How to check coolant with a refractometer
Colour and age tell you nothing about how much freeze protection you actually have. The one simple tool that gives an exact answer in seconds is a refractometer. It measures how the fluid bends light and derives both the mix percentage and the freezing point from that.
Why a refractometer beats a hydrometer
A hydrometer, the glass tool with floating balls or a floating scale, measures density. Density depends on temperature, so a reading taken on hot fluid differs from one taken cold.
A refractometer needs two drops, and most of them work with automatic temperature compensation. The result is both more accurate and repeatable.
There is a practical advantage too: a hydrometer needs a large sample, a refractometer needs one pipette. Taking two drops from the tank is a five-second job.
Taking the reading, step by step
- The engine must be cold. Opening the tank on a hot system risks a burn.
- Clean the prism and the flap with a soft, clean cloth.
- Take two drops from the expansion tank with a pipette and place them on the prism.
- Lower the flap so the fluid spreads evenly and no air bubble is trapped.
- Hold it up to the light and read the line where the bright and dark fields meet.
- Use the right scale. A refractometer normally carries two, one for ethylene glycol and one for propylene glycol. Valvoline Zerex Asian is ethylene glycol based, so you need the first.
- Clean the prism afterwards so residue does not distort the next reading.
The same sample reads differently on the two scales. If you read the propylene scale by mistake, the protection will look worse than it really is.
How to read the result
A refractometer shows the freezing point directly. Read the result like this:
- Around −36 °C. The system holds a 50/50 mix, exactly what lowland Georgia needs.
- Below −45 °C. There is more concentrate than needed. Not dangerous, but heat transfer suffers slightly.
- Between −20 °C and −30 °C. The mixture is weak, most likely water was added. Correct it before winter.
- Above −15 °C. The system is mostly water and the first hard frost is a real risk.
Correcting it is simple: drain a litre or two, add the same amount of concentrate and measure again. For the exact ratio see our dilution calculator.
What a refractometer cannot tell you
This caveat matters. A refractometer measures the amount of glycol and the freezing point that follows from it. It tells you nothing about the inhibitors, the additives that protect the metals from corrosion.
And the inhibitors are exactly the part that depletes first. A fluid can show a perfect −36 °C and have no protection package left.
So a refractometer answers one question, and the rest is answered by age and by looking. If the fluid is more than five years old, or the colour is murky, or there is sediment at the bottom, change it whatever the reading says. We have a separate page on that: when to change your coolant.