Level sensors for tanks
Keeping a tank from running dry or overflowing is one of the most common PLC tasks. The sensor you choose depends on the material (water, oil, powder), the tank and whether you need a point signal or the actual level.
Point level vs continuous level
Point level sensors switch at one height: "tank high", "tank low". Continuous sensors measure the level all the time and send it as 4–20 mA, 0–10 V or over a network.
Point level sensors
| Type | How it works | Good for |
|---|---|---|
| Float switch | a float moves a magnet past a reed switch, or tilts a contact | clean liquids, cheap and simple |
| Capacitive | senses the liquid through a plastic or glass wall, or with a probe | liquids, granules, powders |
| Conductive probes | current flows between probes when a conductive liquid touches them | water, pump control with high/low probes |
| Vibrating fork | a tuning fork changes frequency when covered | almost any liquid, foam-insensitive, very reliable |
| Optical | a prism reflects light inside unless immersed in liquid | small tanks, oil |
Continuous level sensors
| Type | Notes |
|---|---|
| Ultrasonic | non-contact, from the top; affected by foam, vapour and the blind zone |
| Radar | like ultrasonic but with microwaves; works with vapour, dust and pressure; more expensive |
| Hydrostatic (pressure) | a pressure sensor at the bottom: level = pressure ÷ (density × g). Simple and accurate for liquids. |
| Guided wave radar | a rod or cable into the tank; works in difficult conditions |
Pump control with two level points
A classic PLC example: start the fill pump when the level drops below LOW, stop it when it reaches HIGH. That is a seal-in circuit (see ladder logic basics):
|--+--[/ Low_level ]--+--[/ High_level ]--( Pump )--| | +--[ Pump ]--+
Add a separate HIGH-HIGH switch wired to stop the pump directly, so a failed sensor cannot overflow the tank.
Choosing
- Clean water, low cost: float or conductive.
- Sticky, foamy or unknown liquid: vibrating fork.
- Need the actual level on the HMI: hydrostatic, ultrasonic or radar.
- Powders and granules: capacitive, vibrating rod or radar.
The PLC Sim Lab Playground includes tanks and a liquid dispenser with level signals to practise fill control.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
Open the Playground →Guided lessons →