Level sensors detect the level of liquids and other fluids and fluidized solids, including slurries, granular materials, and powders that exhibit an upper free surface. Substances that flow become essentially horizontal in their containers (or other physical boundaries) because of gravity whereas most bulk solids pile at an angle of repose to a peak. The substance to be measured can be inside a container or can be in its natural form (e.g., a river or a lake). The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low. There are many physical and application variables that affect the selection of the optimal level monitoring method for industrial and commercial processes. The selection criteria include the physical: phase (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric constant of medium, density (specific gravity) of medium, agitation (action), acoustical or electrical noise, vibration, mechanical shock, tank or bin size and shape. Also important are the application constraints: price, accuracy, appearance, response rate, ease of calibration or programming, physical size and mounting of the instrument, monitoring or control of continuous or discrete (point) levels. In short, level sensors are one of the very important sensors and play very important role in a variety of consumer/ industrial applications. As with other types of sensors, level sensors are available or can be designed using a variety of sensing principles. Selection of an appropriate type of sensor suiting to the application requirement is very important.
Point and continuous level detection for solids A variety of sensors are available for point level detection of solids. These include vibrating, rotating paddle, mechanical (diaphragm), microwave (radar), capacitance, optical, pulsed-ultrasonic and ultrasonic level sensors.
Vibrating point
These detect levels of very fine powders (bulk density: 0.02–0.2 g/cm3), fine powders (bulk density: 0.2–0.5 g/cm3), and granular solids (bulk density: 0.5 g/cm3 or greater). With proper selection of vibration frequency and suitable sensitivity adjustments, they can also sense the level of highly fluidized powders and electrostatic materials. Single-probe vibrating level sensors are ideal for bulk powder level. Since only one sensing element contacts the powder, bridging between two probe elements is eliminated and media build-up is minimized. The vibration of the probe tends to eliminate build-up of material on the probe element. Vibrating level sensors are not affected by dust, static charge build-up from dielectric powders, or changes in conductivity, temperature, pressure, humidity or moisture content. Tuning-fork style vibration sensors are another alternative. They tend to be less costly, but are prone to material buildup between the tines.
Rotating paddle Rotating paddle level sensors are a very old and established technique for bulk solid point level indication. The technique uses a low-speed gear motor that rotates a paddle wheel. When the paddle is stalled by solid materials, the motor is rotated on its shaft by its own torque until a flange mounted on the motor contacts a mechanical switch. The paddle can be constructed from a variety of materials, but tacky material must not be allowed to build up on the paddle. Build-up may occur if the process material becomes tacky because of high moisture levels or high ambient humidity in the hopper. For materials with very low weight per unit volume such as Perlite, Bentonite or fly ash, special paddle designs and low-torque motors are used. Fine particles or dust must be prevented from penetrating the shaft bearings and motor by proper placement of the paddle in the hopper or bin and using appropriate seals.
Admittance-type An RF admittance level sensor uses a rod probe and RF source to measure the change in admittance. The probe is driven through a shielded coaxial cable to eliminate the effects of changing cable capacitance to ground. When the level changes around the probe, a corresponding change in the dielectric is observed. This changes the admittance of this imperfect capacitor and this change is measured to detect change of level.
Point level detection of liquids Typical systems for point level detection in liquids include magnetic and mechanical floats, pressure sensors, electroconductive sensing or electrostatic (capacitance or inductance) detectors—and by measurement of a signal's time-of-flight to the fluid surface, through electromagnetic (such as magnetostrictive), ultrasonic, radar or optical sensors.
Magnetic and mechanical float
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