Consider an immersion tank waiting to restart after a cable replacement. The fluid report says its breakdown-voltage result passed. The electrical test has answered one question. Whether the new cable materials suit that fluid remains a separate one.
For single-phase dielectric immersion fluids, the useful reading of a pass starts with the sample, method and requirement behind it. Once those are clear, you can see which parts of the maintenance decision still need attention.
Breakdown voltage measures electrical strength under set conditions
In the laboratory, two electrodes sit a fixed distance apart in a cell filled with the fluid sample. The voltage rises steadily until the fluid stops insulating and a spark crosses the gap. That is the breakdown voltage. Standard methods repeat the measurement and report an average.
The set-up is part of the result. IEC 60156 describes an empirical test procedure rather than a basic property of the liquid.[1] Change the method, electrode shape or gap and you change the conditions behind the number.
Water, particles and other contaminants can lower the result. A high value, however, does not establish that all contaminants are absent, as ASTM D1816 cautions.[2] Nor does the test measure water content, ageing products, viscosity, heat transfer or compatibility with tank materials. Those questions require other observations or tests.
Compare results only when method, set-up and units match
“Breakdown voltage” on two reports is not enough to make their values comparable. Even IEC 60156 shows test cells with two different electrode shapes.[1] Before placing a result beside a limit or an earlier result, check:
- the method and its edition
- the electrode shape and the gap between the electrodes
- sample preparation and test temperature
- whether the value is a single breakdown or the mean of a series
- the units, exactly as reported
A value in kV gives the voltage at breakdown; kV/mm expresses it per millimetre of gap. Retain the laboratory's units. Dividing or multiplying by a gap does not establish what a test at a different gap would produce: IEC 60156 notes that the result depends strongly on the measurement conditions.[1]
The Open Compute Project's immersion requirements show why these details matter. Revision 2.10 specifies ASTM D1816 at a 1 mm gap, expressed in kV/mm, and uses that same basis for its minimum requirements. Its amendments remove an earlier option to estimate the 1 mm value from a 2.5 mm measurement. Revision 2.0 also named IEC 60156 in brackets after ASTM D1816; revision 2.10 does not.[3]
OCP gives no reason for either change. For a result judged against those requirements, use the stated method and gap, and ask the laboratory to report on the specified kV/mm basis. Do not make the conversion yourself and assume the measurement is equivalent.
IEC 60156 and ASTM D1816 are separate methods. Comparing across methods, electrode set-ups or gaps requires a basis documented by the laboratory and equipment manufacturer. VeriFluid's immersion-fluid testing page lists IEC 60156 as a representative electrical test, with the final method confirmed after reviewing the fluid. If the equipment limit cites D1816 at 1 mm, resolve that requirement before sending the sample.
Check that the method suits the fluid as well. IEC 60156 covers insulating liquids with nominal viscosity up to 350 mm²/s at 40 °C.[1] That is a scope boundary, not a recommended viscosity or equipment operating limit. ASTM D1816's public scope names petroleum oils, silicone fluids, high fire-point mineral insulating oils, and synthetic and natural esters used in cables, transformers, oil circuit breakers and similar apparatus.[2] Neither public scope names immersion cooling fluids. Ask the laboratory to confirm method applicability for the product you use.
A pass means something only against a named requirement
The method tells you how the laboratory measured the value. The requirement tells you whether that value is acceptable for this fluid, equipment and purpose. A sound measurement can arrive without a named requirement; until that gap is resolved, its acceptance status is undetermined.
Purpose matters too. OCP's table separates unused and lifetime fluid, with its dielectric-strength minimum appearing only in the lifetime column.[3] A commissioning limit, a new-hardware qualification criterion and an in-service action level may serve different decisions. A typical value on a product data sheet is another thing again. Our 8 October coolant article explains why new-fluid and in-use limits should not be swapped.
Get the governing document and revision, and check that it covers the named fluid and equipment. For a pass/fail report, agree the decision rule with the laboratory before testing, including the treatment of results close to the limit. If the applicable requirement is unclear, resolve it with the fluid supplier, equipment manufacturer and laboratory first.
The conclusion you can support is specific: “This sample met the stated electrical criterion under the recorded test conditions.”
Other questions need their own evidence
Choose additional checks for the concern that brought you to the report. The fluid supplier and laboratory can help match the method to the fluid, system history and decision.
Water. Moisture can lower breakdown voltage, as can particles and other contaminants. The electrical result cannot separate their effects or tell you the water concentration. For a moisture concern, use a water-content method suited to the fluid and compare it with the product's criteria.
Chemical change. Choose indicators that suit the formulation. Acidity may be useful where applicable, but read it against a comparable baseline for the same fluid. A confirmed rise beyond expected sampling and measurement variation supports a change in the fluid; it does not give its remaining service life.
Viscosity and cooling. A viscosity result needs its test temperature. To investigate cooling performance, put it beside tank temperatures, flow and heat-rejection records. The viscosity number alone cannot establish whether the installed tank is cooling adequately.
Particles and other contamination. Match the check to the suspected contaminant and fluid type. A clear-looking sample can still contain maintenance residues or another fluid, even when its electrical result passes.
Materials and handling. Check compatibility for the actual seals, cables, coatings and thermal interface materials exposed to the named fluid. Use the maintenance record to trace cleaning solutions, residual fluids and handling changes. A routine bottle sample cannot establish long-term material compatibility or reconstruct that history.
OCP likewise treats dielectric strength, flash point and acidity as separate properties. Revision 2.10 notes that specifications can differ by application and technology, and requires suppliers to identify tests and intervals for monitoring fluid quality.[3]
Example: a replacement cable assembly after maintenance
Return to the tank in the opening. For this worked example, assume the replacement cable came from a different supplier without a compatibility record for the fluid. A sample taken after maintenance met the electrical criterion, using the agreed test method.
What the result covers. The sample's electrical strength under the recorded test conditions.
What remains open. Whether the cable's jacket, insulation, seals and connectors suit long-term exposure to this fluid. Materials may change or release substances into the fluid over time. An early sample may not show those effects.
That gives the next review a clear focus:
- Ask the cable, equipment or fluid supplier for compatibility evidence covering the named fluid, installed materials, temperatures and exposure time. It should address changes to the fluid as well as to the parts.
- Check that the part fitted is the part that evidence describes.
- Check the maintenance record for cleaning solutions, residual fluids or handling changes.
- If records are missing or a specific concern emerges, agree targeted tests and a sampling plan with the fluid supplier and laboratory.
A missing record is not proof that the cable is unsuitable. It leaves compatibility unresolved. Castrol's checklist for its own single-phase immersion fluids treats material compatibility, system cleanliness, fluid handling and operating checks as separate steps.[4]
Record the decision and what remains open
Keep the report with the requirement used to judge it and the decision it supported. Someone returning to the file should be able to see what was settled, what is still open, who owns the remaining checks and when they are due.
Before resampling, identify what was wrong with the first sample or what the next one needs to establish. The test describes the fluid in the bottle; its sampling point, container and handling matter. IEC 60156 calls for rigorous handling when certified results are needed, and IEC 60475 covers sampling insulating liquids.[1] Choose extra tests by asking which result would change the decision.
Keep the operating records in that file too. Reliability Engine's Virtual Chemist offers a related example: reading coolant chemistry alongside flow, temperature and GPU workload in liquid-cooled AI data centres, in addition to periodic laboratory tests.[5] The page concerns coolant loops, not immersion tanks, and describes no VeriFluid integration. Any sensor considered for an immersion tank would need separate confirmation that it suits the fluid.
What to send with the next sample
When you request immersion-fluid testing, send:
- the product name, fluid family and supplier
- the equipment and how the fluid is used
- the reason for testing and the decision the result will support
- recent changes, including new parts, cleaning and top-ups
- the requirement the result will be judged against, with its revision
- the sampling point and date, noting whether the fluid had circulated since any work
- the previous comparable report, if there is one
Those details help the laboratory select a method and identify any supporting tests before the bottle arrives.
Common questions
Can a breakdown-voltage result tell me the water content?
No. Moisture, particles and other contaminants can affect the result, so it cannot be read as a water concentration. Ask for a water-content method suited to the fluid.
Can an IEC 60156 result be judged against a limit written for ASTM D1816?
Not directly. The methods and test conditions differ. Confirm which method the equipment manufacturer's limit assumes, then use that method or obtain an applicable limit for the method used.
Does a low result mean the fluid is wet?
Not necessarily. ASTM D1816 lists water, dirt, cellulose fibres and conducting particles among possible causes.[2] Contamination from the container or sampling point can also affect the result. Check how the sample was taken, then select tests for the suspected cause.
References
Sources reviewed on 11 October 2026. The IEC and ASTM entries rely on publisher abstracts, previews and public scope text; the full paid standards were not consulted. Citing a method does not establish VeriFluid accreditation or method availability.
- IEC 60156:2025: Insulating liquids, determination of the breakdown voltage at power frequency, test method. Edition 4.0, published 31 January 2025. Public abstract and preview: introduction, clause 1 (scope) and clause 2 (normative references). A test method, not limits; the public abstract does not mention immersion cooling fluids.
- ASTM D1816-12(2019): Dielectric breakdown voltage of insulating liquids using VDE electrodes. Active edition when reviewed. Public scope (1.1 to 1.3) and significance (3.1) only.
- Open Compute Project: Immersion Requirements, revision 2.10. Labelled rev 2.1 in OCP's URL and on its immersion project page, where it was the latest revision listed when reviewed. First amendment of revision 2.0, last updated 18 August 2023. Table of amendments (p. 7), SR 4.1-1 (p. 17), SR 4.2-1 (p. 19) and SR 4.3-3 (p. 22). Compared with revision 2.0 of 21 January 2022. In OCP's words, a requirements document and not a specification.
- Castrol ON single phase immersion cooling fluids: Checklist and general troubleshooting. No printed date or revision; PDF metadata 15 October 2024. Sections 1, 2 and 4, pages 1 to 4. Guidance for Castrol's own fluids; it names no test methods or limits.
- Reliability Engine: Virtual Chemist. Product page for coolant-loop monitoring. Related reading only; not evidence of a VeriFluid integration.