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
www.PAControl.com
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.
www.PAControl.com
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.