Kunke Instrument responded how to measure the liquid level value of the density change medium container through the pressure transmitter

  Do you know how to measure the density change and the liquid level value of the medium container through the pressure transmitter? As soon as the editor comes out on this topic, I have something to say, and I hope to give some help to the instrument people.

  As a common measurement method for on-site measurement in industrial production, liquid level measurement is one of the important conditions to ensure the stability of industrial production automation. Liquid level gauge, magnetic flap level gauge, capacitive liquid level sensor, radar level gauge, ultrasonic level gauge, bubble method, etc., each liquid level measurement method and principle are different, and each has its own advantages and disadvantages , There is no liquid level measurement method that is perfect and capable of all conditions. The user should choose the actual working condition before the specific use, and the ultimate goal is to measure the liquid level value of the measured medium as accurately as possible.

  Using the pressure difference of the liquid to be measured to measure the liquid level value is a commonly used measurement method in automated industrial measurement. It has the advantages of high measurement accuracy, simple structure and small maintenance, so it is widely used in liquid level measurement. Of course, we mentioned above that no measuring device can measure all, and the pressure transmitter is no exception. Its disadvantage is that when the density of the measuring medium changes, it is impossible to measure the The density of the measured medium can be adjusted and modified at any time, so there is a large error between the measured liquid level value and the actual liquid level value, especially when the measured medium changes at any time.

  Because industrial production sometimes has high requirements on the accuracy of liquid level measurement values, in such measurement occasions, the measurement data we obtain cannot meet the requirements of the use of industrial systems. How to use the pressure transmitter to obtain an accurate measurement liquid level when the density of the medium changes Improve the liquid level measurement value of the density change medium container, put forward insights, correct the liquid level value measured by the pressure transmitter through a reasonable method, improve the measurement accuracy, and analyze to help friends solve such problems encountered in practical work. problem.

  1. The working principle of the pressure transmitter to measure the liquid level

  The run-in pressure transmitter is usually flange-mounted. The transmitter is connected with the mounting flange and the matching flange on the tank with bolts. By measuring the pressure value of the measured point, it can be known from fluid mechanics that P= ρgh, where ρ is the density of the measured medium, g is the gravitational acceleration, constant, h is the liquid level height of the measured medium, the pressure transmitter converts the pressure signal into a 4-20mA DC output electrical signal, and the control system outputs the The magnitude of the current value is used to calculate the liquid level value h of the measured medium.

  2. Existing problems

  When setting the range of the pressure transmitter, the range of pressure value Pmin-Pmax is usually calculated according to the liquid level variation range hmin-hmax and the density ρ of the measured medium. It is generally considered that the density ρ of the measured medium is fixed throughout the measurement process. Therefore, the measured liquid level value is accurate for the working condition that the density of the measured medium does not change. But for the slurry tank of the wet mill, since the slurry prepared by the mill needs to meet the requirements of the slurry supply system, the density change of the slurry needs a process, and the density of the slurry is not fixed even in normal operation. of. Moreover, the liquid level value of the slurry tank of the mill needs to be referred to the control system of the entire mill. When the liquid level of the slurry tank of the mill is lower than the liquid level, the mill will be shut down. When the liquid level is higher than the upper liquid level, the mill will be stopped. The slurry in the slurry tank of the machine will overflow. The density of the slurry varies from 1.0kg/m3 to 1.4kg/m3 during the entire slurry preparation process.

  Due to the large variation range of slurry density, there is a large error between the liquid level value measured by the pressure transmitter and the actual liquid level value of the slurry tank of the mill, which is easy to cause the malfunction of the mill control system, so how to effectively It is particularly important to improve the measurement accuracy of the liquid level in the mill control system.

 Three, the solution

  It can be seen from the existing problems that the main reason for the inaccurate measurement of the liquid level value is that the density of the slurry is not fixed during the operation process, but is a variable that changes at any time with the operating conditions of the system. The change of the variable corrects the error formed by the liquid level measurement, so that the measured liquid level is consistent with the actual liquid level. In view of this, the density control unit is introduced into the liquid level control unit, the density value measured by the density control unit is compared with the density value set in the pressure transmitter, the liquid level difference due to the density change is calculated, and the difference is calculated. Eliminated by operation to ensure the truth of the output liquid level value.

  It can be seen from the working principle that the density measurement link is added to the tank body of the measured medium. The density measurement is also measured by the differential pressure transmitter to measure the change of the pressure difference between the two fixed points of the tank body, and the real-time measurement of the measured medium is obtained. density value. The pressure difference value ΔP=ρg (h1-h2), where ρ is the density of the measured medium, g is the acceleration of gravity, and h1-h2 is the height difference between the two measurements of 1 and 2. The differential pressure transmitter also converts the change value of the pressure into the corresponding 4-20mA DC output, which is used by the control system to calculate the density value of the measured medium, and use the actual density in the liquid level calculation unit to correct the set density difference. , so as to obtain a liquid level value that is closer to the actual liquid level, providing reliable data guarantee for the precise measurement of the control system.

  The relatively simple correction principle is: According to the difference Δρ between the actual measured density ρ of the measured medium and the set density ρ, the pressure change value ΔP caused by the change of Δρ is approximately calculated, and then the liquid level change value is calculated. Δh. ΔP=Δρgh measurement, Δh=ΔP/(ρ real g), control system output liquid level value h=h measurement-Δh.

  4. Correction results

  As exemplified in the existing problems, after adding this link, the output value of the liquid level after the operation of the control system is h output = h measurement - Δh = 2.4-0.4 = 2m, the output liquid level is consistent with the measured liquid level, which overcomes the problem of The phenomenon of error between the output liquid level and the actual liquid level caused by the density change of the measured medium ensures the authenticity of the measurement data of the entire control system.

  V. Conclusion

  In the liquid level environment where the pressure transmitter is used to measure the density change of the measured medium, by adding a density measurement link, the liquid level value measured by the pressure transmitter can be corrected under the condition that the actual measured medium density is obtained. , the actual liquid level value of the measured medium can be obtained more accurately.

 I don't know if the above summary can really help you solve practical problems. If the sea has any ideas or opinions that need to be added, please feel free to call Weihai Kunke Flow Instrument Co., Ltd. to help you solve it. For more details, please log in www.kunkeandao .com/en The company's official website inquiries, online we have a 24-hour customer service online to help you answer in real time. Come and consult.