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The Art of the Stilling Well

Illustration of a stilling well

For both contact and non-contact methods of liquid level measurement, surface disruption can cause problems. Stilling wells are a great way to dampen disruptions, and provide a clean target surface for an accurate measurement.

Stilling wells are used most often with ultrasonic sensors, as they dampen waves and turbulence, and reduce foaming. However, level probes and some float switches can also benefit. When installed properly, they can be a great way to improve your sensor's performance, increase accuracy, and tame difficult applications.

Chris White, OEM Sales Manager at APG, offers up a few tips for artfully using stilling wells:

  1. Make sure to put a hole in the well above the blanking distance. This lets the fluid move up and down the tube to reflect the true level in the vessel. A stilling well with no hole to equalize pressure at the top will capture the air above the liquid and will push down on the surface of the liquid.
  2. The stilling well should be at least as long as the deepest measurement required, with sufficient openings below the deepest level measured to allow the fluid to equalize inside the tube. If the stilling tube touches the bottom of the tank, make sure there are holes large enough to allow the liquid to flow freely.
  3. Use stilling tubes with float level sensors, or level probes, to minimize bounce that might occur thanks to agitation of the vessel contents. This relatively inexpensive addition can dramatically improve measurement accuracy and consistency.
  4. If you are using a stilling well with an ultrasonic, the inside diameter should be smooth and without ridges or joints. These can cause artificial reflections that impair measurement accuracy.
  5. Make sure your ultrasonic sensor compensates for the temperature within the well. APG ultrasonics measure the temperature at the face of the ultrasonic transducer, inside the stilling well. If the temperature is measured outside of the stilling tube, then the compensation will be wrong and the measurement will be inaccurate.
  6. For applications where there is the potential for biologic growth on the stilling tube walls or for the accumulation of deposits, an alternate sensing method may be required.

Keep these tips in mind when the need for a stilling well arises. You'll be glad you did.

Do you have any success stories about stilling wells? Or nightmares instead? Tell us about what happened and how you fixed it.


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