1 Overview
In today's rapidly changing technology, level measurement technology has evolved from the initial weight, cast foot and other traditional mechanical measurement (Fig. 1) is a measurement technology includes high-tech radar, nuclear radiation, covering industrial production the vast majority of level measurement needs, nuclear radiation technology because of its specificity, the application has its limitations, which I will not repeat them here, but comes originally developed by the military radar radar level measurement technology is becoming an increasingly level measurement mainstream trend.
The so-called radar measurement technology contains two: microwave radar (non-contact) and guided wave radar, wherein the microwave radar technology with lower costs and excellent measurement performance in many applications in harsh conditions of the broad masses of user acceptance, but It does not mean that microwave radar technology is a universal measurement technology can measure all of the media; in relative terms, the new guided wave radar technology (Guide Wave Radar) just to make up for the shortcomings of microwave radar.
2. Comparison of radar technology and the principle of guided wave radar
As the name implies, microwave radar level gauge refers to the space launch, transmission and reception of electromagnetic microwave level measuring instruments (Figure 2), guided wave radar is through some form of waveguide to transmission, transmission and reception of electromagnetic microwave, level measurement instrument.
Microwave radar level measuring instrument has the following advantages:
（1）, no contact with the media;
（2）, high-frequency electromagnetic signals transmitted over long distances is easy, a lot of the process can be measured;
（3）, is not affected by changes in the gas phase space
Microwave radar by transmitting and receiving a high frequency (GHz) electromagnetic wave level is calculated to achieve the level of electromagnetic waves reflected back to the surface and the time to perform the receiving antenna level measurement, and less susceptible to electromagnetic energy transfer properties of propagation space limitations, it can at high / low pressure (vacuum) or with a vaporization medium propagation conditions, and fluctuations in the gas does not affect the propagation of electromagnetic waves.
Radiation ordinary microwave radar level measurement instrument antenna is about 1mW, is a weak signal, when this signal is spread through the air, the energy decreases rapidly when the level reaches the surface of the microwave signal and the reflected signal strength That is the amplitude, have a direct relationship with the dielectric constant of the dielectric constant with very low non-conductive medium such as hydrocarbon liquids, the reflected signal is very weak, which has been weakened signal is returned to the tank mounted on top the way of the receiving antenna, the energy further decay, microwave radar level gauge the return signal energy received approximately only 1% of the signal energy issue! When the above conditions will be used for media, contacts, micro wave radar level meter performance indicators decreased or even not work properly.
To compensate for these shortcomings contact radar level meter, guided wave radar level instruments emerged, guided wave radar works very similar to the foregoing, the foundation GWR is time-domain electromagnetic reflective TDR (TimeDomainRefectory) and ETS isochronous sampling principle. Over the years TDR has been detected for buried cables and wall laying cable breakage.
When measuring cable breakage, electromagnetic pulse propagation along the cable TDR generator issued at arrival decapitated measuring the reflected pulse is generated (Figure 3); at the same time, in the receiver preset and total cable length of the corresponding impedance changes also led to a reference pulse, the reflected pulse compared with the reference pulse, can pinpoint the location of breakage. The principles for level measurement, the TDR generator generates hundreds of thousands of energy per pulse conduction along the waveguide body, when it reaches the surface of the medium to produce a level reflecting the original pulse; at the same time at the top of the probe pre-setting a fixed impedance value, for generating a reliable reference pulse, which is also called the baseline pulse reflected pulse. Radar level gauge to check the level reflecting the original pulse, and the reflected pulse is compared with the baseline to give the value of level measurement, which is guided wave radar level meter works.
ETS and other sampling theory when used to measure speed, low-power electromagnetic signals, is the key to TDR level measurement technique based applications. As we all know, a short distance measuring electromagnetic signal speed is very difficult, ETS real-time capture of electromagnetic signals (UIS), and reconstructs them in equivalent time, to facilitate the better use of advanced technology to measure.
Level measurement technology to today, there have been many types of mature and reliable level measurement instrument, with its different properties and specific scope plays an important role in the level measurement in different conditions: pressure / differential pressure measurement solution counterpoint; RF Admittance / capacitance level gauge; ultrasonic level meter and float level gauge, etc. have had several years of successful experience in the industry, but the guided wave radar level gauge its obvious use advantages and performance more worthy of our attention:
1) low energy consumption. GWR signal energy output waveguide probe is small, about the non-contact radar transmitters 10% energy. This is because the waveguide to provide a fast and efficient channel for the signal to the surface of the propagation medium, attenuation of the signal is maintained at a minimum level.
2) energy intensity. Since the signal transmitted in the waveguide and is not affected by fluctuations in the liquid level in the tank and other obstacles, and thus return the instrument to the received signal energy corresponding to a strong, approximately 20% of the emitted energy.
3) a wide range. Based on a characteristic for low dielectric media GWR have satisfactory performance, Siemens LG200 product (Figure 4), for example, the lowest dielectric constant which can be actually measured 1.4!
4) the dielectric constant change has no effect on the measurement performance. Signal from hydrocarbons (dielectric constant of 2 to 3) or a water surface of the liquid (dielectric constant 80) as reflected by time, except that the difference signal amplitude, the microwave radar to be considered before the medium further screened to obtain accurate measurements, Filter change is assumed that the received signal strength, the signal will be affected; guided wave radar can avoid a lot of energy concentration due to the interference no such concerns.
5) Density change had no effect on the measurement. Impact of changes in the media immersed in the medium density object suffered buoyancy, but does not affect the electromagnetic wave propagation in the waveguide.
Minimal impact 6) media on the probe / cable hanging material for level measurement. Linked material there are two: membranous and bypass, a film linked material is decreased when the material level, high-viscosity media in covering layer formed on the probe, which hang on the probe material in a uniform distribution, so almost no measurements influence; But hanging material able to form bridges lead to significant measurement error, so in the selection of double-pole / cable type conductor, need careful, fully taking into account the viscosity of the medium!
3. Siemens guided wave radar level gauge Introduction and Applications
Siemens AG as the world's leading suppliers of electrical equipment automation, on-site measuring instruments in the field is also the forefront! Especially after former Canadian Milltronics instrument company incorporated in 2000, Siemens, Siemens process instrumentation is in level measurement in a leading position!
In 2007, Siemens Totally Integrated Automation uphold (TIA) concept, the grand launch of a new guided wave radar level gauge SITRANSLG200, it is short-range level for a liquid and a solid, level / interface and volume Guided Wave Radar level Meter measurement. It is not changing process conditions, high temperature and pressure, influence and steam. have:
1) Up to 16 kinds of forms of measuring probes / cable;
2) the accuracy of 2.5mm;
3) low dielectric constant of 1.4
3) up to 427 ° C temperature and 431Bar pressure;
4) Manual button and SIMATICPDM two ways to set parameters.
The products in question soon Shougang Jingtang project limelight in one fell swoop! Shougang Jingtang United Iron & Steel Co., Tangshan jointly by the Shougang Iron and Steel Group Co., Ltd. jointly launched the new site is located in Caofeidian between Tanggu New Port and Qinhuangdao Port, Bohai Bay has the only natural deep-water channel. Caofeidian water, electricity and transportation and other supporting facilities, with the end of 2007 in Beijing Shougang steel output compression of 400 million tons, the new Shougang Caofeidian will be put into operation. The main scale construction projects: raw materials field, pellet systems, sintering system, coking system, iron / steel system, cold system, hot-rolled
Systems for power generation systems, water supply systems and other supporting public auxiliary facilities, the construction scale of 9.8 million t / a. Product target is hot-rolled, cold-rolled, galvanized and cold-rolled silicon steel sheet and other high-end products, including high-strength, ultra-deep drawing of car panels, high strength steels and structural steel for the product logo, mainly to meet the automotive, electronics, home appliances , building and construction, machinery and other industry demand for high-quality sheet metal varieties.
Shougang Jingtang project, Siemens SITRANSLG200 guided wave radar level gauge is mainly used for seal tank, condensate tank level measurement site, and in the installation and construction phase, they also sent professional and technical personnel for technical guidance this product is simple parameter settings easy to operate, easy to install way to get our project approved use in the field of personnel, since April this year, Siemens has products installed LG200 performance is stable and reliable!
