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Submersible Hydrostatic Level Sensors & Probes

Hydrostatic level probes designed for permanent immersion. Select materials (SS, Ti, PVDF) & vented cables for water, wastewater, & chemical applications.

Submersible hydrostatic level sensors provide a dependable solution for continuous liquid level monitoring through direct immersion and precise hydrostatic pressure measurement. Designed for long-term deployment in diverse media ranging from potable water and wastewater to seawater and aggressive chemicals, these IP68-rated probes incorporate specific material selections for housing, diaphragm, and cabling, alongside critical features like vented reference tubes for barometric compensation. Understanding these design variations is key to selecting the optimal sensor for challenging applications such as deep boreholes, chemical storage tanks, groundwater wells, or marine ballast systems, ensuring accurate data for process control and resource management.

Find out more about Submersible Hydrostatic Level Sensors & Probes to determine which product options and capabilities will best meet your application requirements.

Submersible hydrostatic level sensors, often referred to as level probes, are specifically engineered for continuous immersion within a liquid medium. Their primary function is to determine the liquid level or depth by precisely measuring the hydrostatic pressure exerted by the fluid column above the sensor diaphragm. This method provides a reliable way to monitor levels in various static or slow-moving liquid bodies.

The construction of these sensors is critical for long-term reliability in submerged conditions. Housings are typically manufactured from materials like 316L stainless steel for general water and wastewater applications, Titanium for superior corrosion resistance in seawater or brackish water environments, or specialized polymers like PVDF for compatibility with aggressive chemicals such as acids and alkalines. Diaphragm materials, the core sensing element interface, often include robust ceramics known for their excellent chemical resistance and mechanical stability, or stainless steel variants chosen for specific pressure ranges and media compatibility.

A crucial design element for many applications is the vented cable. These cables incorporate a small ventilation tube running alongside the signal wires, terminating typically at the instrumentation end. This tube allows the sensor’s reference pressure chamber (the back side of the diaphragm) to sense ambient atmospheric pressure. This ensures the sensor measures gauge pressure, providing a true level reading unaffected by barometric pressure fluctuations, which is essential for open tanks, rivers, reservoirs, and boreholes. Protecting this vent tube from moisture ingress using filters or desiccant cartridges is vital for long-term accuracy and sensor lifespan, particularly in humid or wet installations. Alternatively, absolute pressure sensors (non-vented) may be used in sealed tanks or when atmospheric compensation is handled externally.

The electrical cable itself requires careful consideration based on the installation environment and liquid medium. Polyurethane (PUR) cables offer good mechanical strength and flexibility suitable for general water applications. For chemically aggressive media or higher temperatures, FEP (Fluorinated Ethylene Propylene) cables provide superior chemical resistance. The cable must not only be waterproof (typically rated to IP68 for permanent immersion) but also resistant to abrasion, tension, and the specific chemical makeup of the liquid it passes through, especially relevant in wastewater lift stations or chemical storage tanks.

Application-specific challenges dictate sensor selection and installation practices. In wastewater or leachate monitoring, robust designs that resist clogging and chemical attack are necessary; features like larger diameter diaphragms or protective nose cones can be beneficial. For groundwater boreholes, considerations include potentially long cable runs requiring adequate tensile strength and strain relief, protection against sediment abrasion, and often the need for narrower sensor diameters. Installing sensors in rivers or open channels might necessitate additional mechanical protection against debris and flow-induced forces, along with lightning surge protection for the electrical connections, especially in remote locations.

Ship ballast tank monitoring demands sensors built with materials highly resistant to saltwater corrosion, like Titanium or specific marine-grade stainless steels, and often requires marine certifications (e.g., DNV, ABS). For potable water applications in tanks, towers, or reservoirs, materials must comply with drinking water standards (e.g., WRAS, NSF), and high accuracy might be needed for resource management. When dealing with concentrated acids or bases in industrial storage tanks, meticulous verification of material compatibility (housing, diaphragm, seals, and cable) against the specific chemical, concentration, and temperature is paramount to ensure safety and sensor longevity.

These sensors typically provide industry-standard output signals such as 4-20mA analog current loops, voltage outputs, or digital protocols like Modbus RTU or SDI-12. This allows for straightforward integration with PLCs, SCADA systems, telemetry units, or dedicated data loggers used in process control, environmental monitoring, and water management systems. Proper installation, including securing the cable to prevent sensor movement or strain, and periodic checks or calibration where feasible, contribute significantly to achieving reliable long-term level measurements.

Product Help

Securing inside a tank

What is the best way to secure the submersible hydrostatic level sensor inside a tank?

The sensor is often simply dangled into a tank from the top. If the contents of the tank are in motion it is then best to use a perforated stilling tube or a C Clip in the tank to prevent the sensor moving around.

Orientation effects

What is the best orientation for the submersible hydrostatic level sensor?

Most are designed and calibrated to be installed with the end of the probe pointing down, although it is okay to use them in any orientation, as long as you are aware of the effects on calibration.   In particular very low pressure ranges are more susceptible to changes in orientation where a thinner diaphragm will move slightly due to its own weight.  Also low pressure range devices which are susceptible to trapping air, such as the volume inside a threaded pressure sensor port, will exhibit calibration shifts at different orientations as the quantity of trapped air changes.

Acceptable offset

What is an acceptable offset for submersible level sensors?

The zero & span offset can vary between products of the same type, but they should all fall within an acceptable limit, which is sometimes indicated on the product data sheet, as a +/- small output signal value e.g. +/-0.2mA or a percentage of full scale e.g. < 1% FS.

Some times submersible level sensors can be configured with reverse outputs or large offsets to suit the application, e.g. measuring over a narrow range of level with a large base head of water, or to measure high signal when a well is empty rather than full.

Mounting sensor to measure river or canal depth

What way would you suggest to mount the sensor to be able to measure the depth of a river or a canal?

cable clamp for mounting submersible hydrostatic liquid level sensors

The sensor can be installed by lowering it slowly into a stilling well. A plastic tube open at both ends and mount to a bridge, jetty or pier is often used to protect the sensor and stop it moving around, whilst still allowing the water to rise and fall with the tides, with an opening at the bottom to ensure the level in the tube equalises with the surrounding water level.

The sensor signal cable should be fixed at the top of the tube using a clamp or junction box, so that the sensor position is fixed and cannot move.

Cable junction box with moisture filter for installing vented cable

Typically the cable will contain a vent tube at the end that is used to allow the sensor reference to vent to atmospheric pressure, which is very important for accurate level measurement.

To ensure no moisture does not enters the vent tube it is often fitted with a filter, but it is also important to terminate the cable in a ‘dry’ but vented enclosure such as a purpose built junction box  with a built-in air moisture trap or breather filter.

Specifying submersible hydrostatic level sensor and probe requirements

To avoid fitting an unsuitable submersible IP68 pressure sensor for measuring the depth of a liquid, it pays to consider all the factors that will influence the final specification you choose.

If you follow the tips described below you will ensure improved level measurement performance, and a longer service life for the submersible pressure sensor you eventually install.

Liquid type

What type of liquid is to be measured?

This will help select a device with the most compatible construction materials. Submersible pressure sensors are comprised of a sensing module, body, cable and seals, all of which must be compatible with the substance in which the sensor will be immersed.

Density of liquid

What is the specific gravity or density of the liquid compared to freshwater?

The hydrostatic pressure generated by a fixed height of liquid is proportional to the specific gravity. 10 metres of water is a very close approximation to a 1 bar hydrostatic pressure, but if the specific gravity is say 0.9 of water (fresh water has a specific gravity of 1.0) a 10 metre head of this liquid would generate a 0.9 bar hydrostatic pressure instead. The output of some submersible pressure sensors can be scaled to correct for specific gravity without the need for the installer to compensate for it within the signal conditioning instrumentation.

Viscosity of liquid

How viscous is the liquid media?

This will help in determining whether a flush profile is required for the sensing element. High viscosity liquids can choke the protective caps and pressure ports that are fitted to the end of the sensor.

Max/Min level

What is the maximum and minimum level of liquid that will be measured from the intended transducer location?

The total depth of liquid is not necessarily the required hydrostatic range. To improve accuracy of measurement the sensor range should be specified for the change in level. For example a 30 metre deep reservoir may only vary in level by 5 metres throughout the year, so only a 5 metre range is necessary if it is possible to suspend the sensor 5 metres below the maximum water level.

Vented or pressurised liquid

Is the air above the liquid surface open to atmosphere or sealed to a fixed or varying gas pressure?

On occasions it is necessary to cap the top of a well or tank to control the pressure or type of gas present directly above the liquid. In these applications the liquid level reading should be compensated by using an additional pressure sensor to monitor the gas pressure on top of the liquid so it can be subtracted from the level reading to obtain the true reading.

Output signal

What signal output is required by the monitoring instrumentation?

4 to 20mA current loop is the most robust signal and can be used over long distances without degradation from wire losses. O to 10 or 5 volts dc is convenient for use with data acquisition cards and low millivolt signals are best for remote sites with limited battery power. SDI and Modbus are standard digital protocols used mostly by hydrologists for high precision data collection.

Cable length

What is the length of cable required from the deepest transducer position to the termination point?

This may seem like an obvious question, but it is not always possible to terminate the sensor cable at the point where the sensor is suspended from and instead it may need to be connected some distance away from the drop point. When extending a submersible cable it is important to ensure the vented reference is kept dry and open to ambient air pressure. The cable will need to be long enough to reach a dry vented enclosure such as a termination junction box.

Temperature range

What will be the highest and lowest temperature of the liquid?

All submersible sensors have an operating temperature range which should cover the variation on temperatures for most applications. In more extreme environmental conditions or where there are high process generated temperatures, the operating limits of the sensor maybe exceeded. The electronics housing and cable are completely immersed in the liquid and since they tend to be the lowest rated components for temperature, they should be checked for a compatible temperature rating. Although it is less likely to occur there may also be situations where the temperature is too low for the sensor, which may cause damage to the electronics or cable due to thermal contraction or brittleness brought on by the low temperatures.

Diameter limitation

Is there any restriction on the diameter of the transducer that can be used?

For some installations such as hydrological surveys a small diameter borehole is drilled which precludes the use of many submersible pressure sensor probes which are typically more than 1 inch (25.4mm) in diameter. Also since many submersible sensors are relatively long compared to the diameter they can become snagged in the hole if there is not sufficient room to allow free movement when installing the probe. Where the liquid level is very dynamic the sensor can also restrict the rise and fall time of liquid in the borehole if the probe has only a slight clearance tolerance with the borehole.

Hazardous area

Will the transducer be located in an area classed as a hazardous zone?

In Europe many sites such as refineries, waste water treatment plants and landfill sites are required by health and safety authorities to ensure that installations are ATEX approved to reduce risks of explosions from flammable gases.

Lightning protection

Is the region where the installation is going to be located prone to lightning strikes?

Installations outdoors will always be at risk at some time or another from lightning strikes, but some regions are at more risk than others due to regional geography and weather characteristics. In these areas where strikes on instrumentation are numerous, some form of lightning protection maybe necessary to reduce the risk of failures. Although nothing is likely to withstand a direct lightning strike, many failures are caused by voltage surges from local strikes rather than direct hits, so high voltage surge protection circuitry can reduce the likelihood of failure significantly.

Checklist for submersible hydrostatic level sensor and probe requirements

Define your submersible hydrostatic level sensor and probe requirements using this checklist:

  1. Liquid type?
  2. Density of liquid?
  3. Viscosity of liquid?
  4. Max/Min level?
  5. Vented or pressurised liquid?
  6. Output signal?
  7. Cable length?
  8. Temperature range?
  9. Diameter limitation?
  10. Hazardous area?
  11. Lightning protection?