Perhaps the simplest (and oldest) form of electrical level detection is where a set of metal electrodes contacts the process material to form a complete electrical circuit, actuating a relay. This type of switch, of course, only works with granular solids and liquids that are electrically conductive
(e.g. potable or dirty water, acids, caustics, food liquids, coal, metal powders) and not with nonconducting materials (e.g. ultra-pure water, oils, ceramic powders).

A legacy design for conductive level switches is the model 1500 “induction relay” manufactured
by B/W Controls (now part of Ametek), using a special transformer/relay to generate an isolated
AC probe voltage and sense the presence of liquid:

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Line voltage (120 VAC) energizes the primary coil, sending a magnetic field through the laminated iron core of the relay. This magnetic field easily passes through the center of the secondary coil when the secondary circuit is open (no liquid closing the probe circuit), thus completing the magnetic “circuit” in the core. With the magnetic circuit thus completed, the armature will not be attracted to the core. However, when a circuit is completed by liquid level rising to contact both probes, the secondary coil’s resulting current “bucks” the magnetic flux4 through its center, causing more magnetic flux to bypass to the end poles where it attracts the iron armature toward the core frame. This physical attraction actuates switch contacts which then signal the presence of liquid level at the probes.


The following pair of illustrations shows the two conditions of this level switch, with the magnetic
lines of flux highlighted as dashed lines through the core :

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The “transformer” design of this particular conductive level switch not only provides electrical isolation between the probes and the energizing (120 VAC) circuit, but it also enables a wide range of detection voltages to be generated for the probes just by altering the number of wire “turns” in the secondary coil. The B/W Controls model 1500 inductive relay is available in a variety of AC voltage output levels, ranging from 12 VAC (for detecting metallic substances) to 800 VAC for use with demineralized water such as that found in steam boiler systems.

More modern variations on the same design theme use much lower AC voltages5 to energize
the probes, employing sensitive semiconductor amplifier circuits to detect probe current and signal
liquid level.

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