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E-Zine September 2008Level measurement systems are used to sense the location of the interface between two or more materials in a fluid containment system. Level measurement systems generally consist of a level sensing device (sensor) and a level transmitter, although some designs have an integral sensor and transmitter. These devices are used to measure one or more interfaces. Common examples of interfaces where material levels can exist include liquid/gas, solid/gas, and liquid/liquid. Liquid/gas interfaces are commonly found in level measurement systems for tanks and other containment vessels. Solid/gas interfaces generally found in solids handling systems are indicative of the inventory of solids in the vessel. Liquid/liquid interfaces are often found in processes where a separation of the liquids occurs due to the different specific gravities of the liquids above and below the liquid/liquid interface. Other interfaces and multiple interfaces, such as slurry/liquid, slurry/gas, slurry/liquid/gas, and liquid/liquid/gas can exist in some processes. The material level is important to determine the inventory of the material in the vessel. These levels may be well defined or not. Liquid level in a tank is an example of a well-defined interface because (assuming no agitation) the liquid/gas interface arranges itself to be horizontal. The gas/solid interface in a vessel would usually not be well defined because solids generally do not arrange themselves to be horizontal, so a solids level measurement may not be indicative of the inventory in the vessel. Therefore, accurate measurement of level is dependent upon the process and the level measurement system. Level measurement systems generally consist of a level sensor and its associated electronics. As defined herein, level sensors are the devices that are used to sense the interface between two or more materials. Level sensors generally exploit one of a number of properties of the material or its interface. Level sensors may be in contact with the material or not. Some level sensors are designed to be in contact with the material and may or may not have moving parts. Others are designed to be in contact with the environment of the material. Still other level measurement systems allow the level sensors to be located externally and have no contact with the material or its environment. The electronic circuitry associated with the level sensor is the level transmitter. The level transmitter processes the signal(s) produced by the level sensor. The level sensor and level transmitter may be separate and distinct pieces of equipment, or both may be integrated into one assembly. Nonetheless, level measurement systems that produce an electronic output will have both level sensor and level transmitter functionality. Excerpted from Excerpted from The Consumer Guide to Non-Contact Level Gauges ISSN 1538-5280 |
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