This article introduces a users guide to level-measurement technology for the water and wastewater treatment industry, with regard to the instrumentation for open channel flow or level measurement.
It has been proven that several applicable level measurement technologies are viable solutions for a broad range of industrial and municipal water treatment applications. As a result of the variety of existing applications and the changing application conditions, there is no technology that is ideal for all conditions.
Point Level Versus Continuous
Regardless of the application, level-measurement instrumentation can be classified into two major types: point-level and continuous-level measurement.
- Point-level (On/Off) measurement indicates the presence or absence of level at a particular threshold (point) inside a vessel. Point-level switches are used as high-level and spill prevention alarms, low-level and pump protection alarms, as well as for pump control.
- Continuous-level (Proportional) measurement indicates the level in a vessel over the entire span of measurement. Essentially, these devices are employed for both process control and inventory control and management.
As a result of the changing process conditions, the technologies employed for level measurement are influenced differently. Following is a brief description of the distinct technologies usually used in a water treatment facility:
- RF Admittance/Capacitance uses a radio frequency signal and detects for a change in capacitance suggesting the presence or absence of material or the amount of material in contact with the sensor, rendering it highly versatile and a perfect choice for a wide range of materials and conditions for point- or continuous-level measurement.
- Radar employs either Frequency Modulated Continuous Wave (FMCW) or Pulsed Wave through-air transmission that allows precise non-contact reading of reflected electromagnetic signals.
- Magnetostrictive uses an electric pulse from ferromagnetic wire for the accurate detection of the position of a float with embedded magnets. When the magnetic field from the float is intersected by the pulse, a second pulse is reflected back to an electric circuit that determines the exact distance and hence the level position.
- Conductivity Switch measures the drop-in resistance that occurs when a conductive liquid is brought into contact with the help of two probes or a single probe and a vessel wall.
- Ultrasonic (Point Level) measurement involves electronically resonating a crystal at a preset frequency to generate sound waves that pass through an air gap toward a second crystal. As the gap between the two crystals is filled with liquid, the second crystal begins resonating along with the first crystal.
- Ultrasonic (Continuous Level) measurement involves producing an ultrasonic pulse with the help of a transmitter and measuring the time taken for a reflected signal to return to the transducer to determine the level of a liquid.
- Guided Wave Radar (GWR) employs a Time Domain Reflectometry (TDR) technique that involves sending a highly focused electronic signal through a flexible cable waveguide or metallic rod. When the surface of a liquid is intersected by the transmitted signal, it is reflected back along the cable or rod to ascertain the distance traveled. Then, it would be feasible to easily deduce the level position.
- Hydrostatic measurement involves sinking a pressure transmitter with a sensing diaphragm and a sealed electronic circuitry that transmits an analog signal proportional to the liquid level beyond the sensor.
- Float Switch is based on a low-density float fixed to a vessel that is magnetically coupled to a limit switch. A change in fluid level results in activation of a switch by moving the float.
- Vibration/Tuning Fork is piezoelectrically energized and vibrates at a frequency of about 1200 Hz. There is a shift in the frequency when the process media covers the fork. This shift in frequency is detected by the internal oscillator, which converts it into a switching command.
The advanced RF Admittance/Capacitance point-level devices are the most versatile point-level technologies, specifically with process media with the ability to coat the sensor. They provide exceptional overfill/spill protection. They can be easily set up and have no moving parts, rendering them nearly maintenance-free. The robust design and circuitry of the RF Admittance/Capacitance point-level devices make them an ideal solution for various water-treatment applications.
Tuning forks as well as ultrasonic gap switches enable reliable high- or low-level measurement in a wide variety of applications. In the case of non-coating conductive liquids, conductivity switches enable economical measurement, and float switches can be used for various basic applications at an extremely low cost.
Mechanical systems like floats and bubblers necessitate extensive maintenance and are less accurate and less reliable than electronic systems. Hydrostatic systems are user-friendly, offer greater reliability, and have the ability to transmit data to another receiver for remote monitoring, recording, and control.
RF Admittance/Capacitance level is one of the best available technologies for indication and control. RF technology intrinsically offers the maximum accuracy and repeatability during interface measurements. There is no significant impact of the variations in the composition of upper and lower phases of a liquid on the accuracy of the system. The need for recalibration is eliminated.
RF Admittance technology enables one of the most desirable measurements for short span measurements. The RF technology turns more suitable when there is a decrease in the level of measurement span. In the case of spans of only a few inches, RF systems have the ability to repeatedly achieve accuracies of 1/32ths of an inch. One more advantage of RF is that it is not limited by “dead zones” that are intrinsic to different established technologies that are usually selected for measurement ranges of over 5 feet.
No technical challenges are presented by non-metallic tanks for Magnetostrictive, Ultrasonic, Radar, Guided Wave Radar (GWR), and Hydrostatic Pressure technologies. The GWR approach is perfect for vessels with internal obstructions and employs lower energy levels than airborne radar technologies. The measurement ranges of non-contact technologies, like Ultrasonic and Radar, can be up to 130 feet.
For long-range measurements or headroom limitations, flexible sensors offer insertion lengths of up to several hundred feet for Hydrostatic Pressure and RF Admittance technology products. Loop-powered GWR (TDR)-based products allow measurement ranges of up to 115 feet in selective applications. Magnetostrictive technology allows an accuracy of 0.1% of measurement span in flexible sensor designs up to a maximum range of 70 feet.
Typical Applications — Point Level
- LS-1 Chemical Storage: RF Admittance or Vibration (Tuning Fork)
- LS-2 Chemical Slurries: RF Admittance or Vibration (Tuning Fork)
- LS-3 Pump Control/Protection: RF Admittance/Capacitance
- LS-4 Mixing Tanks: RF Admittance/Capacitance or Vibration (Tuning Fork)
Typical Applications — Continuous Level
- LT-1 Clarity Monitor: Ultrasonic
- LT-2 Water Filtration: Magnetostrictive or RF Admittance/Capacitance
- LT-3 Chemical Slurry Storage: RF Admittance/Capacitance or Radar
- LT-4 Water Wells: Hydrostatic Pressure or RF Admittance/Capacitance
- LT-5 Mixing Tanks: Ultrasonic, RF Admittance/Capacitance or Radar
- LT6 Chemical Slurries: Radar or RF Admittance/Capacitance
This information has been sourced, reviewed and adapted from materials provided by Ametek Factory Automation.
For more information on this source, please visit Ametek Factory Automation.