Ultrasonics can be used to measure the level of many materials. Common liquid applications are water, slurry, sewerage, oils and corrosive liquids. Common solid applications include sand, rock, coal, lime, ash, pellets and ore.
With new gain and pulse amplifier technology, ultrasonic level now works in far dustier environments than ever before. This new software technology allows the return echo to be analysed, for example, in a dusty silo and then adjust the appropriate firing power of the transducer and the gain of the receiving amplifier circuit. See ALIEN Intelligence for more information.
Myths of ultrasonics
There have been many misconceptions about ultrasonic level measurement. This has been mostly due to a lack of knowledge of ultrasonics, competitive half-truths with misleading information and reference to outdated technology.
In the laser versus ultrasonic scenario, many do not fully appreciate that lasers do have limitations. Lasers make use of a beam of light. If two people had to be standing in a very dusty room or silo, would they be able to hear each other better or see each other better? They would obviously hear each other better. Another example is fog or mist out at sea. Ships use fogs horns instead of flashing lights for guidance. The light, after a fairly short distance, would become diffused by the dust or mist. With sound, even if the airborne dust were to rise to extreme levels, this can be allowed for by increasing the listening ability (gain amplifier return echo) or talking/shouting louder (pulse/power firing level). Laser level measurement is not suitable for clear liquids or dark, matte materials (eg coal). Laser light would travel straight through the surface of clear liquids to reflect of the bottom of the vessel. In a coal silo situation, much of the beam would be almost totally absorbed and diffused by the black surface, with the level of airborne dust further ruining any chance of a reliable reflected signal.
Ultrasonics - background
The term ultrasonics refers to sound vibrations - variations of density in elastic media (eg air) - whose frequencies are beyond the auditory limit, ie above approx 20 kHz. The highest ultrasonic frequencies hitherto attained are of the order 10 MHz. Such high-frequency elastic vibrations are produced in various ways, based on different physical principles.
Sound is energy propagated by means of longitudinal pressure waves in air caused by the motion of an object. An electrical signal may be used to cause something to vibrate (Diaphragm of a loudspeaker, ultrasonic transmitter, etc), which in turn would cause fluctuations in the local air pressure, ie sound waves. These waves travel through air at a speed dependent on the temperature of the air. The fluctuating air pressure in a sound wave can also cause objects to vibrate. If the wave strikes something solid, the wave will bounce back – an echo. Objects that the sound wave causes to vibrate may be used to convert the sound energy into electrical energy (microphones, ultrasonic sensor, etc).
Self-cleaning
Most modern day ultrasonic level transmitters and transducers incorporate self-cleaning solutions. The SUMPI and KAB 10 transducer manufactured by KAB Instruments has special software algorithms that continually check the echo strength. Should the echo strength be reduced to a predetermined level, the software then increases the pulse rate which causes the face of the transducer to vibrate at a much higher rate. This works to shed the debris off the face of the transducer. This routine continues indefinitely until the face of the transducer has become clean again. The software checks the echo strength continually until the signal has returned to its normal strength. As soon as this occurs the system reverts back to the normal running mode. This method is particularly useful in many hostile industry applications where there could be continuous splashing of the transducer, or where dust which sticks to the transducer. Since the vibrating sensor is normally facing down towards the material being measure, with gravity assisting to keep the face of the transducer free of build up. As can be seen from the picture, the sides and top of the transducer are covered in dust and dirt, but the face of the transducer is clean and functional. This application is in a pulp and paper plant, where the previous level measurement system would only work for an hour or two between the inconvenient manual cleaning that was needed.
The above is a typical example where some other level measurement systems would just not be suitable. A laser option could never work - even a small amount of dust would stop the system from working. Pressure transmitters would also be a bad choice - a small layer of dirt could stick to the transducer and causing false readings. Air purge systems would be vulnerable to problems caused by blocked pipes.
ALIEN Intelligence
When attempting to extract the correct level information we need to ensure that the correct echo is extracted from the echo profile. This software has many commercial names. KAB Instruments call it ALIEN Intelligence. Figure 1 shows a typical echo profile waveform received off a flat surface. In this example there are three echoes that could be the 'actual' level being measured.
The first echo can be ruled out as this falls within the transducer blanking time. The blanking time is the time taken for the output of the transducer to subside after the electrical drive pulse has ended. (Think of it as being just like the time it takes for sound from a struck bell to fade, in this instance the 'ring' is of course far shorter.) The blanking time of a transducer could be specified as the time taken for the voltage level of this first pulse to fall below and remain below a typical level of 0,5 V. The measurement of this parameter may vary from manufacturer to manufacturer. To measure the correct echo, the first echo is simply 'blanked' out and, hence, the term 'blanking'.
Not every transducer is made the same and therefore the blanking of each individual transducer will differ. The manufacture of a transducer is a fairly complicated task, which goes beyond the scope of this article. It is fair to mention though, that a 'critically damped' transducer will yield the shortest blanking distance. The marrying of the electrical and mechanical components are critical to the success or failure of a transducer, and need to be such that there is a very little or no oscillations (critically damped) generated after the transducer has been pulsed with energy. A generally accepted norm is to have an instrument setup with a blanking time that equates to a distance of 0,5 m for the sound wave. The blanking setup of any given instrument is a function of the transducer as well as the electronics in the instrument connected to the transducer. Blanking time settings equating to as little as 0,3 m are possible. Care should be taken when setting an instrument to lower blanking distances. Transducer blanking increases with increased power injected into the transducer. (The harder you strike a bell, the longer it will ring) For this reason the blanking is kept at 0,4 to 0,5 m.
An echo received from a far distance (close to the limit of operation) will be very small and probably appear insignificant. This is because the ultrasonic energy has to travel so far. Raising the power applied to the transducer during the transmit pulse would result in a larger echo, but, as stated earlier, this will also increase the blanking time. KAB calls this 'high-pulse-blanking'. A distance of 1,5 m is a typical blanking distance when using high power.
KAB has developed a variable threshold technique that it employs in its instruments (see Figure 1). The red line indicates the threshold for the current setup. The vertical line indicates the current blanking at 0,5 m. The steep slanting line assists KAB to decrease the blanking of any particular setup. This is usually in the 0,5 to 1,0 m region. The shallow slanting line is the main variable threshold of the instrument. KAB decreases this threshold over distance to enable reception of a small echo from a target near the instrument's limit.
The first echo after the blanking and above the variable threshold is most likely to be the echo of the required target. Using a technique that employs a differentiating action ensures that the instrument does not indicate a reading at random, but rather follow the most likely target echo at the set rate of change. Multiple echoes do sometimes cause false readings, but in most instances, the first echo is always above the threshold and therefore, the multiple echoes are ignored. Employing the differentiating function eliminates a sudden jump to a multiple echo.
Future technician
The computer age has made a significant impact on process control. The plant environment is now fully computerised from PLC to scada to the various bus systems on offer. The technician of today should be carrying a laptop computer.
KAB Instruments has seen the need for computerised set-up and diagnostic testing of its range of instrumentation. Due to the increased measuring range and applications available when using the KAB range of ultrasonics, it is vital for the transmitter and transducer to be configured and set up correctly.
KABScope was developed with this in mind. This software enables the user to view the bin map with an on-screen oscilloscope. This on-screen image can offer the user vital information so that his ultrasonic pulse can be maximised to gain the correct output signal to his process. Set-up changes can be made to the ultrasonic transmitter via the RS232 connection to the KABScope KAB believes that this form of diagnostic software will remove previous doubts cast over the legitimacy of ultrasonics in the measuring of various materials for which ultrasonics was not considered a viable option.
A further benefit of using software to configure instrumentation is the service aspect. Accuracy of process measurement is vital. Although the KAB range of ultrasonics is easy to configure, there may be requirements for configuration assistance for the user.
Physical distance between the supplier and the user is no longer a problem. A recent application problem on a plant in Brazil enabled KAB Instruments to solve the problem first hand. The application error was copied to KABScope, including the parameters, which were then emailed to KAB's offices. The necessary parameter changes were made in the KABScope set-up utility which was then emailed back to the user who then downloaded the settings from his laptop onto the transmitter. The application continues to work.
Use of the software and information technology available enables ultrasonic technology to keep ahead of other forms of level measurement. KAB maintains that if one compares the cost versus the range of applications of ultrasonic instrumentation then the realisation is that ultrasonics will continue to dominate the level marketplace.
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