How to Measure Glycol Concentration in a Chiller or HVAC Loop

Measuring glycol concentration takes four steps: circulate the loop, draw a sample, read it on a matching glycol refractometer, then compare against the protection target. The method holds for a loop of water and one glycol type, ethylene or propylene, read on that glycol's scale or the fluid maker's table (ASTM D3321-26, 2026; OCP, 2026).

• ASTM D3321-26 is the active ASTM method for using handheld refractometers to find the approximate freeze point and % glycol of ethylene and propylene glycol coolants (ASTM D3321-26, 2026).

• Dow recommends at least 25% glycol for most applications and at most 60% for efficient heat transfer in its propylene glycol fluids (Dow, 2008).

• Dow advises choosing a protection temperature at least 3 °C (5 °F) below the lowest expected ambient temperature (Dow, 2008).

• One maker's pocket propylene glycol refractometer compensates for sample temperature from 10 to 40 °C and reads concentration to ±0.4% (V/V) (ATAGO, accessed 2026).

These steps assume a closed loop of water and one glycol. A loop with an unknown or mixed glycol, or a salt brine, falls outside these steps because each refractometer scale assumes one fluid; a laboratory test is the safer route there (inference).

What do you need before measuring glycol concentration?

Before measuring glycol concentration in a chiller or HVAC loop, you need four things: the glycol type, the design freeze point, a refractometer with the matching scale, and the fluid's Safety Data Sheet (SDS). The glycol type, ethylene or propylene, decides the scale, because ASTM D3321-26 warns that the wrong glycol scale makes the freeze point and % glycol readings incorrect (ASTM D3321-26, 2026). For its propylene glycol fluids, Dow advises selecting a protection temperature at least 3 °C (5 °F) below the lowest expected ambient temperature (Dow, 2008). The Open Compute Project (OCP) guideline for propylene glycol fluids in liquid cooled racks says the refractive index from ASTM D3321 should be compared against the manufacturer's published data for the specific fluid, because inhibitors can alter the refractive index (OCP, 2026). ASTM D3321-26 also says the handheld refractometer should be calibrated before use (ASTM D3321-26, 2026). For protective equipment, in the US, the Hazard Communication Standard, 29 CFR 1910.1200(g), requires the manufacturer, distributor or importer to provide an SDS for each hazardous chemical, and SDS Section 8 lists exposure controls and personal protection (OSHA, 2012).

How do you measure glycol concentration step by step?

Measuring glycol concentration in a chiller loop takes four steps.

1. Circulate the loop. After glycol or water has been added, Dow's procedure is to circulate for at least 24 hours to ensure complete mixing, then check the concentration with a refractometer (Dow, 2008).

2. Draw the sample from a port on circulating pipework, not from an expansion tank or a low-point drain (OCP, 2026). Before opening a port on a pressurized loop, put on the gloves and eye protection listed in Section 8 of the glycol's SDS, which 29 CFR 1910.1200(g) requires the supplier to provide in the US (OSHA, 2012). Flush the port first and dispose of the flush fluid as SDS Section 13, Disposal Considerations, and local waste rules direct (OSHA, 2012).

3. Read the sample. Calibrate the refractometer, place 2 to 3 drops on the prism (ATAGO, accessed 2026), and read % glycol or freeze point on the scale for the glycol in the loop (ASTM D3321-26, 2026).

4. Compare the result with the design basis. For freeze protection, the freeze point should be at least 3 °C (5 °F) below the lowest expected ambient temperature (Dow, 2008). Burst protection suffices only for a loop left dormant below the solution's freezing point with room for ice and slush to expand; compare its concentration with the maker's burst table (Dow, 2008). If the concentration is below target, adjust it following the fluid maker's guidance (OCP, 2026).

Which glycol refractometer scale should you use for ethylene or propylene glycol?

Use the ethylene glycol scale for an ethylene glycol loop and the propylene glycol scale for a propylene glycol loop, because the same refractometer reading means a different concentration for each glycol. ASTM D3321-26 covers handheld refractometers for the approximate freezing point and % glycol of ethylene and propylene glycol-based coolants, and states: "Care must be taken to use the correct glycol freezing point scale for the glycol type being measured." (ASTM D3321-26, 2026). ATAGO sells the PAL-91S for ethylene glycol and tells propylene glycol users to choose the PAL-88S instead (ATAGO, accessed 2026). Both pocket models read 0.0 to 90.0% (V/V) glycol and a freeze point of 0 to -50 °C, with concentration accuracy of ±0.4% (V/V) (ATAGO, accessed 2026). The OCP guideline also notes that corrosion inhibitors and additives can alter the refractive index, so the correlation between refractive index and freeze point may vary from one manufacturer to another (OCP, 2026). For a branded inhibited glycol, check the reading against that maker's table; Dow, for example, publishes refractive index at 22 °C and degree Brix against concentration for its propylene glycol fluids (Dow, 2008).

Why is a hydrometer unreliable for propylene glycol?

A float hydrometer is a poor fit for propylene glycol because propylene glycol solutions differ little in density from water, and at high concentrations density falls again, so one specific gravity reading can match two concentrations (Dynalene, 2020). Dynalene, a heat transfer fluid maker, lists this among the disadvantages of hydrometers: "The density of propylene glycol decreases at high concentrations, meaning two different concentrations may confusingly have the same specific gravity." (Dynalene, 2020). Dow lists the specific gravity of its DOWFROST concentrate, 95.5% propylene glycol by weight, at 1.046 to 1.056 at 60/60 °F (Dow, 2008). By inference, that leaves only about 0.05 specific gravity units between water and concentrate, so small reading errors move the result a long way. Dynalene notes that manual adjustments must be made when a hydrometer measures specific gravity at a temperature other than its reference temperature, while refractometers compensate for temperature automatically (Dynalene, 2020). For a propylene glycol chiller loop, use a propylene glycol refractometer scale and confirm doubtful results by ASTM D1177, the lab method for the freezing point of aqueous engine coolants (ASTM D1177-22(2026)).

Where should you take a glycol sample from a chiller loop?

Take a glycol sample from a port on well-mixed, circulating pipework, after flushing the port, and not from a tank, a low-point drain or a dead leg. The OCP guideline for propylene glycol fluids says the sample should be collected from a location that reflects well-mixed, circulating fluid, and names expansion tanks, reservoirs, drain valves at low points and dead legs as places to avoid (OCP, 2026). The same note says it is best to flush some fluid through the sample port to eliminate stagnant fluid, and to discard that fluid before taking the final sample (OCP, 2026). Dow gives the same reason for circulation: before withdrawing a sample, the fluid should be well circulated so the sample is representative (Dow, 2008). OCP warns that residual water from commissioning or flushing can lead to over-dilution, and recommends samples from multiple representative locations, where practical, to confirm the system is well mixed (OCP, 2026). Label each bottle with the unit, fluid identity and date, as OCP recommends for start-up samples (OCP, 2026).

How do you check a glycol concentration reading?

A glycol concentration reading is trustworthy when it passes four checks: matching scale, sample temperature inside the compensation range, agreement with makeup history and design protection basis, and agreement with a second method. The table below decides whether to accept a field reading or act on it before adjusting the loop.

The first row decides most results: a reading on the wrong glycol scale is incorrect for both freeze point and % glycol (ASTM D3321-26, 2026), so the other checks only mean something after the scale is right. The second row matters in hot and cold seasons: a sample drawn from a hot or chilled line can sit outside the compensation range, which for one pocket model is 10 to 40 °C (ATAGO, accessed 2026). For the makeup row, Dow notes that glycol concentration sometimes has to be raised to replace fluid lost through leakage or moisture absorbed from the atmosphere (Dow, 2008).

Glycol concentration testing mistakes to avoid

The most common glycol concentration testing mistakes are reading too soon after a fill, sampling the wrong place, and using the wrong instrument or scale. Reading right after adding glycol or water gives a local value; Dow's procedure is to circulate for at least 24 hours before checking (Dow, 2008). Sampling from an expansion tank, reservoir, low-point drain or dead leg gives stagnant fluid, which OCP tells users to avoid (OCP, 2026). Reading an ethylene glycol loop on a propylene glycol scale, or the reverse, makes the freeze point and % glycol incorrect (ASTM D3321-26, 2026). Using a float hydrometer on propylene glycol risks matching one specific gravity to two concentrations at high concentrations (Dynalene, 2020). Ignoring the inhibitor package is another: inhibitors can alter the refractive index, so a generic scale may not match a branded fluid (OCP, 2026). Dow also notes that diluting below 25% to 30% glycol may reduce inhibitor concentration below the level that protects against corrosion, that solutions under 25% may be at risk for bacterial contamination, and that the maximum recommended concentration is 60% for efficient heat transfer (Dow, 2008).

When should you call a specialist about chiller glycol concentration?

Call a heat transfer fluid specialist or laboratory about chiller glycol concentration when field readings disagree, keep falling, or come from a loop whose glycol type is unknown. A field refractometer gives approximate values (ASTM D3321-26, 2026), so a laboratory freeze point by ASTM D1177 is the usual tiebreaker; ASTM notes that the freezing point may be used to determine the approximate glycol content, provided the glycol type is known (ASTM D1177-22(2026)). A falling concentration with water makeup points to a leak or a dilution source that needs tracing (inference). The OCP guideline sets annual testing of each loop by a qualified lab, at a minimum, covering propylene glycol concentration, fluid pH, reserve alkalinity, appearance, inhibitor concentration, chloride, sulfate, total hardness, degradation acids and dissolved metals (OCP, 2026). Dow recommends on-site fluid analysis for systems holding less than 950 L (250 US gal) of its fluid, and offers a laboratory analysis, typically performed annually, for larger systems (Dow, 2008). When the readings, the fluid's TDS and SDS, and the makeup records do not line up, Lubinpla can organize the case from your own documents and site history and return one cited document for your review.

FAQ

How often should glycol concentration be checked?

Check it with a refractometer after every fill, top-up, flush or leak repair, once the loop has circulated; Dow's procedure is at least 24 hours of circulation after adding fluid (Dow, 2008). For laboratory analysis, Dow recommends samples once a year after the system is in operation (Dow, 2008), and the OCP guideline sets annual lab testing of each liquid cooling loop as a minimum (OCP, 2026).

What glycol concentration should a chiller loop have?

The target comes from the freeze or burst protection the site needs. For its propylene glycol fluids, Dow recommends a minimum of 25% glycol for most applications and a maximum of 60% for efficient heat transfer (Dow, 2008). In liquid cooled racks using PG25 fluids, the OCP guideline gives a propylene glycol range of 24.5% to 27.5% by volume, measured by ASTM E202 or ASTM D3321 (OCP, 2026).

How do you raise glycol concentration after a low reading?

Dow gives the formula A = V(D - P)/(100 - P), where A is the quantity of solution to drain and of concentrate to add, V is the total system volume, D is the desired and P the present volume percent of DOWFROST fluid in the system, not of pure glycol, so convert a % glycol reading with Dow's Table 3 first (Dow, 2008). Read the concentration again after the loop has circulated.

Working a case like this? Lubinpla works it from your own documents and site history, step by step with your approval. Free to start, no card.

Sources checked 2026-10-14.

Sources

1. ASTM International. ASTM D3321-26, Standard Test Method for Use of Handheld Refractometers for the Determination of the Freeze Point and % Glycol of Aqueous Engine Coolants. 2026. https://store.astm.org/d3321-26.html

2. ASTM International. ASTM D1177-22(2026), Standard Test Method for Freezing Point of Aqueous Engine Coolants. Reapproved 2026. https://store.astm.org/d1177-22r26.html

3. Dow. Engineering and Operating Guide for DOWFROST and DOWFROST HD Inhibited Propylene Glycol-based Heat Transfer Fluids, Form No. 180-01286-0208. February 2008. https://cdn.shoplightspeed.com/shops/621040/files/64538514/dowfrost-inhibited-propylene-glycol-heat-transfer.pdf

4. Open Compute Project. Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Liquid Cooled Racks, Revision 2.0, Version 1.0.0. September 15, 2026. https://www.opencompute.org/wp-content/uploads/2026/10/OCP-PG25-Guidelines-V1.0.0.pdf

5. Dynalene. Which one, Refractometer or Hydrometer, to use for Glycol Concentration Testing? May 2020. https://www.dynalene.com/concentration/

6. ATAGO. Propylene Glycol Refractometer PAL-88S, product information. Accessed 2026. https://www.atago.net/en/atagodirect-index.php?key=ADc42592

7. ATAGO. Ethylene Glycol Refractometer PAL-91S, product information. Accessed 2026. https://www.atago.net/en/atagodirect-index.php?key=ADT59550

8. US Occupational Safety and Health Administration. Hazard Communication Standard: Safety Data Sheets, OSHA Brief DSG BR-3514. February 2012. https://www.osha.gov/sites/default/files/publications/OSHA3514.pdf

Written by the Lubinpla team from the sources listed and checked against them before publication.