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What Is Diesel Exhaust Fluid (DEF)?

May 2010


Diesel Exhaust Fluid (DEF) is a precise blend. It’s 32.5% highly pure, automotive grade urea and 67.5% de-ionized water. The ISO 22241 Standard defines the clear-cut manufacturing process of DEF, along with proper handling and storing practices.

DEF is non-toxic, non-hazardous, non-flammable, and non-explosive. As such, there are no limitations or regulations placed on those who haul it in bulk quantities.

DEF is corrosive to most metals and coatings, including steel, iron, zinc, nickel, copper, aluminum and magnesium. If it’s spilled, it can be rinsed with water or wiped up.

DEF has the ability to freeze. It freezes and expands at 12°F (-11°C). This can cause damage if, say, someone were to leave DEF handling equipment outside for the winter without purging the system [we’ve seen this before.] Related: here’s a short video on how to winterize a Thunder Creek DEF system.

On average, DEF is consumed at a rate of 2%-10% of fuel consumption. We get this question frequently as we work with people sizing out the right fuel and DEF systems for their fleet’s needs. These numbers are most impacted by the brands of Tier 4 machinery you operate, as each OEM has varying engine designs. Some rely more heavily on Selective Catalytic Reduction technology (thus using more DEF) and others more on Exhaust Gas Recirculation technology (hence, less DEF). Engine load, temperature and humidity also play a lesser, yet still influential, role in DEF consumption rates.

DEF is easily contaminated. It only takes a trace of many common elements to contaminate DEF. Think the residue that remains in a container rinsed with tap water, a non-ISO compliant plastic funnel, or the dirt and dust on the fill port. Contaminates will cause corrosion in the catalyst, clog spray nozzles or form a gummy deposit in an SCR system. Repairs are likely not covered under the manufacturer’s warranty. DEF is most susceptible to contamination in “the last mile.” DEF has been used in Europe, where it’s known as AUS32, for over two decades. They have found the vast majority (70%) of SCR system failures are caused by customers mishandling the fluid. Source: AMB International, Oct. 2016. This is especially concerning for those who operate off-road machinery and are responsible to transfer the fluid between the bulk supply to the field.

DEF is best protected in a closed, ISO-compliant systems. To protect your Tier 4 investment, look for handling systems that are not only made in compliance with the ISO 22241 Standard but are also able to maintain that initial purity for life.