Base Specification for Immersion Fluids, Revision 1.0, Version 1.0 (December 1, 2022)

Overview

The specification rests on the argument that immersion fluid must be standardised because it contacts IT equipment directly. Server makers and suppliers of CPUs, GPUs, memory, SSDs, cables and power supplies cannot verify compatibility with every liquid on the market, nor reliable operation over a design life of about 5 years while immersed. Single-phase liquids fall into four groups (hydrocarbons, fluorochemicals, natural esters, synthetic esters); two-phase liquids used for immersion are mostly fluorochemicals. The document describes itself as a de-facto standard with no hardware product.

For thermal performance it uses combined values instead of individual properties (specific heat, conductivity, viscosity, density). FOM1 covers natural convection through vertical-fin heatsinks, FOM2 developing laminar forced convection for single-phase fluids, FOM3 viscosity for pressure drop and FOM4 two-phase pool boiling. Fluid vendors are expected to provide properties as a function of temperature rather than at one reference temperature; the summary table is evaluated at 25 C and the electrical specifications must be met across 20 to 70 C.

The specification also limits lifetime behaviour: total acid number (TAN) should not exceed twice the unused-fluid limit (hydrocarbons 0.01/0.02, natural esters 0.06/0.12, synthetic esters 0.03/0.06, fluorocarbons 0.001/0.002 mg KOH/g); dielectric properties must be measured at the water solubility limit; dielectric strength must exceed 6 kV/mm, dielectric constant be at most 2.3, loss tangent at most 0.05 and volume resistivity exceed 1e11 ohm-cm. On failure, TAN and water content must be reported to the IT vendor.

The safety and environment section covers toxicity (persistence, bioaccumulation, aquatic and terrestrial toxicity, carcinogenicity), ODP, GWP and PFAS regulatory risk. ODP must be zero and GWP low, while stressing that an immersion deployment has several sources of climate impact and fluid choice is only one. Material compatibility and reliability requirements are not in this version and are deferred to later versions.

Key points

1. Single-phase fluids fall into four groups: hydrocarbon, fluorochemical, natural ester and synthetic ester; two-phase fluids are mostly fluorochemicals.
2. FOM2 (developing laminar forced flow) must exceed 19 for single-phase fluids; FOM1 (natural convection) above 35 (Tier 1) or 45 (Tier 2); 2P FOM4 above 1.1e6.
3. Single-phase vapour pressure below 0.8 kPa; dynamic viscosity below 0.015 N.s/m2; density below 2000 kg/m3.
4. Single-phase closed-cup flash point and boiling point above 150 C at sea level or 155 C at 1500 m; two-phase boiling point 45-55 C at 1 atm.
5. Pour point below -30 C; critical temperature above 155 C.
6. Total acid number (unused / lifetime, mg KOH/g): hydrocarbons 0.01/0.02, natural esters 0.06/0.12, synthetic esters 0.03/0.06, fluorocarbons 0.001/0.002.
7. Lifetime electrical limits: dielectric strength above 6 kV/mm, dielectric constant at most 2.3 and loss tangent at most 0.05 (20 MHz-40 GHz), volume resistivity above 1e11 ohm-cm.
8. ODP shall be zero and GWP low; PFAS chemistries are flagged as an added regulatory risk for some fluids.
9. Values are evaluated at 25 C but must be reported at 25-70 C; future revisions will move the reference temperature to 50 C.
10. Carcinogenicity: for hydrocarbon fluids no ingredient at 0.1% or more may be classed as a carcinogen by IARC, NTP or ACGIH.

Using it in design

• Use the summary table as validation rules for immersion-fluid catalogue entries: flash point, pour point, viscosity, density and dielectric properties.
• Add an altitude-dependent flash and boiling point check for high-altitude sites (150 C versus 155 C).
• Feed fluid density (up to 2000 kg/m3 for fluorocarbons) into tank floor-loading calculations.
• Flag PFAS-based two-phase fluids with a regulatory-risk warning and record GWP and ODP for sustainability reporting.
• Add TAN and water-content monitoring to the O&M plan and define a sampling and reporting procedure for failures.
• Verify that catalogue entries present properties across 25-70 C rather than at a single temperature.

Who should read it

Engineers designing immersion cooling systems, fluid and tank suppliers and server manufacturers.

Document contents

1. License
2. Compliance with OCP Tenets
3. Scope
4. Overview
5. Introduction of Cooling Technologies
6. Need for standardizing Immersion Liquids
7. Immersion Fluid Requirement
8. Thermal Performance Requirement
9. FOM1 - Natural Convection for Single Phase Immersion
10. FOM2 - Forced Convection Figure of Merit for Single Phase Immersion
11. FOM3 - Viscosity for Pressure Drop
12. Density
13. FOM4 - Pool Boiling for Two-Phase Immersion
14. Boiling Point Requirement for Immersion Liquids
15. Pour Point
16. Vapor Pressure Requirement for Immersion Liquids
17. Total Acid Number (Guidance)
18. Water Solubility
19. Electrical and Signal Integrity Requirements
20. Dielectric constant Er (Dk) data
21. Safety, Environmental and Chemistry requirements
22. Toxicity
23. Atmospheric Impacts (ODP and GWP)
24. PFAS Regulatory Risk
25. Additional Chemistry Requirements
26. Final Summary Table of Fluid Specification
27. References

Related standards

• OCP Material Compatibility in Immersion Cooling
• OCP ACS Immersion Requirements Rev 2
• ASTM D93
• ASTM D97
• IEC 60156
• IEC 62368-1

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Publication details

Authors

Open Compute Project Foundation

Publisher

Open Compute Project Foundation

Year

2022

Document number

Rev 1.0

Document language

English

Source type

Industry body

Access

Open licence — downloadable

Licence

OWFa 1.0 — Open Compute open specification licence; redistribution permitted

Licence evidence

www.opencompute.org

Citation