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1. OPERATING PRINCIPLE 1.1

The principle of microwave type level sensors are based on an antenna system transmits and receives ultra-microwaves that are reflected in the surface. The signals are transmitted at the speed of light, being the time between sending and receiving the same proportion to the level of fill of container. Thanks to the propagation procedure specially developed for this purpose can be measured reliably and accurately even extremely short time intervals. Radar sensors operate in low power frequency bands C (when the frequency below the 6GHz) and K (frequency greater than 20GHz).

Microwaves (radar waves) are reflected by the impedance mismatch between air and product from the same sensor to detect it again. The return time of the signal is a measure of the height of the empty section of the tank. If this distance is subtracted from the total height of the tank, you get the product level. Turnaround time becomes an analog output signal.
Thanks to the latest microcontroller technology, these sensors are able to reliably recognize the correct signal, eliminating false echoes to measure the levels with great precision.

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MICROWAVE MEASUREMENT OF LEVELS IN LIQUID

Micropilot The level transmitter operates on the principle of echo. A rod antenna or a horn antenna directed short microwave pulses of 0.8 ns in duration to the product, they are reflected on its surface, and the same antenna detects your back, this time acting as a receiver (see fig. 1). The distance to the surface area is proportional to the flyback pulse microwave:

D = Ct / 2 D: Distance from the transmitter / surface.

C: Speed \u200b\u200bof Light.

t: transit time in seconds.

Figure 1. microwave level measurement in liquids.

The level L is the height total empty tank, E, minus the distance measure, D: L = E - D. The level L can be up to just below the rod antenna or horn antenna, so that there is no distance restrictions. This instrument, known as Micropilot, operates in the frequency band for industrial, scientific and medical, or ISM band (6 GHz to 26 GHz). Its low power radiation allows secure installation in metal and non metal without any risk to humans or the environment. The microwave level measurement is virtually independent of temperature, pressure or under high vacuum, and the presence of dust or vapor. Microwave pulses travel at the speed of light and are unaffected by the presence of steam.
level measurement in ultrasonic sound wave pulses are generated mechanically by vibration of the air and reflected due to the mechanical properties of the medium. In contrast, the microwave measurement electromagnetic radiation is reflected due to a change in impedance (ie, a change of dielectric constant of the medium in which is reflected). The dielectric constant Er of the product must have a minimum value. Er values \u200b\u200bshould be above 1.4 for applications in still-pipes, or 1.9 in any other case. Above these values, changes in Er no afectan a la medición. Para productos conductores (> 10 mS/cm), la medición resulta totalmente independiente de er.

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Un oscilador de alta frecuencia (HF) genera una señal de microondas básica de 6 GHz. Un generador de impulsos de onda que genera una frecuencia de 3,6 Mhz transmite por la antena la alta frecuencia original. La señal se refleja en el producto y regresa de nuevo al receptor por la antena. Luego, la señal recibida y la señal de referencia, con la misma pulsación de 3,6 MHZ, pero reducida en 43,7 Hz, se envían a un mezclador de frecuencias. Cuando el impulso de referencia se mezcla con el transmitido (incluyendo el impulso reflejado), la señal increases by a factor of 82,380, which gives a boost 'sample' of 43.7 Hz (= 3.6 MHz / 82 380) with a carrier frequency of 70 kHz (= 6 GHz / 82 380). This signal 'sample', lower frequency and easier to handle, has the same form and contains the same information to reflect the original signal transmitted (see fig. 2). After processing the signal, only 44 curves per second is used for the statistical analysis and become an output signal.

The quality of the reflected microwave signal may depend on the amount and type of foam present. If the foam is dense conductive reflection will occur in the foam, rather than on the surface. If, however, the foam is non-conductive and not too thick, the signal is still reflected in the surface.

Figure 2. microwave signals used in the process of transmission and reception.

The use of agitators or wave existence reduces the amplitude of the reflected signal, if it was enough. A microwave level gauge is a measuring time of return of the waves and therefore does not depend on the extent of the return signal. The gauges use of pulse wave has certain advantages over the gauges known as FMCW (Frequency Modulated Continuous Wave), frequency modulated continuous wave). FMCW instrument emits a high frequency signal on a continuing basis, which means that its effective power is about 6 times the effective power of a level indicator pulse wave. For this reason, some countries do not allow the use of instruments in applications FMCW open to the atmosphere and / or its use is covered by purchasing licenses. By contrast, the use of industrial gauges based on microwave pulse signals are allowed freely and not subject restrictions or licensing arrangements. The power of a single impulse (of any pulse level indicator) is two hundred times, so that the signal is more effective in measuring conditions unfavorable.

Selection criteria

The choice of instrument for measuring microwave pulse levels appropriate for each application depends on the following factors:

l. Chemical resistance, the process pressure and temperature determine the antenna of the antenna material and flange.

2. The measuring range desired and the process conditions such as Er, waves or eddies and foam determine the size of the antenna.

3. The type of tank (storage, standpipes or process) and type of installation (open space, still pipe or bypass).

4. The required precision and quality requirements for Ex

The first aspect is usually considered relates to the field of measurement desired for a particular application (for example, in a quiet storage tank in a tank controller or a processing tank with agitator). A still-pipe or a referral practice can be seen also as a storage tank present Calm maximum measurement range. Low values \u200b\u200bof dielectric constant and the presence of waves favor the signal attenuation, and they reduce the measurement range up to 35 m. Each type of instrument has its own list of applications for each type of tank and product assembly (see the example in figure 3 for two different diameters).

Figure 3. Example of different sizes of trumpets to the sensor.

indications B, C and D represent various liquids: B = non-conductive liquid such as petrochemicals with dielectric constant Er = 1.9 ... 4, C = concentrated acids, organic solvents, esters, alcohols and ketones with a dielectric constant Er = 4 ... 10; YD = conductive liquids, solutions in water and dilute acids with a dielectric constant Er> 10. Electromagnetic pulses emitted by an antenna, which can be a trumpet of diameters DN80, DN100, DN150, DN200 and DN250, or rod, 390 mm or 540 mm in length. The horn antenna of stainless steel can withstand a maximum temperature of 400 º C and 100 bar pressure (although not simultaneously), while the rod antenna (PTFE or PPS) can withstand a maximum temperature 150 ° C and 40 bar pressure (although not simultaneously).

Important properties of horn antenna and when used:

• When can form condensation on the antenna
• For use in socket> 250 mm
• For fields greater
• To measure pressure and temperature conditions over
· Increased robustness
· The applications best suited to lead
• In case of weak reflection (er low or waves).

Important properties of the antenna rod and when used:

chemical

• Resistance (fully PTFE)
· Pair a small process
· sockets close
• When can cause adhesions in the tubing.

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Signal Conditioning Installation Applications

The microwave pulse should easily reach the surface of the product whenever possible. Each object is lodged with the beam produces an echo signal, the more the closer the object (see fig. 4 (A)). If you can not choose a different installation position to avoid strong echoes, they interfere with the measurement and must be suppressed for calibration. This is done similarly to the ultrasonic measurement with the option TDT (Time Dependent Threshold, threshold function time). Since microwaves are polarized, the radiation beam has to be perfectly parallel to the vessel wall, so that the alignment mark on the coupling flange or cable should be placed parallel to the vessel wall (see fig. 4 (B)). Avoid a central position in the tank and keep it separate (at least 300 mm.) Fittings, posts, rings, pipes and walls. E] axis of the transmitter should be perpendicular to the surface. The peak occurs just below the antenna or horn antenna, so that there is no distance restrictions.

Figure 4. emission angle and position within the curvature respectively.

The front edge of the rod antenna and the conical horn antennas must be pointed towards the tank (see fig. 5). Avoid as measured through the curtain of disposal of the product or in the whirlpool. Observe the mounting instructions sheet specifications for mounting the still pipe and conduit pipe (dip) in parallel. The pipes must be metal, have a constant diameter and mounted along with the diameter of the horn antenna. For the best product mix, in the still-pipe can be opened slots of a certain size (see fig. 6 (A)). You can use a ball valve full bore to close the measuring point for maintenance (see fig. 6 (B), and in practice, fig. 8). For cases where there are high or even sockets inclined nozzles do not conform to specified requirements for assembly, there are areas of special antenna and extension curves.

Figure 5. Details proper installation.

Figure 6. Details proper installation.

Electrical and electronic

The microwave level indicator must be connected to a power source or AC, either. The high frequency energy radiated is low (about 0.2 mW effective power, which is 500 times smaller than a cell phone, and 2,500 times less than the rate of spread of radiation from a microwave oven). Therefore, industry-level indicators based on microwave signal pulses are safe and, moreover, are not subject to acquisition licensing or other regulation. The output signal is 4-20 mA (active or passive) and some instruments have additional relay output for alarm function. For remote operation, the Micropilot can be deployed with one of the four communication modules include:

• Protocol • Protocol HART PROFIBUS

· Interface RS - 185
· Bus Field Foundation

versions not equipped with digital interface is configured using a portable display module. For Ex (ATEX II 1 / 2 G), this unit is intrinsically safe. This compact unit calibration, which includes a display, a keyboard and an error indicator (see fig. 10), is introduced into the connection compartment of the transmitter. A short cable allows you to manipulate with ease, even when the transmitter is in an awkward position. The display, four lines of text, displays what is being done and what to do next. It also shows the curve of the Seine] envelope when the instrument is detected. There is also a model ATEX EEx ia IIC T6/T4 (safe).

Figure 10. measured calibration Unit Endress + Hauser.

Whether

is set to the display module, with the configuration software (ToF-Tool), from a portable terminal or a bus system, the Micropilot always handled from a setup menu with input fields and output of information .

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indication instruments and measurement of microwave levels used in situations where the benefits of other indicators of level of no contact with the product as, for example) ultrasound, are obvious, but can not work due to working conditions, for example, high pressure, low pressure (vacuum), high air temperature or composition. Therefore, Microwaves are often used in chemical processing and storage applications (see fig. 7 and 8).

Figure 7. Example of a level indicator used in a tank.

in Fig. 7, a microwave level gauge is used in a storage tank by condensing high-pressure oil. The instrument is mounted on a still pipe that has a ball valve) so that you can access it without altering the process. Of course, the microwave level indicator of this application meets the requirements of Ex in Fig. 8, the level indicator Microwave is used in a mercaptan storage tank. For security reasons, it adds a level switch pitch, or L1QUIPHANT (in front), as independent alarm high level detection (HLA, High Leve! Alarm).

Figure 8. level meter used in tank mercaptan.

in Fig. 9, uses a microwave level gauge for a Cuba of slurry in a seemingly simple application that, in principle) could have been resolved with an ultrasonic level indicator. But conductive foam absorbs all the acoustic energy and instead, reflects the electromagnetic energy.

Figure 9. Using level meter on an open tank of liquid manure.

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There are four types of microwave level indicators (see fig. 10): 1) Standard type, 10 mm accuracy. with rod antenna or horn antenna for general use. 2) A model of higher accuracy (± 3 mm.) With a small horn antenna. 3) planar type sensor with an accuracy of 1 mm. still-pipes for applications in, for example for use in fluid storage and goods in custody expensive. 4) The same three but with a planar satellite measurement in open spaces.

Figure 10. Types of indicators level microwave.

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1.2 Measurement Models LEVELS IN SOLID MICROWAVE

The cable telecommunications industry developed the principle of measuring levels guided microwave, or TDR (Time Domain Reflectometry, time domain reflectometry) to detect a break in its extensive cable network. When transmitting a radar pulse through the cable, as long as the feedback signal for your travel needs, varies as open or short circuit has different impedances (ie, distance). Annually, this method is used for measuring levels (see fig. 11) in the presence of large amounts of dust, because the ultrasonic measuring tools may fail because of high dust content and level indicators sounding free microwave can not be used due to the dry solids do not reflect the emitted energy.

Figure 11. Example on sound level meter.

TDR technology differs from conventional radar that emits microwave pulses in a wide frequency band (200 kHz to 1.2 GHz). TDR technology uses electromagnetic micro-impulses that travel through cable (guided waves) and are reflected due to a sudden change in dielectric constant. In level measurement, the change in the value of the dielectric constant corresponds to the air / product. The distance D to the product surface is proportional to the flyback pulse, where C is the speed of propagation. It is important that the pulses emitted power is very low (less than 1 microwatts) and are guided by what little energy is dissipated. For this reason, the intensity (amplitude) of the signal that returns will be almost the same if the product level between 3 m and 10 m from the flange.
Just as in measuring ultrasonic and microwave method is used envelope curve in the measurement technique each wire TDR probe is represented by 512 digital points, regardless of the length of it. Collect a sample of the behavior of reflected pulses at each point of the probe and the accumulated information about the sampling cycle is processed and converted into information about the level of the product. The flyback principle guided microwave is used for solid level indicators of up to 20 m and is known for Levelflex. The low power of the radar energy can be installed securely in containers and silos both non-metallic and metallic without risk to humans or the environment. Not require any licenses. Another difference between the microwave level gauges in open space and TDR instruments is that in the first, the same structure, other possible assemblies in the bin and the end of the wire probe also detected and properly return more impulses the product level. For this reason we carried out the so-called "map probe that confirms that the pulse received is the product level (see fig. 14). This "mapping" is usually done at the factory for specified lengths of the probe and it is only necessary if special assemblies or reduced the length of the probe.
measuring instruments guided microwave level serve a wide variety of solids up to a grain size of 20 mm and dielectric constant values \u200b\u200bof 1.8 or higher. Not affected by moisture or by changes in the composition or by pressure or temperature. The presence of large amounts of dust, steam, adhesions, acoustic noise or angle of repose of the product does not affect the measurement. The indicator has no moving parts to wear out or break down. The output is usually 4-20 mA, but the system can be integrated with the RS 485, HART and PROFIBUS protocols or with the Fieldbus Foundation. The indicator is capable to make measurements over a range extending from 30 cm. from the process connection up to 35 cm from the end of the weight of the sensor or the mounting point (see fig. 12).

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The instrument is mounted by BSP 1 ½ "or NPT process connection, or a flange. There are two models, one with a 4-mm cable. diameter (maximum probe length: 10m) and one with a cable of 8 mm. diameter (maximum length of the tube: 20 m). During filling and emptying the product has a load that is limited to a maximum value should not exceed: 10 ki '\\ l for 4 mm cable. and 40 kN for the 8 mm. and of course, mounting on top of the silo must be capable of withstanding these forces. To prevent the probe tip to a hose or a link to Excel, the counterweight end of the string should be facing toward the silo and the diameter of the tubing should be at least as high as wide (see fig . 13 (A)). To prevent false changes in impedance, the cable must hang freely at least 30 cm. wall or any other object in the silo. You can mount this system partially filled silo, after making a map of partial adjustment, the system is ready for measurement. During unloading, silo cable fully stretched (see Fig 13 (B)). If the product is expected to move the cable to less than 30 cm. silo wall, the tip of the probe can be anchored to the bottom (or tapered) of the silo. If, moreover, is tied with no insulation will act as a ground.

Figure 13. Forms installation of the instrument.

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Mount Models

There are two types of models available for various lengths of maximum measurement (see fig. 14). The first type, with cable 4 mm., Comprises a maximum effective measuring length of 19m35, and withstand a maximum pulling force of 10 kN. The second type has a cable of 8 mm. in diameter that allows effective measurement range of 19m35 and resists a tensile force of friction up to 40 kN, therefore, has a special mechanical coupling mechanism (see the left side of Fig. 14) between the head and thread process connection or flange. Both types can be used with a probe or weight tension clamps according to the above cases. The probes bound with tension clamps are recommended if the movement of product may cause unwanted motion sensor cable. Sometimes the tube can be coated with a corrosion resistant material. Longer probes are available as special version.

Figure 14. probes to solid material.

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measuring instruments guided microwave level used in all types of silos, containers, other containers you dry or raw materials whose grain size is less than 20 mm. and the measurement range does not exceed 20 m. Even in powder materials like fly ash and cement as well as grains, granules or flakes, these instruments provide a good result if the product has a dielectric constant as low as 1.8. Even tall and slender silos or in silos of special shapes with studs or obstacles, these gauges provide a reliable measurement. Can also be used in applications with powdery materials such as sawdust) fabrics or bright reflective products. Assembly is easy (see fig. 15) and their adjustment tends to come from the factory.

Figure 15. Example applications on powdery materials.

Set

As mentioned, the setting of indicators guided microwave level to the specified lengths is usually done at the factory. It is only necessary to make a new setting ("map" of the probe) if there are obstacles to less than 30 cm. probe cable, or if it cuts. The "map" of the probe is carried out in an empty silo by adjusting the matrix of four keys that are in electronic or remote calibration by intelligent communication, for example, HART.