Flash point is the lowest temperature at which a liquid gives off enough vapor to ignite momentarily when exposed to a flame. ASTM D92 and ASTM D93 are the two most referenced ASTM methods for petroleum products-but they are not interchangeable. The cup design alone can shift results by 3 °C to 10 °C on the same sample.
What each standard measures
ASTM D92 - Flash and Fire Points by Cleveland Open Cup (COC)
The sample sits in an open vessel exposed to ambient air. As temperature rises, vapors disperse freely above the liquid surface. A test flame passes over the surface at intervals until a momentary flash occurs-that temperature is the flash point. Heating continues until the sample sustains combustion for at least five seconds; that temperature is the fire point.
D92 applies to petroleum products with flash points above 79 °C (175 °F) and below 400 °C (752 °F), except fuel oils. Typical materials include lubricating oils, asphalt, waxes, and heavy industrial fluids.
ASTM D93 - Flash Point by Pensky-Martens Closed Cup (PMCC)
The sample is heated inside a closed cup with a small lid opening. Vapors accumulate in the headspace, simulating confined storage conditions-a fuel tank, drum, or process vessel. A flame passes through the opening at set intervals until the vapor ignites.
D93 covers petroleum products from 40 °C to 370 °C. It includes three procedures:
Procedure A: distillate fuels (diesel, kerosene, heating oil, turbine fuel), biodiesel blends, and homogeneous lubricating oils
Procedure B: residual fuel oils, used oils, viscous liquids, and samples that form surface films
Procedure C: biodiesel B100 (automated electronic detection recommended)
D93 determines flash point only-not fire point.
Why results differ: open cup vs closed cup
The difference comes down to vapor containment.
In an open cup (D92), vapor escapes into the air. Concentration above the liquid builds slowly, so ignition requires a higher temperature. In a closed cup (D93), vapor is trapped and concentrates faster, producing a lower, more conservative flash point.
For the same lubricating oil, D92 might read 210 °C while D93 reads 203 °C. Both readings are valid-but only within their respective methods. Never compare or convert between them without a validated correlation study.
Side-by-side comparison
| Feature | ASTM D92 (COC) | ASTM D93 (PMCC) |
|---|---|---|
|
Cup type |
Open |
Closed |
|
Measures |
Flash point + fire point |
Flash point only |
|
Temperature range |
79 °C – 400 °C |
40 °C – 370 °C |
|
Typical result |
Higher |
Lower (more conservative) |
|
Fuel oils |
Not for flash point |
Standard method |
|
Low-flash contaminants |
Less sensitive |
More sensitive |
|
Regulatory preference |
Rare for compliance |
Preferred for SDS, DOT, UN transport |
|
Common applications |
Lubricants, asphalt, waxes, heavy oils |
Diesel, biodiesel, kerosene, heating oil |
When ASTM specifies which method
ASTM D92 itself states two critical rules:
For fuel oil flash point, use ASTM D93-not D92.
Use D93 when you need to detect small concentrations of lower-flash-point substances that an open-cup method may miss-for example, gasoline contamination in used engine oil showing an abnormally low flash point.
For flash points below 79 °C, neither D92 nor D93 is the primary choice; consider ASTM D56 (Tag closed cup) or ASTM D3828 (Setaflash small-scale).
How to choose the right method
Use ASTM D93 if you are testing:
Diesel, kerosene, heating oil, or turbine fuel
Biodiesel or biodiesel blends
Residual or used fuel oils (Procedure B)
Any sample where SDS, OSHA, DOT, or UN dangerous goods classification requires closed-cup data
In-service oil where fuel dilution is a concern
Use ASTM D92 if you are testing:
Lubricants, bitumen, or asphalt with flash points above 79 °C
Samples where the product specification or contract explicitly cites D92
Materials where fire point data is also required
High-viscosity fluids that are difficult to test in a closed cup
Default rule for procurement and QC: read the product datasheet or contract first. If it says "flash point per ASTM D93," a D92 result will not satisfy the requirement-even if the number looks close.
Instrument selection note
A Cleveland open cup flash point tester runs D92. A Pensky-Martens closed cup flash point tester runs D93. Some automated laboratory units cover one method only; confirm the standard before purchase. Automated D93 instruments with electronic flash detection are recommended for biodiesel and dark or difficult-to-observe samples.
Key takeaways
D92 and D93 measure the same property-flash point-but under different vapor conditions.
Closed cup (D93) gives lower values and is the compliance default for fuels and transport.
Open cup (D92) adds fire point capability and suits high-flash industrial liquids.
Results are not directly equivalent-always follow the method written in your specification.