Abstract: the feasibility of the micro-pressure level transmitter in the water tank level detection was demonstrated through experiments, and the advantages and disadvantages of the two types of level transmitter and the matters needing attention were analyzed and compared, which provided a basis for the application of the micro-pressure level transmitter instead of the input level transmitter in the secondary water supply.
In the field of secondary water supply, the level data of the water supply tank is an important signal to control the water supply of the tank and the safety protection of ultra-low water level.At present, widely used in the field of secondary water supply input type liquid level transmitter collection level signal, due to the effect of its installation, maintenance level fluctuations and inconvenience caused big problem, so to explore use side decay pressure liquid level transmitter instead of inputting type liquid level transmitter collection liquid level signal of more significance.
The plunge-type level transmitter is mounted on the top of the water tank, and the probe extends to the bottom of the water tank through a cable, as shown in figure 1.Because of its stable performance, simple structure and convenient installation, the plunger level transmitter has been widely used.However, the space between the top of the water tank and the top of the pump room is small. When the plunge-type liquid level transmitter needs to be repaired or replaced, the operators need to work on the top of the water tank, which is very inconvenient to operate.In addition, in order to facilitate the replacement, the probe is not fixed when installing the input level transmitter, and it is easy to cause the deviation of the liquid level value due to the disturbance caused by water flow impact, thus causing the system to misoperate, which is extremely unfavorable to the system control.
2.1 principle of micro pressure level transmitter
The essence of micro pressure level transmitter is to use pressure transmitter to indirectly measure the level of liquid, as shown in figure 1.In fact, the pressure transmitter is mounted on the side of the water tank at a certain height from the bottom. According to PASCAL's law, the essence detected by its probe is the pressure value at the detection point.The pressure at the detection point on the side wall of the water tank has the following relationship with the height from the point to the liquid level:
H=P/ ρ g （1）
Where, H is the height (m) of the detection point from the liquid level;P is the pressure at the detection point (Pa);Rho is the density of the liquid (kg/m 3);G is the acceleration of gravity (m/s 2), which is 9.8.The current value can be obtained by converting the pressure value at the detection point
The actual liquid level value of the water tank can be obtained by the height of the detection point with the liquid level and the distance between the detection point and the bottom of the water tank (H 0).
Comparison of installation structure between input type and micro pressure level transmitter
2.2 feasibility analysis of micro pressure level transmitter
In order to verify the feasibility of the micro-pressure level transmitter, an experimental platform was set up. The signal acquisition principle of the experimental platform is shown in figure 2.With the method of side wall installation at the same height of installation accuracy of three different micro pressure liquid level transmitter (substance for pressure transmitter), the tank top mounted 1 piece of input type liquid level transmitter, acquisition of signal transmission to the control cabinet, all through the data from the program operation will pressure transmitter converted to the corresponding level value, also can use standard ruler measured the actual level of the tank.The corresponding liquid level value of each transmitter can be read and downloaded from the man-machine interface.
Set the system to collect one set of data every 1 s. Collect the liquid level value of the water in the tank in two states of rest (when the pump is still) and shock (when the pump is replenished), and randomly intercept the data of 30 s to draw a curve, as shown in figure 3-4.
The broken line of the liquid level in the static state of 1.25m water level is the broken line of the liquid level in the replenishment state of 1.25m water level
The following conclusions can be drawn from the curve in figure 3-4:
(1) it is completely feasible to measure liquid level with high accuracy and stability by means of micro-pressure level transmitter.
(2) under the static state of the water tank, the stability and accuracy of the detection values of the micro-pressure level transmitter and the input level transmitter are relatively high.
(3) in the case of water tank shock, the probe of the input level transmitter is not fixed and fluctuates greatly. The numerical fluctuation of the level transmitter with different precision and micro-pressure is obviously lower than that of the input level transmitter.
Considering the above data analysis results and practical application needs, the method of micro pressure level transmitter is adopted to measure the liquid level. Its advantages and disadvantages are as follows:
3.1 The advantages
(1) the measured liquid level value is less affected by water flow impact and has high accuracy and stability.
(2) adopt the installation method of the side wall of the water tank to facilitate maintenance and repair.
(3) the micro-pressure level transmitter with the same accuracy level has lower cost than the input level transmitter.
If the pressure transmitter is directly used to measure the liquid level, it is necessary to increase the calculation step from the pressure value to the liquid level height value in the system program, and consider the installation height value H 0;If the pressure transmitter with built-in conversion function (display liquid level height value) is used to measure the liquid level, no conversion step is required, but the installation height value H 0 still needs to be considered.
The application of micro-pressure level transmitter can fundamentally solve the problem of inaccuracy and maintenance difficulty caused by the large fluctuation of monitoring value of traditional input level transmitter.Although the monitoring value needs to be converted through the control program, for engineering units, the problem can be solved by unifying the installation size and standard.As the key component of the secondary water supply control system, the change of the monitoring mode is of great significance to ensure the stable operation of the secondary water supply system.In the second water supply increasingly intelligent today, to ensure the accuracy and stability of monitoring level data is an inevitable requirement, so the use of micro-pressure level transmitter to replace the input level transmitter is also an inevitable trend.