Briefing of Level sensor

Level sensors detect the level of liquids and other fluids as well as fluidized solids,including slurries, granular materials and powders with an upper free surface.A flowing substance becomes essentially horizontal in its container (or other physical boundary) due to gravity,while most bulk solids pile up to a peak at some angle of repose.The substance to be measured can be in a container or in a natural state (such as a river or lake).Level measurements can be continuous or point values.Continuous level sensors measure the level within a specified range and determine the exact amount of material at a specific location,while point level sensors simply indicate whether material is above or below the sensing point.Usually the latter detects levels that are too high or too low.There are many physical and application variables that affect the selection of the best liquid level monitoring method for industrial and commercial processes.Selection criteria include physical: phase (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric constant of the medium, density (specific gravity) of the medium, agitation (motion),acoustic or electrical noise, vibration , machinery, electric shocks, size and shape of tanks or litter boxes.Also important are application constraints: price,accuracy, appearance,speed of response,ease of calibration or programming,physical size and mounting of the instrument,monitoring or control at continuous or discrete (point) levels.In short,liquid level sensor is one of the very important sensors that play a very important role in various consumer/industrial applications.Like other types of sensors,liquid level sensors are available or can be designed using various sensing principles.It is important to choose the right type of sensor for the application requirements.

Point and continuous level detection for solids 

A variety of sensors are available for point level detection of solids.These include vibration,rotary paddle,mechanical (diaphragm), microwave (radar),capacitive,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 adjustment, they can also sense levels of highly mobile powders and electrostatic materials.Single probe vibrating level sensors are ideal for bulk powder levels. Since only one sensing element contacts the powder,bridging between the two probe elements is eliminated and media build-up is minimized.The vibration of the probe tends to remove material buildup on the probe element.Vibrating level sensors are unaffected by dust, static charge buildup from dielectric powders, or changes in conductivity, temperature,pressure,humidity or moisture content. A tuning fork vibration sensor is another option.They tend to be less costly, but tend to build up material between the tines,

Rotating paddle

Rotary paddle level sensors are a very old and well-established technology for point level indication of bulk solids.This technique uses a low-speed geared motor that spins the paddle wheel.When the paddle is stalled by solid material, the motor rotates on its shaft through its own torque until the flange mounted on the motor contacts the mechanical switch.The paddles can be made of a variety of materials, but sticky material must not be allowed to build up on the paddles.Buildup can occur if the material being processed becomes tacky due to high moisture content in the hopper or high ambient humidity.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 entering the shaft bearings and motor by properly positioning the paddles in the hopper or hopper and using proper seals.

Admittance-type

RF admittance level sensors use a rod probe and an RF source to measure changes in admittance.The probe is driven through a shielded coaxial cable to eliminate the effect of changing the cable's capacitance to ground.When the liquid level around the probe changes,a corresponding change in the dielectric is observed.This changes the admittance of this defective capacitor and this change is measured to detect level changes.