Immersion Cooling Fluids for Data Centers
A practical guide to dielectric fluids for single-phase immersion cooling of servers and AI hardware – fluid types, OCP requirements, material compatibility, fire safety and qualification. Written by the ester fluid specialists of IsoTec.
Updated 9 November 2026 · Published by IsoTec
Contents
- Immersion cooling at a glance
- What is immersion cooling?
- Why data centers are moving to immersion cooling
- Which fluids are used?
- Why esters for immersion cooling?
- What does the OCP require of an immersion fluid?
- Material compatibility
- Fire safety
- Operation and fluid monitoring
- How IsoTec supports immersion cooling projects
- All articles on data center immersion cooling
Immersion cooling at a glance
- In immersion cooling, servers are submerged in an electrically non-conductive (dielectric) fluid that absorbs heat directly from all components.
- Single-phase systems use fluids that stay liquid – synthetic hydrocarbons (PAO, GTL), synthetic esters or natural esters. Two-phase systems use low-boiling, mostly fluorinated fluids.
- The Open Compute Project (OCP) Immersion Requirements set minimum values for dielectric strength, resistivity, flash point, sulphur content and acidity, among others, and advise against mineral oils.
- Esters combine high flash and fire points, high moisture tolerance and good heat transfer. Their higher polarity makes material compatibility testing essential.
- Every fluid should be qualified with the actual server components. OCP guidance describes a 14-day soak at 80 °C for single-phase fluids.
- A dedicated ASTM specification for single-phase data center immersion fluids is under development (committee D27, work item WK97110).
What is immersion cooling?
Immersion cooling is a liquid cooling method in which IT hardware is submerged in a dielectric fluid. Because the fluid does not conduct electricity, it can be in direct contact with processors, memory, power supplies and circuit boards. It absorbs the heat of all components – not only of the processor – and carries it to a heat exchanger. From there the heat passes to a water circuit and on to dry coolers, cooling towers or a heat reuse system.
Three designs are common:
- Tank immersion (open bath): servers stand vertically in a tank filled with fluid and covered by a lid. Most single-phase deployments use this design.
- Precision or chassis-level immersion: each server sits in its own closed enclosure filled with fluid, so the rack format is kept.
- Two-phase immersion: the fluid boils on hot surfaces and condenses on a coil above the bath. See single-phase vs two-phase immersion cooling.
Immersion cooling differs from direct-to-chip (cold plate) cooling, where a water-glycol coolant flows only through cold plates on the processors, while the other components are still cooled by air.
Why data centers are moving to immersion cooling
- Rack power density: AI accelerators and high-end CPUs push rack power far beyond what air cooling handles efficiently.
- No server fans: in immersion, fans are removed or switched off, which saves fan power and reduces noise.
- Water use: depending on climate and heat rejection design, the need for evaporative cooling can be reduced.
- Heat reuse: the heat leaves the tank in a liquid at a higher temperature than the exhaust air of an air-cooled room, which makes reuse easier – see waste heat reuse.
- Protection: the fluid shields components from dust, humidity and airborne contaminants.
- Regulation of fluorinated fluids: PFAS restrictions and 3M’s exit from PFAS production push users towards single-phase fluids – see PFAS and immersion cooling.
Which fluids are used?
| Fluid family | System | Typical characteristics |
|---|---|---|
| Synthetic hydrocarbons (PAO, GTL) | Single-phase | Low viscosity possible, low permittivity, good material compatibility; flash point rises with viscosity |
| Mineral oils | Single-phase | Low cost; the OCP Immersion Requirements advise against mineral oils |
| Synthetic esters (polyol esters) | Single-phase | High flash and fire points, very good oxidation stability, high moisture tolerance, higher permittivity |
| Natural esters (vegetable-oil based) | Single-phase | Very high fire point, high moisture tolerance; lower oxidation stability – needs covered or inerted tanks and antioxidants |
| Fluorinated fluids | Two-phase and single-phase | Non-flammable, low permittivity (most grades), high cost; most are PFAS |
A detailed comparison: immersion cooling fluids compared. Choosing between the two ester types: natural vs synthetic esters for immersion cooling.
Why esters for immersion cooling?
Advantages
- Fire safety: transformer-grade esters are K-class liquids with a fire point above 300 °C (IEC 61039). Low-viscosity grades for server tanks have lower values – check the data sheet.
- Moisture tolerance: esters dissolve far more water than hydrocarbons before free water forms, so their dielectric strength is less sensitive to moisture.
- Heat transfer: esters typically have a higher thermal conductivity and specific heat than hydrocarbons of comparable viscosity.
- Chemistry: esters are not fluorinated. Many are readily biodegradable in OECD 301 tests – the test report of the specific fluid is decisive, and readily biodegradable does not mean harmless.
Points to manage
- Permittivity: esters have a relative permittivity of about 3, hydrocarbons of about 2. This can matter for high-speed signals – OCP asks suppliers to report the dielectric constant at 20 and 40 GHz.
- Compatibility: esters are more polar than hydrocarbons. Plastics, elastomers, cable sheaths, thermal interface materials and labels must be tested.
- Water and hydrolysis: at elevated temperature, water can split esters into acids and alcohols. Keep the fluid dry and the tank covered.
- Oxidation: natural esters age faster in contact with air than synthetic esters.
- Viscosity: esters are usually more viscous than the lightest hydrocarbons, which has to be considered in pump and flow design.
IsoTec specialises in ester-based dielectric fluids – synthetic esters (IsoTec SE 300), natural esters (IsoTec NE 350) and ester fluids for immersion cooling.
What does the OCP require of an immersion fluid?
The OCP Immersion Requirements define what suppliers must report for a fluid and which minimum dielectric values apply. Selected requirements from Rev. 2.0:
| Property | OCP minimum requirement (Rev. 2.0) |
|---|---|
| Dielectric strength (1 mm gap) | ≥ 6 kV over the fluid lifetime |
| Resistivity | ≥ 2.00 GΩ·m unused, ≥ 0.20 GΩ·m over the lifetime |
| Flash point (COC) | ≥ 150 °C |
| Auto-ignition temperature | ≥ 250 °C |
| Sulphur content | < 10 ppm |
| Acidity | Limit depends on the fluid class (hydrocarbons, natural esters, synthetic esters, fluorocarbons) |
The document also states that mineral oils should be avoided and that synthetic oils, synthetic esters or processed natural esters should be used. Always check the current revision. More: OCP immersion fluid requirements explained.
Material compatibility
The fluid touches every part of the server. Plasticisers can leach out of PVC cable sheaths, thermal greases can wash out, elastomers can swell and label adhesives can fail. The OCP compatibility guideline (v1.5, June 2026) describes a heat soak of components in single-phase fluids at 80 °C ± 2 °C for 14 days, followed by checks of the materials and the fluid. Details and a component checklist: material compatibility in immersion-cooled servers.
Fire safety
Flash point, fire point and auto-ignition temperature describe how a fluid behaves in a fire. They are safety properties, not operating limits. Insurers and local authorities may set requirements beyond the OCP minimum. More: fire safety of immersion cooling fluids.
Operation and fluid monitoring
A fluid that is clean and dry when filled can pick up water, particles and extracted substances in operation. Regular analysis of water content, acidity, dielectric strength, resistivity and particle count shows when filtration, drying or replacement is needed. More: immersion cooling fluid maintenance and testing.
How IsoTec supports immersion cooling projects
- Requirements: tank design (open, covered or sealed), operating temperatures, hardware, region and applicable regulations.
- Fluid selection: synthetic or natural ester, matched to viscosity, fire safety and oxidation requirements.
- Laboratory testing: compatibility soak with your server components and analysis of the fluid before and after.
- Samples and trial quantities for tank tests.
- Supply with a certificate of analysis for each batch and support with fluid monitoring.
A step-by-step checklist: how to qualify an immersion cooling fluid.
All articles on data center immersion cooling
- Single-phase vs two-phase immersion cooling – how they work, fluids, pros and cons
- Immersion cooling fluids compared – hydrocarbons, esters and fluorinated fluids
- Natural vs synthetic esters for immersion cooling
- OCP immersion fluid requirements explained
- Material compatibility in immersion-cooled servers
- Fire safety of immersion cooling fluids
- Immersion cooling fluid maintenance and testing
- How to qualify an immersion cooling fluid – checklist
- PFAS and immersion cooling
- Waste heat reuse from immersion-cooled data centers
- Immersion cooling glossary
Frequently asked questions
What fluid is used for immersion cooling in data centers?
Single-phase systems use synthetic hydrocarbons (PAO, GTL), synthetic esters or natural esters. Two-phase systems use low-boiling fluids, mostly fluorinated. The OCP Immersion Requirements advise against mineral oils.
Does an immersion fluid damage electronics?
A dielectric fluid does not conduct electricity, so live components can be submerged. However, every material in the server must be compatible with the fluid. Fans are removed or switched off, and hard disk drives that are not hermetically sealed cannot be immersed.
What is the difference between single-phase and two-phase immersion cooling?
In single-phase systems the fluid stays liquid and is pumped through a heat exchanger. In two-phase systems the fluid boils on hot components and condenses on a coil above the bath.
Are esters suitable for data center immersion cooling?
Yes. Esters offer high flash and fire points, high moisture tolerance and good heat transfer. Because they are more polar than hydrocarbons, compatibility with the server materials must be tested, and the fluid must be kept dry.
Natural ester or synthetic ester?
Synthetic esters are more resistant to oxidation and suit tanks with air contact. Natural esters are vegetable-oil based and have a very high fire point, but need covered or inerted tanks and antioxidants.
What flash point does an immersion cooling fluid need?
The OCP Immersion Requirements (Rev. 2.0) specify a flash point of at least 150 °C (COC). Insurers or local authorities may require more. Transformer-grade esters reach fire points above 300 °C.
How long does an immersion cooling fluid last?
Service life depends on temperature, air contact, water ingress and contamination from materials. Filtration, covered tanks and regular fluid analysis extend it. Ask the supplier for the expected life under your conditions.
Do immersion cooling fluids contain PFAS?
Most fluorinated immersion fluids are PFAS. Hydrocarbons and esters are not fluorinated chemistries.
Is there a standard for immersion cooling fluids?
The OCP Immersion Requirements are the main industry reference. ASTM is developing a specification for single-phase data center immersion fluids (WK97110). Test methods are taken from IEC and ASTM standards for insulating liquids.
Where does IsoTec supply?
IsoTec products are supplied to customers outside Germany. Contact us with your application and location.
Sources and further reading
- OCP Immersion Requirements, Rev. 2.0 (Open Compute Project)
- OCP Server Component Immersion Material Compatibility Testing, v1.5 (June 2026)
- ASTM WK97110 – New Specification for Single Phase Liquids used in Immersion Cooling of Data Centers
- Directive (EU) 2023/1791 on energy efficiency (EUR-Lex)
- ECHA – Per- and polyfluoroalkyl substances (PFAS)
Planning an immersion cooling project?
Tell us about your tank and hardware – we select an ester fluid and test it with your components in our laboratory. IsoTec products are supplied to customers outside Germany.