POINT VERSUS CONTINUOUS LEVEL MEASUREMENT
THE VARIOUS PROCESS CONDITIONS PLAY A MAJOR PART IN THE TECHNOLOGIES USED TO MEASURE LEVEL. DONALD KOENEMAN AND WILLIAM SHOLETTE TAKE A LOOK AT THE TECHNOLOGY AVAILABLE FOR THE WASTEWATER INDUSTRY, AND RECOMMEND PRACTICAL SOLUTIONS
However, because of the many different applications that exist in liquids measurement and the varying application conditions, no single technology is best suited in all cases.
The purpose of this article is to broaden end-users' understanding of the available technologies, whether they are selecting instrumentation for level, open channel flow, or sludge blanket monitoring.
Regardless of application, there are two major classifications of level-measurement instrumentation: point level and continuous level measurement.
Point level (on/off) measurement indicates the absence or presence of level at a certain threshold (point) within a vessel. Point-level switches are used as high-level and spill-prevention alarms, low-level and pump-protection alarms, and to turn pumps on and off.
Continuous level (proportional) measurement, on the other hand, indicates the level in a vessel over the full span of measurement. These devices typically are used for process control as well as inventory control and management.
The technologies used to measure level are affected differently by the varying process conditions. Here is a brief description of each of the different technologies commonly used in a wastewater facility:
- RF admittance uses a radio frequency signal. A change in RF admittance indicates either the presence or absence of material or how much material is in contact with the sensor, making it highly versatile and a good choice for a wide range of conditions and materials for point or continuous level measurement
- Radar uses frequency modulated continuous wave (FMCW) through-air transmission, which allows for accurate non-contact reading of reflected electromagnetic signals
- Magnetostrictive uses an electric pulse from ferro-magnetic wire to accurately detect the position of a float with embedded magnets. As the pulse intersects the magnetic field from the float, a second pulse is reflected back to an electric circuit that accurately reads the level
- Conductivity switch measures the drop in resistance that occurs when a conductive liquid is brought into contact with two probes or a probe and a vessel wall
- Ultrasonic (point level) measurement electronically resonates a crystal at a fixed frequency to generate sound waves that travel across an air gap to a second crystal. As liquid fills the gap between the two crystals, the second crystal begins to resonate with the first
- Ultrasonic (continuous level) measurement uses a transmitter to generate an ultrasonic pulse and measures the time it takes for a reflected signal to return to the transducer to determine the level of a liquid
- Time domain reflectometry (TDR) takes a highly focused electronic wave guided by a metallic rod or flexible cable to the surface of a liquid and reflects it back along the rod or cable to determine the level
- Hydrostatic pressure immerses a two-wire transmitter with a sensing diaphragm and a sealed electronic circuitry that transmits an analog signal proportional to the liquid level above the sensor
- Float switch relies on a low-density float mounted in a vessel that is magnetically coupled to a limit switch
- Vibration / tuning fork is piezo-electrically energised and vibrates at a frequency of about 1,200hZ. When the fork is covered in material, the frequency shifts - this is detected by the internal oscillator and converted into a switching command.
Both tuning forks and ultrasonic gap switches provide reliable high- or low-level measurement in a wide variety of liquids. For non-coating conductive liquids, conductivity switches provide economical priced measurement, while float switches can be used in many basic applications at cost-effective prices.
With continuous level solutions, mechanical systems such as floats and bubblers require extensive maintenance and are less reliable and accurate than electronic systems. Hydrostatic systems afford greater reliability, are simple to use and can transmit data to another receiver for remote monitoring, recording and control.
RF level is the time proven, best available technology for indication and control.
RF technology inherently provides the greatest accuracy and repeatability in interface measurements. Variations in the make-up of upper and lower phases of a liquid have no appreciable effect on system accuracy. Recalibration is not required.
For short span measurements, RF admittance technology provides one of the most preferred measurements. As the level of measurement span decreases, the more appropriate RF technology becomes.
In spans of only a few inches, RF systems can repeatedly produce accuracies of 0.8mm.
RF has the added benefit of not being limited by dead zones that are inherent with many popular technologies that are typically selected for measurement ranges greater than 1.5m.
Non-metallic tanks pose no technical problems for ultrasonic, magnetostrictive, hydrostatic pressure, radar and TDR technologies. The TDR approach is suitable
for vessels with internal obstructions and uses lower energy levels than airborne radar technologies. Non-contact technologies, such as radar and ultrasonic can have measurement ranges up to 39m.
For long-range measurements or headroom limitations, flexible sensors offer insertion lengths up to several hundred feet for hydrostatic pressure and RF admittance technology products. Loop-powered TDR-based products allow measurement ranges up to about 35m in selected applications.
Magnetostrictive technology allows accuracy of 0.1% of measurement span in flexible sensor designs up to a maximum range of 12m. n
Donald Koeneman and William Sholette are with Ametek Drexelbrook
T: +1 215 674 1234