Level Switch

SOR offers level switches using different technologies to handle the diversity of applications.  Float and Displacer operated level switches are considered mechanical devices because of their moving parts, the buoyancy principle of operation and because they don’t require a power supply.  RF and Ultrasonic level switches are referred to as electronic products as they require a power supply, have no moving sensor parts and work on the principles of capacitance and sound transmission.

Learn more on Float Operated Level Switch Operation and Displacer Operated Level Switch Operation

SOR recognizes that there is no industry convention with respect to terminology and definitions pertinent to level switches. This glossary applies to SOR level switches.

Admittance – The measure of the ability for current to flow through an electrical circuit.

Adjustable Differential – Also known as ‘adjustable dead band’ – The ability to precisely set the actuation and deactuation points of a switching element or relay.

Attraction Sleeve – A magnetic component that travels with the stem movement and actuates/deactuates a magnetically coupled switch mechanism with changing liquid levels.

Capacitance – The ability of a device to store an electrical charge.

Capacitor – A component that stores an electrical charge; it consists of two conductive surfaces separated by an insulator (dielectric).

Chamber – A metal externally mounted vessel that contains the liquid level sensing components and exposes them to the same liquid levels of a main process vessel.

Continuous Level Measurement –Indicating level changes over a varied distance or height.

Dielectric Constant – A measure of the non-conductivity of an insulating material

Differential – The difference in level between the actuation and deactuation points; also referred to as hysteresis, reset value or dead band.

Displacer Operated – The sensor is a weight (displacer) heavier than the liquid, which is suspended by a spring.  When liquid contacts the displacer, a buoyant force is produced, which causes the spring to retract slightly to a new equilibrium position.  When the spring retracts the attraction sleeve moves upward into the field of the external magnet, actuating the switch.

DPDT Switching Element – Double-Pole, Double-Throw (DPDT) is two SPDT elements operated by a common lever assembly so simultaneous actuation/deactuation occurs at both increasing and decreasing level set points. If desired, two independent circuits can be switched.

Dry Heat –Heat that transfers from the process through direct conduction, only where it contacts the enclosure.

Dual-Stage – A level instrument that detects two discrete level points.

Enclosing Tube – A protective tube that allows the attraction sleeve to engage the magnetic field of the switching element while keeping the switch isolated from the process media.

Failsafe – Refers to the safe mode of operation upon failure or loss of power to an instrument.  A de-energized relay will have continuity between the Normally Closed and Common terminals.

Float-Operated – A float rides on the process liquid surface, precisely tracking liquid surface motion. Rising liquid level lifts the float, moving the attraction sleeve upward into the field of the external magnet, actuating the switch.

Hazardous Area – Refers to an area containing a mixture of flammable liquids, gasses or dusts capable of ignition in the presence of an electrical spark.

Hermetically Sealed – The method of factory sealing an electrical switching element so that it is unaffected by ambient conditions. The enclosure is completely sealed from the environment using techniques such as welding, soldering, brazing or glass-to-metal fusion.

Integral – Term for mounting a level instrument directly into the primary process vessel (no external chamber).

Level Instrument – A device that senses a level change in a process media and provides an indication of position.

Level Switch – A device that senses a change in process level then actuates a switching mechanism or relay when a pre-determined level set point is reached.

Line Power – Sometimes referred to as ‘four-wire’ – where the power supply and output signal is transmitted on different pairs of wires.

Loop Power – Commonly referred to as ‘two-wire’ – where the power supply and output signal is transmitted on the same pair of wires.

Manual Check – A method of actuating a switch mechanism or relay to verify functionality without physically changing liquid level in the vessel.

Normally Closed Contact – The electrical contact of a switching element or relay that current can flow through when in the non-actuated, ‘shelf position’.

Normally Open Contact – The electrical contact of a switching element or relay that current cannot flow through when in the non-actuated, ‘shelf position’.

Point Control – Level indication at a specific point.

Reset Point – The level at which the switching element  returns to its normal operation position (deactuates after actuating).

Remote – A type of instrument where a portion of the unit is some distance from the process vessel, usually electronics mounted separately from the sensor in the process.

Sealed Chamber – A welded vessel assembly where the internal sensor components cannot be accessed for maintenance or repair.

Serviceable Chamber – A vessel that can be opened for sensor component maintenance or repair, usually through flanged connections.

Set Point – The measured level height at which a switching element or relay actuates, typically expressed as increasing or decreasing level.

Single-Stage – A level instrument that detects only one discrete level point.

SPDT Switching Element – Single Pole, Double Throw elements have three electrical connections; C = Common, NO = Normally Open, NC = Normally Closed.

Specific Gravity – The ratio of the density of a liquid to the density of water.

Steam Heat – Heat that is carried with the vapor from the process to every internal portion of the enclosure.

Tandem Floats – Utilization of two independent floats in a level instrument to actuate two independent switch mechanisms.

Temperature Extension – Components designed to reposition the electrical switch elements or relays away from damaging process temperatures.

Time Delay – A method or device to delay the actuation of a switch or relay after the sensor’s initial detection of level change; can be used to minimize the effect of process turbulence.

Trim – The float or displacer, attraction sleeve and other related parts that make up the sensor in a mechanical level switch.  These components are exposed to the process liquids and vapors.

Triple-Stage – A level instrument that detects three discrete level points

Wetted Parts – Components of the instrument that come in contact with the liquid or gas process media.

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