Dry material level switches are used to detect the level of dry materials, especially fly ash, cement, plastics, flour, powders, sand, grain, carbon black, and wood chips, along process lines, or in tanks or vessels. A probe within each dry material level switch is used to read the level of the media, and when a predetermined limit is reached, the switch snaps closed electronically. This action allows the flow to maintain an appropriate rate by making sure that extra material does not clog the system. When the media level drops below a second predetermined level, the switch opens, allowing the flow to continue.
When determining which of the dry material level switches will best fit the needs of the application, it is important to consider the following: Will the switch be used to process only one media type, or will there be numerous powders being transferred? Additionally, will it be used to pass slurries or liquids? Will the media be of elevated temperature, corrosive, or reactive? Once this is determined, questions should focus on whether a contact or noncontact probe would fit best with the application, and what sensing technology is most appropriate.
Dry material level switches are available using many sensing technologies. Listed below are some of the most common technology types and their methods of level detection. Differential pressure sensors are used to measure head pressure in a vessel due to height of material in vessel. Ultrasonic devices measure the length of time it takes for a reflected sound wave to return to a transducer. Transmission time changes are detected as proportional to the level of the media as it rises. Radar and microwave dry material level switches emit a microwave pulse toward the process material. The pulse is reflected back by the surface of the material and picked up by a receiver. Level is inferred from the time of flight. Optical devices use the principle of optical refraction to detect the presence or absence of media. Typically LEDs are used as the source of light. Pressure membrane devices use a pressure sensitive switch to read and transmit pressure levels to an internal sensor via an organic, or thin metal, membrane. Electrical conductivity or resistance switches use a low-voltage power source applied across separate electrodes. A conductive liquid contacting both probes completes a conductive circuit. Capacitive and RF admittance dry material level switches use a radio frequency technique based on differing dielectric constants. When materials approach the sensing region, impedance is generated within the RF signal. Air bubblers use a constant volumetric rate of gas flow (usually air) fed through a bubble tube submerged in the flow. The amount of pressure required to force the bubble gas out of the bottom of the bubble tube is equal to the hydrostatic pressure at that point. Mechanical floats use sealed reed switches in a stem with a permanent magnet installed in the float. As the float rises or falls, the switch is activated. Rotation paddles are driven by a small electric motor, which rotates freely in the absence of material. When the paddle is impeded by material, the motor senses this and triggers the switch. Tuning forks use various technologies (often piezoelectric) to vibrate probe, then constantly monitor the presence or absence of that vibration. The presence of media alters the vibration.
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