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Fill level measurement – what you need to know

Guest post by Christian Halper – medon

The methods for level monitoring at a glance

Fill level measuring or level monitoring is an important procedure in the process industry, food technology as well as in the area of water & waste. Various methods are used by which the fill level of a medium in a container is captured and converted into an electronic signal. The fill level signal is then either displayed directly on-site or integrated into a process control system to switch a relay.

But what methods are there and what differentiates them? Christian Halper, CEO of TELE partner medon, summarises the most important level detection procedures:

The most important processes and their strengths/weaknesses

Float

They are the epitome of level detection. Usually a basic design for a simple YES/NO determination. Versions with several consecutive switches for measuring with a resolution of less than 5 mm are possible.

  • Application: Direct MIN/MAX level switch, usually in water.
  • Advantage: Inexpensive.
  • Disadvantage: Susceptible to contamination, the specific weight of the liquid must be observed.

 

Optical

Old and well-proven method for level detection. This is usually used with bypass lines and/or together with floats.

  • Application: Checking of oil, food and chemical tanks.
  • Advantage: Inexpensive and virtually error-free.
  • Disadvantage: Sometimes difficult to connect to a control system.

 

Scale

One of the oldest measuring methods. Basically simple and easy to understand. The problem is in the details, as clean decoupling between conveying system and measuring container is absolutely essential.

  • Application: Meter, usually for foodstuffs, chemicals and granulates.
  • Advantage: Clear quantification.
  • Disadvantage: Complex design required due to the weight of the pipework.

 

Pressure

Every liquid has a specific weight. If this is known, then the fill level is proportional to the pressure. Basically, there are two types of hydrostatic level detection. 1. The pressure transmitter is attached to the lowest possible point of a container. 2. The submersible probe is fitted with a correspondingly long cable and is simply “thrown” into the medium to be measured. Both systems require pressure equalisation, so that the ambient pressure is compensated for. Absolute pressure cells are variants of this

  • Application: Level measurement for water, waste water and chemicals.
  • Advantage: Relatively inexpensive, good measurement stability.
  • Disadvantage: Abrasive media can destroy the membranes, not necessarily frost-proof.

 

Differential pressure

Basically same functionality as the pressure cell. Here, the ambient pressure does not play a role. This means that this measuring system can also be used in closed pressure vessels.

  • Application: level detection or density measurement, usually for foodstuffs and chemicals.
  • Advantage: High precision, can also be used for calculation by weight.
  • Disadvantage: Abrasive media can destroy the membranes, not necessarily frost-proof.

 

Ultrasound

Here, the duration of the airborne sound until it reaches the reflective boundary is measured. If the container geometry is known, the level or the content can be directly displayed. It is also possible to detect the level through the pipe wall by using ultrasound.

  • Application: Water, waste water and chemicals with low vapour formation.
  • Advantage: No media contact, precise.
  • Disadvantage: Sensitive to turbulent surfaces, vapours and foam which can alter or prevent the measurement.

 

Radar

Similar functionality as ultrasound, but a higher frequency is used.

  • Application: Water, waste water and chemicals with low vapour formation.
  • Advantage: No media contact, precise, less sensitive to foam formation.
  • Disadvantage: Difficulty detecting empty state, expensive.

 

X-rays

The principle is comparable to ultrasound, but it operates at a much higher frequency and can penetrate walls. A very controversial measurement method for precisely this reason due to possible harmful health effects.

  • Applications: Chemicals through metal tanks.
  • Advantage: No media contact, precise, suitable for non-Newtonian fluids.
  • Disadvantage: Expensive, complex safety rules.

 

Vibration

A “tuning fork” is set in vibration and the contact with the medium or air causes a frequency change. This is sufficient to receive a full or empty signal.

  • Application: Liquids and granulates.
  • Advantage: Can detect nearly every medium.
  • Disadvantage: Danger of mechanical damage.

 

Capacitive

An electrical oscillating circuit is generated, where the counter electrode is the medium to be detected. This means that limit levels but also analogue measurements can be attained depending on the design of the probe.

  • Applications: Mostly foodstuff and chemical applications.
  • Advantage: Analogue output corresponding to the measured level, not dependent on position and pressure.
  • Disadvantage: In some instances, prone to contamination and mechanical loads.

 

Conductive

Two or more rods are put into a conductive liquid and fed with alternating current. If current flows, the level has been reached. A very simple but also cost-effective solution.

  • Application: Electrically conductive liquids.
  • Advantage: Inexpensive and precise switching points.
  • Disadvantage: Prone to contamination, only possible in electrically conductive liquids.

 

The author

Ing. Christian Halper is the CEO of medon, one of Austria’s leading fill level measurement specialists and a partner of TELE-Haase.

TELE has various offerings for conductive fill level measurements in its product range. If you have unique requirements, our experts will gladly develop industry-specific custom solutions together with you.