Showing posts with label level detection. Show all posts
Showing posts with label level detection. Show all posts

Wednesday, September 30, 2015

Ultrasonic Level Measurement


Ultrasonic level instruments measure the distance from the transmitter (located at some high point) to the surface of a process material located farther below using reflected sound waves. The frequency of these waves extend beyond the range of human hearing, which is why they are called ultrasonic. The time-of-flight for a sound pulse indicates this distance, and is interpreted by the transmitter electronics as process level. These transmitters may output a signal corresponding either to the fullness of the vessel (fillage) or the amount of empty space remaining at the top of a vessel (ullage).

Ullage is the “natural” mode of measurement for this sort of level instrument, because the sound wave’s time-of-flight is a direct function of how much empty space exists between the liquid surface and the top of the vessel. Total tank height will always be the sum of fillage and ullage, though. If the ultrasonic level transmitter is programmed with the vessel’s total height, it may calculate fillage via simple subtraction:

Fillage = Total height − Ullage

If a sound wave encounters a sudden change in material density, some of that wave’s energy will be reflected in the form of another wave in the opposite direction. In other words, the sound wave will “echo” when it reaches a discontinuity in density20. This is the basis of all ultrasonic ranging devices. Thus, in order for an ultrasonic level transmitter to function reliably, the difference in densities at the interface between liquid and gas must be large. Distinct interfaces of liquid and gas almost always exhibit huge differences in density, and so are relatively easy to detect using ultrasonic waves. Liquids with a heavy layer of foam floating on top are more difficult, since the foam is less dense than the liquid, but considerably denser than the gas above.

A weak echo will be generated at the interface of foam and gas, and another generated at the interface of liquid and foam, with the foam acting to scatter and dissipate much of the second echo’s energy.

The instrument itself consists of an electronics module containing all the power, computation, and signal processing circuits; plus an ultrasonic transducer to send and receive the sound waves. This transducer is typically piezoelectric in nature, being the equivalent of a very high-frequency audio speaker.

The ISA-standard designations for each component would be “LT” (level transmitter) for the electronics module and “LE” (level element) for the transducer, respectively. Even though we call the device responsible for transmitting and receiving the sound waves a transducer (in the scientific sense of the word), its function as a process instrument is to be the primary sensing element for the level measurement system, and therefore it is more properly designated a “level element” (LE).

This photograph shows a typical installation for an ultrasonic level-sensing element (LE), here sensing the level of wastewater in an open channel:


If the ultrasonic transducer is rugged enough, and the process vessel sufficiently free of sludge and other sound-damping materials accumulating at the vessel bottom, the transducer may be mounted at the bottom of the vessel, bouncing sound waves off the liquid surface through the liquid itself rather than through the vapor space. As stated previously, any significant difference in material densities is sufficient to reflect a sound wave. This being the case, it shouldn’t matter which material the incident sound wave propagates through first:

This arrangement makes fillage the natural measurement, and ullage a derived measurement (calculated by subtraction from total vessel height).

Ullage = Total height − Fillage

As mentioned previously, the calibration of an ultrasonic level transmitter depends on the speed of sound through the medium between the transducer and the interface. For top-mounted transducers, this is the speed of sound through the air (or vapor) over the liquid, since this is the medium through which the incident and reflected wave travel time is measured. For bottom-mounted transducers, this is the speed of sound through the liquid. In either case, to ensure good accuracy, one must make sure the speed of sound through the “timed” travel path remains reasonably constant (or else compensate for changes in the speed of sound through that medium by use of temperature or pressure measurements and a compensating algorithm).

For more information, check out this online document or visit Ives Equipment at www.ivesequipment.com.


(Attribution to Tony R. Kuphaldt under Creative Commons Attribution 3.0 United States License)

Wednesday, August 19, 2015

Level Phase Split Detection and Measurement

Phase Level Detection
Phase level detection.
Need a system for continuous interface measurement? This system will monitor the emission phase and detect when phase A/B occurs, avoiding any flow of the emulsion into the incorrect area. When phases are separated, the system allows for tight control, which increases efficiency of separation of liquids A&B.

This system provides the ability to remotely view a process that may not normally be watched. Multiple viewing stations may be linked to the system output so various departments may monitor a process. Customers may purchase video monitors, amplifiers or screen splitters to enhance the system.

 
  • Ethernet systems allow the additional functionality of being able to remotely view through a Gigabit network system. Users can have access to live system images from their office networked computer. 
  • Software is available for customers that require additional functionality over simple viewing of a live image. Liquid level, color of different phases, and visual verification. 
System verifies the color of fluid in phase split and sends an output signal locating specific points of the interface. All measurements can be recorded and archived for a historical record. 


Monitor phase split in batch mode on organic droplets for increased product recovery. As the droplets appear the system warns of organic phase and the upcoming emulsion. The amount of early droplets and their retraction time will indicate the completeness of the separation. Once the split is identified and stopped, the operator has a visual verification from the video monitor. The color can then be analyzed to ensure there is no inversion.

For more information, contact:

Ives Equipment
601 Croton Road
King of Prussia, PA 19406
(877) 768-1600
www.ivesequipment.com