When selecting sensors for liquid level measurement applications, engineers are often faced with numerous technologies and measuring principles to choose from. Simon Dear provides an overview of five common level sensing technologies and the applications these are best suited to
Many industrial processes require devices that are able to sense the level or depth of liquids stored within various types of tank, chamber or container. The signals from these level sensors can be used to control a production process or to provide feedback and indication of the status.
The level measurement itself can be either continuous or point values. Continuous level sensors measure levels within a specified range and determine the exact amount of liquid in a certain location, while point-level sensors indicate whether the liquid level is above or below the sensing point.
There are many physical and application variables that affect the selection of the most appropriate level sensing method for industrial processes. This article considers five common types of level sensing technology and where these are best used:
Float switches are one of the most cost effective, but also well-proven and reliable technologies for liquid level sensing. A float switch comprises a magnet contained within a float, as well as a magnetic reed switch contained within a fixed housing. The movement of the float, due to the changing liquid level, will cause the reed switch to operate (ie, close or open) at a particular level. This technology is based on a relatively simple mechanical design that offers long-term reliability without the need for the user to calibrate the switch.
Single switch points are normally provided for upper and lower level sensing. The choice of switch style that may be suitable for an application depends on the physical arrangement of the container or tank, available mounting positions and whether access is available to the inside of the tank. Typically, suppliers will offer a variety of different mounting options, with the main styles being horizontal/side mounting and vertical mounting.
Float switches are best suited to relatively small tanks or chambers and are provided in a variety of different materials to suit most fluids, chemicals, temperatures and pressures. Float switches can also be ATEX-approved for installation in potentially explosive atmospheres such as offshore oil & gas platforms or onshore petrochemical plants.
Optical level switches
Optical level switches are solid-state devices used for point level sensing of liquids or suspended solids. These switches measure the decrease or change in transmission of infrared light emitted from an infrared diode (LED). With the correct choice of construction materials and mounting location, these switches can be used with aqueous, organic or corrosive liquids. Typically, optical level sensors offer high reliability and are resistant to shock and vibration and other mechanical influences and are less affected by solids in suspension. Optical level sensors will usually incorporate some form of circuit logic and electronics such as anti-splash technology in order to eliminate any false readings.
Cable-suspended level switches are relatively simple, cost effective mechanical devices that are typically installed in large tanks or containers, as well as in natural open spaces such as reservoirs. For example, cable-end float switches are operated by changing fluid levels, which cause the angle of the switch to vary. This, in turn, causes a ball bearing in a housing to move from one end of a sealed chamber to the other, thus operating a micro switch, which opens/closes the contacts depending on the type of switching action required for the application. ATEX-certified switches can normally be provided for installation in potentially explosive gas atmospheres.
Cable-suspended pressure/depth sensors
Hydrostatic pressure level sensors are submersible or externally mounted pressure sensors suitable for measuring the level or depth of liquids in large/deep tanks or water in reservoirs. For these sensors, using chemically compatible materials is important to ensure high performance.
As these devices sense increasing pressure with depth and because the specific gravity of different liquids will vary, the sensor must be correctly calibrated for each application. Large variations in temperature can also cause changes in specific gravity, which should be accounted for when pressure is converted to level.
For use in open-air applications, where the sensor cannot be mounted to the bottom of the tank or pipe, special versions of a pressure level sensor can be suspended from a cable into the tank to the bottom point that is to be measured. Cable-suspended pressure sensors are solid-state devices that provide continuous depth/level measurements via analogue output signals. This type of technology is impervious to surface movements and so is suitable for installation in either small or large tanks, as well as in rivers and reservoirs. These devices are available in a wide range of outputs and measuring ranges.
Fixed pressure/depth sensors
Similar to cable-suspended pressure sensors, fixed pressure sensors are permanently mounted to the side wall or base of small or large tanks. These solid-state, analogue output devices provide continuous depth/level measurement and are impervious to surface movements and agitation. Compared to cable-suspended versions, fixed pressure sensors are less affected by suspended solids.
Simon Dear is with Cynergy3 Components Ltd, Wimborne, Dorset, UK.