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Discrete Process Measurement - Level Switches

A level switch is one detecting the level of liquid or solid (granules or powder) in a vessel. Level switches often use floats as the level-sensing element, the motion of which actuates one or more switch contacts.

Recall that the “normal” status of a switch is the condition of minimum stimulus. A level switch will be in its “normal” status when it senses minimum level (e.g. an empty vessel).

level switch symbols showing normally open and normally closed contacts

Two water level switches appear in this photograph of a steam boiler. The switches sense water level in the steam drum of the boiler. Both water level switches are manufactured by the Magnetrol corporation:


Level switches by Magnetrol used in steam boiler

The switch mechanism is a mercury tilt bulb, tilted by a magnet’s attraction to a steel rod lifted into position by a float. The float directly senses liquid level, which positions the steel rod either closer to or further away from the magnet. If the rod comes close enough to the magnet, the mercury bottle will tilt and change the switch’s electrical status.

This level switch uses a metal tuning fork structure to detect the presence of a liquid or solid (powder or granules) in a vessel:


Level switch uses a metal tuning fork structure to detect presence of liquid or solid

An electronic circuit continuously excites the tuning fork, causing it to mechanically vibrate. When the prongs of the fork contact anything with substantial mass, the resonant frequency of the structure dramatically decreases. The circuit detects this change and indicates the presence of material contacting the fork. The forks’ vibrating motion tends to shake off any accumulated material, such that this style of level switch tends to be resistant to fouling.

A more primitive variation on the theme of a “tuning fork” level switch is the rotating paddle switch, used to detect the level of powder or granular solid material. This level switch uses an electric motor to slowly rotate a metal paddle inside the process vessel. If solid material rises to the level of the paddle, the material’s bulk will place a mechanical load on the paddle. A torque-sensitive switch mechanically linked to the motor actuates when enough torsional effort is detected on the part of the motor. A great many level switches of this design sold in the United States under the trade-name Bindicator (so-called because they detected the level of solid material in storage bins).

Yet another style of electronic level switch uses ultrasonic sound waves to detect the presence of process material (either solid or liquid) at one point:


electronic level switches using ultrasonic sound waves

Sound waves pass back and forth within the gap of the probe, sent and received by piezoelectric transducers. The presence of any substance other than gas within that gap affects the received audio power, thus signaling to the electronic circuit within the bulkier portion of the device that process level has reached the detection point. The lack of moving parts makes this probe quite reliable, although it may become “fooled” by heavy fouling.

Another electronic liquid level switch technology is capacitive: sensing level by changes in electrical capacitance between the switch and the liquid. The following photograph shows a couple of capacitive switches sensing the presence of water in a plastic storage vessel:


Capacitive Switches for high and low level


Click here to continue reading to the next page, Temperature Switches

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Comments (1)Add Comment
Turning fork
written by Turning fork, April 30, 2012
The phrase “discrete” indicates personal or unique. In technological innovation, a “discrete” diverse or statistic represents a true-or-false situation. Thus, a unique Guide Weblink indicator is one that is only able to indicate whether the calculated diverse is above or below a specified setpoint.

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