This article presents a users guide to level-measurement technology for the water and wastewater treatment industry, whether that may be the instrumentation for open channel flow or level measurement.
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A number of applicable level-measurement technologies have been proven to be viable solutions for a wide variety of municipal and industrial water treatment applications. Due to the range of existing applications and the varying application conditions, no technology is ideal for all situations.
Point Level versus Continuous
Irrespective of the application, level-measurement instrumentation has two major classifications: point level and continuous level measurement.
- Point Level (On/Off) measurement denotes the presence or absence of level at a specific threshold (point) within a vessel. Point level switches are employed as low-level and pump protection alarms, high-level and spill prevention alarms, and for pump control.
- Continuous Level (Proportional) measurement denotes the level in a vessel over the complete span of measurement. Typically, these devices are used for both process control and inventory control and management.
Technology Choices
Due to the varying process conditions, the technologies used to measure level are affected differently. A brief description of the different technologies generally used in a water treatment facility is given below:
- RF Admittance/Capacitance uses a radio frequency signal and monitors for a variation in capacitance indicating the presence or absence of material or the amount of material in contact with the sensor, making it highly versatile and an ideal choice for a broad variety of materials and conditions for point- or continuous-level measurement.
- Radar makes use of either Frequency Modulated Continuous Wave (FMCW) or Pulsed Wave through-air transmission that enables accurate non-contact reading of reflected electromagnetic signals.
- Magnetostrictive employs an electric pulse from ferromagnetic wire to accurately detect the position of a float with embedded magnets. When the pulse intersects the magnetic field from the float, a second pulse is reflected back to an electric circuit that accurately determines the distance and therefore the level position.
- Conductivity Switch measures the drop-in resistance that emerges when a conductive liquid is brought into contact using two probes or a single probe and a vessel wall.
- Ultrasonic (Point Level) measurement electronically resonates a crystal at a fixed frequency to produce sound waves that surpass an air gap to a second crystal. Since the gap between the two crystals is filled with liquid, the second crystal starts resonating with the first.
- Ultrasonic (Continuous Level) measurement to produces an ultrasonic pulse using a transmitter and measures the time taken for a reflected signal to return to the transducer for determining the level of a liquid.
- Guided Wave Radar (GWR) makes use of a Time Domain Reflectometry (TDR) method by sending a highly focused electronic signal down a flexible cable wave guide or metallic rod. As the transmitted signal intersects the surface of a liquid, it reflects back along the cable or rod to determine the distance traveled. It is then possible to easily infer the level position.
- Hydrostatic measurement sinks a pressure transmitter with a sensing diaphragm and a sealed electronic circuitry that sends an analog signal proportional to the liquid level beyond the sensor.
- Float Switch depends on a low-density float attached to a vessel that is magnetically coupled to a limit switch. A variation in fluid level activates a switch by moving the float.
- Vibration/Tuning Fork is piezoelectrically energized and vibrates at a frequency of around 1200 Hz. When the fork is covered by the process media, the frequency shifts. The internal oscillator detects the frequency shift and converts it into a switching command.
Point-Level Solutions
The most versatile of the point level technologies is the advanced RF Admittance/Capacitance point level devices, particularly with process media that can coat the sensor. They offer excellent overfill/spill protection. They are easy to set up and there are no moving parts, making them almost maintenance free. Thanks to their sturdy design and circuitry, RF Admittance/Capacitance point level devices are a perfect solution for several water treatment applications.
Both tuning forks and ultrasonic gap switches offer reliable high- or low-level measurement in a range of applications. For non-coating conductive liquids, conductivity switches offer economical priced measurement, while float switches can be employed in a number of basic applications at a very low cost.
Continuous Level Solutions
Mechanical systems such as bubblers and floats need extensive maintenance and are less reliable and less accurate when compared to electronic systems. Hydrostatic systems provide greater reliability, are user-friendly, and can transmit data to another receiver for remote monitoring, recording, and control.
RF Admittance/Capacitance level is time-proven and one of the best available technologies for indication and control. RF technology innately offers the greatest accuracy and repeatability in interface measurements. Changes in the composition of upper and lower phases of a liquid do not have any significant effect on system accuracy. Recalibration is not needed.
RF Admittance technology offers one of the most preferred measurements for short span measurements. The RF technology becomes more appropriate with the decrease in the level of measurement span. In spans of just a few inches, RF systems can repeatedly yield accuracies of 1/32ths of an inch. RF has the additional benefit of not being restricted by “dead zones” that are innate with various established technologies that are typically chosen for measurement ranges above 5 feet.
Non-metallic tanks do not present any technical challenges for Magnetostrictive, Ultrasonic, Radar, Guided Wave Radar (GWR), and Hydrostatic Pressure technologies. The GWR strategy is ideal for vessels with internal obstructions and uses lower energy levels when compared to airborne radar technologies. Non-contact technologies, such as Ultrasonic and Radar, can have measurement ranges up to 130 feet.
For headroom limitations or long-range measurements, flexible sensors provide insertion lengths up to several hundred feet for RF Admittance and Hydrostatic Pressure technology products. Loop-powered GWR (TDR)-based products enable measurement ranges of up to 115 feet in chosen applications. Magnetostrictive technology enables an accuracy of 0.1% of measurement span in flexible sensor designs up to a maximum range of 70 feet.
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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 STC.
For more information on this source, please visit Ametek STC.