Quick answer
PG25 is a heat-transfer fluid of about 25% propylene glycol and 75% water, plus corrosion inhibitors (and often a dye), used in direct-to-chip secondary loops. It balances good heat transfer (closer to water than stronger glycol mixes) with biological growth control, corrosion protection and some freeze protection. Suppliers quote around −10 °C freeze protection for their PG25 products.
Key takeaways
- PG25 means 25% propylene glycol with inhibitors.
- The water-rich mix gives better heat transfer than 50% glycol.
- Inhibitors protect copper and other wetted metals.
- Shell and Dow both quote −10 °C freeze protection for their PG25 products.
- OCP publishes PG25 guidance. Several suppliers market fluids against it.
PG25 composition
| Component | Role |
|---|---|
| Propylene glycol (~25%) | Freeze protection, microbial control |
| Water (often deionized) | Primary heat carrier |
| Inhibitors | Corrosion protection |
| Dye (optional) | Leak detection (e.g. Shell green, Dow yellow) |
Why PG instead of EG?
Propylene glycol is less toxic than ethylene glycol, which matters in occupied buildings. EG transfers heat slightly better. See EG vs PG.
Freeze & boil point calculator
Interpolated from published glycol/water tables at 1 atm [EG] [PG]. Commercial coolants and pressurised systems differ.
PG25 vs water vs higher-glycol mixes
| Fluid | Heat transfer | Freeze protection | Notes |
|---|---|---|---|
| Water (treated) | Best | None | Needs strict biocide and corrosion control |
| PG25 | Good | About −10 °C (supplier data) | Common DLC choice |
| PG40–50 | Lower | Much lower freeze point | Used where freezing is a risk |
Freeze points for PG mixes are on the freeze & boil calculator.
Suppliers of PG25 fluids
Maintenance
Monitor concentration with a refractometer, plus pH, conductivity and inhibitor levels per the supplier. See CDU maintenance.
Why inhibitors matter in PG25
A water-rich fluid like PG25 would quickly corrode copper and other metals without inhibitors, and could support microbial growth. Suppliers use inhibitor packages (organic acid, inorganic or hybrid) chosen for the wetted materials in cold plates and CDUs, and several advertise freedom from specific additives such as 2-EHA, borates or silicates (see the Shell and Arteco profiles). Each supplier's datasheet lists its own maintenance limits.
Can you make PG25 on site?
Technically you can dilute propylene glycol concentrate with water, but most suppliers sell PG25 pre-diluted with purified water to control purity, inhibitor concentration and contamination. Hardware approvals typically name a specific ready-to-use product. Mixing your own risks voiding approvals and introducing poor-quality water. Check with your hardware and fluid vendors.
What the OCP PG25 guideline covers
The Open Compute Project's Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Liquid Cooled Racks (Revision 2.0, Version 1.0.0, 15 September 2026) focuses on PG25 inhibited fluid in the closed TCS between the CDU and cold-plate-based IT equipment. It explicitly excludes the facility water system. Contributors include people from Dow, Castrol, PFX Group, Cool Filtration and others.
The document gives reasons for choosing PG25 inhibited fluid: thermal stability, corrosion protection for mixed-metal loops (for example copper cold plates and stainless piping), a biostatic nature, and compatibility with common loop materials. It then covers the full lifecycle: typical fluid properties, wetted-material selection and validation of unlisted materials, cleaning, flushing and commissioning, filtration and contamination control, and fluid monitoring and maintenance. It presents these as a baseline framework of recommended practices, not rigid requirements.
Comparing data-center fluids?
Open comparator →The OCP PG25 base specification
Separately, OCP has published a Base Specification for Single-Phase PG25 fluids (v1.0.0), aimed at fluid suppliers, to define a common baseline for production requirements. OCP's Cold Plate sub-project also runs a PG25 Mixing workstream. When a supplier says its fluid "meets OCP PG25 specifications", as Shell does for DLC Fluid S3, that's the reference to check. Ask suppliers which OCP document and version they mean, because the documents have been revised.
PG25 lifecycle: from fill to replacement
- Specify: choose a fluid on your cold-plate and CDU vendors' approved lists.
- Clean and flush: ASHRAE TC9.9 guidance says flush water should meet the TCS water-quality guidelines or ASTM D1193 Type II, using clean, non-contaminating materials and filtration before the final fill.
- Fill: use the pre-diluted fluid and avoid air entrainment.
- Baseline: record initial pH, conductivity, glycol concentration and appearance.
- Monitor: ASHRAE bulletins list pH, conductivity, glycol concentration and inhibitor reserve as minimum measurements.
- Maintain: filter, top up with the same fluid, and treat or replace per the supplier.
PG25 vs water-glycol mixes in buildings
HVAC systems have used inhibited glycol for decades, typically at concentrations chosen for freeze protection in outdoor coils. Data-center TCS loops sit indoors, so PG25 is chosen less for freeze protection than for its biostatic nature and inhibitor delivery, with modest freeze protection for transport and storage. Higher concentrations would reduce heat transfer.
What to ask a PG25 supplier
- Does it meet the OCP PG25 base specification, and which version?
- What are the pH, conductivity and inhibitor-reserve limits?
- What dye, and is it UV-detectable?
- Which additives does it exclude?
- What sampling service and lab turnaround do you offer?
Common mistakes
- Diluting PG25 on site. Use the ready-to-use fluid.
- Reading PG25 on an EG scale. Use the PG scale.
More questions answered
Is PG25 food-grade?
Not necessarily. It depends on the product. Check the SDS and label.
Reader questions
How is PG25 shipped?
Usually ready to use in drums or totes. Ask suppliers about packaging.
Key point to remember
Specify, monitor and maintain PG25 to the supplier's limits.
In practice
In practice, keep a baseline sample.
Next step
Read the OCP PG25 guideline.
Frequently asked questions
What does PG25 stand for?
Propylene glycol at about 25% in water, with inhibitors.
What is the freeze point of PG25?
Suppliers such as Shell and Dow quote about −10 °C for their PG25 products.
Related guides
Sources
- Data Centre Magazine: NVIDIA qualifies Shell DLC fluid
- Dow: DOWFROST LC 25 product page
- BASF news release, Oct 2026
- OCP: Guidelines for Using Propylene Glycol-Based Heat Transfer Fluids in Liquid Cooled Racks (Rev 2.0, Sep 2026)
- OCP: Base Specification for Single-Phase PG25 fluids v1.0.0
- OCP Cold Plate sub-project: PG25 Mixing workstream
- ASHRAE TC9.9 white paper: Water-Cooled Servers: Common Designs, Components, and Processes
- ASHRAE TC9.9 Technical Bulletin (TCS cleaning and commissioning)
- ASHRAE TC9.9 Technical Bulletin (TCS fluid monitoring)