Commonly used liquid level measurement methods

1. Glass tube method, glass plate method, two-color water level method, manual gauge method Glass tube method: This method works on the principle of connecting device, and the liquid level is directly read from the indicating scale. Two-color water level meter method: This method uses the optical principle to make the water display green and the water vapor to display red, thereby indicating the water level. Manual gauge method: This method is used to measure the level of the oil tank. When measuring, the surveyor puts the dipstick into the oil, and lifts the dipstick when the dipstick is in contact with the bottom of the tank. According to the oil traces on the dipstick, read the oil level height; determine the height of the water cushion according to the change of the color of the water test paste at the end of the dipstick, so as to determine the oil height and water height. The above four methods are all manual measurement methods, which have the advantages of simple measurement, high reliability, intuitiveness and low cost. 2. Air blowing method, differential pressure method, HTG method Air blowing method: the working principle of this method: since the pressure in the blowing pipe is approximately equal to the static pressure of the liquid column, P=ρgH differential pressure method: the working principle of this method: for Open container or atmospheric pressure container, valve 1 and gas phase pressure piping can be omitted. The relationship between the pressure difference and the liquid level is ΔP=P2-P1=ρgH In the formula: ΔP-the pressure difference between the positive and negative pressure chambers of the transmitter; P2, P1-the pressure of the impulse pipe; H-liquid level. The differential pressure transmitter converts the pressure difference into a DC signal of 4 to 20 mA. If the corresponding output signal is greater than or less than 4 mA when the pressure is at the lower limit of the measurement range, it is necessary to use zero-point migration techniques such as adjusting the migration spring to make it equal to 4 mA. HTG method: This method is used in the differential pressure liquid level measurement of oil tanks, as shown in Figure 2-3. In the figure: P1, P2, P3 - high precision pressure sensor; RTD - temperature detection element; HIU - interface unit. P1 is at the shell near the bottom of the tank, P2 is 8 feet higher than P1, and P3 is at the shell near the top of the tank. For atmospheric oil tanks, the pressure sensor P3 can be omitted. Assuming that the pressures measured by the pressure sensors P1, P2, and P3 are respectively p1, p2, and p3, the RTD is used to measure the temperature of the oil to perform temperature compensation on the measured values. The price of HTG measurement system is lower, but the accuracy of liquid level measurement is lower, and the installation must be opened in the tank wall. The above three methods all use the pressure difference of the liquid to measure the liquid level. 3. Float method, buoy method, float method, servo method, sinking method float method: This method uses a float as a liquid level measuring element, and drives a display device such as a code disc or a code belt, or connects an electronic transmitter for remote control. Distance transmission measurement signal. Float method: This method uses a magnetic float with a hole in the middle as a liquid level sensing element. The stainless steel sleeve passes through the middle hole of the buoy and is fixed between the tank top and the tank bottom. The change of liquid level drives the hollow magnetic float (built-in permanent magnet) to move up and down along the casing, and attracts the magnet in the casing to move up and down along the inner wall of the casing. The secondary instrument calculates the liquid level according to the movement of the magnet. Floating ball method: This method works on the principle of lever. The floating ball rotates around the rotating shaft with the change of the liquid level, drives the pointer on the rotating shaft to rotate, and balances with the balance weight at the other end of the lever, and at the same time indicates the liquid level value on the dial. There are two types of floating ball method: inner floating ball type and outer floating ball type, as shown in Figure 3-2. The float method is mainly used to measure the liquid level with high temperature and high viscosity, but the range is small. Servo method: This method adopts the wave integral circuit to eliminate jitter, prolong life and improve the accuracy of liquid level measurement. The measurement accuracy of modern servo liquid level meters is high, which has reached the accuracy of less than 1 mm in the range of 40 m, and generally has the function of measuring density distribution and average density. Sinking method: The position of the sinking cylinder changes with the change of the liquid level, but the amount of change is not equal to that of the liquid level. The above five methods all use the principle of buoyancy to work. 4. Capacitance method, resistance method, inductance method Capacitance method: Capacitance method for measuring non-conductive liquids. The capacitor is composed of two concentric cylindrical plates. Capacitive liquid level meters are low in price, easy to install, and can be applied In high temperature and high pressure occasions. However, the measurement repeatability of the capacitance liquid level meter is low, requiring regular maintenance and re-calibration, and the working life is not very long.
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