Radio frequency (RF) admittance-based level sensing has been one of the best technologies for indicating and controlling interface applications. The very nature of most interface applications (conductive vs. nonconductive liquids) plays to the technological strengths of RF admittance technology. Recent improvements in ultrasonic level-sensing technology have been leveraged to enhance these abilities (see Fig. 1).

A typical interface application often consists of a nonconductive liquid over a conductive one, for example, a hydrocarbon over an aqueous solution. If there is a "clean" and defined separation between the conductive and nonconductive phases most RF- or capacitance-based liquid level systems produce an accurate indication of the interface's location. If the interface develops an emulsion, or "rag" layer that separates the phases, measurement becomes more complicated. In this situation most RF- or capacitance-based technologies will indicate that the interface is "somewhere" undefined in the rag layer.

To accurately define the liquid interface and the rag layer, RF admittance technology must be used. By careful selection of the sensing-element capacitance and transmitter frequency, users select whether they want to see the top or bottom of the rag layer. This is important if the user plans on "pulling off" product without including contamination that may be present in the rag layer.

In contrast, interface applications that have similar electrical characteristics, such as a layer of sand under water, cannot be measured effectively by RF-based systems. These systems see both water and sand as conductive materials and cannot differentiate between them. For these types of interfaces, an alternate technology is required. Interfaces with good separation of either acoustical or physical properties can be measured with an ultrasonic technology (Fig. 2).

In these applications, an ultrasonic sensor can be submerged in the upper phase. The sensor then uses the return echo to detect a heavier, dissimilar phase below it. This is particularly effective when there are enough solids in the lower phase or enough emulsion in a rag layer to allow a return echo. Primary uses of this technology are liquid/solid separation in water/wastewater treatment and refinery operations. It is also used in applications where liquids (water, crude oil, etc.) are extracted from wells or from settling (gravity separation) operations.

A method that combines both technologies can be used to indicate the top and bottom of a rag layer. For example, in a phosphorus storage application, the vessel must be flooded with water to prevent the element's exposure to air. By keeping the ultrasonic transducer in contact with the water, a device can be used to sense the return echo off the top of the rag layer. An RF admittance system can then be used to determine the bottom of the rag layer, providing complete information for interface control and rag layer thickness (Fig. 3).

In operation, the ultrasonic system ensures that as level changes, the rag layer does not become exposed to air and self ignite. The RF system watches the bottom of the rag layer to ensure that as the phosphorus is removed from the vessel bottom the rag layer is not pulled off with the phosphorus, causing possible contamination.

Another method of ensuring interface control uses a dual-input RF admittance system. The dual input "smart" transmitter works with two sensing elements—one element measures the material interface, the other corrects for changing process parameters that are not directly measured (Fig. 4).

In a typical interface application, as the electrical characteristics of the upper and lower phase become close to each other, the more difficult it is to recognize the difference between the two and provide an accurate level measurement. Dual-input systems leverage RF admittance technology and use the reference input to monitor the changing electrical characteristics of the semiconductive phase.

The reference sensor is mounted in a location where it will never see the interface level change and gathers data solely on the changing characteristics (or composition) of the semiconductive phase changes. As long as the conductivity of the semiconductive phase remains below a conductivity threshold determined by sensing element selection, it will accurately track the interface. This method makes real-time adjustments to the reference point based on its analysis of the semiconductive phase changes.

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