Interface measurement in liquids

Guided once – reflected twice

As a member of the plics family, the Vegaflex 67 measuring instrument offers considerably simplified detection of liquid interfaces. It utilises the guided microwave principle, also known as TDR, to carry out measurement. Vegaflex 67 thus operates completely independent of the density of the medium, and is easy to install and set up.

Dipl.-Ing. (FH) Wolfgang Perenthaler

Measuring the respective levels of two liquids that separate in a vessel at a more or less distinct line of separation, i.e. interface, is known as interface measurement. Oils or liquid gases on water, solvent traps as well as recovery of raw materials from production processes are typical areas of application for measuring systems that detect two levels in a single vessel. The applied measurement technology is always based on the physical or chemical differences between the two mediums. Float and displacer systems utilise the differing medium densities, laser systems the medium-specific refractive index of light, and radiometric systems the absorptance as well as difference in density. If the aforesaid material properties change, these measuring technologies quickly reach their limits.
The guided microwave measuring principle
The guided microwave (TDR = Time Domain Reflectometry, also called guided radar) is taking the lead alongside traditional measuring principles. Completely independent of medium density, refractive index and absorptance, Vegaflex 67 operates on the basis of the difference in dielectric value of the two mediums.
In the process, a microwave pulse (approx. 0 to 2 GHz) is transmitted to a rod, a cable or a rod in a coaxial system. Due to the high frequency technology, the so-called skin effect, the pulse runs not on the inside of the metallic conductor, but on its outer surface. Just like with conventional, free radiating radar instruments for level measurement, the microwave pulse is reflected when it comes into contact with a different impedance (the dielectric, i.e. the measured medium). The reflected signal is stronger or weaker depending on the dielectric value i.e. the conductivity. If the medium is nonconductive, as is the upper medium in the vessel, only a small part of the signal energy is reflected at the surface. The remaining energy passes through this medium. At the interface to the second medium another reflection takes place. Conductive mediums, however, reflect all the energy and are not penetrated by the microwave pulse.
Requirements on the measuring application
The reflective properties of conductive and nonconductive media determine the first rule for the application of microwave technology for interface measurement: the upper medium must not be conductive. Because the specific dielectric constant of air is 1, microwaves travel at the speed of light in air. In materials with a higher dielectric constant the transit velocity is accordingly lesser. The change in running time of the microwave can thus be calculated. If the upper medium is homogeneous and stabile with respect to the dielectric constant, a further criterion for this measuring principle is fulfilled.
The third requirement on the application is also derived from the reflective properties of media. A reflection at the interface can only happen when the lower medium has a considerably higher dielectric constant or is electrically conductive. In order for TDR to clearly separate the two echoes (absolute level and interface level), the dielectric constant of the lower medium has to be 10 higher than the upper medium, which must have a minimum thickness of 80 to 100 mm.
Application advantages
Compared with traditional interface measurement techniques, the advantages of this method are immediately obvious. In contrast to float and displacer systems, the guided microwave is entirely independent of the density of the medium. In normal operation, Vegaflex 67 enables not only the measurement of absolute level and layer thickness – it can also determine which of the two media is in the vessel if only one of them is momentarily present. With Vegaflex 67, Vega offers a measuring instrument that is not subject to wear and has no moving parts. It can be installed in open or closed vessels as well as in standpipes or bypass tubes. Setup is as easy as one, two, three. The sensor comes factory-adjusted and re-quires no further calibration in most applicati-ons. To achieve reliable measuring results, only the dielectric value of the upper medium needs to be entered during setup. Changing process parameters, such as other physical properties of the medium, for example, represent no problem for this sensor. A simple adjustment of the dielectric value is all that’s required to tailor the sensor to new process conditions.
Vegaflex 67 is part of Vega’s plics concept – a universal system that encompasses instruments and services. Easy planning, installation and setup, as well as easy servicing and ultrafast repair, are the major advantages that characterise this new product concept.
Signal processing with Hart, Profibus, FF
When the Vegaflex 67 two-wire sensor is digitally evaluated (Hart, Profibus PA or Foundation Fieldbus), it delivers information on the absolute level in the vessel as well as the position of the interface between the two mediums. It thus combines conventional level measurement and interface detection in one sensor. To convert both measured values into analogue current output, the Vegamet 625 signal conditioning instrument is applied. This Hart communication-based device with three current outputs (4…20 mA) can also output, beside level and interface, the layer thickness as a standard current signal on the third measuring channel.
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