GB/T 21789-2025 Determination of flash point — Abel closed-cup method

Summary page. This page is a compiled excerpt/summary of the standard — it is not the full standard text and does not replace it. Verify every value against the official standard and the product certificate before use.

GB/T 21789-2025 specifies the Abel closed-cup determination of flash point; this page covers the scope and precision ranges, apparatus and pressure measurement, the ignition and heating sequence, pressure correction, precision and reference-material verification of the 2025 edition.

Evidence note: original method study material for the 2025 edition, not the full text of the standard, a controlled operating procedure or a calculator. Method precision is not the uncertainty of a product batch, and independent technical review is pending.

Identity, version and scope of study: Designated original PDF physical pages 1, 4. This study material corresponds to GB/T 21789-2025, "Determination of flash point — Abel closed-cup method", issued 2025-10-05, implemented 2026-02-01, replacing the 2008 edition and adopted with modification from ISO 13736:2021. Two specific official records show it as current; the adoption relationship does not imply equivalence or product applicability. This text is for understanding the method and the boundaries of its results in this edition and cannot replace the controlled standard, instrument documents or safety procedures.

Scope and precision ranges: Designated original PDF physical pages 4, 9. The method applies to flammable liquids with a flash point of −30.0 °C to 75.0 °C and does not apply to water-borne paints. The scope of the method and the precision range of the cooperative study are separate: automatic equipment −8.5 °C to 75.0 °C, manual equipment 20.0 °C to 70.0 °C; ranges wider than these may affect precision, and the whole method range cannot be regarded as having established r/R. The manual thermometer range must also be checked against the particular thermometer and the original annex. Range | Temperature range °C | Note Scope of the method | −30.0 to 75.0 | excludes water-borne paints Precision, automatic equipment | −8.5 to 75.0 | under the conditions of original clause 14 Precision, manual equipment | 20.0 to 70.0 | not extrapolated to the whole method range

Principle and recognition of the flash: Designated original PDF physical pages 5, 8. The ignition source is introduced into the closed cup at the specified temperature intervals; stirring is stopped at ignition, the temperature at which a flash appears is recorded and the specified pressure correction applied. A blue halo near the ignition source is not a flash. The reference pressure for the result is 101.3 kPa, and the original criteria are not replaced by a screen colour or an automatic prompt.

Apparatus, ignition source and pressure measurement: Designated original PDF physical pages 6, 15. The original edition covers manual and automatic equipment; where nothing else is agreed, disputes are arbitrated by the manual method. The barometer measures absolute pressure and must be accurate to 0.5 kPa. Annex A.2.5 gives a heating rate of 0.75 °C/min to 1.25 °C/min for automatic equipment. The complete cup, ignition source, installation dimensions and software control must be verified against the original annexes and instrument documents; this text does not provide a complete set of manufacturing or installation specifications.

Sample reception and representativeness: Designated original PDF physical page 7. The sample container is filled to 85 %–95 % of its capacity; if a second test is needed, at least 50 % of the sample should remain in the container. Free water is observed on a separate portion of sample taken before mixing. Where free water cannot be separated physically, the original directs the user to GB/T 5208, and no substitute operation is guessed here. Sampling, volatile loss and the condition of the container may affect the result.

Cooling and preparation before the test: Designated original PDF physical pages 6, 7. The water bath depth is at least 38 mm. Original 8.4.1 (water bath) and 8.4.2 (solid bath) require cooling to the expected flash point minus 9 °C or −35 °C, whichever is higher; original 9.4/10.2 for the sample uses the expected flash point minus 17 °C or −35 °C, whichever is higher. The heading of 8.4.3 and the wording "sample temperature" must be verified separately, and bath, cup and sample must not be reduced to a single cooling condition. These are method conditions and cannot serve as product storage temperatures.

Ignition, heating and stirring: Designated original PDF physical page 8. The specified pre-ignition is carried out before heating; if a flash occurs, the run is stopped, discarded and restarted with a lower expected flash point. From the first ignition to the end the heating rate is about 1 °C/min, with stirring clockwise at 30 ± 5 r/min, stopped during ignition. The first ignition temperature is the expected flash point minus 9 °C or −35 °C, whichever is higher, with the shutter open for about 2 s and closed for about 1 s; a flash at the first ignition means the run is discarded and repeated. Original 11.5 refers to 11.6 for the first ignition, whereas 11.6 actually covers stirring and 11.7 covers ignition; the cross-reference connection is left for verification.

Subsequent observation and end point: Designated original PDF physical page 8. Ignition is then applied at each 0.5 °C rise; a genuine flash must be distinguished from a blue halo. If a flash occurs below −30 °C, it is recorded, reported and the run stopped as in the original, and the result is not forced into the precision range of this edition. The sequence of operations and the handling of anomalies must be read together with the controlled procedure.

Pressure conversion and result correction: Designated original PDF physical page 8. hPa or mbar multiplied by 0.1 gives kPa, and mmHg multiplied by 0.133322 gives kPa. The original equation is Tc = Td + 0.25×(101.3 − P), where Tc is the corrected result and Td the observed temperature. Original 12.2 defines P as the pressure at 0 °C in kPa, while the pressure-recording note states that no ambient-temperature correction is made; the connection between the two conventions still awaits technical verification, and no temperature-correction algorithm is added. The pressure applicability range of 98.0 kPa to 104.7 kPa noted with the correction equation and the gas-ignition exception are retained. No calculator is provided. Original unit | Conversion to kPa hPa | × 0.1 mbar | × 0.1 mmHg | × 0.133322

Precision and expression of results: Designated original PDF physical pages 9, 19. Original clause 14 gives, within the specified range, a repeatability r = 1.4 °C and a reproducibility R = 3.2 °C at the 95 % precision level. An F-test at the 95 % level found no significant difference between manual and automatic or between gas and electric ignition results, which does not mean that any equipment and sample are unbiased. Routine results are rounded to 0.5 °C and verification determinations recorded to 0.1 °C. The method r/R is not the certificate uncertainty of a particular product batch. Use | Original accuracy or rounding Routine flash point report | 0.5 °C Verification determination record | 0.1 °C

CRM, SWS and verification frequency: Designated original PDF physical pages 19, 20. Annex D is normative. A CRM should have a certified value established by the corresponding method, an uncertainty and a statement of traceability; the assigned value and the way an SWS was established should be verified, and a material must not be treated as a CRM because of its file name or material name. Verification with a CRM is required at least every 12 months, with continued checks in between; the original recommends two CRMs/SWS covering the measurement range and repeated determinations. No particular model, catalogue number or batch document is promised to exist, be obtainable or be available.

Verification acceptance limits and anomalies: Designated original PDF physical pages 7, 19, 20, 21. Annex D.4 uses |x−μ| ≤ R/√2 for a single result and |x̄−μ| ≤ R1/√2 for the mean of several results, where R1 = √[R²−r²(1−1/n)], n being the number of repeats, and with the premise that the uncertainty of the reference value is small relative to R. The connection between original 8.5, "the result obtained shall be equal to or less than … R/√2", and the difference definition of D.4 awaits review; the root in the denominator of the acceptance limit cannot be omitted, and the verification difference must not be used as a flash point bias correction. An SWS failure requires the CRM, the records and the SWS to be re-checked and handled as specified, and the failure must not be ignored automatically. Original D.3.3 cites SWS (D.2.2) whereas D.2.2 is the CRM and D.2.3 the SWS; the reference awaits review.

Reporting and informative quality control: Designated original PDF physical pages 9, 20, 22, 23. The report includes the method, sample identification, ambient pressure, result, deviations and test date. Annex E is informative, and the quality-control material selection and approximate flash points it gives are not assigned values or product guarantees. Annex D, Table D.1 prints "propylene glycol 12.1 °C", and the technical consistency between the material name and the listed value still awaits verification; the name must not be replaced from experience or a product recommended. Annexes A–D are normative and E informative, and the two must not be confused.

Source range, unverified items and limits of use: Designated original PDF physical pages 4, 15, 16, 17, 18, 19, 20, 21, 22, 23. The 24 physical pages of the designated 2025 original were used for orientation, with particular checks on the scope, ignition/pressure, precision and Annexes D/E; a review of the range is not a full technical audit. The complete annex instrument dimensions, the cross-page thermometer tables, the electrodes/sensors and automatic equipment software, safety, reference-material values and conflicts with the original references still require technical review and independent acceptance. The original is read-only and scans are not published; no product applicability, storage, supply or batch certificate value is inferred.

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