Powder and Bulk Engineering
Tuesday, September 02, 2014
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Powder and Bulk Engineering is a technical publication devoted to readers in the bulk solids market, worldwide.
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Publication Date: 12/2005
Pages: 4
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What to consider when selecting a continuous level monitor — Part II

This two-part article explains what factors you need to consider when selecting a continuous level monitor for your storage or process vessel. Sections describe types of available monitors, how to consult a monitor manufacturer for selection advice, and material characteristics and vessel construction details you need to consider when choosing a monitor. Part I appeared in November; Part II concludes the article.

Considering your material characteristics

Moisture content and dielectric constant. Understanding how changing moisture content can affect your material’s dielectric constant during storage can also help you select a level monitor. For example, changes in moisture content can cause a capacitance monitor to give an incorrect level reading. This monitor uses a probe mounted parallel to its ground reference (typically the vessel sidewall), and the probe and sidewall form a capacitance circuit. The material level’s rise or fall causes a capacitance change in the circuit, which the monitor converts to a level measurement. To measure accurately, the capacitance monitor requires that the vessel contents have a consistent dielectric constant, which is a measure of the material’s ability to resist the formation of an electric field within it. (Dry air has a dielectric constant of about 1; bulk materials have a dielectric constant greater than 1, ranging from about 1.2 to about 100; and water’s dielectric constant ranges from about 40 to 80.) Introducing moisture into the vessel, or changing the moisture content in it, will change the material’s dielectric properties, in turn creating an error in the capacitance monitor’s level reading. For this reason, you should select the capacitance monitor only if your material has a consistent moisture level. Other continuous level monitors, such as microwave

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