Liquid Level Sensors for AdBlue/Diesel Exhaust Fluid

Why there is such a demand.

Off-highway vehicle manufacturers have to comply with increasingly stringent emissions legislation. The chief objective of which is to reduce NOx gases, hydrocarbon gases, and particulate matter (PM) released from the exhaust of diesel engines.

From the first emission standard, Euro I, to the current iteration, Euro VI, engine design changes have seen NOx levels and PM levels reduced by 95% and 97% respectively.

The introduction in engines of Selective Catalytic Reduction (SCR) and Diesel Particulate Filter (DPF) technology is responsible for the improvements. A typical SCR system includes a storage tank for Diesel Exhaust Fluid (DEF), the SCR chamber with catalyst, an injector, sensors, NOx analyzer, wire connections, and a control unit.

During operation, exhaust gas after filtering through the particulate filter flows to the SCR chamber where a dose of DEF is injected. A reaction occurs to convert NOx to harmless nitrogen and water.
AdBlue, composed of 32.5% high-grade urea and 67.5% distilled water, is the most popular diesel exhaust fluid in the market today. More significant is that vehicle manufacturers have designed their engines to limit performance should the AdBlue tank run empty. This is to prevent users from driving and emitting NOx and particulates against regulations.

SCR Systems Require Level Detection Capabilities

SCR systems have created a paradigm shift for vehicle operation. Put simply, the way vehicles cannot function when their fuel tanks are empty is the same way they also cannot function when their Adblue tanks are empty.

The ability to detect the presence or absence of AdBlue in the storage tank is, therefore, critical to the SCR system’s performance and to ensure compliance.

AdBlue level sensors take away any guesswork. These sensors determine the level of fluid in the tank and relay the information to the control unit. Depending on the input, the control unit:

  • Allows the vehicle to operate normally
  • Returns a warning message to the driver in a visual (gauge) or audible alert, or
  • Limits the vehicle’s performance, thereby stalling operation or reducing productivity until the AdBlue tank is topped up.
Since AdBlue consumption in heavy-duty vehicles is about 3-5% of total fuel consumption levels, a simple guideline is for users to fill up their AdBlue tank whenever the diesel tank is filled. And where there are no AdBlue filling stations, it’s recommended to carry along a top-up container.

AdBlue Level Sensors for Minimum/Low-Level Detection

A level sensor detects when a liquid, here AdBlue, has reached a predefined minimum level and it acts as a switch to activate a response. Integrated with the electronic control unit of the vehicle, the response could be the sounding of an alarm or the limiting of the vehicle’s performance.

Liquid Level Low
Liquid Level Mid
Liquid Level High

PST's Optical Liquid Level Sensors Compared with Other AdBlue Level Sensors

There are a variety of AdBlue level sensors (level sensor for adblue tank) available in the market. Here’s how popular technologies compare in general to PST’s optical level sensor technology.

Float Switch (vertical and horizontal)

In general, floats have a mechanism that rises or falls with the liquid in the tank. Once the liquid reaches a prespecified level, the switch is activated to produce an appropriate response. A disadvantage of these switches is their need for substantial amounts of fluid to be present to register a signal.

Float Switch

Ultrasonic Point Level Switch

In general, floats have a mechanism that rises or falls with the liquid in the tank. Once the liquid reaches a prespecified level, the switch is activated to produce an appropriate response. A disadvantage of these switches is their need for substantial amounts of fluid to be present to register a signal.

Ultrasonic Switch

PST’s Optical Level Sensor

The primary component of PST’s optical level sensor is a polymer or glass sensor tip that houses an infrared (IR) light source and an optical detector. The sensor utilizes the principle that infrared light is refracted out of the sensor tip when in a liquid, instead of reflected back into the optical detector when in air.
The sensor is solid-state with no moving parts. It is compact, protruding a small distance into the application, and capable of detecting the smallest amounts of fluid in a tank for high reliability. The digital sensor includes a configurable microcontroller with alarm relay capabilities that processes the input from the sensor.
Finally, the sensor is capable of surviving harsh liquid media and a wide range of operating temperatures.

Liquid Level Sensor

Benefits of PST's Optical Liquid Level Sensors

1. Provide immediate notification about the Adblue tank level to induce the user to fill up when necessary.
2. Easy to install due to its simple and flexible design, saving assembly time.
3. Easy to integrate into a wide variety of materials and structures without requiring significant changes to a tank’s design or placement due to their 3. cylindrical geometry and compact size.
4. Have high sensitivity and sensor accuracy and can detect small amounts of fluid.
5. Are solid state with no moving parts, and completely passive having no chemical interaction with Adblue.
6. Are digital with complete separation between fluid and electrical parts for longevity.
7. Are available with probe style sensors, ideal for automotive connectors (superseal).
8. Can be configured to suit your unique application, please contact us for further details including:

  • High pressure options
  • Customization for AC supply
  • OEM sensors for very high volumes
  • High power output drive options for direct control of machinery (pumps, relays, indicators, motors) to eliminate the use of an intermediate controller

Suggested Sensor for AdBlue Systems: Optomax Digital LLC210D3SH

The Optomax digital range of liquid level sensors are ideal for applications with restricted space, such as a level sensor for adblue tank, that require a miniature and low power solution.


  • Operating voltage: 4.5VDC to 15.4VDC
  • Operating temperature range: Standard (-25°C to 80°C) or Extended (-40°C to 125°C)
  • Can be configured to output a high or low signal when in either a wet or dry state.
  • Can sink and source up to 100mA
  • Digital TTL/CMOS compatible push-pull or load-switching outputs
  • Pre-wired connectors available
  • 3 and 4 wire options available
  • Reverse polarity, over voltage, short circuit, and transient protection
  • Available in two chemically resistant housing materials: Polysulfone (Standard) or Trogamid
  • Housing options for internal or external sensor mounting
  • Sensor comes with power supply and microcontroller


< Back to Knowledge Base

Want to see more information like this?

Sign up to one of our Industry newsletters and you’ll receive our most-recent related news and insights all directly to your inbox!

Sign Up