Você realmente entende o princípio da medição do nível de líquidos usando transmissores de pressão?

Do you really understand the principle of liquid level measurement using pressure transmitters?

I. Basic Principles

DP transmitters are a commonly used liquid level measurement technology and are easy to understand. They are not only simple and easy to use and install, but also can be used in different application scenarios and various working conditions. If the liquid level in an open or vented container is to be measured, a pressure (GP) or differential pressure (DP) transmitter should be used. If it is a storage tank or pressure tank, a differential pressure transmitter must be used to compensate for the container pressure. In addition to basic liquid level measurement, DP transmitter can also be set for density measurement and interface position measurement.

Liquid Level Measurement in Open Containers

In an open container, the liquid level is calculated by measuring the static pressure of the liquid. Due to its own weight, the liquid column exerts a force on the base of the column. This force, known as the liquid static pressure or head, can be measured in pressure units. The liquid static pressure is calculated using the following formula: Liquid static pressure = Liquid column height x Specific gravity. If the liquid level (height) changes, the liquid static pressure will change proportionally. Therefore, a simple method to measure the liquid level is to install a pressure gauge at the lowest point of the container. By rearranging the above formula, the height can be calculated, and the liquid level above the measurement point can be determined from the liquid static pressure. If the pressure is not in units of liquid height, a conversion is required.

Liquid Level Measurement in Storage Tanks

If it is a pressure vessel, a GP transmitter is not suitable. When a GP transmitter detects a pressure change, it cannot distinguish whether it is caused by a change in liquid level or a change in the pressure of the container. To solve this problem, differential pressure measurement should be used in the application of closed containers to compensate for the container pressure. When using differential pressure measurement, since the overall container pressure change affects both the upper and lower pressure tapping valves, the pressure effect is cancelled out.

The pressure tap near the bottom of the container measures the sum of the hydrostatic pressure of the liquid and the pressure in the vapor space. The low-pressure pressure tap connected near the top of the container only reads the pressure in the vapor space. The liquid level is determined by the pressure difference (differential pressure) between the two pressure taps. Differential pressure measurement can be performed using a differential pressure transmitter with impulse lines or capillaries and seals, or by using two gauge pressure or two absolute AT3051 series transmitters to calculate the differential pressure in an electronic remote sensor configuration. Liquid level = Differential pressure / Specific gravity

II. Advantages

Generally speaking, pressure transmitters are economical, easy to use, and simple to understand. Moreover, These transmitters can be used in most storage tanks and liquids, including slurry applications. They have a wide range of applicable pressures and temperatures and are suitable for applications with foam and turbulence.

III. Limitations

The accuracy of liquid level measurement by AT3051 series pressure transmitters is affected by changes in fluid density. Special attention should be paid when used in thick, corrosive, or dirty liquids. In addition, as the concentration increases, some liquids (such as papermaking raw materials) will gradually solidify. Pressure transmitters cannot accurately measure this solidified state.