Ultrasonics - an accurate and cost-effective solution

With powerful microprocessing and improved echo recognition, ultrasonic level instruments are essential for today's water industry applications, says Peter Ward.

ULTRASONIC MEASUREMENT techniques are an accepted method in the water and effluent treatment industry, and are applicable to a diverse range of applications, including measurements of level, open channel flow, density and interface.

Ultrasonics have proven to be accurate and reliable, and are an economic solution for level applications that do not require the precision of techniques such as radar.

Today's ultrasonic level instruments also feature powerful microprocessors and sophisticated signal processing. Improved echo recognition and the ability to filter out unwanted information have made ultrasonics a high reliability, cost-effective solution ideal for most applications in the water and effluent industry.

Ultrasonics are most typically used for:
  • Continuous level measurement in open and closed tanks, sumps and wetwells, using a non-contact ultrasonic transmitter
  • Flow in open channels, using non-contact ultrasonics in conjunction with flumes and weirs
  • Sludge density measurement using attenuation of ultrasound
  • Measurement and control of sludge blanket level, using discrete on/off level sensors or ultrasonic continuous measurement
Ultrasonic pulses sent from the transmitter face are reflected from the surface of the liquid and received back by the same transmitter. Measurement of the time delay between transmitted and received signals enables the microprocessor-controlled electronics to calculate distance to the liquid level. Once the transmitter is programmed with the bottom reference for the application - usually the bottom of the tank - the liquid level is calculated automatically.

Using this principle, the microprocessor in the Mobrey range of MSP products controls the loop current to give a 4-20mA or a HART digital output signal proportional to the level, with the liquid level reading also available on an LCD display inside the enclosure. All the transmitters have integral temperature compensation to maintain accuracy across the operating temperature range.

Solar gain
Advances in temperature compensation methods mean ultrasonic transmitters can now be reliably used on open channel flow applications where their onboard software converts level in the channel to flow.

The introduction of the Environment Agency's MCERTS (Monitoring Certification) scheme has driven manufacturers to improve their measurement devices to provide the precision required to meet the EA's standards.

Typically, such systems comprise an ultrasonic level transmitter mounted over the liquid, upstream of a flume or weir. Used in the open air, conventional ultrasonic level transmitters are prone to suffer from inaccuracies caused by solar gain when the sun shines directly on them and heats the air between the transmitter and the liquid surface.

Products such as the MSP900FH transmitter have been designed to overcome this problem and help water and industrial processing sites comply with MCERTS legislation by using a remote temperature sensor wired directly into the transmitter.

The MSP900FH temperature sensor can be located in an appropriately shady spot between the liquid surface and the transmitter, ensuring that the air temperature measurement used in the speed of sound calculation is truly representative.

The sensor is hard-wired into the transmitter with a 2m cable so it requires no additional cabling or set-up and only a single connection to the controller, keeping installation costs to a minimum. The signal from the transmitter is fed back to the controller, which converts the level reading into a flow reading.

The flow controllers can be programmed to handle all the calculations needed. For example, the Mobrey ultrasonic system features pre-configured flow curves for the most popular flumes and weirs as well as user-defined curves for non-standard structures. An onboard logger records up to 7,000 flow readings and displays both daily and cumulative flows.

Many years of practical experience have shown that the ultrasonic attenuation in a slurry is directly proportional to the percentage of suspended solids. Measurement products use this principle to enable users to determine the percentage of suspended solids.

The use of ultrasound for this application has the advantage over optical infrared sensors, which are prone to blockage and fouling, resulting in unreliable measurement.

Discrete on-off sensors are typically used to detect high or low sludge levels in primary settlement tanks (PSTs), where reliable control of level is important to maintain sludge transfer to other parts of the process. They can also be used in final tanks or sludge thickeners.

The sludge density is measured between the gap of the sensor, which can be varied to suit the application - typically 150mm for a primary tank, or 450mm for a final tank, where greater sensitivity is required. Products such as the Mobrey MSM433 are available in a range of sizes depending on the percentage of solids to be measured.

The sensor can either be located at the top of the tank to monitor the upper parts of the settled blanket, or close to the discharge point at the bottom of the tank to monitor the density leaving the tank.

Mobrey sensors are of an all-welded 316 stainless steel construction, with an IP68 submersible rating for the sensor which can be simply mounted either by suspending it from a tube or conduit by means of the BSPT mounting thread.

Ultrasonic echoes
Another option, particularly suited to blanket measurement of the return activated sludge (RAS) in the final tank, is to use the sonar principle to provide continuous measurement. An ultrasonic pulse is transmitted vertically under water and reflects from the surface of the sludge blanket.

The ultrasonic echoes reflected from the blanket interface are captured and analysed by sophisticated echo processing. By knowing the speed of sound in the supernatant, the depth of the sludge blanket can be determined from the time of flight of the echo.

The advantage of this method is that it provides more precise control of the process, and provides pre-warning of any solid carry-over if the blanket is becoming too high.

This principle is used in products such as the MSL600 sludge blanket monitor, a microprocessor based system that includes an ultrasonic transducer and a control unit designed for mounting on a clarifier or settlement tank bridge and handrail.

The monitor can be configured to suit the user's needs using a menu system, with local programming via a membrane keyboard. The control unit includes an integral air compressor that provides an air purge across the transducer face to prevent fouling and build up of deposits on the face of the transducer.

Today's ultrasonic measurement products are more sophisticated in terms of reliability, accuracy and the level of detail they can provide. Powerful microprocessors and signal processing mean that they can be used for applications such as open channel flow, providing a reliable and economic way for water and effluent companies to meet environmental.

Peter Ward is product manager, marketing, at Emerson Process Management.
T: 01753 756649

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