Level switches

 

Level measurement

Level switch

Types of level switch

Different types of level switch are used in industries as requirements and functioning in the process control

some mostly uesd types of level switches are listed here

1. Floats type level switch

2. Rotary type level switches

3. Vibrating level switch 

4. Capacitance type level switches

5. Radio frequency type level switches

Point level

It is used to measure whether level of material is belowbor high than a specific point.

   outputs of level switch is used as alarm or tripping purposes of pump, screw conveyor and belt conveyor.

Float switch

Float Switches Applications & Installations

 A simplex (one pump) system will use a single switch wired in series with the motor leads so that the switch directly starts and stops the pump motor.

Three main classification of mounting the float level switch are

1. side-mounted

2. Top-mounted

3. external-mounted 

float switches.

Displacer Switches

 Displacer remains partially or completely submerged. The apparent weight of the displacer is reduced as it becomes covered by more liquid.

 When the weight drops below the spring tension, the switch is actuated.

 Displacer switches are more reliable than regular floats on turbulent, or foamy 

applications.

 These switches are interchangeable between tanks because differences in process density can be accommodated by changing the tension of the support spring.

2. Rotary types level switch

The purpose of the rotary paddle level switch is to detect the presence of solid/powdery material in most kinds of tanks, bins and containers. The unit is usually located through a bin wall at the top, middle or low level of a bin. During normal operation (no material present) a synchronous motor rotates the paddle at RPM. When this paddle rotation is impeded by material, surrounding the paddle, motor will stall and cause the Micro-switch to change state (indicating an alarm or control).

3. Vibrating level switch

 vibrating tuning fork liquid level switch is an instrument for monitoring the height of a liquid and tripping a relay contact switch or transmitting a solid state relay switch output when a maximum, minimum or intermediate level has been reached. The liquid level switch output can then be utilised by other instrumentation to open a valve, illuminate a warning lamp, activate an audible alarm, or switch on a pump.

Vibrating tuning fork liquid level switches are threshold contact trip devices which typically consist of a tuning fork which extends out from the switch housing.

In operation a piezoelectric oscillator is used to vibrate the tuning fork at its natural frequency for air.

A second piezoelectric device is used to pick-up the frequency of the vibrating tuning fork. When the liquid level rises and covers the tuning fork it will vibrate at a lower frequency.

This change in vibration frequency is used to detect when the required liquid level has been reached.

A vibrating tuning fork liquid level switch makes direct contact with the liquid to be measured, and therefore material compatibility has to be a consideration.

Since there are no pressure sensitive components exposed to the inside of the tank it is possible to fit tuning fork level switches to sealed pressurised tanks.

Also, since there is no sensitive components immersed in the fluid it is possible to use them on higher temperature media that would not be tolerated by other types of liquid level switch.

Vibrating tuning fork liquid level switches are generally mounted through the top or side of the tank with the tuning fork protruding horizontally into the tank.

Because they're often tied to protective logic rather than just process control, level switches get treated with more scrutiny during commissioning and maintenance than you might expect from such a simple-looking device.

Main Types You'll Run Into

Float switches
A float rides on the surface of the liquid and moves a magnet or mechanical linkage past a reed switch as level rises or falls. Simple, reliable, and cheap — still widely used for water, oils, and other clean liquids. Mounting styles include side-mounted (bracket on the vessel wall at the set point), top-mounted (float hangs down from the top on a rod or cable), and external/chamber-mounted (float sits in a small external chamber connected to the vessel, useful when internal obstructions make direct mounting impractical).

Capacitance (RF) switches
A probe extends into the vessel; the material itself acts as the dielectric between the probe and the vessel wall (or a reference probe). As material covers the probe, the capacitance changes, and the switch trips at a calibrated threshold. Works on liquids, slurries, and bulk solids — a common choice for powders and granular material where a float wouldn't function. Sensitive to material dielectric properties, so it needs to be specified correctly for the actual material, not just "solids in general."

Vibrating fork (tuning fork) switches
A small fork vibrates at its natural resonant frequency when uncovered. Once material contacts the fork, the vibration is dampened and the frequency shift is detected electronically, triggering the switch. Extremely popular for bulk solids and powders — bins, silos, hoppers — because they have no moving parts to jam or wear, and they're largely unaffected by material dielectric properties (unlike capacitance types), dust buildup, or minor coating.

Rotary paddle switches
A motor-driven paddle rotates continuously inside the vessel. When material covers the paddle and stops it from rotating, a torque switch detects the stall and trips the output. A traditional and rugged choice for high or low level detection in silos and hoppers carrying dry bulk solids like cement, clinker dust, or raw meal.

Point Level vs. Continuous

Worth being clear on this distinction: point level switches only tell you whether material is above or below one fixed point — that's it. If you need to know the actual percentage or height of material continuously, you need a level transmitter (radar, guided wave radar, capacitance-type transmitter, etc.), not a switch. Plants commonly use both together — a continuous transmitter for normal control, and independent switches as high/high-high or low/low-low alarm and trip points, wired separately from the transmitter loop for genuine redundancy.

Common Field Issues

• Material buildup/coating on the probe or fork — especially with sticky or fine powders, buildup can cause false-high indication even when the vessel is genuinely low. Periodic physical inspection and cleaning is the fix; for chronic buildup, a technology less sensitive to coating (vibrating fork over capacitance, for example) may be worth switching to.
• Float sticking in guide tubes — mineral deposits, corrosion, or debris in the guide tube can cause a float to hang up at one position instead of moving freely. Field cleaning or guide tube replacement resolves it.
• False trips from turbulence or splashing — a float or capacitance probe positioned too close to a fill point can see momentary contact from splash and trip prematurely. Repositioning or adding a stilling well usually solves this.
• Rotary paddle motor burnout — continuous stall against packed material (rather than a clean stop-on-contact) can overheat and damage the drive motor over time. Confirm the torque/stall sensitivity setting matches the actual bulk density of the material.

Bottom Line

Level switches are unglamorous but critical — they're often the last line of defense before an overflow, a dry-run pump, or a blocked chute. Picking the right technology for the actual material (liquid vs. solid, clean vs. sticky, dielectric properties) matters more than it might seem for a "simple" on/off device, and a little attention during commissioning saves a lot of nuisance trips — or worse, missed trips — down the line.