Design/Selection Of A Level Measurement In Liquids

Liquid level measurement is an important part of many industrial processes. It is used to monitor the level of liquid in containers to ensure that processes run efficiently and safely. There are a variety of liquid level measurement methods, which must be selected according to the application and the specific requirements of the process.

Free-running time measurement

Let's start with the simplest principle:

the classic free jet transit time measurement – simple, relatively inexpensive and surprisingly accurate!

Measuring the transit time of free jets is a simple and reliable method for determining the speed of fluids. It is cost-effective and can be easily integrated into a wide range of applications.

Examples:

• measuring the speed of water in pipes
  
• Measuring the speed of air in aircraft engines
• Measuring the speed of gases in chemical reactors
• Measuring the speed of currents in the atmosphere

The time of flight of a pulse (sensor <-> water surface) is measured, calculated based on the distance to the surface and converted into a volume. This can even be used to measure the flow rate of an open channel (e.g. a stream).

But: “If you have steam and foam, you can't trust a free jet!”

The guided radar

This leads us on to guided radar: the principle is the same, but the pulse is sent on its way in a rod (or cable, if necessary) and is therefore, to put it simply, better protected by the ambient conditions.

Fields of application:

• Level measurement in containers with liquids and solids
• Level measurement in pipelines
• Level measurement in silos and warehouses
  
• Level measurement in tanks and boilers
• Level measurement in process containers

The obvious disadvantage: If the tank is five meters high, we need a five-meter-long rod! Ropes are easier to thread, but as soon as there is some movement in the tank, the ropes swing and an accurate measurement is not possible. A good rule of thumb (with a wink) is “If the tank is stirred vigorously, don't use a radar!”

In this context, the magnetostrictive floats, a rod with a float, must also be mentioned. Advantage: Absolutely reliable measurement result, disadvantages: Poor cleanability and if the rod is bent, it must be disposed of.

A brief digression on capacitive measurements

It is the preferred measurement method in many industries and works by measuring the capacitance between a sensor and the container wall. The capacitance changes when the fill level changes, as the liquid has a different dielectric constant than the container wall. The more liquid touches the rod, the higher the capacitance (like a changing electrolytic capacitor) Important: the liquid to be measured must still have a residual conductance.

Hard cut, on to the differential pressure

This method measures the pressure difference between two points. We are in the fortunate position that the Earth’s gravitational force is relatively constant and gravitational waves currently only play a role in the Nobel Prize. Hence the very simple measurement: pressure at the bottom minus pressure at the top is the fill level. 1bar is then quasi the 9.81m. Disadvantage: “If it’s cold at the top and warm at the bottom, you’ll be poor with differential pressure.” Pressure measurements must therefore be temperature-decoupled or compensated, depending on the measuring principle.

Differential pressure also includes hydrostatic measurements. The hydrostatic measurement method for level measurement is a method for determining the level of liquids in containers by measuring the pressure exerted by the liquid on the bottom of the container. To determine the fill level using the hydrostatic measuring method, a pressure sensor is immersed in the tank. The pressure sensor measures the pressure at the bottom of the container. The pressure is directly proportional to the height of the liquid in the tank. The upper measurement is not used, but the tank must be operated at atmospheric pressure.

Level measurements with the aid of load cells

Here, the fill level in containers is determined by measuring the force that the liquid or solid exerts on the load cell. Each cell contains a strain gauge that changes its electrical resistance the more load is placed on it. The force is directly proportional to the mass of the medium in the container. Of course, this only works well if the tank is properly “decoupled”. An example from everyday life: if I put my head on the washstand when I weigh myself, my BMI looks better, but unfortunately it doesn’t change the reality!

Level measurement using optical measurement

This method is being used more and more. The container contents are filmed and a camera analysis calculates the fill level. An example of an optical fill level measurement with a camera is the use of a 2D camera to capture an image of the container. The image is analyzed by a computer to determine the fill level. The analysis of the image can be carried out using a variety of methods, such as:

• detecting certain features in the image, such as the edge of the liquid
• measuring the height of the liquid in the image
• using image processing software to determine the fill level

Disadvantage: condensate

If the camera cannot detect anything, it cannot evaluate it.

Radiometric measurements

The absorption of gamma radiation by the medium is measured here. The gamma radiation is emitted by a radioactive source and passed through the medium into the sensor. The sensor measures the intensity of the radiation that reaches the sensor. The intensity of the radiation is directly proportional to the height of the medium in the container.

An example of a radiometric level measurement is the use of a gamma probe to determine the level of a container with a liquid substance. The gamma probe is immersed in the container and the gamma radiation is guided through the medium into the probe. The probe measures the intensity of the radiation that reaches the sensor. The intensity of the radiation is directly proportional to the height of the liquid in the container.

Finally, the laser measurement

Distance between the laser and the surface of the filling material. The laser beam is directed into the container and the time it takes for the laser beam to travel from the source to the surface of the filling material and back is measured. The distance between the laser and the surface of the filling material is directly proportional to the height of the filling level in the container. This method is not yet widely used and is rarely applied.

That was a brief overview of some level measurement principles. We are happy to assist in the design, procurement, installation, commissioning, maintenance and calibration of the solution that best suits your applications!