Liquid level static level and differential pressure static level

1. Measurement principle

The static level settlement monitoring principle uses the principle of the connector to connect the reference point and the monitoring point with the liquid tank, measure the relative pressure change  or liquid level change between the monitoring point and the reference point in real time, and calculate the relative pressure change of the monitoring point relative to the reference point. Subsidence. The formula for calculating the settlement of the monitoring point is,

∆Hi=∆ℎ0−∆ℎ

In the above formula, ΔHi is the sedimentation amount of the i -th monitoring point, Δℎ0 is the liquid level height change of the reference point, and ∆Hi is the liquid level height change of the i -th monitoring point.

At present, various static level settlement monitors adopt different technologies in obtaining the liquid level height, and the settlement calculation methods are all the above formulas.

1  Direct liquid level measurement method

A downward ranging sensor is installed on the top of the static level to measure the liquid level change in a non-contact way. Non-contact measurement distances include laser rangefinders and ultrasonic rangefinders. The laser distance meter needs the liquid surface to reflect the laser point back, and has strict requirements on the reflectivity of the liquid (transparent liquid is not acceptable), while the ultrasonic liquid surface distance measurement does not have the requirement of liquid reflection performance, but because the ultrasonic wave is a horn divergent type, it requires There is enough lateral space inside the monitor to avoid the reflection of sound waves from the side walls.

2  Float method

A floating ball with a lower density than the liquid is placed on the surface of the liquid in the monitor, and the height change of the floating ball is measured in real time. Magnetostrictive technology is a typical application of this principle. In addition, there is a measuring mechanism that drives the pulley to rotate through the change of the float height, and the application of indirectly calculating the change of the float height by measuring the change of the pulley angle is also relatively common.

3  buoyancy method

Similar to the floating ball method, a floating ball float with a density lower than that of the liquid is placed on the liquid surface of the monitor, and the change in the liquid level is indirectly calculated by measuring the change of the buoyancy on the float by a force sensor. There are many sensing technologies for measuring force, such as vibrating wire stress sensor, fiber grating stress sensor, capacitive stress force sensor, etc.

4  Hydraulic method

A liquid pressure sensor is installed at the bottom of the monitor, and the liquid level height value is indirectly calculated by measuring the liquid pressure. There are many sensing technologies for liquid pressure sensors, such as strain gauge pressure sensors, silicon resistance pressure sensors, etc.

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Second, the performance comparison

Discussing the performance of the static level settlement monitoring system is nothing more than the measurement accuracy of settlement, and the accuracy of settlement depends on the measurement accuracy of the liquid level change at each monitoring point. 

2.1 Definition of liquid level height

The essence of measuring relative settling with a communicator is a pressure change, not a liquid level change. For example, the temperature rise at a certain point causes the liquid volume to increase and the liquid level to rise, but in fact no settlement deformation occurs at this point. The conclusion is clearly wrong. The correct processing method should be to correct the liquid height according to the current temperature. The corrected liquid level height is called "representative liquid height". When calculating the sedimentation amount, the representative liquid height should be used instead of the actual measured physical meaning. liquid height.

 Example: The density of water will change by about 0.5% from 5 ℃ to 35 ℃ , that is, the volume height will change by 0.5% .

If the liquid level is 1000mm , it will become 1005mm at 35 °C . If this physical liquid level change is directly used to calculate the settlement, the conclusion is that " 5mm of settlement occurred at this point", which is obviously wrong. 

2.2 Influencing factors when measuring the height of "characterized liquid level"

 1  Direct liquid level measurement

Whether it is laser diffuse reflection or ultrasonic ranging, the measurement is the "physical liquid level height" defined in physics. Before calculating the settlement amount, it needs to be corrected to "characterized liquid level height" to calculate the settlement amount. This measurement method "seems" to be straightforward on the surface, but in fact it is self-evident in the static leveling system. In addition, laser ranging needs to use a liquid with good reflective performance, and ultrasonic ranging requires enough space inside the instrument to ensure the vertical range, resulting in a larger diameter of the instrument. 

 2  Float method

There is inevitable contact between the floating ball and other internal components in the monitor, and there must be frictional resistance caused by physical contact. The size of the frictional resistance is related to the contact form, contact area and contact angle between the floating ball and other components, and the resistance is random. The sensitivity of the floating ball following the movement of the liquid surface is reduced and uncontrollable, and the random error increases. Another measuring mechanism that utilizes a float + pulley, the pulley bearing also has the problem that the friction force changes with time, resulting in a systematic error of unknown magnitude. At the same time, the float method also has the above-mentioned problems of "physical liquid level height" and "characterized liquid level height". 

 3 Buoyancy methodFloat method

The buoyancy method is actually measuring the change in the buoyancy of the float caused by the change of liquid height.

"Force", the change of liquid level caused by liquid temperature will not cause the change of buoyancy, so this method avoids the problem of inconsistency between "physical liquid level height" and "characterized liquid level height" from the measurement principle level. This measurement method has the problem of friction between the float and other components. 

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 4 Pressure method

Directly based on the basic principle of measuring settlement by the communicating device, the pressure change at each point is measured by the pressure sensor. After the pressure change is converted into the change of liquid height, it is the "characterized liquid level difference", which can be directly used for settlement calculation. This advantage is completely similar to the buoyancy method. same. In addition, because the pressure is measured, there is no real change in the liquid level inside the instrument, so there is no requirement for the volume of the monitor, and it can be made into a small device that is easy to carry and install. The above three types of static level must have enough height to meet the principle level of the internal liquid level change during settlement and deformation. 

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2.3 Performance comparison

The following is a list of liquid level measurement principles to compare the advantages and disadvantages of various measurement methods and static level monitoring systems. 

2.4 Several "false conclusions"

1  Liquid level static level is more accurate than differential pressure static level

The liquid level type is divided into two categories: direct measurement of the physical liquid level and characterization of the liquid level. Each category has different measurement mechanisms, which can only be said to have their own advantages and disadvantages. From the measurement link, the pressure method is the most direct. Compared with the pressure method, through various liquid level measurement methods, it shows that there are too many links, and the accuracy is difficult to guarantee. 

2  XXX sensing technology is better than others

Any kind of sensing technology can meet the accuracy requirements of the system through certain technical means. The statement that "the accuracy of XXX sensing technology is higher than other sensing technologies" is itself a subjective expression without scientific basis. The choice of sensor is only "fit" and not "best".  For example, the magnetostrictive sensor can choose the accuracy of 1mm and 0.1mm . It is only a matter of cost. The pressure sensor can also have different accuracies such as 0.5% , 0.1% and 0.05% . 

3  Differential pressure static level gauges inversely calculate the liquid level through pressure, so the accuracy is low

This is obviously stealing the concept. The so-called "direct measurement of liquid level" of the liquid level method is to measure the physical liquid level and temperature and then indirectly calculate the "characterized liquid level" required by the settlement formula, while the pressure method is to measure the pressure directly. The "Characteristic Level" is calculated. People who can say such things either do not understand the principles of static leveling and pretend to understand, or they are deliberately passing on the wrong information to people, low-level fooling. 

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3. Precautions for the purchase of static level

The only reason why there are so many different types of static levels is the cost. But more importantly, each type of instrument has its own advantages and disadvantages. Even monitoring instruments from different manufacturers based on the same measurement principle also exist. The difference in production and design should be analyzed from the perspective of principle in combination with the specific type of monitor when purchasing, and some simple tests should be carried out if necessary. 

For a type of static level that directly measures the "physical liquid level", it is necessary to know whether the monitor has the function of correcting the physical liquid level to characterize the liquid level and how well it performs. The simple way to judge is whether the manufacturer allows filling of a variety of different liquids, and the correction parameters dedicated to the "temperature - density" characteristics of each liquid. I don't think the manufacturer's words can be trusted. The test is the most convincing evidence. The simple test method is to inject liquids with the same height difference but different temperatures, and observe the reading of the level. 

For a type of static level that uses a float or buoyancy to measure the liquid level, it is necessary to understand how it solves the friction between the float and other internal components. It can test the measurement sensitivity of the monitor when it is not absolutely horizontal. Generally, the friction of the float will increase when it is not horizontal, resulting in a decrease in the sensitivity of the float when the liquid level changes. 

The above purchase principles are only based on the measurement principle and the level of the measurement mechanism, on how to check the sensitivity, stability, and accuracy of the liquid level sensing element. 