Level switches are used to detect liquid or powder levels, or interfaces between liquids. These level measurements are indicated via an electrical switching action. Level switches can be used either as an alarm device or as control switches, turning something on or off, such as a pump, or sending a signal to a valve actuator. What makes level switches special is that they have a switched output and can be either electromechanical or solid state, either normally open or normally closed.

Specifications

There are several important factors to consider when selecting a level switch. These specifications are related to the function of the switch in the desired application.

Configuration

When selecting a level switch, the user needs to determine if the electric circuit requires a normally open or normally closed switch.

• Normally open (NO) switches do not allow current to pass through in the free position. They need to "make" a contact to be activated.
• Normally closed (NC) switches allow current to pass through in the free position and need to "break" contact (open) to be activated.

Point/Continuous

There are two basic level-measurement types:

Point level sensors or multi-point sensors mark a specific level and communicate to the user if the media is above or below that level. They are used generally as a high alarm or switch. These level sensors can also be integrated in a single device to add a low alarm, or to serve as a stepped version of a continuous level.

Example of point level measurement

Video Credit: madisonsensors / CC BY 3.0

Continuous level sensors measure liquid or dry material levels within a specified range and provide outputs which continuously indicate the level. They are more sophisticated devices than point level sensors because they measure over a range of levels instead of at one point to let the user know the exact amount of a substance. This produces an analog output that directly correlates to the fluid level in the container.

Example of continuous level measurement

Video Credit: madisonsensors / CC BY 3.0

Level switches are typically point level switches, measuring whether the level of a container has reached or gone over a predetermined point. However, switching mechanisms can be integral to continuous level switches as well. Multiple point switches can be integrated together to give a stepped version of continuous level.

Switch Type

Electromechanical switch- Electromechanical switches have mechanical contacts or relays. These types of switches can control a wider range of current and voltage options. They are not affected by dirt, mist, magnetic fields or temperature ranges from near absolute zero to 1000°). Electromechanical switches can adapt to misalignment in installation/application to ensure there is no leakage current and making it available in many circuitry, actuator, and housing styles. Disadvantages include their price, limited contact life cycle, large and switch slowly. 

Solid-state switch - Solid- state switches are electric devices that do not have moving parts to wear out. They are able to switch faster without sparking between contacts or problems with contact corrosion. Their disadvantages include a high cost to build in very high current ratings.

Measurement Technology

There are several technologies available that can measure the level of media in a system. A full description and application of each technology is available in the Level Sensor Specification Guide.

Video Credit: kansaiautomation / CC BY 3.0

Pole/Throw

The typical options for poles and throws are available. Most switches have one or two poles and one or two throws, but some manufacturers will produce custom level switches for special applications. The number of poles describes the number of separate circuits which can pass through the switch at the same time. The number of throws describes the number of circuits each pole can control. This is noted by the configuration of the circuit (NO/NC). Breaks are interruptions to the circuit caused by the separated contacts the switch introduces into each circuits it opens, or interrupts in the circuit.

Single Pole, Single Throw (SPST) - Single pole, single throw (SPST) switches make or break the connection of a single conductor in a single branch circuit. They usually have two terminals and are referred t name="idPrompt_switch_spdt">o as single-pole switches.

Single Pole, Double Throw (SPDT) - Single pole, double throw (SPDT) switches make or break the connection of a single conductor with either of two other single conductors. They usually have three terminals and are commonly used in pairs. SPDT switches are sometimes name="idPrompt_switch_dpst">called three-way switches.

Double Pole, Single Throw (DPST) - Double pole, single throw (DPST) switches make or break the connection of two circuit conductors in a single branch circuit. They typi name="idPrompt_switch_dpdt">cally have four terminals.

Double Pole, Double Throw (DPDT) - Double pole, double throw (DPDT) switches make or break the connection of two conductors to two separate circuits. They typically have six terminals and are available in both momentary and ma name="idPrompt_switch_other">intained contact versions.

Other- Other, special throw types with more than two poles. Examples include switches that are designed to split loads into separate circuits (e.g., headlamp switch).

Image Credit: Digi-Key

Measuring Range

The measuring range is probably the most important specification to examine when choosing level switches. It is defined as the level range that devices can measure. Measuring range is determined by the maximum and minimum media levels. The user should also account for anticipated media level fluctuations. If the proper measuring range is selected, less data will be lost during event which exceeds the measuring range.

Devices with field adjustable measurement ranges allow for the user to adjust the measuring range depending on the media container and the environment conditions.

Electrical Specifications

Also of critical concern are the ratings for current and voltage the switches require. Analog outputs from level switches can be current or voltage signals. Also possible is a pulse or frequency.

• Maximum current- The device's maximum nominal (name plate) current-carrying capacity.
• Maximum AC voltage- The device's maximum AC voltage rating.
• Maximum DC voltage- The device's maximum DC voltage rating.
• Transistor- transistor logic (TTL) - TTL driver circuitry enables the status of the switch to be controlled by the level of the TTL logic input. Power can be applied across a pair of designated power terminals and then control the switch operation with a 5 volt control circuit.

Output Options

Computer signal outputs that are possible are usually serial or parallel.

Serial- Serial communication means that data bits are sent in a serial (one after another) way over a single line. The advantages of serial communication are that the data can be sent further, and the cable connection is simpler and uses fewer wires. Serial interfaces use a serial communication protocol such as universal serial bus (USB), RS232, or RS485. 

Parallel- Parallel communication means that a number of data bits are sent at the same time. More information can be sent quickly and more reliably, but the processing time is longer. Parallel interfaces use a parallel communication protocol such as general-purpose interface bus (GPIB).

User Interface

Level switches can have displays that are analog, digital or video displays. Control for the devices can be analog with switches, dials and potentiometers; digital with menus, keypads and buttons; or controlled by a computer.

• Non-electrical visual or audio outputs which are non-electrical visual indicators such as a simple float, or an audible click. Non-electrical visuals are inexpensive and the simplest to use and install but they are subject to user error in level reading.
• Analog meters or indicators which use an analog meter with a needle or an LED indicator to identify the level of fluid in the system.
• Digital readouts which feature numerical or application specific readouts. These displays are easy to read and can display significant digits.
• Video display terminals (VDT) which include cathode ray tubes and flat panel display. VDT is a term used for the computer display consisting of a computer output surface and projecting mechanism that shows data.

System Specifications

• Maximum operating pressure- The maximum operating pressure of the material for which the device is rated.

• Material temperature range- The maximum material temperature for which the device is rated.

Media

Media is the material that the sensor needs to measure. Level switches can be responsible for measuring a wide variety of media in fluid level systems in many industries. The two general media types include liquid and dry materials.

• Liquid media can include
  • Water (hot or cold, clean or dirty, fresh or salt)
  • Gasoline (diesel fuel)
  • Hydraulic fluid
  • Highly viscous or gummy fluids
• Dry materials can include
  • Bulk solids
  • Powders

Mounting Options

Depending on the needs of the application, level switches can be mounted a few different ways. These switches can be mounted on the top, bottom or side of the container holding the substance to be measured. Some factors to consider when selecting a mounting option is the package size of switch or sensor, and the space available in the application. The switch should be rugged to handle the environment of the system.

Features

Some features that can make level switches more desirable are being programmable, having controller, recorder or totalizer functions and a built-in alarm indicator, whether audible or visible.

Also important for some applications are sanitary ratings and the ability to handle slurries with suspended solids, such as wastewater or sewage.

• Rated for Sanitary Applications- Devices are rated for sanitary applications such as those found in the pharmaceutical and foo class="lm-text" name="idPrompt_feat_slurry">d processing industries.
• Handles Slurries /Suspended Solids-Devices can handle slurries or suspended solids class="lm-text" name="idPrompt_feat_alarm">such as sewage and waste water.
• Built-in Alarm / Indicator- Devices have a built-in audible or visual alarm that is more than a switch or relay closure.
• Programmable- Devices can be programmed for different ranges, materials, outpu class="lm-text" name="idPrompt_feat_record">ts, etc.
• Recorder /Totalizer Functions- Devices are data loggers which record/totalize and i class="lm-text" name="idPrompt_feat_control">ntegrate measured values.
• Controller Functions- Devices have or take sensor inputs, provide control functions (e.g., limits, PID, logic), and output a control signal.

Applications

Level monitoring is important in a wide variety of applications. Any industrial process involving the filling or storing a liquid in a tank or vessel would benefit from the use of a level switch. They are also an important part of process control systems which manages flow rate into and out of a storage vat or reactor.

Image Credit: cmcontrols.com

Level switches can be used for high or low level sensing and alarms, leak detection, overfill shutoff and regulating the interface between levels of different media. They are found in a wide variety of industries-- not limited to-- manufacturing, food and beverage, chemical and pharmaceutical, marine, medical, and fuel/energy management. 

Image credit:

Alfa Laval Inc.