Liquid level control relays and sensors

Liquid level control relays are designed to monitor liquid levels in tanks, reservoirs, wells and other storage or process systems. In response to a signal from a connected level sensor or electrodes, the relay can control a pump, valve, alarm device or another automation component.

The category may include DIN rail level control relays, liquid level sensors, probes and connection electrodes. Important selection criteria include the number of monitored levels, filling or draining control function, supply voltage, output contact ratings, sensor type and the electrical conductivity of the liquid.

Automatic liquid level monitoring

A liquid level control relay processes signals from level sensors or electrodes and switches its output contact when the liquid reaches a specified limit. Such a system can be used for automatic filling or draining of a tank, protection of a pump against dry running, and minimum or maximum level indication.

Conductive level control devices detect liquid by measuring electrical conductivity between electrodes. It is therefore important to confirm that the monitored liquid is suitable for this measuring principle. The length and position of the probes or electrodes determine the levels at which the relay changes the state of its output.

How to choose a liquid level control relay

• Control function: determine whether the system will fill or drain a tank, prevent overflow or monitor dry-running conditions.
• Number of levels: monitoring one or several thresholds may require different quantities of electrodes or sensors.
• Supply voltage: it must match the voltage available in the automation system.
• Output contacts: check the contact configuration and permissible switching load.
• Sensitivity range: the conductivity setting must be suitable for the specific liquid and the distance between the electrodes.
• Sensors and electrodes: they must be compatible with the relay, liquid, tank and intended installation method.
• Mounting: consider the relay mounting method, available panel space and required sensor cable length.

The relay settings, electrode positions and wiring arrangement must be selected according to the operating principle of the particular system. If the relay contact is not designed to switch the pump load directly, a suitable contactor or another switching device should be used to control the pump.