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  • Aug 28, 2019

Measuring Diesel Exhaust Fluid by Refractometer or Hydrometer

How can Bellingham + Stanley help you measure DEF easily and effectively?

Reducing Environmental Pollution

In an effort to reduce environmental pollution, the European Union has established certain rules about emissions from heavy and light goods vehicles. One method adopted by vehicle manufacturers to allow compliance is SCR (Selective Catalytic Reduction), which chemically reduces NOx emissions using a urea additive in the exhaust flow. 

AdBlue® (DEF in the USA) is such an additive. They are stored as a concentration in a separate tank prior to mixing with the exhaust gases as they leave the engine. In order to maintain efficiency and to be sure that systems are not compromised, it is essential that the concentration is accurately maintained. Measuring devices such as refractometers and hydrometers may perform such checks and depending on the location and accuracy required, a choice can be made from the following instruments available from Bellingham + Stanley.

Portable Devices - Hydrometer

A hydrometer is a scaled device that measures the specific gravity of a liquid, which in turn is proportional to concentration. The DEF Hydrometer is low cost and offers excellent accuracy for measurements in the field. 

Operation: Sufficient sample is syphoned from the tank using the integral suction pump. Concentration is subsequently checked against the point where the hydrometer crosses two calibrated target marks. The hydrometers are also available with an alternative Specific Gravity Scale (SG).

Portable Devices - Digital handheld refractometer

For systems to perform efficiently, the AdBlue® solution mixed with the exhaust gases must be precisely 32.5% with little margin for error. Although the low cost optical refractometers may be used to monitor AdBlue®, many people find its compressed field of view too difficult to work with on a regular basis and so opt for a digital model instead. The OPTi DEF/AUS 32 digital refractometer negates all the problems normally associated with the optical measurement by providing a simple “drop-on & press-read” measurement solution; as well as applying an accurate temperature correction in order to further reduce potential errors.

The OPTi refractometer is specifically recommended for use where large numbers of AdBlue® tests are to be made by more than one person and in a short amount of time. These environments include vehicle hire reception areas, fleet haulage yards, municipal depots and the like.

Oil in Water Test Strips

It is sometimes a requirement to check if there has been contamination to the diesel exhaust fluid tank. One method for testing if diesel has been accidentally or even purposefully mixed with the DEF is to use simple test strips that detect oil. The DEF is simply removed from the tank using a syphon prior to the strip being immersed and agitated. Presence of oil causes a change in the colour of the strip. 

Laboratory Devices 

Manufacturers of Aqueous Urea Solutions (AUS) within the European Union are required to conform to ISO 22241-2 in respect of refractive index measurement. This norm dictates that the measurement temperature must be extremely stable (±0.02 °C) and the refractive index resolved to 5-decimal places. Bellingham + Stanley has produced a specific refractometer called the RFM990-AUS32 that meets this requirement.

Continuous DEF Refractometer for Car Manufacturers 

Car manufacturers that pre-charge DEF systems with fluid may continually monitor the concentration of AdBlue® using an RFM refractometer fitted on a bypass line. The concentration may be read from the display or more conveniently, output via a network connection to remote display, PLC and/or data capture device, where alarm signals may be pre-set for AdBlue® concentration at 32-33% urea.


To learn more about our range of handeld refractometers specifically for the automotive industry, please visit Refractometer Shop, our online home of the OPTi. 

DEF is commonly used to describe a diesel exhaust fluid such as AdBlue®. DEF is also used to describe an aqueous urea solution scale for measuring diesel exhaust fluids such as AdBlue®. AdBlue® is a registered trademark of the VDA Verband der Automobilindustrie e.V.

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