Reasons for errors in the measurement of tap water flow and how to deal with them

Abstract: The reasons for the errors in the measurement of tap water flow meters and the processing methods are provided by the excellent flowmeter and flowmeter manufacturers and quotation manufacturers. There are problems The electromagnetic flowmeter is used to measure the flow of tap water, which is accurate most of the time of the year, but in the high temperature period, the sub-meter runs faster than the total meter. A set of tap water flow meters are installed on the fifth and third floors of the podium of a building in Shanghai, and there is no meter on the fourth floor, so the difference method is used for calculation. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following are the reasons for the errors in the measurement of tap water flowmeters and the details of the treatment methods. There are problems The electromagnetic flowmeter is used to measure the flow of tap water, which is accurate most of the time of the year, but in the high temperature period, the sub-meter runs faster than the total meter. A set of tap water flow meters are installed on the fifth and third floors of the podium of a building in Shanghai, and there are no meters on the fourth floor. The difference method is used to calculate the water consumption: Q4= Q5-Q3 in the formula Q4—Total water consumption by users on the fourth floor, m3; Q5—The total amount of total surface water on the fifth floor, m3; Q3—The total water consumption of users on the third floor, m3. The system diagram is shown in the figure. Calculated from the statistical data of the computer metering data collection system, the statistical results of the total water consumption in the past few years were normal, but in the hottest days in Shanghai in the summer of 2010, the Q4 had a negative value. The reason was that the Q3>Q5, this conclusion is wrong. Analysis and Diagnosis (1) Why is the electromagnetic flowmeter on the third floor high? Relevant personnel inspected the site and found that the water tank located on the fifth floor had no direct sunlight. Since the water tank is an open container, the dissolved air in the water should be saturated. The water pipes absorb heat from the atmosphere along the way, and the water temperature rises, especially on the third floor. There is a section of about 20m long laid outdoors, and the temperature rises even higher. The tap water flows through here and enters the flow meter on the third floor. The gas comes out of the water and collects in the upper part of the horizontal pipe. When this section of the pipeline reaches the third floor, there is no chance for it to go up, so the gas accumulated in the horizontal pipe cannot be discharged. There is inevitably gas in the measuring pipe of the tap water flowmeter installed on the horizontal pipe on the third floor, occupying a certain flow cross-sectional area, so that the flow indication value is on the high side. (2) Suggestions for rectification Add a gas collector and an exhaust valve at the appropriate position of the horizontal pipe on the third floor to regularly discharge the accumulated gas in the pipe. There will be no accumulation of gas in the pipe in autumn, winter and spring, and generally no exhaust is required. (1) Reasons for the negative value of Q4 There are two reasons for the negative value of Q4. One is that the air is accumulated in the horizontal pipe on the third floor, and it cannot be discharged by itself, and there is no exhaust port, so the indication value of the tap water flowmeter on the third floor is too high. ; Second, in formula (4.22), Q4 is a very small value, that is, the fourth floor is a very small user, and the monthly statistics report shows that its water consumption is less than 2% of that of users on the third floor, so the error between Q5 and Q3 In the end, it will be digested by Q4, which will bring large errors to the measurement data of the fourth floor. If the meter is refitted on the fourth floor and the water consumption on the third floor is obtained by subtraction, there will be no negative value for users. This lesson can be drawn“measure small”“reckoned large”is a useful experience. (2) Install an exhaust valve where necessary It is also an important experience to install an exhaust valve where gas may accumulate and cannot be discharged by itself. (3) Another type of non-full pipe flow measurement object A For flow measurement objects such as municipal drainage and wastewater treatment, although the pipe is not full, it is completely different from the above situation, and the treatment method is also completely different. For measurement objects that are often partially filled and cannot be made full, a partially full flow meter should be used. The following is a type of non-full pipe flowmeter. The working principle of the electromagnetic flow meter that is not full pipe The pipeline type tap water flow meter is usually applied to the liquid full pipe flow of the closed pipe. For the situation where the flow rate varies greatly, sometimes the pipeline is full, and sometimes the pipeline is not full, the pipeline type tap water flowmeter is not suitable. At this time, a non-full pipe water flow meter is required. Non-full pipe electromagnetic flow sensor can be consistent with the installed pipe diameter, work in a wide range, and can be used for full pipe flow in closed pipes and non-closed pipes or free surface flow measurement of open pipes, and does not produce head loss, such as municipal Drainage, wastewater treatment, agricultural irrigation, fluids are measured by the flow of natural flow. The tap water flowmeter is based on the constant cross-sectional area of ​​the sensor, and the average flow rate is measured to obtain the flow rate. The cross-sectional area of ​​the fluid in the non-full pipe varies, and the flow measurement should not only measure the average flow rate flowing through the sensor, but also measure the cross-sectional area of ​​the fluid flowing through the sensor. That is to say, the flow measurement of the non-full pipe water flow meter At least two variables, flow rate and level, are required. Figure 4.24 shows the structural principle of a non-full pipe flow sensor model TIDALFLUX4000. It is an electromagnetic flow sensor with an integrated capacitive liquid level measurement system and is designed for conductive liquids. The flow q(t) flowing through the pipeline is: q(t)=v(t)A(t) where v(t)—Flow rate; A(t)—Fluid cross-sectional area. The flow rate is determined based on the known principles of electromagnetic flow measurement. In order to measure 10% of the liquid level reliably, two measuring electrodes are arranged in the lower part of the measuring tube about 10% of the height. There is a pair of excitation coils above and below the measuring tube, which generates a magnetic field when an excitation current flows through the coils.

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