Level measurement in liquids is an important part of many industrial processes. It is used to monitor the fill level of containers with liquids to ensure that processes run efficiently and safely. There are a variety of methods for level measurement in liquids, which must be selected depending on the application and the specific requirements of the process.
Free radiator transit time measurement
Let’s start with the simplest principle:
The classic free-radiator transit time measurement – simple, (relatively) inexpensive and surprisingly accurate!
The transit time measurement of free jets is a simple and reliable method for determining the velocity of fluids. It is cost-effective and can be easily integrated into a wide range of applications.
Examples:
- Measuring the velocity of water in pipes
- Measuring the speed of air in aircraft engines
- Measuring the velocity of gases in chemical reactors
- Measuring the speed of currents in the atmosphere
The transit time of a pulse (sensor <-> water surface) is measured, calculated for the distance to the surface and converted into a volume. This can even be used to measure the flow of a free channel (e.g. 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 journey in a rod (or possibly a cable) and is therefore, to put it simply, better protected from the ambient conditions.
Areas of application:
- Level measurement in containers with liquids and solids
- Level measurement in pipes
- 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 pole! Ropes are easier to thread, but as soon as there is some movement in the tank, the ropes vibrate and an accurate measurement is not possible. A good mnemonic (with a wink) is “If the tank is stirred a lot, don’t use radar!”
The magnetostrictive float, a rod with a float, should also be mentioned in this context. Advantage: Absolutely reliable measurement result, the disadvantages: Poorer 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, up to 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 by means of optical measurement
This method is being used more and more frequently. The contents of the container are filmed and a camera analysis calculates the fill level. An example of optical 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 performed by using different methods, such as:
- The detection of certain features in the image, such as the edge of the liquid
- Measuring the height of the liquid in the image
- The use of image processing software to determine the fill level
Disadvantage: Condensate
If you can’t capture anything with the camera, you can’t analyze 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 product. 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 product and back is measured. The distance between the laser and the surface of the product is directly proportional to the height of the fill level in the container. This method is not yet widespread and is rarely used.
That was a brief overview of some level measurement principles. We are happy to assist you with the design, procurement, installation, commissioning, maintenance and calibration of the solution that best suits your application!