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Non-contact measurement & inspection


Contactless fluid-level measurement using a reflectometer chip


In this article, Bruce Hemp, senior applications engineer at Analog Devices, explains why the development of the ADL5920, a single-chip reflectometer device, brings new types of applications, such as fluid-level instrumentation


luid-level measurements can be accurately measured through the wall of a nonmetallic tank by placing an air-dielectric transmission line up against the side of the tank and sensing the RF impedance. This article provides an empirical design example that illustrates how a reflectometer device such as the Analog Devices ADL5920 can simplify the design. Compared to traditional methods of fluid-


F


level sensing that might involve mechanical floats, the reflectometer-based solution offers several benefits, including:


Fast, real-time fluid-level measurements


Extensive electronic postprocessing becomes possible


Contactless design (no contamination of the liquid)


No moving parts Minimal radiated RF field (far field cancels)


No holes in the tank for an internal sensor (reduce possibility of leaks)


Intrinsic safety, due to no electrical wires or parts in the tank


FLUID-LEVEL MEASUREMENT OVERVIEW Figure 1 shows a block diagram of the overall system, consisting of an RF signal source driving a balanced and terminated air-dielectric transmission line with a reflectometer located inline.


PRINCIPLE OF OPERATION Transmission lines suspended in air can be designed for precise characteristic impedance and low RF loss as a result of low loss conductors and the lack of solid dielectric material. Classical plots of E and H vectors show that the electric and magnetic fields are concentrated around the conductors, and their magnitude decays quite rapidly with distance, where distance is measured relative to the size and spacing of the transmission line structure itself. Any nearby dielectric material such as a fluid tank wall and the fluid within will alter the transmission line’s electrical characteristics, which can be summarily measured with a reflectometer such as the ADL5920.


DETAILED DESCRIPTION Consider the case of an air-dielectric, low loss transmission line designed for a specific


characteristic impedance ZO in air. Any added dielectric substance such as a fluid in the near field of the transmission line will:


Lower the characteristic impedance of the transmission line,


Reduce the velocity of propagation, thus increasing the effective electrical length of the line, and


Increase attenuation of the line.


All three of these effects can combine to create a reduction in return loss, which is directly measurable with a reflectometer device or instrument. With careful design and calibration, return loss can be correlated to fluid level. To simplify the analysis, consider the air-


Above the fluid level, the transmission line is air dielectric, except for the tank wall


material. Transmission line impedance ZOA is changed little from its air dielectric value,


ZO. The same is true for transmission line velocity of propagation.


Below the fluid level, the transmission line impedance ZOF becomes lower compared to ZOA. Electrical length effectively increases, as does attenuation, all because of the extra


dielectric material present in the near field of the transmission line.


The impedance of the termination ZO at the far end of the transmission line will be transformed


when measured by the reflectometer at the source end of the transmission line. The transformation is depicted graphically,


Figure 1. Fluid-level measurement system block diagram.


44


approximately as shown in Figure 2. Because ZOF is lower than ZO, a clockwise Smith chart rotation is created, as shown by the arrows.


June 2020 Instrumentation Monthly


Figure 2. Expanded, normalized Smith chart representation of transmission line input impedance. Trace endpoints depict how fluid level translates to a return loss measurement.


dielectric transmission line of Figure 1 with


impedance set equal to ZO before attaching the line to the tank. Because the line is terminated


with ZO, theoretically, there is no reflected energy, and return loss is infinite. After the transmission line is affixed to the side of


a tank, what was one transmission line now behaves as two separate transmission lines, cascaded in a series configuration:


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