Choosing a level sensor sounds straightforward until you start reading datasheets. Hydrostatic, radar, ultrasonic, capacitive, guided-wave, float… each technology has loud advocates and good reasons to exist. The honest answer is that the right technology depends entirely on the application, and picking the wrong one is one of the most common causes of unreliable level data in the field.
This guide compares the three most widely deployed level technologies, hydrostatic, radar and ultrasonic, and gives you a practical framework for picking between them.
How each technology actually works
Hydrostatic level measurement uses the simple physics of fluid pressure: the pressure at the bottom of a column of liquid is proportional to the height of that liquid. A submersible pressure transmitter sits at (or near) the bottom of the tank or borehole, measures the pressure above it, and converts that into a level reading. As a practical guide, roughly 1 metre of water = 100 mbar — and from there, the Stork level measurement range translates pressure into a calibrated level output.
Radar level measurement sends a microwave signal down from the top of the tank, measures the time taken for the reflection to return from the liquid surface, and calculates the distance, and therefore the level.
Ultrasonic level measurement does the same thing as radar but using high-frequency sound waves instead of microwaves. A transducer mounted above the liquid emits a pulse and times the echo.
Three different physical principles, three different sets of strengths and limitations.
Hydrostatic: the workhorse for water and remote sites
Strengths
- Unaffected by foam, vapour, turbulence or surface conditions — the sensor sits below the surface, so what’s happening on top is irrelevant.
- Ideal for deep, narrow installations — boreholes, wells, sumps, and tall tanks where top-down line-of-sight is impossible.
- Excellent low-power options for remote sites — particularly when paired with SDI-12.
- Long-term stability and straightforward calibration.
- Performs well in water, wastewater and many chemicals with the right wetted materials.
Limitations
- Requires knowledge of fluid density — a change in density changes the apparent level.
- The sensor is in contact with the media, so material compatibility matters.
- Cable management for very deep installations needs planning.
When to specify hydrostatic: groundwater and borehole monitoring, surface water level, sumps and wet wells, water and wastewater treatment, irrigation reservoirs, fire hydrant pressure-to-level conversion, and most remote/telemetry-driven sites.
Stork’s submersible level sensors and transmitters range is built around exactly these applications:
- SLS-D Digital Submersible Level Transmitter – SDI-12 – fully SDI-12 v1.4 compliant, 1–100 mWG, ideal for environmental monitoring, flood defence and remote telemetry.
- SLS-A Submersible Level Transmitter – Analogue Output – the workhorse 4–20 mA submersible for rivers, reservoirs and boreholes.
- SLS-LC Submersible Level Transmitter – Ceramic Sensor – ceramic Al₂O₃ sensing element for excellent corrosion resistance.
- LMK 382 Stainless Steel Level Transmitter – for sewage, viscous and pasty media.
- LMK 806 Plastic Level Transmitter for Aggressive Media – slim 21 mm probe for acids, lyes and aggressive wastewater.
- LMK 458 H Level Transmitter with HART® for Marine and Offshore – marine-approved (LR, DNV, CCS, ABS) for ballast and shipboard tanks.
Where a fixed mounting is preferred over a suspended probe, the screw-in level transmitters range uses the same hydrostatic principle with a threaded process connection.
Radar: the right answer for tough top-down installations
Strengths
- Non-contact — never touches the media, so material compatibility largely disappears as a concern.
- Generally unaffected by density, dust, vapour and temperature.
- Performs well in pressurised vessels and at high temperatures.
- Strong choice for bulk solids as well as liquids.
Limitations
- More expensive than hydrostatic per measurement point.
- Requires clear top-down access to the liquid surface.
- Foam, heavy agitation and complex tank internals can confuse the signal — proper antenna selection and installation matter.
- Generally higher power draw than a low-power hydrostatic + SDI-12 setup.
When to specify radar: sealed industrial vessels, high-temperature or high-pressure tanks, bulk solids silos, fuel and chemical storage where non-contact measurement is preferred, and applications where the sensor must never touch the media.
Ultrasonic: a cost-effective non-contact option for simpler tanks
Strengths
- Non-contact like radar, but typically at a lower price point.
- Simple to install on open tanks and channels.
- Well-suited to bulk solids in clean conditions and to open-channel flow measurement.
Limitations
- Strongly affected by vapour, foam, dust, temperature gradients and turbulence, the speed of sound varies with all of these.
- Limited range compared with radar.
- Doesn’t perform well in vacuum or pressurised vessels.
- Needs a clear, calm surface and stable atmosphere above the liquid for best results.
When to specify ultrasonic: open water channels and weirs, simple open tanks with low vapour, bulk solids in dry, dust-controlled environments, and budget-sensitive non-contact installations.
Quick comparison
Factor | Hydrostatic | Radar | Ultrasonic |
Contact with media | Yes | No | No |
Best for deep / narrow installs (boreholes) | Excellent | Limited | Limited |
Affected by foam / vapour | No | Some | Yes |
Affected by density change | Yes | No | No |
Power consumption | Low (esp. SDI-12) | Medium–High | Medium |
Typical cost per point | Low–Medium | High | Medium |
Pressurised / sealed vessels | Suitable | Excellent | Poor |
Bulk solids | No | Excellent | Good |
How to actually choose
A practical decision tree:
- Is it a borehole, sump, well or deep narrow tank? → Hydrostatic is almost always the right answer. Start with the SLS-D or SLS-A.
- Is it a remote, battery- or solar-powered site needing daisy-chained sensors? → Hydrostatic on SDI-12 (SLS-D + TPT-SDI + USB-SDI12-PRO for configuration).
- Is it a sealed, high-pressure or high-temperature vessel where non-contact is essential? → Radar.
- Is it an open tank or channel with simple, calm conditions and a tight budget? → Ultrasonic.
- Is it sewage, viscous media or aggressive chemicals? → Hydrostatic with the right material — LMK 382 or LMK 806.
- Is it marine or offshore with class approvals required? → LMK 458 H.
A note on density and temperature
The one Achilles’ heel of hydrostatic measurement is density. If your liquid’s density changes — with temperature, concentration, or composition — the level reading will drift unless you compensate. Two practical approaches:
- Density correction in the sensor — the SLS-D supports density correction via its SDI-12 configuration software.
- Combined level + temperature measurement — many of the LMK-series probes and the screw-in range offer integrated temperature output, allowing dynamic specific gravity compensation in the data logger or PLC.
For radar and ultrasonic, density doesn’t affect the measurement, but they have their own compensation needs (temperature for ultrasonic, dielectric properties for radar).
The takeaway
Radar and ultrasonic each have their place, but for the majority of water, wastewater, environmental and process-tank applications, particularly anything involving boreholes, remote sites, low-power telemetry or aggressive media , hydrostatic remains the most cost-effective, robust and well-understood option.
Browse the full level measurement portfolio, the submersible level transmitter range, or the screw-in level transmitters, or get in touch with Stork Solutions to discuss your application before you commit to a technology.
