temperature water tank needs to be collected. The platinum resistance PT1000 temperature sensor is used to measure the water temperature. The temperature acquisition circuit of this system samples the constant current drive mode, as shown in Figure 4. In order to avoid the self-heating of the PT1000 sensor under long-term operation and affect the temperature measurement accuracy, the output current of the constant current source is designed to be within 5mA. The op amp on the left of Figure 4 forms a constant current source circuit. The op amp on the right forms a differential amplifier to increase the common mode rejection ratio, and the voltage gain is R17/R16 (where R15=R17, R14=R16).

System Software

Design of MCU Software

After the system is powered on, the microcontroller is initialized first, mainly including the working mode selection of the timer, serial communication module and basic input and output ports and the initialization of related variables. After the initialization is completed, the number of flow points, flow rate and test time data transmitted by the host computer are read through the serial communication interface and saved in the external memory so that the system can be started without the host computer. After the data is stored, the control unit first tests whether the communication of the heat meter to be tested is normal. If there is a heat meter that cannot communicate, the control unit will upload the detailed information to the host computer, and the user will decide whether to continue the calibration work. After the entire calibration process is completed, the control unit writes the flow correction coefficient and system time of different flow points into the corresponding heat meter, and then uploads it to the host computer. The host computer determines whether the heat meter is qualified and displays the detailed information of the calibration on the work interface.

Since the photoelectric interface may cause abnormal communication under strong light, the following processing is done to avoid deadlock: once the control unit finds a heat meter with communication problems during the calibration process, it immediately notifies the host computer and asks the user through the human-machine interface whether to continue the test. If the user chooses to continue the test, the control unit will no longer read the heat meter with problems.

Host computer software

The host computer software adopts the programming method of MFC (Microsoft Foundation Class Library), which fully utilizes the advantages of object-oriented technology. The powerful functions of various objects in the MFC class library are sufficient to complete most of the functions required in the program. The software operation interface is shown in Figure 5. Through the setting options in the interface, the number of calibrated flow points, flow rate of flow points and specific calibration time of different flow points can be set.

After the user starts the calibration process with the host computer, the host computer software transmits the calibration information to the control unit through the RS-232 interface. The control unit is responsible for the automatic control of the calibration process. After the calibration is completed, the data is uploaded to the host computer through the RS232 interface.

System operation results and analysis

This system was used to calibrate the heat meters produced by Dalian Ruigong Microelectronics. According to the National Metrology Verification Regulations of the People's Republic of China, the selection of the 5 flow points q1-q5 for verification should be: qmin≤q1≤1.1qmin, 0.1qp≤q2≤0.11qp, 0.3qp≤q3≤0.31qp, 0.9qp≤q4≤1.0qp, 0.9qmax≤q5≤1.0qmax, where qmin is the lower limit of the heat meter flow measurement, qp is the nominal flow of the heat meter, and qmax is the upper limit of the heat meter flow measurement. Table 1 shows the calibration results of 10 heat meters randomly selected from the production site of Dalian Ruigong Microelectronics by this system. According to the National Metrology Verification Regulations, the accuracy of Class III heat meters is 3%. In the on-site calibration, the correction coefficient of adjacent flow points exceeding 3% is considered unqualified. The correction coefficient of heat meter 3 at flow points 4 and 5 does not meet the requirements, the correction coefficient of heat meter 6 at flow points 1 and 2 does not meet the requirements, and the remaining heat meters are qualified.

In order to verify the calibration results of this system, the 10 heat meters were sent to Dalian Metrology Bureau for calibration, and the results are shown in Table 2. Comparing Table 1 and Table 2, it can be seen that the calibration results of the heat meters using this system are completely consistent with the calibration results of the heat meter standard calibration device of the Metrology Bureau.

Conclusion

In view of the fact that most traditional heat meter calibration devices are manual systems, which are cumbersome to operate, take a long time to calibrate, and are prone to human errors, a heat meter flow automatic calibration system is designed. The system automatically completes the entire calibration process without human intervention, and the calibration time is short, only about 30 minutes. The flow correction coefficient and related information are also automatically written, avoiding unnecessary human errors.

The system has been successfully applied in Dalian Ruigong Microelectronics Co., Ltd. After more than one year of production verification, the test results of the heat meter flow automatic calibration system are completely consistent with the test results of the national standard measurement unit. The system has the advantages of high efficiency, low cost, energy saving, safety and reliability, easy operation and simple maintenance. Its development meets the needs of heat meter manufacturers for batch automatic calibration of products and can greatly improve labor productivity.