Choosing the Right Level Switch

There are several types of technology used for level switches, and some of them are also used for level sensors. The different types of technology available are more or less suitable according to the type of product you want to measure the level for, whether it is a liquid, a pasty product or a solid in bulk.

The main technologies used for level switches are:

Float level switch: a float follows the movement of the surface of a liquid and triggers a switch.

Capacitive level switch: the detector forms an electrical capacitor with the tank, whose capacity changes when the level reaches the detector.

Conductive level switch: when a conductive liquid comes into contact with the electrodes of the switch, current is able to flow and trigger the switch.

Vibrating level switch: a rod or blades vibrate at their resonance frequency, when the product reaches them, the vibrations are damped and the detector switches.

Rotary paddle level switch: a motor rotates a paddle, when the product reaches the paddle, the rotation is stopped and the detector switches. 

Electro-optical level switch: the light coming from an infrared LED is reflected towards a receiver by a prism, when the level of liquid reaches the prism the beam is not reflected anymore but refracted in the liquid, which causes the detector to switch.

Float level switches are based on a simple principle: the combination of a float and a mechanical or magnetic switching device (reed switches). Naturally a float placed in a liquid tends to stay at the surface, this is the effect that is exploited here by detecting the displacement of a float when the liquid surface reaches its height in the tank.

There are 3 main types of float level switches:

• The “suspended” level switch type, which is simply made up of a float which hangs at the end of its cable. The switching device (usually mechanical) is placed inside the float. When the surface of the liquid reaches the float, the float rises and then tilts, triggering the change of state of the switching device. It is a simple but not very accurate technology, widely used in pumps as a dry run safety device.
• The vertical type where the float slides along a vertical axis. This type of switch uses a magnetic switch (a reed switch) integrated into the axis, the float is equipped with a magnet and the detector changes state when the liquid makes the float rise to the level of the reed switch.
• The broken finger float type consists of a float fixed at the end of a horizontal articulated shaft. The rise of the float under the effect of the liquid causes a change in the inclination of the hinged axis which triggers the switching device of the detector.

Advantages:

• Float level switches are suitable for all liquids, provided that the switch materials are compatible with the liquid (plastic for acids, stainless steel for water and oils, etc.)
• They are reliable, economical and easy to use
• No calibration is required
• Float level switches are unaffected by the nature of the liquid, its electrical conductivity, the pressure and temperature conditions in the tank, the presence of vapors, foam and boiling at the surface. They are however sensitive to the wave phenomenon.

Capacitive level switches are based on the properties of electrical capacitors. The detector and the walls of the tank form the two electrodes of a capacitor, whose capacity (the electrical capacity measured in Farad) depends on the height of the product in the tank.

Changes in the level lead to changes in the capacitance, which are converted into a switching signal by the integrated sensor electronics.

Advantages:

• Very robust and reliable technology
• No maintenance required
• Cost-effective technology
• The switches can be installed in any position
• Works with both conductive and non-conductive liquids and solids and is not affected by viscous or sticky products
• Resistant to high temperatures and operating pressures

Conductive level switches are only used with conductive liquids. In their basic configuration, they are made up of two electrodes separated by a gap forming an open electrical circuit (current cannot flow between the two electrodes). When the liquid level reaches the electrodes, the conductive liquid closes the electrical circuit and a current flows between the electrodes. The electronics of the sensor converts the current flow into a switching signal.

This technology only works with conductive liquids such as acids, lyes and solutions containing water. Hydrocarbons, oils and solvents, which are generally not very conductive, are not compatible with this technology.

These switches can be used with aggressive liquids if they are made of suitable materials.

Advantages:

• Simple, robust and cost-effective
• Multiple setup and installation options
• No calibration is required
• The option to have several switching points if you choose a detector with several electrodes.

Vibrating level switches and vibrating rod level switches are based on the same principle: a rod or a pair of blades are set in vibration at their resonance frequency (usually by a piezoelectric system). When the product level reaches the rod or the pair of blades, the vibrations are damped by the presence of material around the sensing element.

The electronics of the sensor analyze the amplitude and frequency of the vibrations and convert the variations due to damping by the product into a switching signal.

This technology is not sensitive to the nature or properties of the product in the tank. It can be used with liquids as well as with granular and powdery solids with fine or coarse grains. Another benefit is that these level switches can be used in explosive areas, so they are advantageous for the storage of raw materials, the chemical industry and the food industry.

Advantages:

• Unaffected by the product and its properties (conductivity, viscosity and particle size), operating conditions (temperature, pressure), turbulence, foam, vibration and clogging
• Abrasion-resistant
• Easy to install and operational without calibration
• Self-monitoring
• No wear or maintenance

The principle of rotary paddle level switches is based on an electric motor that turns at a low speed, via a transmission and a shaft equipped with one or more paddles.

This type of switch is used in silos, bins and hoppers to detect high and low levels of bulk solids. When the product level reaches the paddles, the material slows down the rotation and stops the motor, which triggers the switching signal of the sensor. 

Rotary paddle level switches are widely used in silos for grain, sugar, cocoa, animal feed, lye, chalk, plaster, cement, wood chips, etc.

Advantages:

• Measuring principle for simple applications
• Robust technology
• No calibration is required

The operation of an electro-optical level switch is based on an infrared LED and a photoreceptor. The light from the LED is sent to a glass prism forming a tip that constitutes the sensor probe. As long as this prism is surrounded by gas, the light is reflected from the prism to the receiver. When the liquid level reaches the prism, the light beam is refracted into the liquid and the receiver registers a decrease in the received light intensity, which triggers the switch.

These switches work with all liquids. The measurement is independent of the physical characteristics of liquids such as density, dielectric constant, conductivity and refractive index or color.

These switches are robust, they use resistant materials (stainless steel for the case, borosilicate glass or quartz for the prism), they have no moving parts and technically only the glass tip of the prism needs to be in contact with the liquid. This allows for extremely compact detectors with a minimum tank footprint and the ability to operate with small volumes of product.

Advantages:

• Detection independent of liquid characteristics (density, dielectric constant, conductivity, color and refractive index)
• This method also works on small volumes
• No calibration is required
• Some models can be used in hazardous environments.

To choose a level switch, you must first determine which technology to use for the product to be measured (liquid, pasty or bulk solid), the properties of the product (electrically conductive or not, aggressive, abrasive, etc.) and the presence of foam or turbulence on the surface.

Once you’ve chosen the technology, you will need to select a switch capable of withstanding the operating conditions of the process (pressure, temperature, etc.) and made of materials compatible with the product to be measured.

Setup is another essential factor: switches can be vertical or horizontal, threaded or equipped with a flange and of course more or less compact.

Lastly, the output signal of the level switch is another important point to consider. There are different switching signals: dry contact or an NPN or PNP transistor.