An electrical conductivity sensor cannot independently determine the glycol percentage in a formulated coolant. For routine concentration checks, use a refractometer with the correct propylene glycol (PG) or ethylene glycol (EG) calibration. Use online conductivity to follow changes in the coolant's ionic condition.
Routine concentration checks: Use a coolant-compatible refractometer and the correct PG or EG scale.
Density measurements: Convert to concentration only with a validated relationship for the actual fluid and temperature.
Continuous monitoring: Combine periodic concentration checks with conductivity and other measurements selected for the coolant.
For data center CDUs, BESS cooling circuits and other closed-loop systems, these measurements serve different maintenance decisions. Concentration checks verify the mixture; online sensors help operators detect changes between sampling visits.
Choosing a Glycol Concentration Measurement Method
Choose the method according to the coolant formulation and the result you need.
| Method | What It Measures | Best Use | Main Limitation |
|---|---|---|---|
| Refractometer | Refractive index, interpreted as glycol concentration or freeze point | Routine field concentration checks | Needs the correct glycol scale or fluid-specific conversion |
| Hydrometer or density meter | Density or relative density | Checking a known formulation with a validated chart | Temperature and additives affect the conversion |
| Laboratory analysis | Glycol composition or other specified analytes | Unknown mixtures, disputed results and troubleshooting | Requires a suitable analytical method and sampling |
| Conductivity sensor | Ionic conductivity in µS/cm or mS/cm | Continuous coolant-condition trends and alarms | Does not uniquely identify glycol percentage |
Refractometers for Routine Checks
A refractometer needs only a small sample and is convenient for maintenance rounds. Before testing, identify the coolant, calibrate the instrument as instructed, and select the correct PG or EG scale. Record whether the result is a percentage or an estimated freeze point. ASTM D3321-26 describes handheld refractometer testing for glycol-based aqueous coolants.
A Brix reading needs the coolant manufacturer's conversion relationship. Temperature compensation also needs to suit the instrument and fluid; additives and mixed glycols can affect interpretation.
Density Measurements for Known Formulations
Density can be useful with a validated fluid-specific chart, but temperature and inhibitors affect it. Some PG mixtures are difficult to distinguish by density. Use a defined measurement temperature and conversion relationship; ASTM D1122-22 describes relative-density testing by hydrometer, rather than a universal glycol-percentage conversion.
Laboratory Analysis for Uncertain Samples
Use a laboratory when coolant identity is uncertain, PG and EG may have been mixed, or field readings conflict. Specify glycol analysis and any separate tests needed for inhibitors, corrosion metals or degradation products.
Glycol Concentration and Freezing Protection
Glycol percentage affects freeze protection, viscosity and pumping demand. The approved concentration must balance cold-weather exposure with system performance and the coolant's inhibitor requirements.
See Dow's Data Center Liquid Cooling Solutions & Technologies page, in the FAQ on glycol concentration, freeze protection and pumping efficiency.
The following values illustrate how PG concentration changes freezing behavior.
| Propylene Glycol Concentration by Mass | Approximate Freezing Point in °C | Freezing Point in °F |
|---|---|---|
| 20% | −7.1 | 19.2 |
| 30% | −12.7 | 9.2 |
| 40% | −21.1 | −6.0 |
| 50% | −33.5 | −28.3 |
Source: 2025 ASHRAE Handbook—Fundamentals, Chapter 31, Table 5, I-P edition, for aqueous inhibited PG. Celsius values are converted from the published Fahrenheit values and rounded to one decimal place. These are reference properties, not recommended operating concentrations. Mass percentage, volume percentage and commercial concentrate percentage are different; use the supplier's current formulation data for design and dosing.
Freeze Protection and Burst Protection
Freeze protection prevents ice formation so the coolant can remain pumpable at the specified minimum temperature.
Burst protection can allow ice or slush during shutdown, provided the system accommodates expansion. It does not establish suitability for circulation or cold startup.
Select the protection requirement for the actual operating scenario. ASHRAE Chapter 31.
Why Conductivity Cannot Independently Determine Glycol Percentage
Conductivity responds to dissolved ions and temperature. In a formulated glycol coolant, the reading can be influenced by makeup-water quality, ionic inhibitors, contamination and chemical changes during service. Two coolants with the same glycol percentage can therefore have different EC readings.
A conductivity change is useful evidence that fluid conditions have changed, but it does not identify the cause.
Review temperature, maintenance records and coolant additions, then select further tests. A stable EC reading does not confirm that concentration and inhibitor condition are correct.
RIKA explains this interpretation in its guide to conductivity monitoring in liquid cooling.
Even a locally developed EC-to-concentration correlation requires controlled chemistry and validation. It should not be carried over to a different coolant formulation or relied on after contamination.
What to Do When Glycol Concentration Is Too Low
Treat a low reading as a maintenance finding to verify and investigate before changing the mixture.
Check instrument calibration, PG or EG selection, sample temperature and the percentage basis. Retest a representative, well-mixed sample.
Review water top-ups, coolant leaks, fluid replacement and residual flushing water. Investigate unexplained dilution or possible cross-contamination.
For routine replacement of lost, correctly mixed coolant, use the specified compatible premix. Correcting an already diluted system may require an approved concentrate or partial fluid replacement, based on a dosing calculation.
Circulate sufficiently, resample and check concentration against the approved target. Record conductivity under comparable temperature conditions, together with other required fluid checks.
Correct leaks, topping-up practices or service procedures that caused the change. Arrange laboratory testing if the fluid condition remains uncertain.
For example, if a loop contains 20% glycol by mass, adding a 30% premix by mass leaves the mixture below 30%.
Calculate the correction from the existing fluid inventory, measured concentration and approved correction-fluid strength. Dynalene's LC Series Engineering Guide, 2024, pp. 7–8 discusses residual-water dilution, concentration adjustment and checks after circulation.
Do not add untreated tap water or mix coolant products without approval. Dilution water and correction fluids must meet the coolant supplier's quality and compatibility requirements.
Combining Concentration Checks With Online Coolant Monitoring
Use continuous measurements to decide when investigation is needed, alongside scheduled concentration and fluid-health checks.
| Measurement | Maintenance Purpose | Instrument or RIKA Option |
|---|---|---|
| Glycol concentration | Verify the mixture and associated freeze protection | Suitable refractometer or validated laboratory method |
| Conductivity | Follow changes in coolant ionic condition | RK500-13LC conductivity sensor |
| pH where specified for the fluid | Follow acid-base condition against approved limits | RK500-12LC pH sensor |
| Turbidity | Follow optical changes associated with suspended matter | RK500-07LC turbidity sensor |
The RIKA liquid-cooling sensor specifications list dual output interfaces, 4–20 mA and RS485, with deionized water, PG25 and EG25 among the listed media. Confirm the exact commercial coolant, materials and operating conditions before selection. These online sensors complement concentration testing. RIKA sensor collection.
Establish a commissioning baseline after filling and mixing. Record the coolant product, concentration basis, temperature and approved monitoring results. Set alarms from the coolant and equipment requirements, then compare trends under consistent conditions. Check that EC temperature compensation is appropriate for the actual coolant.
Select pH monitoring only where the supplier specifies a meaningful method and limits; some very low-conductivity formulations use other fluid-health criteria. Turbidity does not identify particle composition or size, and conductivity does not quantify inhibitor depletion.
Eight Checks Before Selecting Instruments
Confirm the following requirements before selecting coolant concentration and online monitoring instruments.
PG or EG, commercial product and inhibitor package.
Required percentage and whether it is by mass or volume.
Minimum and maximum temperature, pressure and freeze-protection requirement.
Correct refractometer scale or validated analytical conversion.
Expected range, resolution, calibration and temperature compensation.
Supplier-approved pH, turbidity and laboratory tests.
Wetted materials, seals, process connection and controller inputs.
Baseline, sampling frequency, alarm investigation and approved correction procedure.
Frequently Asked Questions
Can a Brix Reading Be Used as Glycol Percentage?
Only after conversion using a relationship validated for the coolant. A sugar-based Brix scale is not a direct PG or EG percentage scale.
Is Density More Accurate Than a Refractometer?
There is no universal ranking. Compare the instrument, temperature control and validated conversion for the specific coolant. For routine field work, a correctly configured refractometer is often convenient.
Can PG and EG Use the Same Freeze-Point Chart?
No. Use the correct chart for the glycol and formulation. Mixed PG and EG samples may require laboratory analysis.
Is Lower Coolant Conductivity Always Better?
No. Different inhibitor formulations have different normal EC levels. Compare readings with the approved coolant specification and its operating baseline.
Is 25 to 30 Percent PG the Standard for Every Data Center?
No. The required concentration depends on the approved fluid, system design, minimum temperature and project requirements. Confirm the percentage basis as well as the value.
Can Conductivity Confirm Corrosion Inhibitor Depletion?
No. Ionic inhibitors can influence EC, but the reading does not uniquely quantify their concentration. Use the supplier's approved inhibitor analysis.
Sources & references
- ASHRAE, 2025 ASHRAE Handbook—Fundamentals, Chapter 31, Physical Properties of Secondary Coolants (Brines), Table 5.
- ASTM International, ASTM D3321-26, Handheld refractometer testing for glycol coolants.
- ASTM International, ASTM D1122-22, Relative-density testing by hydrometer.
- Dynalene, LC Series Engineering Guide (PDF), 2024, pp. 7–8.
- Dow Inc., Data Center Liquid Cooling Solutions & Technologies, FAQ on glycol concentration, freeze protection and pumping efficiency.
- RIKA SENSOR, Liquid Cooling Water Quality Sensor Specifications.
- RIKA SENSOR, Why Conductivity Monitoring Matters in Data Center Liquid Cooling.
Discuss Your Coolant Monitoring Requirements
Planning water quality monitoring for a CDU, BESS cooling circuit or other glycol loop? Share your coolant product, concentration, operating temperature and pressure, expected conductivity range and controller interface with RIKA SENSOR.
