Fire Safety of Immersion Cooling Fluids: Flash Point, Fire Point, Auto-Ignition
A data hall with tanks full of fluid raises questions from insurers and fire authorities. The key properties and how to read them.
Updated 9 November 2026 · Published by IsoTec
Three properties
| Property | Meaning | Test method |
|---|---|---|
| Flash point | Lowest temperature at which the vapours above the liquid ignite briefly when an ignition source is applied | Open cup: ASTM D92 / ISO 2592 (COC); closed cup: ASTM D93 / ISO 2719 |
| Fire point | Lowest temperature at which the liquid keeps burning for at least 5 seconds after ignition | ASTM D92 / ISO 2592 |
| Auto-ignition temperature | Temperature at which the liquid ignites in air without an ignition source | ASTM E659 / DIN 51794 |
Closed-cup flash points are usually lower than open-cup values for the same fluid. Always check which method a data sheet uses.
What OCP requires
The OCP Immersion Requirements (Rev. 2.0) require a flash point of at least 150 °C (Cleveland open cup) and an auto-ignition temperature of at least 250 °C, both for the unused fluid and over its lifetime. See OCP immersion fluid requirements.
K-class: classification by fire point
IEC 61039 classifies insulating liquids by fire point:
| Class | Fire point |
|---|---|
| O | ≤ 300 °C |
| K | > 300 °C |
| L | No measurable fire point |
Transformer-grade synthetic and natural esters are K-class. Low-viscosity fluids designed for server tanks – esters as well as hydrocarbons – usually have lower flash and fire points, because lighter molecules evaporate more easily. Fluorinated fluids are generally non-flammable.
Viscosity versus flash point
For heat transfer, a low viscosity is desirable; for fire safety, a high flash point. For hydrocarbons, both pull in opposite directions. Esters are less volatile than hydrocarbons of similar viscosity and therefore reach higher flash points at the same viscosity. This trade-off is one of the main reasons esters are used in immersion cooling. A comparison: immersion cooling fluids compared.
Flash point is not an operating limit
Normal fluid temperatures in immersion tanks are far below the flash point. Fire risk comes from faults: electrical arcs, short circuits or local overheating. OCP therefore asks operators to avoid local overheating and to check flash point and resistivity after such an event.
Regulations and insurers
- Fluids with a flash point well above 60 °C – such as fluids meeting the OCP minimum of 150 °C – are not classified as flammable liquids under the EU CLP Regulation.
- Building and fire codes may still treat them as combustible liquids – in the US, for example, liquids with a flash point of 93 °C (200 °F) or more are Class IIIB under NFPA 30.
- Insurers assess fire load, detection, spill containment and suppression. Involve them early and provide flash point, fire point and auto-ignition data.
IsoTec ester fluids
Our transformer-grade esters IsoTec SE 300 and IsoTec NE 350 are K-class fluids with fire points above 300 °C. For immersion cooling projects we provide the fire safety data of the selected fluid. More in the data center immersion cooling guide.
Frequently asked questions
What is the difference between flash point and fire point?
At the flash point the vapours ignite only briefly; at the fire point the liquid keeps burning for at least 5 seconds. The fire point is higher.
Are immersion cooling fluids flammable?
Hydrocarbon and ester fluids are combustible but, with flash points of 150 °C or more, not classified as flammable liquids under EU CLP. Fluorinated fluids are generally non-flammable.
What does K-class mean?
Under IEC 61039, K-class insulating liquids have a fire point above 300 °C.
Fire safety data for your project
We provide flash point, fire point and auto-ignition data for the ester fluid selected for your tanks. IsoTec products are supplied to customers outside Germany.