Level Sensor

Level sensors are used to measure liquid level. The liquid 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: It measures level within a specified range and are used to know the exact amount of liquid in a certain place and
Point level sensors: It measures a specific level; generally this is used to detect high level alarms or low level alarms.

There are many physical and application variables that affect the selection of the optimal level monitoring solution for industrial and / or commercial processes.
The selection criteria include the physical: state (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric constant of medium, density or specific gravity of medium, agitation, acoustical or electrical noise, vibration, mechanical shock, tank or bin size and shape; and 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.

Level measurement methods

The most commonly used liquid-level measurement methods are:
• RF capacitance
• Conductance (conductivity)
• Hydrostatic head/tank gauging
• Radar
• Ultrasonic

RF Capacitance
RF (radio frequency) technology uses the electrical characteristics of a capacitor, in several different configurations, for level measurement. Commonly referred to as RF capacitance or simply RF, the method is suited for detecting the level of liquids, slurries, granulars, or interfaces contained in a vessel. Designs are available for measuring process level at a specific point, at multiple points, or continuously over the entire vessel height.
An electrical capacitance (the ability to store an electrical charge) exists between two conductors separated by a distance, d. The first conductor can be the vessel wall (plate 1), and the second can be a measurement probe or electrode (plate 2). The two conductors have an effective area, A, normal to each other. Between the conductors is an insulating medium—the nonconducting material involved in the level measurement.
With the tank empty, the insulating medium between the two conductors is air. With the tank full, the insulating material is the process liquid or solid. As the level rises in the tank to start covering the probe, some of the insulating effect from air changes into that from the process material, producing a change in capacitance between the sensing probe and ground. This capacitance is measured to provide a direct, linear measurement of tank level.

Conductance
The conductance method of liquid level measurement is based on the electrical conductance of the measured material, which is usually a liquid that can conduct a current with a low-voltage source (normally <20 V). Hence the method is also referred to as a conductivity system. Conductance is a relatively low-cost, simple method to detect and control level in a vessel.
One common way to set up an electrical circuit is to use a dual-tip probe that eliminates the need for grounding a metal tank. Such probes are generally used for point level detection, and the detected point can be the interface between a conductive and nonconductive liquid.
There are two dual-tip probes that detect maximum and minimum levels. When the level reaches the upper probe, a switch closes to start the discharge pump; when the level reaches the lower probe, the switch opens to stop the pump.

Hydrostatic Head
One of the oldest and most common methods of measuring liquid level is to measure the pressure exerted by a column (or head) of liquid in the vessel. The basic relationships are:
P = mHd
where,
P = pressure (pounds per square inch)
m = a constant
H = head (in feet)
d = density (in pounds per cubic feet)
Though the DP transmitter is most commonly used to measure hydrostatic pressure for level measurement. One newer system uses a pressure transmitter in the form of a stainless steel probe that looks much like a thermometer bulb. The probe is simply lowered into the tank toward the bottom, supported by plastic tubing or cable that carries wiring to a meter mounted externally on or near the tank. The meter displays the level data and can transmit the information to another receiver for remote monitoring, recording, and control.
Another newer hydrostatic measuring device is a dry-cell transducer that is said to prevent the pressure cell oils from contaminating the process fluid. It incorporates special ceramic and stainless steel diaphragms and is apparently used in much the same way as a DP transmitter.

Radar or Microwave
It operates on the principle of beaming microwaves downward from a sensor located on top of the vessel. The sensor receives back a portion of the energy that is reflected off the surface of the measured medium. Travel time for the signal (called the time of flight) is used to determine level. For continuous level measurement, there are two main types of noninvasive systems, as well as one invasive type that use a cable or rod as a wave guide and extend down into the tank’s contents to near its bottom.
One type of noninvasive system uses a technology called frequency-modulated continuous wave (FMCW). From an electronic module on top of the tank, a sensor oscillator sends down a linear frequency sweep, at a fixed bandwidth and sweep time. The reflected radar signal is delayed in proportion to the distance to the level surface. Its frequency is different from that of the transmitted signal, and the two signals blend into a new frequency proportional to distance. That new frequency is converted into a very accurate measure of liquid level.
The second noninvasive technology, pulsed radar or pulsed time-of-flight, operates on a principle very similar to that of the ultrasonic pulse method. The radar pulse is aimed at the liquid’s surface and the transit time of the pulse’s re turn is used to calculate level. Because pulse radar is lower power than FMCW, its performance can be affected by obstructions in the tank as well as foam and low-dielectric materials (K < 2).
Guided-wave radar (GWR) is an invasive method that uses a rod or cable to guide the micro wave as it passes down from the sensor into the material being measured and all the way to the bottom of the vessel. The basis for GWR is time-domain reflectometry (TDR), which has been used for years to locate breaks in long lengths of cable that are underground or in building walls. A TDR generator develops more than 200,000 pulses of electromagnetic energy that travel down the waveguide and back. The dielectric of the measured fluid causes a change in impedance that in turn develops a wave reflection. Transit time of pulses down and back is used as a measure of level.

Ultrasonic and Sonic
Both ultrasonic and sonic level instruments operate on the basic principle of using sound waves to determine fluid level. The frequency range for ultrasonic methods is ~20–200 kHz, and sonic types use a frequency of <=10 kHz. A top-of-tank mounted transducer directs waves downward in bursts onto the surface of the material whose level is to be measured. Echoes of these waves return to the transducer, which performs calculations to convert the distance of wave travel into a measure of level in the tank. A piezoelectric crystal inside the transducer converts electrical pulses into sound energy that travels in the form of a wave at the established frequency and at a constant speed in a given medium. The medium is normally air over the material’s surface but it could be a blanket of nitrogen or some other vapor. The sound waves are emitted in bursts and received back at the transducer as echoes. The instrument measures the time for the bursts to travel down to the reflecting surface and return. This time will be proportional to the distance from the transducer to the surface and can be used to determine the level of fluid in the tank.

Vibrating Point Level Sensors
Vibrating level sensors are designed for point level detection of very fine powders (bulk density: 0.02 g/cm3 - 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, the level of highly fluidized powders and electrostatic materials can also be sensed.
Single-probe vibrating level sensors are ideal for highly static bulk powder environments. Since only one sensing element contacts the powder, bridging between two probe elements is eliminated and media build-up is minimized. Vibrating level sensor technology offers other advantages: The vibration of the probe itself tends to eliminate build up of material on the probe element; and they are not affected by dust, static-charge build-up from dielectric powders, or changes in conductivity, temperature, pressure or humidity/moisture content. Tuning fork style vibration sensors are another alternative. They tend to have a lower price point, but are prone to material buildup between the forks.

Rotating Paddle Level Sensors
Rotating paddle level sensors are a very old and established technique for bulk solid point level indication. The technique requires 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 bulk densities (very low weight per unit volume) such as Pearlite, Bentonite or fly ash, the weight of the material is insufficient to stop the paddle. For such difficult applications, special paddle designs and the use of lower-torque motors can be employed. In addition, 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 sealing technology.

Air Bubbler Level Measurement Systems
Pneumatically based air bubbler systems contain no moving parts, making them suitable for measuring the level of sewage, drainage water, sewage sludge, night soil, or water with large quantities of suspended solids. The only part of the sensor that contacts the liquid is a bubble tube which is chemically compatible with the material whose level is to be measured. Since the point of measurement has no electrical components, the technique is a good choice for classified “Hazardous Areas”. The control portion of the system can be located safely away, with the pneumatic plumbing isolating the hazardous from the safe area.
Air bubbler systems are a good choice for open tanks at atmospheric pressure and can be built so that high-pressure air is routed through a bypass valve to dislodge solids that may clog the bubble tube. The technique is inherently “self-cleaning”. It is highly recommended for liquid level measurement applications where ultrasonic, float or microwave techniques have proved undependable.

Pneumatic Level Sensors
Pneumatic level sensors are indicated where hazardous conditions exist, where there is no electric power or its use is restricted, and 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. It has the additional benefit of being a relatively low cost technique for point level monitoring.

Magnetic and Mechanical Float Level Sensors
The principle behind magnetic, mechanical, cable and other float level sensors involves the opening or closing of a mechanical switch, either through direct contact with the switch, or magnetic operation of a reed. 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 (micro) switch.
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 (thick) 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.

What are the parameters to select a right level sensor?

Five types of information commonly define the level-measuring instrument or system needed:
• Area classification. (Nonhazardous, hazardous, or corrosive)
• Material characteristics. (Need to measure a liquid, slurry, solid, interface, granular, or powder)
• Process information. (At normal temperature and pressure)
• Vessel size and material (metallic or nonmetallic)
• Power requirements

Applications

Water level sensor work to maintain a constant water level avoiding material wastage in your process plant. Common applications also include switching pumps on and off to avoid overflow, dry running and indicating water level in an empty tank to avoid wear and tear and production stoppage. Level sensor can be used in the Food, Power, Chemicals, Sugar, Detergent, Steel, Minerals and Textile industry.