How Liquid Level Sensors Work

Level sensors are used to monitor and regulate levels of a particular free-flowing substance within a contained space. These substances are usually liquid, however level sensors can also be used to monitor some solids such as powdered substances. Level sensors are widely used industrially. Cars use liquid level sensors to monitor a variety of liquids, including fuel, oil and occasionally also specialist fluids such as power steering fluid. They can also be found in industrial storage tanks, for slurries, and in household appliances such as coffee machines. Basic level sensors can be used to identify the point at which a liquid falls below a minimum or rises above a maximum level. Many sensors can detail the specific amount of liquid in a container relative to the minimum/maximum levels, to provide a continuous measurement of volume.

There are a number of different types of liquid level sensor used to detect the point level of a liquid. Some types use a magnetic float, which rise and fall with the liquid in the container. Once the liquid, and by extension, the magnet, reach a certain level, a reed magnetic switch is activated. Commonly, there is a switch towards the top and the bottom of the container, allowing detection of minimum and maximum levels of liquid. Many sensors also include a protective shield to protect the magnet from turbulence or interference from direct contact with the liquid.

Another common type of level sensor is known as a Conductive sensor. Only liquids which conduct electricity can be used in this sensor. This type of sensor includes a source of power, usually of a fairly low voltage. At least two electrodes are placed within the container. When a conductive liquid reaches a certain point, it will come into contact with both a longer and a shorter electrode, and thus complete a circuit and activate an internal switch.

Pneumatic sensors are also a fairly common occurrence with particularly hazardous liquids, or in systems where the use of electricity is not viable or possible. This is because the sensor itself does not come into direct contact with the liquid at all. The sensor detects the level of air between the liquid and the pneumatic sensor, then uses this to calculate the amount of liquid used to fill the remainder of the container. These types of sensors are also relatively cost-effective.

There are also other types of level sensors which offer continuous measurement of liquids. Magnetostrictive sensors are similar in design to regular magnetic float sensors, yet the magnet level is measured using a magnetostrictive wire, which will react when its magnetic field is interrupted by the presence of the magnet. The exact point at which this interruption occurs can be determined by the distance between the bottom of the wire and the point of interference. Alternatives to this design include a magnetoresistive sensor, whereby an additional magnet is inserted onto the arm of the float, allowing accurate triangulation of the exact position of the magnets. This type of sensor is commonly used in conjunction with computer programs due to its accuracy.

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