Immersion Cooling Fluids Compared: Hydrocarbons, Esters and Fluorinated Fluids
Six fluid families are used for immersion cooling. This comparison shows where each one is strong and where it needs attention.
Updated 9 November 2026 · Published by IsoTec
The fluid families
- Mineral oils – refined petroleum oils, the oldest dielectric coolants.
- PAO (polyalphaolefins) – synthetic hydrocarbons with narrow molecular distribution.
- GTL fluids – synthetic hydrocarbons made from natural gas (gas-to-liquids).
- Synthetic esters – mostly polyol esters, made from polyols and fatty acids.
- Natural esters – refined vegetable oils (triglycerides), for example from rapeseed, soybean or sunflower.
- Fluorinated fluids – fluorocarbons, hydrofluoroethers and fluoroketones; mostly used in two-phase systems.
Comparison table
| Property | Mineral oil | PAO / GTL | Synthetic ester | Natural ester | Fluorinated |
|---|---|---|---|---|---|
| Relative permittivity | about 2.2 | about 2.1 | about 3 | about 3 | mostly about 1.8–2; some grades higher |
| Flash / fire point | Moderate | Rises with viscosity | High; K-class grades above 300 °C fire point | Very high | Usually non-flammable |
| Moisture tolerance | Low | Low | High | High | Low water solubility |
| Oxidation stability | Moderate | High | Very high | Lower – needs antioxidants and limited air contact | Very high |
| Heat transfer | Good | Good; very low viscosity grades available | Good; higher thermal conductivity and specific heat | Good; higher thermal conductivity, higher viscosity | Single-phase moderate; two-phase very high |
| Material compatibility | Good, some elastomers swell | Good, some elastomers swell | More polar – test plastics, elastomers, TIMs | More polar – test plastics, elastomers, TIMs | Good; extraction of plasticisers possible |
| Fluorinated / PFAS | No | No | No | No | Mostly yes |
| Fluid cost | Low | Moderate | Moderate to higher | Moderate | High |
Values are typical for each family; individual products differ. The data sheet of the specific fluid is decisive.
Mineral oils
Mineral oils are inexpensive and well known from transformers. The OCP Immersion Requirements, however, state that mineral oils should be avoided and that synthetic oils, synthetic esters or processed natural esters should be used. Typical concerns are variable composition, sulphur content and lower flash points at low viscosity.
Synthetic hydrocarbons (PAO and GTL)
PAO and GTL fluids are pure, stable and available in very low viscosities, which helps heat transfer and pumping. Their low permittivity is an advantage for high-speed signals. The trade-off: for hydrocarbons, a lower viscosity usually means a lower flash point, and their low water solubility means that free water can form quickly if moisture enters.
Synthetic esters
Synthetic polyol esters reach high flash and fire points, are very resistant to oxidation and dissolve much more water than hydrocarbons without losing dielectric strength. They typically have a higher thermal conductivity and specific heat than hydrocarbons of comparable viscosity. Their polarity requires compatibility testing of plastics, elastomers and thermal interface materials, and the fluid must be kept dry to prevent hydrolysis. Synthetic esters are well suited to tanks with air contact.
Natural esters
Natural esters are made from refined vegetable oils. They have very high fire points and high moisture tolerance. Because their fatty acids are partly unsaturated, they oxidise faster in contact with air and have a higher viscosity and pour point. In immersion tanks they need tight lids or inert gas, suitable antioxidants and regular monitoring. See natural vs synthetic esters.
Fluorinated fluids
Fluorinated fluids are non-flammable, and most grades have a low permittivity. They enable two-phase cooling, but they are expensive and most of them are PFAS. 3M announced its exit from PFAS production by the end of 2025, and the EU is preparing a broad PFAS restriction. See PFAS and immersion cooling.
How to choose
- Define the tank type (open, covered, sealed) and the operating temperatures.
- Set minimum fire safety values (OCP: flash point ≥ 150 °C; insurer and local requirements).
- Check viscosity against pump and flow design.
- Test compatibility with your server components.
- Check signal integrity if high-speed interfaces are immersed.
- Plan monitoring and fluid care.
The complete process: how to qualify an immersion cooling fluid.
Frequently asked questions
Which immersion cooling fluid has the best heat transfer?
Among single-phase fluids, esters typically have a higher thermal conductivity and specific heat than hydrocarbons of comparable viscosity. Low viscosity also improves heat transfer, which favours light hydrocarbons. The combination of both decides.
Why do esters have a higher permittivity?
Ester groups are polar. This raises the relative permittivity to about 3, compared with about 2 for hydrocarbons.
Can different immersion fluids be mixed?
Not without testing. Mixing changes flash point, viscosity and compatibility, and the warranty of fluid or hardware may be affected.
Comparing fluids for your tank?
Send us your requirements – we compare ester options and test them with your components. IsoTec products are supplied to customers outside Germany.