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Ultrasonic Level Measurement
(Part 3 of 3)

By David W. Spitzer

E-Zine December 2011

Click here to review Part 1
Click here to review Part 2

On the other hand, the sensor on the left emits and receives ultrasonic energy from the same surface on the bottom of the sensor. This design not only reduces the tendency to accumulate material by eliminating the reflector and its potential problems, but it also tends to vibrate and remove material from the receiver surface when ultrasonic energy is emitted.

The ultrasonic energy travels from the sensor to the material and back. The energy can be attenuated in transit due a number of phenomena, including acoustical attenuation caused by nature of the vapors in the energy path, and dust/dirt in the energy path to/from the material. Note that ultrasonic level measurement can often be performed accurately and reliably even when the material cannot be seen visually. The presence of dust/dirt in the energy path can be temporary, such as when filling occurs and causes a dust cloud to form in the energy path.

The material itself can cause the intensity of the reflected ultrasonic energy to degrade when the material exhibits poor reflective qualities, such as when contaminants on the surface of the material cause the ultrasonic energy to reflect poorly.

In addition, accuracy can be degraded based upon the surface on which the ultrasonic energy is reflected. For example, ultrasonic level measurements typically measure the top of a layer of foam by reflecting off the top of the foam. However, the characteristics of the foam, such as its composition, density, bubble size, and the like, can cause the foam to absorb ultrasonic energy instead of reflecting it. In this application, not only will the ultrasonic level measurement system not measure the (desired) liquid level, but varying amounts and consistency of foam can cause the measurement to become erratic.

Excerpted from The Consumer Guide to Non-Contact Level Gauges

ISSN 1538-5280

Spitzer and Boyes, LLC
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