The most reliable way to test glycol concentration is with a refractometer, which measures how light bends through a fluid sample to determine the percentage of glycol in your water mixture. A single drop of fluid on the lens gives you a reading in seconds, and the tool is accurate enough to be the basis of ASTM’s standard field test for engine coolants. Test strips exist but are far less precise, and lab analysis is available for situations where accuracy is critical.
Refractometers: The Standard Field Tool
A refractometer works by measuring the refractive index of your glycol-water mixture. Since glycol changes how light passes through water in a predictable way, the tool can convert that reading into a concentration percentage or a freeze point temperature. ASTM D3321, the industry standard for field-testing engine coolants, is built entirely around this method.
You need to match the refractometer scale to the type of glycol in your system. Ethylene glycol and propylene glycol bend light differently at the same concentration, so using the wrong scale will give you an incorrect reading. Digital refractometers often come pre-programmed with separate scales for each type, letting you read both concentration (as percent by volume, from 0 to 100%) and freeze point. Analog optical refractometers typically have the scales printed on the internal chart, and many have dual scales for both glycol types side by side.
To get an accurate reading, calibrate your refractometer with distilled water before each use. Place a drop on the prism, close the cover plate, and read the display or look through the eyepiece. The fluid sample should be at room temperature, since extreme heat or cold can skew results. For tighter quality control, calibrate with both distilled water and a known 50/50 coolant mixture. Field refractometers are generally accurate to within a few degrees of freeze point, which is sufficient for routine maintenance. If you’re doing factory fill or verifying incoming product shipments, a laboratory-grade refractometer or a dedicated high-precision scale is worth the investment.
Why Test Strips Fall Short
Glycol test strips use color-changing pads that you dip into the fluid and compare against a printed chart. They’re cheap and require no equipment, but their accuracy is poor enough to be a real problem. A study by the FAA tested automotive-style glycol strips against known concentrations and found consistent, large errors. At a 50% ethylene glycol concentration (true freeze point around -25°F), all six test subjects using the strips read the freeze point as 0°F, an average error of 25 degrees. At 60% concentration (true freeze point around -46°F), the average strip reading was only -8°F, off by nearly 38 degrees.
The strips typically offer only five color levels corresponding to freeze points like +10, 0, -10, -30, and -60°F. That leaves huge gaps, and human interpretation of the colors introduces additional variation. For a rough check in a non-critical system, strips can confirm that glycol is present. For anything where freeze protection actually matters, they’re not precise enough to rely on.
Laboratory Analysis for Comprehensive Results
Sending a fluid sample to a testing lab gives you the most complete picture. Beyond concentration, labs can measure pH, corrosion inhibitor levels, dissolved metals, and contamination. This matters because glycol degrades over time: it becomes acidic, and its corrosion inhibitors deplete. A system can have the right concentration but still be damaging your pipes or heat exchangers.
For commercial HVAC and chilled water systems, the recommended schedule is monthly visual inspections (looking for color changes, cloudiness, or debris), quarterly concentration testing with a refractometer, and a full laboratory analysis every six months. Concentration testing should increase to monthly during fall as you prepare for winter conditions.
Freeze Protection vs. Burst Protection
Understanding what your concentration reading means is just as important as getting an accurate number. Glycol-water mixtures don’t freeze solid the way pure water does. Instead, they form a slush that can still expand but won’t rupture pipes the way a solid ice block would. This creates two different protection thresholds you should know about.
Freeze protection is the temperature at which the mixture begins to form ice crystals and turn to slush. Burst protection is the lower temperature at which the system’s pipes or coils would actually be at risk of damage from expansion. Burst protection requires significantly less glycol because the slushy mixture expands very little. For example, to protect against freeze damage at -20°F you need about 44% ethylene glycol concentration, but burst protection at that same temperature only requires about 31%. At -40°F, freeze protection calls for 52% concentration while burst protection needs just 37%.
A 25% ethylene glycol solution expands only 1.5% when cooled to 14 degrees below its freezing point. A 40% solution expands less than 1% at 22 degrees below its freeze point. Solutions above 50% glycol may not expand at all when they freeze. This is why many building systems use glycol concentrations aimed at burst protection rather than full freeze protection, since running too much glycol reduces heat transfer efficiency and increases pumping costs.
Getting the Right Concentration Target
Before testing, you need to know what number you’re aiming for. That depends on your system type and the lowest temperature it could encounter.
- Automotive cooling systems typically run a 50/50 mix of ethylene glycol and water, providing freeze protection down to roughly -34°F and boil protection well above normal operating temperature.
- HVAC chilled water loops often use propylene glycol (since it’s less toxic) at concentrations between 25% and 40%, depending on the minimum ambient temperature the outdoor piping could see.
- Solar thermal systems usually need higher concentrations, often 40% to 50% propylene glycol, because collectors on a rooftop can reach much colder temperatures than ambient air during winter nights.
More glycol isn’t always better. Concentrations above 50% actually start to raise the freeze point again with ethylene glycol, and at any concentration, higher glycol content means the fluid transfers heat less efficiently and is harder to pump. Test regularly, aim for your target range, and add water or glycol concentrate as needed to stay in the right zone.
Practical Testing Tips
Draw your sample from a point in the system where the fluid is well mixed, not from a dead leg or an expansion tank where concentrations can differ from the circulating loop. Let the sample cool to room temperature before testing with a refractometer, since temperature affects the reading. If your system uses an inhibited glycol product (which nearly all commercial glycol products are), the corrosion inhibitors can slightly shift the refractive index compared to pure lab-grade glycol. This is one reason field refractometers are considered accurate “within a few degrees” rather than exact.
Keep a log of your readings over time. A gradual drop in concentration usually means water is being added to the system (through makeup water connections or leak repairs) without proportional glycol. A rising concentration could indicate water loss through evaporation or steam leaks. Either drift signals that the system needs attention beyond just topping off the glycol.