If your head spins after reading how amazing one level mea­surement method is versus another, it may help you to under­stand the key attributes of the technologies and how they apply to specific processes.

In monitoring the level of clean or not-so-clean water — be it in filter beds, wet wells, lift sta­tions, water towers, chemical tanks or open channels — ultra­sonic technology emerges on top, followed by radar.

It’s true that some level measurement challenges can be solved by more than one technology, and yet no single technology can handle every process. Ultrasonic level technology has been proven for decades in the water and wastewater industry. Even though both ultrasonic and radar technologies have the required muscle, ultrasonic instruments also have the intelligence to meet comprehensive needs.

It is not enough to simply compare the attributes of ultrasonic and radar devices; it is necessary to consider their features in the context of the application. For example, if choosing a device to monitor the level in a lift station and control pump operation, it won’t help that radar transmitters can function in a vacuum and withstand high pressures and temperatures; none of these conditions exist in lift stations. 

To gain clarity, it’s best to evaluate whether a newer technology offers tangible benefits. If the current level measurement solution is reliable, and considering that long-term performance should be the goal, why make major changes due to factors like ease of commissioning or cost? And if the price of a newer model is attractive, consider what it might lack in functionality. A lower-priced device may be limited to simpler functions.

Similarities and differences

Ultrasonic and radar are noncontacting technologies: They measure through air using the time-of-flight principle. A contacting technology is equivalent to the oil dipstick in a car. Capacitance and guided-wave radar are contacting level technologies.

A significant benefit of noncontacting instruments is that they are low maintenance. There’s seldom a need to clean the transmitter section (the antenna, emitter, lens or transducer face), as the water level usually remains below the sensor by 10 inches or more. In a wet well or lift station, the clearance between the highest level and the sensor is usually several feet.

In certain applications, however, a buildup can occur. Water treatment facilities must deal with overfill­ing from storms or floods, and in such cases, the measuring devices can be submerged. In addition, condensation can form on the sensing area with large tempera­ture swings, as from warm days and cold nights. In choosing, it is important to understand how ultrasonic and radar technologies deal with these conditions.

Under submergence, the best-case scenario for a level device is to consistently report a maxed-out level measurement, indicating the abnormality. However, many instruments yield random values when submerged and thus aren’t reliable.

With recent advances, some radar level transmitters fare better under these conditions, but the issue of buildup remains. A slight nonconductive buildup is not a problem, but a heavier buildup can trap more con­ductive debris, leading to signal degradation and random operation. Since radar transmitters generate electromag­netic waves with no electromechanical action, there is no inherent mechanism to reduce the accumulation.

Like radar transmitters, ultrasonic transmitters are compact, encapsulating the electronics and sensing mechanism in one instrument. They differ in having a receiver and transducer that can be separated from each other. In the case of a controller, only the fully sealed transducer is placed inside the process; no sensitive electronics are exposed to the conditions in a lift station, wet well or tank.

In addition, unlike radar transmitters, ultra­sonic transducers create electromechanical action during ultrasonic wave generation. This action on the transducer face renders them inherently self-cleaning. Where submergence conditions are prevalent, transducers can be fitted with a shield or hood; the air trapped in the hood keeps debris from reaching the sensor face.

Also important: Due to the mechanical action, the signal signature during submergence can be differentiated from that during normal operation. Thus, the controller keeps the level locked at more than 100% to indicate the submer­gence event. 

Assessing accuracy

Some argue that radar technology is more accurate than ultrasonic because it is not affected by wind and since the speed of sound varies with changing temperatures. However, for more than 20 years, ultrasonic transducers have included integral temperature sensors, allowing the devices to correct for the speed of sound.

Still, it is a good prac­tice to use protective covers to shield them from direct sunlight and avoid heating up the electronics. There is an error in level measurement for every degree change in temperature, but thanks to ad­vances like digital filtering and signal processing algorithms, some ultrasonic controllers can deliver accuracies down to plus or minus 1 mm.

It takes several minutes for an ultrasonic level sensor to acclimate to a drastic temperature change, as when a unit is brought from storage to a significantly hotter or colder place, but thereafter it will consistently provide correct measurements.

Radar transmitter accuracy is also affected by the temperature of the electronics, and it too can be correlated to an error per degree change. Add to the mix the expansion and contraction of vessels and substances being measured, and millimeter differences in accuracy become trivial.

The key question is how crucial accuracy is in the context of the application. In general, tight accuracy is not overly important in water tanks, chemical tanks, lift stations, scum wells, filter beds or clarifiers. Most radar and ultrasonic instru­ments offer accuracies of 0.25% or better — good enough in the water and wastewater industry. 

The cost factor

In the past, radar technology was more expensive than ultra­sonic since radar transmitters can be used in more demanding applications that require longer measurement rang­es. However, the newest compact radar transmitters have decreased in cost and size, and their operating frequencies have increased significantly.

The clear benefit is that radar instruments operating in the 80 GHz range can produce a narrow signal without large antennas. In constricted, crowded spaces, a narrow beam suffers less degradation from interference. Radar transmitters can be set up quickly; in simple applications, they can be up and running in a few minutes.

Many newer radar transmitters can be connected to a controller for additional functionality, but doing so minimizes their cost advantage. A further complication is that some newer controllers designed to interface with low-cost radar transmitters are more basic than their advanced counter­parts for both level and open-channel flow. Functions included with most advanced controllers but often lacking in more basic models include:

Differential level

Gate or screen control

Smart pump control

Volume calculation

Energy-saving algorithms (pump operation during off-peak electricity rates)

Fat ring reduction (in wet wells)

Dual-point operation capabilities (solids and liquids, level and flow, or any combination)

Multiple measurement ranges (transducer dependent)

Before committing to a lower-priced solution, it is impor­tant to consider what will be gained or lost and, thus, avoid creating gaps in functionality.

Choosing with care

There is room for both ultrasonic and radar level measure­ment technologies in the water and wastewater industry. Ultrasonic devices have been relied on for decades and have evolved to meet market demands. Both technologies offer the benefit of low maintenance.

But in level measurement, there are other factors to consider. An understanding of the process and its unique challenges (sub­mergence, buildup, condensation, pump demands) will provide the clues to selecting the ideal instrument.

Enticing attributes such as high accuracy and low cost should not be the sole deciding factors. The key is to be objective about what the challenge entails. When unsure, it is wise to work with a supplier who offers a well-balanced product portfolio, backed by years of experience that can provide much-needed clarity.

About the author

Herman Coello (herman.coello@siemens.com) is a product marketing manager with Siemens Industry, Process Industries and Drives, based in Alpharetta, Georgia.