Immersion Cooling Glossary

Terms A–Z

Acid number (acidity)

Amount of potassium hydroxide needed to neutralise the acids in 1 g of fluid, in mg KOH/g. Rises with oxidation and hydrolysis.

Auto-ignition temperature

Temperature at which a fluid ignites in air without an ignition source. OCP Rev. 2.0 minimum: 250 °C.

Breakdown voltage (dielectric strength)

Voltage at which a fluid between two electrodes fails electrically. OCP lifetime minimum: 6 kV at 1 mm gap.

CDU (coolant distribution unit)

Unit with pumps, heat exchanger and controls that circulates the fluid and transfers heat to the facility water circuit.

Chassis-level (precision) immersion

Design in which each server sits in its own closed enclosure filled with fluid.

Cold plate

Metal plate with internal channels mounted on a processor; used in direct-to-chip cooling.

Dielectric fluid

Electrically insulating liquid that can be in direct contact with live electronic components.

Direct-to-chip cooling

Liquid cooling in which a coolant flows through cold plates on the hottest components only; the rest is air-cooled.

ERF (energy reuse factor)

Share of the energy of a data center that is reused, for example as heat. Reported under EU rules.

Fire point

Lowest temperature at which a fluid keeps burning for at least 5 seconds after ignition.

Flash point

Lowest temperature at which the vapours of a fluid ignite briefly. OCP Rev. 2.0 minimum: 150 °C (open cup).

Fluorinated fluid

Fluid based on fluorocarbons, hydrofluoroethers or fluoroketones; used mainly in two-phase immersion; most are PFAS.

GTL fluid

Synthetic hydrocarbon made from natural gas (gas-to-liquids).

Heat soak test

Compatibility test in which components are kept in the fluid at elevated temperature – OCP: 80 °C for 14 days for single-phase fluids.

Hydrolysis

Reaction of an ester with water that forms acids and alcohols; accelerated by heat.

Immersion cooling

Cooling method in which IT hardware is submerged in a dielectric fluid.

Interfacial tension

Tension between oil and water; drops when polar contaminants or ageing products are present.

K-class

Insulating liquid with a fire point above 300 °C according to IEC 61039.

Kinematic viscosity

Resistance of a fluid to flow, in mm²/s. Lower viscosity improves heat transfer and reduces pump power.

Loss tangent

Measure of dielectric losses at a given frequency; relevant for high-speed signals in immersion.

Natural ester

Dielectric fluid based on refined vegetable oils (triglycerides).

OCP (Open Compute Project)

Industry organisation that publishes open specifications, including the Immersion Requirements for fluids and systems.

Open bath

Immersion tank at atmospheric pressure, closed by a lid, in which servers stand vertically.

Oxidation stability

Resistance of a fluid to reaction with oxygen; tested e.g. according to IEC 61125.

PAO (polyalphaolefin)

Synthetic hydrocarbon used as base fluid for single-phase immersion.

Particle count

Number and size of particles in a fluid, reported as a code according to ISO 4406.

Permittivity (dielectric constant)

Ability of a material to store electrical energy in a field. About 2 for hydrocarbons and about 3 for esters.

PFAS

Per- and polyfluoroalkyl substances; very persistent fluorinated chemicals. An EU restriction is in preparation.

Plasticiser leaching

Migration of plasticisers from PVC and other plastics into the fluid; makes the plastic brittle and contaminates the fluid.

Polyol ester

Synthetic ester made from a polyol and fatty acids; the most common type of synthetic dielectric ester.

Pour point

Lowest temperature at which a fluid still flows.

PUE (power usage effectiveness)

Ratio of total data center energy to IT energy. A value closer to 1 means less overhead.

Readily biodegradable

Result of an OECD 301 test showing fast degradation under test conditions. Does not mean that a substance is harmless.

Resistivity

Electrical resistance of a fluid volume, in GΩ·m. OCP: at least 2.00 GΩ·m unused and 0.20 GΩ·m over the lifetime.

Single-phase immersion

Immersion cooling in which the fluid stays liquid and is circulated through a heat exchanger.

Synthetic ester

Dielectric fluid made by synthesis, mostly a polyol ester; high oxidation stability.

Thermal interface material (TIM)

Material between chip and heat sink. Greases can wash out in immersion; solid TIMs are used instead.

Two-phase immersion

Immersion cooling in which a low-boiling fluid evaporates on hot components and condenses on a cooled coil.

Water content

Dissolved water in a fluid, in mg/kg (ppm), measured by Karl Fischer titration.

WUE (water usage effectiveness)

Water use of a data center per unit of IT energy, in litres per kWh.