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Application Challenge: Level Measurement on Vortices

One of the more difficult and expensive level applications is a vortex in an agitated vessel. There are a variety of issues, and you need to ask yourself the right questions:

Are you measuring level as a safety loop, providing a layer of protection against an overflow?

Are you monitoring inventory and therefore interested in the mass in the vessel?

How strong of a vortex are you dealing with?

Can you momentarily stop the agitation and allow the liquid to settle?

The main difficulty with a vortex is that there is not a true level. What you’re measuring is either the highest point of the liquid, or trying to guess where the liquid would be when not in motion.

For non-contact sensors, such as ultrasonics - the turbulence of mixing can create an inconsistent surface and a poor reflective target. Foam can also form, which is difficult for ultrasonic sensors.

Photo Credit: kaŹstn

A stilling well can sometimes be effective in avoiding a roiling surface or foam. However, the movement of the liquid across the bottom of the well introduces inaccuracies. For example, a high velocity might make the stilling well behave like a pitot tube and the level would be artificially high.

Hydrostatic (submersible) pressure sensors measure the level based on the height of liquid and its density.  Therefore, denser fluids will read higher than low-density fluids. Agitated liquid can change density, and a fast moving vortex can push the liquid to the outer wall of the vessel. The question is, does the pressure at the bottom truly indicate the level?

Stem and float sensors can suffer from similar inaccuracies. A float sensor usually cannot tolerate an agitator unless the measuring range is above the agitator (wouldn’t want to get the stem bent around the agitator). In addition, the turbulence from the agitation will cause unsettled readings. The stilling well might again provide relative calm, and protect from buffeting, but the liquid level will still be affected by the vortex.

In agitated vessels, where the goal is level measurement, these methods allow for some precision:

  1. One could choose to stop agitating intermittently, allowing the liquid to settle enough to provide a measurement. This is the least expensive option, but may not be realistic.
  2. One could choose to meter material in and out of the vessel and do the math. This is a good solution, but is more expensive.
  3. One could mount the whole vessel on load cells and tare the empty vessel to determine the mass of liquid in the vessel. This is the most expensive of the three, and requires lifting up the vessel and installing the proper mounts.

Where safety is the main concern, the use of point level solutions or flow switches in overflow lines are possibilities for additional layers of protection.

But as with all sensing applications – clarify why you are taking the measurement!

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