A new range of level sensors that operate at a much higher frequency are available from Vega Instruments SA. They considerably extend the application spectrum of Vega Radar sensors and, in combination with the existing Vegapuls 50 series, offer an optimum solution for almost all level-measurement problems.
The breakthrough of radar technology in level measurement is largely due to the terrific pace of development in the microwave sector. New manufacturing processes, new materials and above all an ever-increasing degree of integration (miniaturisation) have been the main drivers of this development.
Application of radar sensors to level measurement is growing continually. The increasing interest in radar systems reflects the technological advantages that distinguish radar sensors from other level measurement instruments. Radar technology can be used without hesitation for measurements in liquids, powders and bulk materials. Measurement takes place without contact with the medium and is thus free of wear and is almost independent of process parameters. Application is possible even under the most difficult conditions - conditions under which other methods of measurement cannot be employed.
With these new products in addition to its existing ones, practically all applications are covered. Vega Radar systems are to be found in applications for measurement above ladles of molten steel and in vessels under pressure containing strong and poisonous materials. Vegapuls sensors are also found in sterile environments, for example in the pharmaceuticals and food industries. Whether in the closed processes of the petrochemicals industry or in the open air, keeping tabs on the environment, for example monitoring the level of mountain streams, these sensors find applications everywhere.
To fulfil some special application needs and to improve technical parameters still further, Vega has now produced a new range of radar systems. The new feature is that the operating frequency of well over 20 GHz is more than four times higher than that of the proven Vegapuls 50 series. This higher frequency extends the capabilities of the product range to cover applications and problems that were not adequately addressed by the earlier 5,8 GHz systems. Systems with a higher operating frequency are notable for their greater accuracy, and because of their smaller antenna, can be used in 11/2" fittings. The reliability, and the very low energy consumption (which enables them to be configured as 'loop-powered' sensors) are retained.
Theoretical basis
Radar, an acronym of radio detection and ranging, describes a noncontact process by which objects can be detected with the aid of electromagnetic high-frequency waves (also known as microwaves). Since merely the information that an object is present is seldom sufficient, it is mainly properties such as distance, speed and direction of motion that are determined. Evaluation of a simple microwave signal is, however, relatively complicated. The signals are therefore also modulated.
For level measurement, the most common modulation processes are:
* Amplitude modulation (PULSF radar).
* Frequency modulation (frequency modulated continuous wave - FMCW - radar).
All Vegapuls sensors operate on the PULS-radar principle. With this type of modulation, a short microwave pulse is transmitted, reflected by the material in the vessel, and the signal received again by the sensor. The time that elapses between the signal being sent and received is proportional to the distance between the sensor and the surface material in the vessel. Given knowledge of vessel geometry, the level can then be calculated directly.
Because microwave signals travel at essentially the speed of light, the resultant very short times between transmission and reception are very difficult to measure, and a special time-extension process is employed. The effect of this is to transform the fast microwave signal, which covers a distance of 1 cm in ca. 0,3 µs, so that 1 cm of travel corresponds to 10 µs. It is this time extension that enables rapid evaluation of a level signal with great accuracy and relatively modest technical resources. It operates for a variety of operating frequencies.
Depending on the choice of operating frequency, pulse width, and power transmitted, different properties can be influenced to suit the application. For example, the operating frequency can be chosen to suit the reflective properties of certain materials. The pulse width, and so the band width of the signal, determine to a great extent the accuracy of a system and the ability to separate two signals which are very close to each other (eg useful signal from interference signal). The signal power decides how much energy a system needs, and whether it can be operated as a 'loop-powered' sensor or not. The signal power does not, as is frequently claimed, determine the accuracy of a radar system. It also has no influence on the signal/ noise ratio, because for example, an increase in transmitted microwave power not only increases the useful signal received but also increases the interference signal or system noise at the same time.
The operating frequency, bandwidth and power of a system that transmits microwaves are not, however, a matter of free choice - for civil applications there are precisely defined frequency bands. Those known as the ISM bands (industrial, scientific, medicine) have set limits for the permissible frequencies and the maximum power that may be transmitted.
To the frequency bands generally used for level measurement up to now (5,8, 6,3 and 10 GHz), further bands have been added at 24 and 26 GHz as a result of the demands of technical development. The Vegapuls 40 series exploits the advantages of a higher frequency to the full. Because antenna dimensions are inversely proportional to the operating frequency, the cost of the mechanical parts of a sensor, and the size of an opening in a vessel needed for installation are both reduced considerably. The greater bandwidth ensures higher accuracy and, due to the shortened pulse width, system noise in the immediate vicinity of the antenna has been halved.
On the other hand, with the size of antenna we were accustomed to up to now, better focusing of the microwave signals can be achieved; this significantly reduces sensitivity to interference reflections and improves the signal/noise ratio considerably.
The higher frequency is not always the best
Besides the many advantages of the higher operating frequency of the Vegapuls 40 series, the increased damping of microwave signals in concentrated vapour phases must be taken into account. In these cases, Vegapuls 50 sensors with a lower operating frequency will give better results. It is a consequence of the physics of wave propagation that lower frequencies experience less damping than higher frequencies.
The high degree of focusing of the microwave signal can lead to a reduced signal amplitude if there is a strong disturbance of the surface to be measured. A part of the microwave energy is then not reflected directly back to the receiver but is reflected away in some other direction. Particularly in process vessels with strong disturbance of the surface, Vegapuls 50 series sensors with their lower signal frequencies can exploit their strengths.
Examples of special applications
The small size of the antennae of the Vegapuls 40 series, together with improved focusing of the microwave signals - and the possibility of measuring up to the rim of the antenna, make these new sensors especially suitable for use in small containers and in standpipes or bypass pipes. The presence of interfering devices in a tank, or of holes or lateral connection to standpipes have much less influence on high-frequency sensors than they do on those that operate on lower frequencies.
The Vegapuls 40 series sensors are thus particularly well suited to applications in the process vessels and bypass tubes of distillation columns in the chemical and petrochemical industries. Such applications generally call for measurements in the range from 0,5 to 2,0 m. The containers used almost always have technical process aids such as cooling or heating tubes or stirrers, and cleaning devices often further encumber the space needed for measurement in a vessel.
On the other hand, sensors of the Vegapuls 50 series are a better choice for applications in which foaming may occur, or where a concentrated vapour phase is present; because of their lower transmission frequency they exhibit lower signal damping under such circumstances.
Sensors with a lower operating frequency also have advantages when the surface to be measured is severely disturbed. Because a greater surface area is irradiated, there is always more energy reflected back than with sensors that have a narrower radiation cone.
Conclusions
The Vegapuls 40 sensors that operate at a higher frequency extend the previous field of application. They effectively complement the existing range of Vegapuls sensors, which have proven their effectiveness in practice (over 20 000 in use). Both ranges of measuring devices have their specific features and advantages. They provide solutions for every user. Whether the need is for high accuracy, for example in storage tanks, or optimum resistance to a variety of tank contents, much in demand in the chemical industry, or whether it is for high-resistance to temperature, for example for a blast furnace, or for the smallest antenna size for bypass tubes in the petrochemical industry, for every measurement problem Vega say they have a radar that suits the application.
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