ULTRASONIC LEVEL MEASUREMENT: A MODERN CLASSIC!

Instrumentation & Monitoring

Figure 1: Even harsh winters with icy temperatures don’t affect the Prosonic S. The sensor with flood protection is equipped with a heater.

By Moritz Rohn, Product Manager Level, Endress+Hauser Messtechnik GmbH+Co. KG

Radar technology has long had the reputation of being the best method of level measurement available - but does this also apply to the wastewater market? Based on its physical characteristics, radar technology certainly offers great flexibility and high performance. However, from an objective standpoint, non-contact level measurement using ultrasound is often the first choice and also the right choice due to its cost-effectiveness and application-specific benefits!          

Applications
While we are aware of the physical advantages of the Micropilot, to what extent, if any, does the technology benefit the day-to-day running of a WWTP?

In practice, many typical level applications such as wet wells, rainwater basins and wastewater channels are covered or built underground. Wind and sunlight cannot interfere with ultrasonic measurement here. On the contrary, it is precisely in these applications that the Prosonic S FDU90 scores well due to a higher level of accuracy at close range, more flexible installation options as well as minimal maintenance thanks to the self-cleaning effect (oscillation of sensor membrane).

For larger measuring ranges located outdoors, for example level measurement at bridges across rivers or outgassing media (e.g. level monitoring in digestion tanks), the physical benefits of the Micropilot come to the fore. The use of ultrasonic measurement in a digester is not a viable option, as the gas layers in the digester cause a shift in the time of flight of the ultrasonic pulse, which in turn gives rise to inaccuracies.

Typical Examples of Ultrasound in Use:

Dosing stations and chemical tanks
Dosing stations, in which flocculating or precipitating agents are used for the purpose of phosphate elimination, sludge dewatering or biogas desulfurization, are classic examples of where ultrasonic sensors such as the Prosonic S or T are used. The fully welded FDU9x sensors made of PVDF are resistant to aggressive media and are also particularly suited to small tanks due to the short blocking distance. The compact Prosonic T delivers a high level of reliability at a very affordable price.

Pumps and lift stations
For energy saving reasons, only the necessary number of pumps should be in operation in pump and lift stations. Using the Prosonic S transmitter, it is possible to control up to six pumps directly on an alternating basis. This ensures a uniform load across the screw pumps. An additional PLC is not required.

Flow measurement in open channels or weirs
The first measurement usually carried out in the wastewater inlet of a WWTP involves determining the inlet quantity using a Q/h curve. Endress+Hauser’s ultrasonic sensors contain numerous Q/h curves for channel and weirs. Alternatively, they can be entered individually.

In practice, the typical measurement accuracy of ultrasonic measurement is completely sufficient for flow measurement in open channels or weirs. The use of a radar device that functions independently of temperature is usually not necessary. The accuracy of flow measurement is influenced primarily by the design, construction and the installation of the channel or weir.

Rainwater basin / overflow structures
Rainwater basins are used to prevent the overloading of wastewater networks and WWTPs. In the buffer basins, it is necessary to measure both the level and the volume of overflow into the drainage channel. Both measured variables can be determined at the same time using a transmitter and an ultrasonic sensor that can be mounted up to 300m away from the transmitter. Due to the narrow measuring range, the small dimensions of the device and the option of installing it directly on the ceiling, the FDU90 is particularly suited to measurement in narrow wastewater systems such as these. The use of a flooding protection tube guarantees signal analysis even if the sensor is completely flooded. If the measuring point is located outdoors, the use of the FDU90 with heater is recommended. The heater prevents the formation of ice on the sensor and guarantees reliable measurement at any time of the year.

Bar screen control
Coarse and fine bar screens are components of every WWTP. To optimize their use, the screen should be cleaned only when a sufficient level of contamination has been reached. For this purpose, a Prosonic FDU91 is installed both upstream and downstream from the bar screen in order to measure the difference in contamination level. Analysis and control of the bar screen cleaning process is performed by the Prosonic FMU90 transmitter with two sensor inputs and integrated relays to the pulse output. Easy programming of the FMU90 keeps cost to a minimum and saves the user time.

Summary
A comparison of the pros and cons of both time-of-flight methods, ultrasonic and radar, shows that radar is not necessarily the best method for the user. Endress+Hauser has tailored its range of traditional ultrasonic measurement technology to the requirements of customers in the wastewater industry to ensure an optimum price-performance ratio by current standards.


Figure 2: A good dosing system means good savings: Reliable dosing of precipitating agent at a biogas plant using the Prosonic T


Figure 3: It doesn’t always have to be radar: The Prosonic S family is perfectly suited to the wastewater industry.


Figure 4: Exceeding limits: Even for outgassing media, the Micropilot offers high levels of accuracy and measurement reliability.


Figure 5: The Prosonic T ultrasonic sensor is perfectly suited for use in dosing and storage tanks. With its affordable price tag, easy and safe operation, the compact two-wire measuring device is an impressive choice.


Figure 6: Capacitance probes are tough performers: The Liquicap guarantees high levels of measurement reliability when used in media which generate buildup.


Figure 7: Spotlight on the wastewater industry: Ultrasonic sensors, such as the Prosonic M, measure on a non-contact basis, are maintenance-free and offer an optimum price-performance ratio


Figure 8: Optimized control of pumps and lifting stations is a typical example of how ultrasonic technology, either compact or separate instrumentation, is used


Figure 9: Comprising an analysis unit and sensor, the Prosonic S as a separate instrument offers considerable advantages, particularly in environments which are difficult to access.