Level Sensors

Level sensors are used to detect liquid or powder levels, or interfaces between liquids. There are two basic level-measurement types: continuous and point or multi-point. Continuous level sensors measure liquid or dry material levels within a specified range and provide outputs which continuously indicate the level. Point level sensors or multi-point sensors mark a specific level and are used generally as a high alarm or switch. These level sensors can also be integrated in a single device to add a low alarm, or to serve as a stepped version of a continuous level. For both types of level sensors, choices for output options include analog voltage signal, analog current signal, frequency or modulated frequency, and alarm or visual indicator.   

There are three major device classifications for level sensors: sensor-only, sensor system, and transmitter. Sensor-only devices provide outputs for processors, controllers, or data acquisition systems; however, they do not have a display or user interface. Typically, sensor-only devices are used in applications that require a switch. Level sensor systems are simple instruments or gauges. These level sensors provide more capabilities than sensor-only (transducer) devices, and may include a display and/or user interface. Generally, level sensor systems are designed for applications that require a simple gauge, a locally-programmable (stand-alone) unit, or a controller/totalizer. Sensor transmitters are designed to transmit data from the sensor via a two-wire current loop. The loop has an external power supply, and the transmitter acts as a variable resistor with respect to its input signal. 

Level sensors are available with many different meter technologies. Choices include air bubbler, capacitive or radio frequency (RF) admittance, differential pressure, electrical conductivity or resistance, mechanical or magnetic float, and optical. Level sensors may also use pressure membrane, radar or microwave, radio frequency, and sonic or ultrasonic meters. Level sensors with tuning forks use various technologies (typically piezoelectric) to vibrate a probe and then constantly monitor the presence or absence of that vibration. Other unlisted, specialized, or proprietary meter technologies are also available from some level sensor suppliers. 

Level sensors carry specifications for measuring range, allowable sensor over-range, process operating conditions, mounting style, communication interface, and instrument style. Measuring range and allowable sensor-over range are important parameters to consider when selecting level sensors. There are two process operating conditions to consider: maximum operating pressure and material temperature range. Mounting styles are defined as side-mount, top-mount, and bottom-mount. There are two basic communication interfaces: serial and parallel. In terms of instrument options, level sensors may feature non-electrical visual or audio outputs, analog meters or indicators, digital readouts, or video display terminals (VDT). Choices for instrument user interface are analog front panel, digital front panel, and computer-controllable.  

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