Rosemount 3108 Ultrasonic Level Transmitter used to

measure flow in open channel applications

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Ultrasonic Technology

for Level Measurement

in Water Processing

ApplicationsVijay Vidyasagar at Emerson

Process Management explains how ultrasonic measurement technologies can provide accurate and reliable measurement for many water processing applications

IntroductionUltrasonic measurement techniques have become an accepted mode of measurement 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. Modern ultrasonic level i n s t r u m e n t s fe a t u r e powerful microprocessors and sophisticated signal p r o c e s s i n g w h i c h – combined with improved echo recognition and the ability to filter out unwanted information – have made these instruments a highly reliable and cost-effective solution that is ideal for most applications in the water and effluent industry.

Measuring level using non-contact ultrasoundA n u l t r a s o n i c l e v e l transmitter is mounted on the top of the tank and transmits an ultrasonic pulse down into the tank. This pulse, t ravel l ing at the speed of sound, is reflected back to the transmitter from the liquid surface. Measurement of the time delay between transmitted and received signals enables the device to calculate the distance to the surface. Once the transmitter is programmed with the bottom reference for the application – usually the bottom of the tank – theliquid level is calculated automatically.

The advantage of ultrasonic transmitters is that they are easy to install (even if the tank has liquid in it). Set up is simple and devices with onboard programming ability can be configured in a matter of minutes. As it is a non contacting device, the

measurement is unaffected by changes in liquid density, dialectric or viscosity and performs well in aqueous liquids and many chemicals.

Ultrasonic transmitters rely on the pulse being unaffected during its time of flight, therefore liquids with heavy vapours, steam or vapour layers should be avoided. Vacuum applications are not possible as the pulse needs to travel through air. The condition of the

liquid surface is important. Some turbulence can be tolerated, but foaming will often `damp out’ the return echo. Obstructions in tanks can cause false echoes, but most transmitters have sophisticated software algorithms to mask or ignore these.

Using ultrasonic level instruments in open

channel flow applicationsThere is a specific relationship between the height of inlet water and the flow rate for every open channel that is free flowing through a specific controlled metering structure. For any given inlet height, there is a corresponding flow. Flow therefore can be determined by accurately measuring the water level. Ultrasonic level transmitters can be considered for these applications,

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typically, with a system transmitter mounted over the liquid, upstream of a flume or weir.

However, conventional ultrasonic level transmitters are prone to inaccuracies caused by ‘solar gain’ when the sun shines directly on them and heats the air between the transmitter and the liquid surface. As the speed of sound in air varies with its temperature, it is necessary to compensate for this. For example, a change in air temperature from 20°C to 22°C would translate into a 1.2cm change in distance to surface value. Depending on the size and shape of the channel, this error could have a significant effect on calculated volume flow.

A d v a n c e s i n t e m p e r a t u r e compensation methods mean ultrasonic

transmitters can now be reliably used on open channel flow applications where their on-board software converts level in the channel to flow. Some products, such as the Rosemount 3108 transmitter, have been designed to overcome this problem and help water and industrial processing sites comply with local legislation by using a remote temperature sensor wired directly into the transmitter.

The accuracy can also be affected by variations in surface level caused by surface turbulence or a sudden variation in flow that cause wave effects. In order to take account of these changing conditions, a rapid ultrasonic pulse rate is required. For example, an ultrasonic pulse rate of one per second enables changing flow profiles to be tracked as they occur.

Mobrey 433 Fixed sludge interface level for final and secondary

settlement tanks

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Measuring sludge density and blanket levelMany years of practical experience have shown that the ultrasonic attenuation in 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. In this application, an ultrasonic transmitter/receiver pair is submerged in a tank or mounted in a pipe section. An ultrasonic signal is transmitted between the two and is scattered by suspended solids particles that are present in the slurry. The amount of signal received determines the sludge density. The use of ultrasound for this application has the advantage over optical infra-red 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 (PST’s), 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. For sludge density measurement or automatic control of the de-sludge process in a settlement tank, there are two basic types of sensor:

A pipe section sensor is used in installations where the density is to be measured on the sludge discharge line from the settlement tank or clarifier; alternatively, a tank mounted sensor can be installed directly in the settlement tank to measure density at a fixed point in the tank or to provide control when the sludge density reaches a certain percentage of solids.

Suspended tank mount sensors are

usually available in a variety of sizes depending on the range of density to be measured. 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.

Conclusion Today’s ultrasonic measurement products, while using the same principles as their predecessors, are considerably 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 requirements. WWA