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
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:
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
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238 CHAPTER 8. DISCRETE PROCESS MEASUREMENT
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:
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.
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8.6. LEVEL SWITCHES 239
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:
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.

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Pressure switches

A pressure switch is one detecting the presence of fluid pressure. Pressure switches often use
diaphragms or bellows as the pressure-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 pressure
switch will be in its “normal” status when it senses minimum pressure (e.g. n applied pressure, or
in some cases a vacuum condition)1.

Pressure switch symbols

The following photograph shows two pressure switches sensing the same fluid pressure as an
electronic pressure transmitter (the device on the far left):

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Series and parallel circuits

Impedance in a series circuit is the orthogonal sum of resistance and reactance:
Z =(R2 + (X2L− X2C)) ^1/2
Equivalent series and parallel circuits are circuits that have the exact same total impedance
as one another, one with series-connected resistance and reactance, and the other with parallelconnected
resistance and reactance. The resistance and reactance values of equivalent series and
parallel circuits may be expressed in terms of those circuits’ total impedance:

If the total impedance of one circuit (either series or parallel) is known, the component values of
the equivalent circuit may be found by algebraically manipulating these equations and solving for
the desired R and X values:
Z2 = RseriesRparallel


Z2 = XseriesXparallel

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Electron versus conventional flow

When Benjamin Franklin advanced his single-fluid theory of electricity, he defined “positive” and
“negative” as the surplus and deficiency of electric charge, respectively. These labels were largely
arbitrary, as Mr. Franklin had no means of identifying the actual nature of electric charge carriers
with the primitive test equipment and laboratory techniques of his day. As luck would have it,
his hypothesis was precisely opposite of the truth for metallic conductors, where electrons are the
dominant charge carrier.
This means that in an electric circuit consisting of a battery and a light bulb, electrons slowly
move from the negative side of the battery, through the metal wires, through the light bulb, and on
to the positive side of the battery as such:

Unfortunately, scientists and engineers had grown accustomed to Franklin’s false hypothesis long
before the true nature of electric current in metallic conductors was discovered. Their preferred
notation was to show electric current flowing from the positive pole of a source, through the load,
returning to the negative pole of the source:

This relationship between voltage polarity marks and conventional flow current makes more
intuitive sense than electron flow notation, because it is reminiscent of fluid pressure and flow
direction:

If we take the “+” sign to represent more pressure and the “-” sign to represent less pressure,
it makes perfect sense that fluid should move from the high-pressure (discharge) port of the pump
through the hydraulic “circuit” and back to the low-pressure (suction) port of the pump. It also
makes perfect sense that the upstream side of the valve (a fluid restriction) will have a greater
pressure than the downstream side of the valve. In other words, conventional flow notation best
honors Mr. Franklin’s original intent of modeling current as though it were a fluid, even though he
was later proven to be mistaken in the case of metallic conductors where electrons are the dominant
charge carrier.

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