A refractometer is the standard on-site tool for checking AdBlue®. It's quick, low-cost to run and needs only a few drops. Workshops use them to check a vehicle tank, fleets use them to check deliveries, and they're handy when someone suspects a tank has been topped up with water. But it's important to know exactly what the reading tells you, and what it doesn't.
What a refractometer measures
A refractometer measures how much light bends as it passes through a liquid. Dissolving urea in water changes that, so the reading can be converted directly into urea concentration. ISO 22241-1 sets the refractive index of AdBlue at 20°C at 1.3814 to 1.3843, which corresponds to the urea range of 31.8% to 33.2%. The target is 32.5%.
Most refractometers sold for AdBlue have a scale marked in urea percent, or a scale with a shaded AdBlue band, so you don't need to convert anything. Many dual-purpose units also read coolant and battery acid on separate scales. Make sure you read the right one.
There are two common types:
- Optical (handheld) refractometers. You look through an eyepiece at a scale and read where the light and dark boundary falls. No batteries needed.
- Digital refractometers. Give a numeric reading on a screen, which removes guesswork about where the line sits.
Look for automatic temperature compensation (ATC). Refractive index changes with temperature, and ATC corrects for it within a working range.
How to take a reading
- Zero it first. Put a few drops of distilled or de-ionised water on the prism and adjust the calibration screw (or press zero on a digital unit) so it reads zero. Don't use tap water.
- Clean and dry the prism with a soft lint-free cloth.
- Take a representative sample. From a tank, run a little through the nozzle first rather than sampling the first drops sitting in the hose. From a vehicle, use a clean dedicated pipette.
- Apply two or three drops to the prism and close the cover plate so the sample spreads evenly without bubbles.
- Let it settle for around 30 seconds so the sample and the instrument reach the same temperature.
- Read the scale against a light source, or read the display.
- Clean the prism with de-ionised water and dry it. Dried AdBlue leaves urea crystals that will affect the next reading.
Reading the result
- Around 32.5% (within 31.8–33.2%) – concentration is within specification.
- Below 31.8% – the product has probably been diluted, most often by someone adding water, or by rainwater getting into a tank. Diluted AdBlue freezes at a higher temperature and doses less effectively.
- Above 33.2% – water may have evaporated, for example from a container left open or stored hot. Very high readings can also indicate that the sample was taken from crystallised residue.
If a reading is out of range, repeat it with a fresh sample and re-zero the instrument before drawing conclusions. Out-of-spec concentration should not go into a vehicle.
One thing to do today: if your site has a refractometer, zero it with de-ionised water and check it reads zero. Instruments that have been dropped, left in a hot ute or never calibrated can drift, and a refractometer that reads 1% high will happily tell you watered-down product is fine.
The limits: what it can't detect
This is the part worth remembering. A refractometer only checks concentration. It tells you nothing about purity. It cannot detect:
- Calcium, magnesium and other minerals from tap water, if only a small amount was added
- Metals such as copper, zinc, iron or aluminium from incompatible fittings
- Biuret or aldehydes from poor raw materials or degradation
- Phosphate or insoluble matter
- Small amounts of diesel or oil
ISO 22241 limits most of these metals to 0.5 mg/kg or less, which is far below anything a refractometer could see. AdBlue made from low-grade urea and ordinary water can be mixed to exactly 32.5% and read perfectly while still being unsuitable for an SCR system.
So treat a good refractometer reading as a pass on one test, not a clean bill of health. For purity, you need a lab analysis against ISO 22241-1, which is what a batch Certificate of Analysis provides.
How Liquidia can help
Every batch of Liquidia AdBlue is lab-tested against ISO 22241, and a Certificate of Analysis is available on request. See our certification details, or visit the resources page for SDS and certificates.