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Diferencia Entre Ultrasonico y Radar

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Page 1: Diferencia Entre Ultrasonico y Radar
Page 2: Diferencia Entre Ultrasonico y Radar

Typical applications:

Pumping station

Flow measurement

Chemicals container The measuring methods in detail Both measuring principles, ultrasonic and radar, are so-called “running time” methods and operate by measuring the distance between the sensor and medium surface. Through parameterisation of the 0% and 100% values, the distance value is converted into a level-proportional signal. The two techniques differ in the speed of signal propagation and in the physical influences on the different wave forms. Due to the relatively slow propagation of sound waves, no special requirements are placed on the speed of the evaluation electronics. To measure the distance to the product surface with a resolution of 1 mm, a time measurement of several microseconds – no problem for standard semiconductors – is all that’s necessary.

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A comparison of ultrasonic and radar sensors based on example applications in water management For many years, ultrasonic sensors have been the standard method for measuring liquids in widely different industrial areas. In the water-supply and distribution sector, they are used primarily in places where due to the application conditions a contact less measurement is preferred. Standing opposed to the reasonable price of ultrasonic sensors are the physical influences on this measuring principle, which, depending on the application, can lead to sizable measurement errors. In this article we compare the two measuring principles, ultrasonics and radar, and the physical processes that influence the measuring result. Typical level applications in water supply and distribution Level measurement applications in the areas of drinking water purification and sewage disposal are quite diverse. Beside water level recording in pump shafts, channels and rivers as well as filling level measurement in chemical containers for water treatment, flow measurement in open flumes is also an important field of application for the sensors. Since the plants usually operate without on-site staff, high reliability is essential in all these applications. The required measuring precision is highly dependent on the particular field of application. While an accuracy of 1-2 cm is sufficient for level measurement in pump shafts and rainwater overflow basins, far higher demands are made on sensor precision in other applications. Especially in flow measurement of open flumes, for example, high measurement resolution is an absolute must. The rate of flow is determined by the level in the flume and H/A characteristic curve defined by the shape of the flume. To record the flow quantity as accurately as possible, a highly precise level signal is required. High sensor precision is also vital for level measurement of rivers important for shipping or high water forecasting as well as for volume measurement of reservoirs. The specification of the accuracy of ultrasonic and radar sensors is similar with many sensor manufacturers. This is certainly appropriate for reference conditions, i.e. constant ambient conditions, but in practice these marginal conditions can be maintained only with great difficulty. The following describes the influencing variables in theory and in practice.

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In contrast to this, the requirements on the running time measurement of microwaves are higher by many magnitudes. Because radar signals travel at the speed of light, i.e. approx. 300,000 km/s, running time measurement of the signals is a big technical challenge. To also reach a measurement resolution of 1 mm here, the running time measurement has to be carried out with a precision of approx. 6 picoseconds (6 x 10-12 seconds). This is hardly possible even with the most modern electronic components. For that reason, the search for a way to convert these extremely short running times into a different, more easily measurable quantity began very early. Two measuring techniques, used today in radar sensors for level measurement, resulted. The so-called FMCW method and the pulse method. With the FMCW method, a frequency displacement between the emitted signal and the signal reflected from the product is created by modulating the transmission frequency. This frequency displacement is proportional to the distance and serves as a basis for generating the measured value. With the pulse running time method, the extremely short running times are transferred – by means of a special sampling technique – to another time scale in which time measurement can be carried out in microseconds. Through this avenue, very high precision can also be reached with the pulse method. Thus, the two methods can practically be regarded as equivalent. A comparison of the physical influences In order to examine the physical influences on the measuring principles more precisely, it is necessary to divide them into different categories. One fundamental point is of course the measured medium itself, since it has a great influence on the strength of the reflected signal. To evaluate the physical factors that change the signal running time, a more exact analysis of the process influences is necessary. Installation constraints and mounting location are further important criteria, because a considerable potential for disturbing influences also exists here. The influence of the measured medium The reflection of sound waves depends mainly on the density of the reflecting surface. The denser the surface structure, the stronger the reflection of the sound waves will be. A very strong reflection is always generated at the interface between liquids and air. It an ideal reflector for sound waves. With microwaves, the difference in the dielectric constant and the conductivity, i.e. the electrical properties of the medium, are the determinants of reflectivity. Water has a DK value of over 80, so it has very good reflective properties. Nearly the complete strength of the signal is reflected by the surface. But media with smaller DK values absorb a part of the energy and the reflected signal is correspondingly weaker. Due to the high dynamic range of modern radar sensors, however, this is hardly a restriction – even products with very small DK values (<2), such as mineral oil, can be measured with no problem. The reflective characteristics are determined not only by the medium itself however, but also disturbing effects on the surface, such as foam, floating dirt or waves. Both measuring principles experience signal damping due to energy absorption, more or less according to the consistency of the foam. The degree of damping is largely dependent on the structure and bubble size of the foam. In principle, considerably stronger damping is caused by fine-grained foam with high water content than by large bubbles.

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Because sound waves are reflected strongly by the individual bubble surfaces, ultrasonic systems are more seriously affected than radar systems. Particularly with thick foams, the reflection at the foam surface can be so strong that completely erroneous measurements can result. Radar is considerably less affected due to its high signal sensitivity. To be sure, the strength of its signal is noticeably reduced by the foam, but the water surface can usually still be measured reliably. Strong waves and contamination floating on the surface cause part of the signal energy striking the medium to be deflected. This effect can, however, be compensated in both measuring techniques with the help of high-quality signal evaluation and intelligent signal averaging. The influence of process conditions In a comparison of ultrasonic and radar, the biggest differences most certainly lie in the process conditions, such as temperature and pressure. While the signal running time of microwaves is practically unaffected by changing temperatures, sound waves are subject to considerable influence. The propagation velocity of sound waves changes by 1.6% per 10° Celsius temperature variation. This is a significant measurement error that must be compensated for. Temperature compensation is carried out in the same way in all sensors used for level measurement. A temperature sensor in the electro acoustic transducer measures the ambient temperature and the calculation of the signal running time is corrected accordingly. Since the temperature of the electro acoustic transducer does not necessarily correspond to the temperature of the entire measuring range however, a possible source of error creeps into the ultrasonic measuring principle. An appreciably better compensation is in fact possible by using several temperature sensors, but that would be all out of proportion to the total cost of this measuring technique. The influence of process pressure is also considerably larger with ultrasonic than with radar, but this can be neglected in the areas of use described here – significant pressure changes seldom occur in water and sewage applications. However, it is relevant that sound waves are subject to considerable damping at lower air pressures, i.e. when the number of air molecules present is lower. The practical limit for the implementation of ultrasonic devices is a vessel pressure of about 0.5 bar, depending on the application. When media containing large amounts of solvent are measured, it is quite possible that the signal running time of the sound waves will be influenced by changing gas concentrations or compositions, causing measurement errors to arise. Microwaves are not influenced by such changes. The influence of the mounting location Due to the radiation characteristics of sound and also of microwaves, sensors are also influenced by the mounting location. Installation close to a wall, for example in a pump shaft with a very rough surface or with heavy build-up, can lead to disturbing effects. Ultrasonic devices react more sensitively than radar sensors to disturbances resulting from the strong reflections from hard surfaces. While it is possible to suppress static interfering signals through a false echo storage, it is considerably more difficult to deal with echoes from build-up that changes constantly. Due to the varying shape and distance of the build-up, gating out the signals isn’t easy – good signal focussing is what can help the most here. With ultrasonic sensors, signal focusing is mainly determined by their overall size and frequency – a change of the

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focusing properties through mechanical design is almost impossible. With radar sensors too, signal focusing is all the better the higher the emission frequency, but considerably better signal focusing can be achieved by implementing a larger antenna system. Effects on measurement precision in practical applications As mentioned already, the propagation velocity of sound waves changes substantially under the influence of the ambient temperature. Attempts are made to compensate for this change by incorporating a temperature sensor in the electro acoustic transducer. In many applications this works well only conditionally since the measured temperature often does not correspond to the ambient temperature. A concrete example demonstrates the influence of temperature: An ultrasonic sensor is mounted in an open, 10 m-high vessel to measure the filling level. It is exposed to environmental effects that in no way correspond to the reference conditions. Due to solar radiation the electro acoustic transducer, which is usually black, warms up to an appreciably higher temperature than its surroundings. The temperature measured on the transducer can easily be 20° C above the ambient temperatur as a result of direct solar radiation. With a sound velocity change of 1.6% / 10° C, the resulting measurement error in a 10 m-high vessel is over 30 cm! This may still be acceptable for pump control in a sewage disposal facility, but for precise gauge measurement this doesn't suffice by any means. Especially in flow measurement of open flumes, sizable measurement errors can arise due to the influence of the surroundings, which in most cases the operator will probably not even be aware of. By using a sun shield or shelter these effects can be reduced, but that increases the cost and the amount of work required for installation. Since radar sensors aren't subject to such environmental effects, they deliver in practice far better precision. In many cases, however, the decision on which measuring principle to use is made – for economical reasons – in favour of ultrasonic. New radar sensor for simple process conditions By adapting existing technology to the requirements of the water management sector, VEGA Grieshaber KG succeeded in developing a cost-effective radar sensor that is little affected by ambient conditions. The sensor stands out through its plastic antenna system that is sealed with a polypropylene cover. By means of a simple fastening with an adjustable mounting

bracket, the sensor can be easily positioned in widely differing installation sites. When the sensor is mounted on a vessel, a plastic collar flange provides a pressure tight connection. The approx. 80 mm diameter antenna makes for excellent signal focusing and reduces the disturbing influence of reflections from container wall, fixtures or build-up. Thanks to the large antenna system, the instrument is also insensitive to dirt and condensation. Fig. 2: The radar sensor VEGAPULS 61 works exactly, reliably and also cost-effectively.

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Future prospects With its intelligent, application-oriented solutions, radar technology is asserting itself more and more in areas which in the past were typical for the use of ultrasonic devices. Especially when the costs over the complete life cycle of an instrument are considered, radar sensors score very well in comparison with other measuring principles, since practically no maintenance is necessary. The new radar sensor VEGAPULS 61 offers filling level and gauge measurement an interesting alternative to conventional ultrasonic, fulfils highest requirements on precision and reliability and is not limited to applications in the area of water management alone. Author: Dipl. Ing. (FH) Jürgen Skowaisa Product Management Ultrasonic, Radar VEGA Grieshaber KG Am Hohenstein 113 77761 Schiltach Germany Phone: +49 7836 50-0 Fax: +49 7836 50-8415 E-Mail: [email protected] www.vega.com