Quick answer: PG25 is a nominal 25% propylene glycol / 75% demineralised water mix with a corrosion-inhibitor package — the common working fluid in single-phase direct-to-chip loops. It’s fully compatible with 304/316L stainless steel pipework, pairs with EPDM elastomers for seats and gaskets, and its main demand on the system is that the inhibitor package stays in specification — which clean, passive pipework helps it do.
Why PG25?
Pure water transfers heat best, but glycol buys freeze protection (for external pipe runs and dry-cooler circuits), some biological suppression, and a formulated inhibitor package protecting the loop’s mix of metals. 25% is the common balance point: meaningful protection with a modest penalty to heat capacity and pumping power. Many IT OEM cooling requirements are written around it.
What it means for material selection
- Tube and fittings: propylene glycol itself is benign to stainless steel. 316L is the standard TCS choice — not because of the glycol, but for tolerance of chlorides in makeup water and evolving additive chemistry over a 20-year life (304 vs 316L).
- Elastomers: EPDM is the default seat and gasket material for glycol/water service; FKM and alternatives exist for unusual additive packages. Always confirm against the specific coolant datasheet — inhibitor packages differ by brand.
- What to avoid: uncontrolled mixed-metal loops (galvanic couples consume inhibitors), and any component whose wetted materials the coolant supplier hasn’t seen.
Keeping PG25 healthy
The coolant is a consumable chemistry, not just a fluid. Inhibitors deplete as they do their job; concentration drifts with top-ups. Loops should be baselined at commissioning and sampled periodically — and here pipework quality pays a quiet dividend: a passive, smooth-bore stainless loop contributes almost no metal ions or particulate, so the inhibitor package lasts longer between corrections and filters run cleaner.
Specifying the wet side in one line
316L stainless tube and fittings, EPDM-seated butterfly valves and clamp gaskets, all wetted materials declared to the coolant supplier for sign-off. Neumo stocks that combination end to end: valves · tube · fittings.
Talk to us about your project
Kevin Windsor — Product Manager, Data Centre Liquid Cooling
kw@neumo.co.uk · 01952 583 999