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Sensor to detect the level of substances that flow

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.

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.

These detect levels of very fine powders (bulk density: 0.02), fine powders (bulk density: 0.2), and granular solids (bulk density: 0.5 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 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.

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.

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.

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 (micro) switch. For both magnetic and mechanical float level sensors, chemical compatibility, temperature, specific gravity (density), 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 (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. 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 (usually remote) indication or signalling. The technique is frequently referred to as RF for the radio frequency signals applied to the capacitance circuit. The sensors can be designed to sense material with dielectric constants as low as 1.1 (coke and fly ash) and as high as 88 (water) or more. Sludges and slurries such as dehydrated cake and sewage slurry (dielectric constant approx. 50) and liquid chemicals such as quicklime (dielectric constant approx. 90) can also be sensed. ), similar to ultrasonic level sensors, and Doppler systems employing FMCW techniques. Just as with ultrasonic level sensors, microwave sensors are executed at various frequencies, from 1 GHz to 60 GHz. Generally, the higher the frequency, the more accurate, and the more costly. Microwave is executed non contact technique or guided. The first is done by monitoring a microwave signal that is transmitted through free space (including vacuum) and reflected back, or can be executed as a "radar on a wire" technique, generally known as guided wave radar or guided microwave radar. In the latter technique, performance generally improves in powders and low dielectric media that are not good reflectors of electromagnetic energy transmitted through a void (as in non contact microwave sensors). This technique can use application specific waveguides to get more accurate results or additional information required for sensor application (e. g. some sensors can use tank parts or other equipment as a waveguide or its part). It's common practice to use remote waveguides, when waveguide is distanced from electronic part (commonly for reservoirs with harsh conditions, radiation, or boiling under high pressure liquids/gases, etc.). But with the guided technique the same mechanical constraints exist that cause problems for the capacitance (RF) techniques mentioned previously by having a probe in the vessel. Non contact microwave based radar sensors are able to see through low conductivity 'microwave transparent' (non conductive) glass/plastic windows or vessel walls through which the microwave beam can be passed and measure a 'microwave reflective' (conductive) liquid inside (in the same way as to use a plastic bowl in a microwave oven). They are also largely unaffected by high temperature, pressure, vacuum or vibration. As these sensors do not require physical contact with the process material, so the transmitter /receiver can be mounted a safe distance above/from the process, even with an antenna extension of several meters to reduce temperature, yet still respond to the changes in level or distance changes e. g. they are ideal for measurement of molten metal products at over 1200 °C. Microwave transmitters also offer the same key advantage of ultrasonics: the presence of a microprocessor to process the signal, provide numerous monitoring, controls, communications, setup and diagnostic capabilities and are independent of changing density, viscosity and electrical properties. Additionally, they solve some of the application limitations of ultrasonics: operation in high pressure and vacuum, high temperatures, dust, temperature and vapor layers. Guided wave radars can measure in narrow confined spaces very successfully, as the guide element ensures correct transmission to and from the measured liquid. Applications such as inside stilling tubes or external bridles or cages, offer an excellent alternative to float or displacement devices, as they remove any moving parts or linkages and are unaffected by density changes or build up. They are also excellent with very low microwave reflectivity products like liquid gasses (LNG, LPG, ammonia) which are stored at low temperatures/high pressures, although care needs to be taken on sealing arrangements and hazardous area approvals. On bulk solids and powders, GWR offers a great alternative to radar or ultrasonic sensors, but some care needs to be taken over cable wear and roof loading by the product movement. One perceived major disadvantage of microwave or radar techniques for level monitoring is the relatively high price of such sensors and complex set up. However, price has reduced significantly over the last few years, to match those of longer range ultrasonics, with simplified set up of both techniques also improving ease of use.

Magnetostrictive level sensors are similar to float type sensors in that a permanent magnet sealed inside a float travels up and down a stem in which a magnetostrictive wire is sealed. Ideal for high accuracy, continuous level measurement of a wide variety of liquids in storage and shipping containers, these sensors require the proper choice of float based on the specific gravity of the liquid. When choosing float and stem materials for magnetostrictive level sensors, the same guidelines described for magnetic and mechanical float level sensors apply. Magnetostrictive level and position devices charge the magnetostrictive wire with electric current, when the field intersects the floats' magnetic field a mechanical twist or pulse is generated, this travels back down the wire at the speed of sound, like ultrasound or radar the distance is measured by time of flight from pulse to return pulse registry. the time of flight corresponds to the distance from the sensor detecting the return pulse. Because of the accuracy possible with the magnetostrictive technique, it is popular for "custody transfer" applications. It can be permitted by an agency of weights and measures for conducting commercial transactions. It is also frequently applied on magnetic sight gages. In this variation, the magnet is installed in a float that travels inside a gage glass or tube. The magnet operates on the sensor which is mounted externally on the gage. Boilers and other high temperature or pressure applications take advantage of this performance quality

Resistive chain level sensors are similar to magnetic float level sensors in that a permanent magnet sealed inside a float moves up and down a stem in which closely spaced switches and resistors are sealed. When the switches are closed, the resistance is summed and converted to current or voltage signals that are proportional to the level of the liquid. The choice of float and stem materials depends on the liquid in terms of chemical compatibility as well as specific gravity and other factors that affect buoyancy. These sensors work well for liquid level measurements in marine, chemical processing, pharmaceuticals, food processing, waste treatment, and other applications. With the proper choice of two floats, resistive chain level sensors can also be used to monitor for the presence of an interface between two immiscible liquids whose specific gravities are more than 0.6, but differ by as little as 0.1 unit.

Magnetoresistance float level sensors are similar to float level sensors however a permanent magnet pair is sealed inside the float arm pivot. As the float moves up the motion and location are transmitted as the angular position of the magnetic field. This detection system is highly accurate down to 0.02° of motion. The field compass location provides a physical location of the float position. The choice of float and stem materials depends on the liquid in terms of chemical compatibility as well as specific gravity and other factors that affect buoyancy of the float. The electronic monitoring system does not come in contact with the fluid and is considered intrinsically safe or explosion proof. These sensors work well for liquid level measurements in marine, vehicle, aviation, chemical processing, pharmaceuticals, food processing, waste treatment, and other applications. Due to the presence of a microprocessor and low power consumption, there is also capability for serial communication from to other computing devices making this a good technique for adjusting calibration and filtering of the sensor signal.

Hydrostatic pressure level sensors are submersible or externally mounted pressure sensors suitable for measuring the level of corrosive liquids in deep tanks or water in reservoirs. Typically, fluid level is determined by the pressure at the bottom of the fluid containment (tank or reservoir); the pressure at the bottom, adjusted for the density / specific gravity of the fluid, indicates the depth of the fluid. The technique is used to regulate the level of molten steel in a continuous casting process of steelmaking. The water cooled mold is arranged with a source of radiation, such as cobalt 60 or caesium 137, on one side and a sensitive detector such as a scintillation counter on the other. As the level of molten steel rises in the mold, less of the gamma radiation is detected by the sensor. The technique allows non contact measurement where the heat of the molten metal makes contact techniques and even many non contact techniques impractical. Nucleonic level sensors are often used in mineral crushing circuits, where an increase in gamma ray detection indicates a void, compared to being filled with ore.