Published on 08/25/2009

A typical level switch is a simple on/off indicator showing whether the liquid level in a vessel is above or below the switch's location. Creative use of multiple switches allows a more precise picture of a tank's contents.

Simulating Continuous Level Indicators

Many vessels are designed with only one or two level switches. Depending on the expected operation there might be a switch near the top to signal the tank is full and/or one at the bottom to let operators know the tank is nearly empty.

Instead of one or two level switches, vessels can be fitted with several switches each at a different level in the tank. As the tank is filled, switches are triggered in sequence as the level rises past each one. When the

liquid level starts to fall, switches are deactivated from the top down. Signals from the switches can be routed to a control board to provide a stepped continuous level indicator that gives operators more information than a single switch could.

Mapping Trends

Rather than just knowing what the current level of liquid is, operators often need to know how that level is changing. Installing a series of level switches allows operators to track changes in liquid level as well as the level itself. They can see what direction the level is going and make any response necessary to maintain equilibrium.

These trends can be used in process control systems to make more precise adjustments to flow volumes or other operational parameters. The system can respond not only to the level in the tank but whether that level is increasing or falling. The more information the process control system has, the more efficiently it can respond to changing conditions in the system.

Complex Control Logic

In a simple control system, a safety valve might open once a level switch triggers indicating a tank is getting too full. When the level drops, the valve closes again. With more information, process control engineers can design control systems that can make more complicated decisions than "on/off".

An advanced control system can examine the level and rate of change of liquid in a vessel and make decisions accordingly. To extend the example above, if the control system sees the tank is full but also sees the level is dropping, it is not necessary to open the emergency valve. On the other hand if the level is below the critical threshold but rising quickly, the valve could be opened early to deal with the overflow before it becomes a problem.

A level switch is a simple device but that doesn't mean it isn't capable of complex operations. Incorporating a network of switches into a process control system allows operations to run more efficiently and with finer control.