Capacitance type Level Limit Switch Introduction- RF-Capacitance point level detection is based on the change in capacitance of the capacitor formed by sensing probe and the wall of the container. The material acting as dielectric modifies the value of this capacitor - as explained below in operating principle. In case if wall is non-metallic, a secondary ground probe or reference probe may be required. RF-Capacitance Level Limit switch mostly used for free flowing liquids and free-flowing homogeneous solids. Trumen LMC-RF Capacitance Level switch can be used for multi point level switching up to four independent switching point on the single probe with remote electronic version. LMC range of devices are available in ultra-wide range truly universal power-supply, suitable for AC as well as DC supply operation i.e 12-80V DC or 12-260V AC on same terminal for integral model. Potential free DPDT relay output make your automation easy. IP-68 protection class provided for electronics housing. Another options of interest are PNP/NPN NO types which operates exclusively on DC supply over a wide range. Trumen Technologies is also supplying ultra low power 4/20mA loop powered suitable for various hazardous zone applications A short list of advantages with Trumen LMC: Since Trumen LMC comes with various power-supply and switching output options, users of Trumen LMC have more options at their dispense. Though most common among the industry is potential-free output with universal power supply, there are various other output options which have a sizable use across the process plants in various industries. Trumen offers: LMC-UD Integral Models with Truely Universal Power Supply & DPDT relay output. 12VDC to 80VDC 12VAC to 260VAC LMC-PN PNP DC with 12VDC to 60VDC LMC-TW Two Wire Loop Powered 8/16mA to lessen the wiring cost and implement faster automation IP68 Enclosure for all integral models. LMC-ER Remote Model with 80VAC to 260VAC Universal Power Supply LMC-ER Upto max 4 simultenous SPDT relays when required. All of the Trumen LMC: Integral or Remote, have bar graph LED process indicator to assist in installation and calibration Operating Principle LMC RF Capacitance type level sensing best suited where single probe level sensing is required and material is not sticky. The operating principle is simply that of capacitance of parallel plates. Referring to the figure on left, consider two identical metal plates, plate1 and plate2, facing each-other, having area Aseprated by distance d. The capacitance of such parallel plate capacitor is given by the expression C = ε0 εr A/d where, ε0 is the permittivity of free space (=8.85418782 pF/meter) is a universal constant, and εr is the relative permittivity of the medium separating the plates, also known as the dielectric constant of the material (air = 1.00059 at atmospheric pressure). Since, for any given prallel plate capacitor, ε0 is always a constant and if Area A and sepration d are kept uchanged by the design, then the Capacitance C is only depedent upon relative permittivity or dielectric constant εr of material. For example, for dry Sand, the dielectric constant is approximately 3 to 6 (say 4), it means that the Capacitance of parallel plate capacitor will be 4 times more than the same having air (or no material as it is commonly called in material level sensing terminology). Therefore, presence of sand can be sensed by measuring the change is capacitance caused by rising or falling of material level. The only issue in material sensing using capacitance change method is that, parallel plate capacitors are too inconvenient to be used a sensing device in process plants and field, therefore, a slight modification to the prallel plate is required. The plates may or may not be parallel, but one of the plates is always the container wall, while other plate is shaped as a rod. The separation at fitting end is done using stable dielectric materials like PTFE, ceramics etc, while the process material Trumen RF Capacitance type level limit switch is modeled on the same lines. The process connection e.g.threaded, flanged, sanitary etc, are mounted to the vessel or container wall, become on of the metal plate of the capacitor, and the sensing rod becomes another. The PTFE insulation block used to separate the process connection and the sense rod must be strong enough to endure the process conditions like material bulk, material movement and the sense rod must be innert to the material physical and chemical properties. Now consider an RF oscillator having a Voltage V and frequency f, is feeding its output to the sensing rod and the vessel wall, then an electric current I, due to the capacitance C will flow into the circuit. The value of this RF Current can be calculted simply using ohms law as: I = 2πfCV Suppose a current meter which can measure this RF current is connected in series as shown in the image on left, then, presense or absence of material can be sensed by judging the reading of this current. Since a judgment or a comparision is needed, RF Capacitance type level sensors need a calibration prior to use in order to give proper switching output for a given type of service material used in the process.
Posted on: Tue, 09 Sep 2014 14:48:46 +0000
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