A float switch is one of the simplest and most reliable level devices in the field: a buoyant float that rises and falls with the liquid and mechanically trips a switch at a set height. It gives a discrete on-off signal rather than a continuous reading. This guide explains how a float switch works, where it differs from a level transmitter, and its oil and gas role.
Float Switch in one line: A float switch is a point-level device in which a buoyant float rides on the liquid surface and, when the level reaches a set height, mechanically actuates an electrical switch to open or close a contact. It provides a discrete on-off signal used for alarms, pump control, and interlocks rather than a continuous level value.
A float switch pairs a buoyant float with a switch mechanism. In a common magnetic design, the float contains a magnet and slides on a stem; as the float rises to the trip height, its magnet actuates a sealed reed switch inside the stem, changing the contact state. Other designs use a hinged float arm that mechanically toggles a microswitch, or a mercury-free tilt element in a tethered float. Because the actuation is mechanical and the switch contacts are sealed, float switches are rugged, need no external power to sense, and fail predictably.
A single float switch detects one threshold - level reached or not reached - and reports it as a discrete contact. Installations often stack several floats on one stem, or use several separate switches, to mark distinct setpoints such as pump-on, pump-off, high-high alarm, and low-low alarm. The contacts wire straight into a discrete input on a PLC or RTU, or directly into a motor control or shutdown circuit.
Because they are simple and independent of any transmitter's electronics, float switches are widely used as safety and control point-level devices: high-level and high-high shutdown on tanks and vessels, low-level pump protection, and pump start-stop control on sumps and water tanks. They are frequently used as an independent backup to a continuous level transmitter so that a critical high-level trip does not depend on the same instrument that drives normal control.
Their discrete contact wires into a PLC or RTU digital input, or directly into an interlock. A cloud SCADA such as Merobix reads that switch state from the controller over Modbus or DNP3 as a digital status point, so an operator sees a high-level or pump-fault condition and can be alarmed remotely. The float switch does the sensing and, often, the hard-wired trip; SCADA provides the visibility and notification on top of it.
A level transmitter reports a continuous, proportional level value across a range. A float switch reports only a discrete on-off state at one set height - reached or not reached. Transmitters drive normal level control and trending; float switches drive alarms, pump start-stop, and safety trips, and are often installed as an independent backup to a transmitter.
Independence and simplicity. A hard-wired float switch does not share electronics, power, or firmware with the continuous transmitter, so a critical high-level shutdown does not depend on the same device that runs normal control. Its mechanical, low-complexity design also fails in predictable ways, which is valued for safety interlocks and high-high trips.
Its contact closure wires into a discrete (digital) input on a PLC or RTU, which turns it into an on-off status tag. It may also be wired directly into a motor or shutdown circuit for the actual trip. A cloud SCADA then reads that digital status from the controller over a protocol like Modbus or DNP3 to display and alarm the state remotely.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.