Level sensor
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 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. 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, temperature, pressure or vacuum, chemistry, dielectric constant of medium, density of medium, agitation, 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 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, microwave, capacitance, optical, pulsed-ultrasonic and ultrasonic level sensors.Vibrating point
These detect levels of very fine powders, fine powders, and granular solids. 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 detectors—and by measurement of a signal's time-of-flight to the fluid surface, through electromagnetic, ultrasonic, radar or optical sensors.Magnetic and mechanical float
The principle behind magnetic, mechanical, cable, and other float level sensors often involves the opening or closing of a mechanical switch, either through direct contact with the switch, or magnetic operation of a reed. In other instances, such as magnetostrictive sensors, continuous monitoring is possible using a float principle.With magnetically actuated float sensors, switching occurs when a permanent magnet sealed inside a float rises or falls to the actuation level. With a mechanically actuated float, switching occurs as a result of the movement of a float against a miniature switch. For both magnetic and mechanical float level sensors, chemical compatibility, temperature, specific gravity, buoyancy, and viscosity affect the selection of the stem and the float. For example, larger floats may be used with liquids with specific gravities as low as 0.5 while still maintaining buoyancy. The choice of float material is also influenced by temperature-induced changes in specific gravity and viscosity – changes that directly affect buoyancy.
Float-type sensors can be designed so that a shield protects the float itself from turbulence and wave motion. Float sensors operate well in a wide variety of liquids, including corrosives. When used for organic solvents, however, one will need to verify that these liquids are chemically compatible with the materials used to construct the sensor. Float-style sensors should not be used with high viscosity liquids, sludge or liquids that adhere to the stem or floats, or materials that contain contaminants such as metal chips; other sensing technologies are better suited for these applications.
A special application of float-type sensors is the determination of interface level in oil-water separation systems. Two floats can be used with each float sized to match the specific gravity of the oil on one hand, and the water on the other. Another special application of a stem type float switch is the installation of temperature or pressure sensors to create a multi-parameter sensor. Magnetic float switches are popular for simplicity, dependability and low cost.
A variation of magnetic sensing is the "Hall effect" sensor which utilizes the magnetic sensing of a mechanical gauge's indications. In a typical application, a magnetism-sensitive "Hall effect sensor" is affixed to a mechanical tank gauge that has a magnetized indicator needle, so as to detect the indicating position of the gauge's needle. The magnetic sensor translates the indicator needle position into an electrical signal, allowing other indication or signalling.
Pneumatic
Pneumatic level sensors are used where hazardous conditions exist, where there is no electric power or its use is restricted, or in applications involving heavy sludge or slurry. As the compression of a column of air against a diaphragm is used to actuate a switch, no process liquid contacts the sensor's moving parts. These sensors are suitable for use with highly viscous liquids such as grease, as well as water-based and corrosive liquids. This has the additional benefit of being a relatively low cost technique for point level monitoring. A variation of this technique is the "bubbler", which compresses air into a tube to the bottom of the tank, until the pressure increase halts as the air pressure gets high enough to expel air bubbles from the bottom of the tube, overcoming the pressure there. The measurement of the stabilized air pressure indicates the pressure at the bottom of the tank, and, hence, the mass of fluid above.Conductive
Conductive level sensors are ideal for the point level detection of a wide range of conductive liquids such as water, and is especially well suited for highly corrosive liquids such as caustic soda, hydrochloric acid, nitric acid, ferric chloride, and similar liquids. For those conductive liquids that are corrosive, the sensor's electrodes need to be constructed from titanium, Hastelloy B or C, or 316 stainless steel and insulated with spacers, separators or holders of ceramic, polyethylene and Teflon-based materials. Depending on their design, multiple electrodes of differing lengths can be used with one holder. Since corrosive liquids become more aggressive as temperature and pressure increase, these extreme conditions need to be considered when specifying these sensors.Conductive level sensors use a low-voltage, current-limited power source applied across separate electrodes. The power supply is matched to the conductivity of the liquid, with higher voltage versions designed to operate in less conductive mediums. The power source frequently incorporates some aspect of control, such as high-low or alternating pump control. A conductive liquid contacting both the longest probe and a shorter probe completes a conductive circuit. Conductive sensors are extremely safe because they use low voltages and currents. Since the current and voltage used is inherently small, for personal safety reasons, the technique is also capable of being made intrinsically safe to meet international standards for hazardous locations. Conductive probes have the additional benefit of being solid-state devices and are very simple to install and use. In some liquids and applications, maintenance can be an issue. The probe must continue to be conductive. If buildup insulates the probe from the medium, it will stop working properly. A simple inspection of the probe will require an ohmmeter connected across the suspect probe and the ground reference.
Typically, in most water and wastewater wells, the well itself with its ladders, pumps and other metal installations, provides a ground return. However, in chemical tanks, and other non-grounded wells, the installer must supply a ground return, typically an earth rod.