Water Level Probe (3 electrodes) Symbol
Definition: The Water Level Probe (3 electrodes) symbol represents a three-electrode conductivity-based liquid level sensor used in circuit diagrams to indicate a device that detects High, Low, and Common reference levels in a tank or vessel, as described in IEC 60617 instrumentation conventions.
Also known as: electrode level sensor, conductive level probe, tank level sensor, liquid level electrode, water level sensor.
What the Water Level Probe (3 electrodes) symbol means
The Water Level Probe (3 electrodes) symbol denotes a sensor that uses electrical conductivity between immersed metal electrodes to detect discrete liquid levels. When water bridges two electrodes, current flows and signals a control circuit that a level threshold has been reached or exceeded.
In wiring and control diagrams the symbol identifies the physical probe assembly installed in a tank or sump. Its three terminals—High, Low, and Common—allow a relay or PLC input to distinguish between empty, partially filled, and full states, enabling automatic pump control, fill-valve actuation, or overflow alarms.
How to identify the Water Level Probe (3 electrodes) symbol
The glyph depicts three vertical lines of differing lengths descending from a horizontal mounting bar, representing three electrodes at different immersion depths. The longest electrode is the Common reference, the mid-length electrode marks the Low level, and the shortest electrode marks the High level. Connection lines exit from the top of the symbol toward the control circuit.
Function in a circuit
A three-electrode water level probe detects liquid presence by measuring conductivity between pairs of electrodes. When the liquid surface rises to contact the High electrode, the circuit between High and Common closes, signalling a full condition; when the liquid falls below the Low electrode, the Low-to-Common circuit opens, signalling an empty condition. The probe output drives a relay, contactor, or PLC input that controls pumps or valves accordingly.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 (graphical symbols for diagrams) covers level-measurement instruments in the process instrumentation subset; the probe symbol is typically drawn as three electrode lines of different lengths with a common terminal, consistent with IEC 60617-11 sensor/transducer conventions. |
|---|---|
| ANSI/IEEE 315 | ANSI/ISA-5.1 (Instrumentation Symbols and Identification) identifies level sensors with the function letter 'L' and a tag suffix; a discrete electrode probe is often shown as an instrument bubble with 'LE' (Level Element) connected to electrode lines, equivalent in meaning to the IEC depiction. |
| Key difference | IEC uses electrode line glyphs descending from a horizontal rail; ANSI/ISA-5.1 uses a circular instrument bubble with 'LE' tag letters. The physical meaning is identical in both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| high | High |
| low | Low |
| common | Common |
Typical values
Supply voltage typically 5–24 V DC; probe current < 1 mA per electrode pair; electrode separation 10–200 mm depending on tank geometry; conductivity threshold typically > 20 µS/cm for water detection.
Where the Water Level Probe (3 electrodes) symbol is used
- Sump pump control circuits that start and stop a pump based on pit water level
- Water storage tank automation for automatic fill-valve or pump control
- Cooling tower make-up water systems maintaining minimum basin level
- Industrial process tanks monitoring fill, low-alarm, and high-alarm levels
- Wastewater lift stations controlling submersible pumps via PLC input
- Aquarium or hydroponics reservoir level monitoring and auto-refill circuits
Example
In a basement sump-pump control panel, the three-electrode probe symbol appears connected to a float-relay input module: the Common electrode is permanently submerged, the Low electrode signals pump-off to the relay coil, and the High electrode signals pump-on, so the pump runs only when water is between the Low and High marks.
Key facts
- The Water Level Probe (3 electrodes) symbol has three terminals: High (short electrode), Low (mid electrode), and Common (long reference electrode).
- Detection relies on water conductivity completing the circuit between the Common electrode and either the High or Low electrode; deionised or very pure water may not be detected reliably.
- The symbol designator in process diagrams is typically 'LE' (Level Element) per ANSI/ISA-5.1, with a tag number identifying the specific instrument loop.
- Supply voltage for electrode probe circuits is commonly 5–24 V DC; probe current is kept below 1 mA to prevent electrolytic corrosion of the electrodes.
- A three-level probe provides two discrete setpoints (High and Low) from a single probe body, reducing panel wiring compared to two separate float switches.
- The Common electrode must always be the longest so it remains submerged even at the minimum liquid level, providing a continuous reference path.
- Electrode probes require periodic cleaning in hard-water applications because calcium carbonate scale can insulate the electrode surface and cause false readings.
Diagrams that use this symbol
- circuit diagram of water level indicator
- water level controller circuit diagram
- water level indicator diagram
- fully automatic water level controller circuit diagram
Frequently asked questions
What does the water level probe symbol mean in a wiring diagram?
The water level probe symbol represents a conductivity-based sensor that detects liquid levels at two or more discrete heights using immersed electrodes. In a wiring diagram it identifies where the probe connects to a control relay, PLC, or level-relay module that switches pumps or valves based on the sensed level.
What do the High, Low, and Common terminals on a 3-electrode probe do?
The Common terminal is the always-submerged reference electrode. The Low terminal signals when liquid falls below the minimum level (pump-on or alarm), and the High terminal signals when liquid rises to the maximum level (pump-off or overflow alarm). A control relay monitors conductivity between the Common and each of the other two electrodes.
What is the designator letter for a water level probe in a schematic?
Per ANSI/ISA-5.1, the designator is 'LE' (Level Element) followed by a loop number, for example LE-101. In general-purpose electrical schematics the probe may simply be labelled with a tag name such as 'WLP1' or 'E1' without a standardised letter code.
What is the IEC vs ANSI difference for the water level probe symbol?
IEC 60617 depicts the probe as three electrode lines of different lengths descending from a horizontal mounting rail. ANSI/ISA-5.1 uses a circular instrument bubble labelled 'LE' connected to the probe location. Both convey the same functional meaning; the ISA style is dominant in process-control and PLC documentation.
What type of water can a conductivity electrode probe detect?
Electrode probes detect water and aqueous solutions with conductivity above approximately 20 µS/cm, which covers tap water, process water, and wastewater. Highly purified or deionised water (conductivity < 1 µS/cm) may not complete the circuit reliably; capacitive or ultrasonic sensors are preferred for those applications.
How many terminals does a 3-electrode water level probe have?
A three-electrode water level probe has exactly three terminals: High, Low, and Common. These correspond to the three immersed electrodes at different depths in the tank or vessel.
Where is the water level probe symbol used in circuit diagrams?
The symbol appears in control panel wiring diagrams for sump pumps, water storage tanks, cooling towers, lift stations, and industrial process vessels. It is also found in PLC I/O wiring drawings where the probe outputs connect to discrete input modules.
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