WATER LEVEL SENSOR
Types, Diagram, and Applications
A complete technical guide covering 7 major sensor technologies - with internal
diagrams, key specifications, application guidance, and a practical decision table.
Introduction
Two Types of Measurement
Point-Level Sensors
Continuous Sensors
1. Float Switch
How It Works
Internal Diagram
Water surface
----------------------------------------
^
| Float body (buoyant, rises / falls)
|
[Magnet - permanent]
|
+----------------------------------+
| Threaded stem (NPT / BSP) |
| +----------------------------+ |
| | Glass tube | | <- reed switch sealed inside
| | [Blade A] [Blade B] | |
| +----------------------------+ |
+----------------------------------+
Wire 1 Wire 2
Key Specifications
|
Specification |
Typical Value |
Note |
|
|
Contact rating |
0.5–2 A, 100 V AC/DC |
Reed switch limit |
Do not drive loads directly - use a relay |
|
Float SG |
0.7–0.95 minimum |
Liquid specific gravity |
Check against your liquid's actual SG |
|
Temperature |
−40 °C to +105 °C |
Stem temperature |
Match to your process temperature |
|
Contact type |
NO / NC / SPDT |
Circuit arrangement |
SPDT for dual signal output |
Best Applications
Sump pump control and basement water management
Irrigation storage tanks and agricultural reservoirs
RV and marine freshwater tank monitoring
Sewage lift stations and wastewater sumps
Bilge pump control on boats and vessels
2. Conductivity / Electrode Probe
How It Works
Internal Diagram
Electrode A ───────── Wire A ────┐
|
+--------+
|Sensing | <- low-voltage AC (5–12 V, 50–60 Hz)
|Circuit | prevents electrolysis
+--------+
|
Electrode B ───────── Wire B ────┘
Tank wall (grounded) ────────────────────┘
Water bridging both electrodes -> circuit closes -> ON
One or both electrodes dry -> circuit open -> OFF
Key Specifications
|
Specification |
Typical Value |
Note |
|
|
Min. conductivity |
≥ 20 µS/cm |
Pure water ~1 µS/cm |
Will NOT work with pure or distilled water |
|
Electrode materials |
SS316L, Hastelloy C, titanium |
Match to liquid chemistry |
Corrosion destroys reading accuracy |
|
Max. probe length |
500 mm (standard) |
Longer on request |
Measure your tank height first |
|
Max. temperature |
+200 °C |
High-temp versions available |
Match to your process temperature |
Best Applications
Industrial boiler water level control
Cooling tower basin level monitoring
High-temperature tank overflow protection
Conductive chemical process tanks
Chrome plating and electroplating bath level
3. Optical Liquid Level Sensor (TIR)
How It Works
Internal Diagram
+------------------------------------------+
| Prism tip |
| -> IR LED | <- light enters prism
| -> |
| [Prism] | <- TIR in air (light trapped)
| -> |
| Phototransistor | <- ON in air
| |
| Liquid contact: |
| Refractive index changes -> |
| TIR breaks -> light escapes -> |
| phototransistor dims -> OFF |
+------------------------------------------+
|
Output: ON / OFF
Key Specifications
|
Specification |
Typical Value |
Note |
|
|
Response time |
1–5 ms |
Instantaneous switching |
Ideal for fast-process machinery |
|
Temperature |
−40 °C to +125 °C |
Operating range |
Check prism material for your chemical |
|
Pressure |
Up to 1 MPa (body) |
Process pressure |
Use appropriate process fitting |
|
Output |
NPN, PNP, IO-Link |
Signal type |
Match to your PLC input type |
Best Applications
Medical equipment (dialysis machines, IV fluid bags)
Hydraulic oil level monitoring
Leak detection inside sealed enclosures
Pharmaceutical tank overfill protection
Coffee machine water reservoir
4. Ultrasonic Level Sensor
How It Works
Block Diagram
Transducer fires pulse -> liquid surface -> echo returns
+----------------------------------------+
| Piezoelectric element + temp sensor |
| Time-of-flight processor |
| Output: 4-20 mA / RS-485 / IO-Link |
+----------------------------------------+
|
L = Tank height - (c x t) / 2
Key Specifications
|
Specification |
Typical Value |
Note |
|
|
Range |
0.2 m to 60 m |
Measurement span |
Choose above minimum blind-zone distance |
|
Accuracy |
±0.1–0.5 % of range |
Full-scale accuracy |
Requires empty and full calibration |
|
Frequency |
40–200 kHz |
Higher freq = narrower beam |
40 kHz for long range; 200 kHz for narrow tanks |
|
Blind zone |
50–500 mm |
Near transducer unmeasurable |
Account for this at tank design stage |
Best Applications
Open water storage reservoirs and tanks
Cooling tower basins and wastewater clarifiers
Chemical storage tanks (non-contact - no media contamination)
Underground cistern and borehole level monitoring
Food and beverage process vessels
5. Hydrostatic Pressure Sensor
How It Works
Block Diagram
Tank bottom: diaphragm <- P = rhogh
+----------------------------------------+
| Diaphragm (strain gauge / capacitive) |
| Vented cable (atmospheric reference) |
| Signal conditioning + temp comp |
| Output: 4-20 mA / 0-10 V |
+----------------------------------------+
Key Specifications
|
Specification |
Typical Value |
Note |
|
|
Range |
0–1 m to 0–100 m WC |
Water column range |
Match to your tank depth |
|
Accuracy |
±0.1–0.5 % FS |
Full-scale accuracy |
Includes linearity and hysteresis |
|
Vent tube |
Required |
Atmospheric reference |
Must stay dry - moisture causes drift |
Best Applications
Municipal drinking water tanks (submersible version)
Irrigation reservoir level monitoring
Marine hull bilge monitoring
Hydroponics nutrient tank level
Industrial open-process vessels
6. Radar Level Sensor
How It Works
Block Diagram
Antenna ---- Microwave pulse -> liquid surface
<- reflection echo
+----------------------------------------+
| ToF processor / FMCW analyser |
| Output: 4-20 mA / HART / RS-485 |
+----------------------------------------+
Key Specifications
|
Specification |
Typical Value |
Note |
|
|
Range |
0.1 m to 70 m |
Measurement span |
80 GHz extended range available |
|
Accuracy |
±0.5–3 mm |
High precision |
FMCW > pulsed for accuracy |
|
Frequency |
6 / 26 / 80 GHz |
Higher = narrower beam |
80 GHz best for small-diameter tanks |
|
Beam angle |
4°–20° |
Narrower at high frequency |
Prevents wall reflections in narrow tanks |
Best Applications
High-pressure petrochemical and refinery storage tanks
Steam-generating boilers
Tanks with heavy foam, vapour, or turbulent surface
Hygienic pharmaceutical process tanks
Underground storage tank monitoring
7. Capacitive Level Sensor
How It Works
Block Diagram
+----------------------------------------------+
| Electrode A <------- d -------> Electrode B |
| | | |
| +----------- C = eA/d --------+ |
| er: 1 -> 80 when water fills gap|
| Output: switch / 4-20 mA |
+----------------------------------------------+
Key Specifications
|
Specification |
Typical Value |
Note |
|
|
Mode |
Point-level and continuous |
Two product families |
Check which your system requires |
|
Probe length |
0.1 m to 6 m |
Standard range |
Custom lengths available |
|
Through-wall option |
Non-conductive walls only (PE, PP, glass) |
Wall thickness ≤ 10 mm |
Metal tanks require insertion probe |
Best Applications
Viscous liquids - slurry, adhesive, resin, glue
Plastic tank level (through-wall - no tank modification)
Oil-over-water interface detection
Pharmaceutical and cosmetics mixing vessels
Adhesive and coating process tanks
Choosing the Right Type: Decision Matrix
|
Application Need |
Best Sensor Type |
Mode |
Contact |
Key Advantage |
|
On/off switching, lowest cost |
Float switch |
Point-level |
Yes |
Simplest, cheapest, most reliable |
|
High/low alarm, conductive liquid |
Conductivity probe |
Point-level |
Yes |
Works in high-temperature boilers |
|
Fast switching, hygienic / sterile |
Optical TIR |
Point-level |
No |
1-5 ms response, no moving parts |
|
Continuous, open tank, medium cost |
Ultrasonic |
Continuous |
No |
Best value for open-tank monitoring |
|
Continuous, closed tank, harsh conditions |
Radar |
Continuous |
No |
Unaffected by steam, foam, pressure |
|
Submersible, open tank |
Hydrostatic pressure |
Continuous |
Yes |
Simplest direct level measurement |
|
Through-tank-wall, no modification |
Capacitive (through-wall) |
Both |
No |
No tank opening or draining required |
|
Liquid interface (oil over water) |
Capacitive or conductivity |
Point-level |
Yes |
Detects the interface layer between fluids |
Installation Considerations
Float Switch
Mount vertically for most applications; horizontal for side-entry tanks
Ensure clearance from tank walls - use a stilling well in dirty or debris-laden water
Check the specific gravity rating against your liquid's actual SG before specifying
Keep the magnet stem away from ferromagnetic tank walls
Conductivity Probe
Ground the tank wall or install a separate reference electrode
Keep cable connections sealed - moisture causes false dry readings
Use AC excitation only - DC causes electrolysis within days
Align electrodes perpendicular to flow to prevent false bridging
Optical TIR Sensor
Install with the prism flush with the tank interior wall - no protrusion into flow
Keep the prism tip clean - residue builds up and causes false readings
Check chemical compatibility of prism material (glass vs polysulfone vs PVDF)
Allow a 10–20 mm dead band at the setpoint to prevent output chatter
Ultrasonic Sensor
Mount above the highest expected water level - observe the blind zone specification
Point downward from above the tank at the centre - away from walls and inlet flow
Use a stilling well or bypass tube in turbulent or foamy applications
Keep the transducer face clean and dry - condensation on the face causes errors
Hydrostatic Pressure Sensor
Mount at the lowest point of the tank - at or near the bottom
Route the vent tube upward from the sensor to prevent moisture accumulation
Never submerge the vent tube opening - it must remain open to atmosphere
Avoid mounting near inlet flow or pump outlets - dynamic pressure adds measurement error
Radar Sensor
Mount above the highest expected level with clearance above the beam cone
Use a stilling well in turbulent or foamy tanks to damp surface waves
Keep the antenna clean in dusty or humid process environments
In narrow tanks, aim at the centre - the narrow beam prevents wall reflections
Capacitive Sensor
Through-wall: ensure the tank wall is non-conductive and ≤ 10 mm thick
Insertion probe: keep the probe away from tank walls and inlet flow
Calibrate in the actual tank with the target liquid - dielectric constant varies by fluid
Through-wall mode: allow 24 hours for the sensor to stabilise after first power-on
