GB Standard Vapor Pressure Determination: Test Principles, Methods, and Laboratory Applications

GB standard vapor pressure determination measures the volatility of gasoline, crude oil, light distillates, and liquefied petroleum gas in accordance with Chinese national (GB) test methods such as GB/T 8017 and GB/T 11059. The result, typically expressed as Reid vapor pressure (RVP) at 37.8 °C, is a decisive quality and safety parameter for fuel blending, storage, transport, and regulatory compliance. Accredited third-party laboratories provide professional GB vapor pressure testing with calibrated apparatus, standardized procedures, and rapid turnaround for producers, traders, and regulatory authorities.

Technical Overview

Vapor pressure is the pressure exerted by the vapor of a liquid when it is in thermodynamic equilibrium with the liquid phase at a specified temperature. For volatile petroleum products, vapor pressure at 37.8 °C (100 °F), commonly reported as the Reid vapor pressure (RVP), is the primary indicator of evaporation tendency, boil-off losses, vapor-lock risk, and pressure build-up inside storage vessels and pipelines.

In the Chinese national standard system (GB, abbreviated from "Guobiao"), vapor pressure determination is governed primarily by the following test methods:

• GB/T 8017 — Determination of vapor pressure of petroleum products, Reid method, technically aligned with ASTM D323, applicable to liquids with a vapor pressure below 180 kPa.
• GB/T 11059 — Determination of the vapor pressure of crude oil using pressurized sampling and expansion-type measurement.
• GB/T 12576 — Determination of the vapor pressure of liquefied petroleum gas (LPG) calculated from compositional analysis.
• SH/T 0794 — Automatic mini-method for the vapor pressure of gasoline and light distillates, comparable to ASTM D5191, reporting the dry vapor pressure equivalent (DVPE).

Vapor pressure limits are embedded directly in Chinese product specifications, notably GB 17930 (motor gasoline, China VI) and GB 18351 (ethanol gasoline E10), which set seasonal maxima and minima to balance cold-start performance, hot-weather vapor lock, and evaporative emissions control.

Testing Samples

Motor Gasoline and Ethanol Gasoline

Finished gasoline and E10 ethanol blends are the most frequently tested materials. Representative sampling is critical: samples must be collected in sealed containers with a fill ratio of approximately 70–80%, leaving an air headspace, and a minimum volume of about one liter is recommended to allow for repeat determinations. Blend components such as reformate, alkylate, butane, and oxygenates are also routinely tested to support blending control.

Crude Oil

Stabilized crude oils, and in some cases live or pressurized crude samples, are tested to evaluate the concentration of light ends and the pressure behavior during storage and shipment. Because crude oil contains dissolved gases and light hydrocarbons, pressurized cylinders and strict sample-transfer procedures are required to prevent the loss of volatile components before measurement.

Light Distillates, Naphtha, and Solvents

Naphtha, light solvent oils, pentane and hexane fractions, and other low-boiling hydrocarbon streams fall within the scope of GB/T 8017 or the mini-method, provided their vapor pressure does not exceed 180 kPa. These materials are often tested to verify suitability for specific processing, packaging, or transport conditions.

Liquefied Petroleum Gas (LPG)

Commercial propane, butane, and propane-butane mixtures are evaluated against GB 11174 product requirements, in which the gauge vapor pressure at 37.8 °C is a limiting specification parameter. LPG vapor pressure is determined from composition using GB/T 12576 or by direct manometric measurement following ISO 4256.

Aviation Gasoline and Specialty Volatile Fuels

Aviation gasoline and certain specialty volatile fuels are tested using the appropriate procedure of the Reid method, with particular attention to air saturation requirements specified in the standard for these product classes.

Testing Items

Typical vapor pressure test items offered under the GB framework include:

• Reid vapor pressure (RVP) at 37.8 °C, expressed in kPa, by GB/T 8017.
• Dry vapor pressure equivalent (DVPE) at 37.8 °C, determined by the automatic mini-method SH/T 0794.
• Saturated vapor pressure of crude oil at 37.8 °C or other specified temperatures and vapor-liquid ratios, by GB/T 11059.
• Gauge vapor pressure of LPG at 37.8 °C or 45 °C, by GB/T 12576 or ISO 4256.
• Supporting determinations such as gauge calibration corrections, bath temperature verification, and sample air-saturation checks required by the methods.

Testing Methods

GB/T 8017 — Reid Method

The classical Reid method uses a two-chamber pressure bomb consisting of a lower liquid chamber (typically 125 mL) and an upper air chamber with an approximate volume ratio of 4:1, connected to a calibrated pressure gauge or transducer. The measurement is performed in a constant-temperature water bath maintained at 37.8 ± 0.1 °C. The general procedure is as follows:

• The sample is cooled to between 0 °C and 1 °C, and the sample container is shaken vigorously to saturate the air in the headspace, as required for gasoline-type samples.
• The chilled liquid chamber is filled brim-full with the cooled sample, and the pre-heated air chamber is attached; the assembled bomb is immediately inverted so that the sample wets the liquid chamber walls.
• The bomb is immersed in the 37.8 °C bath and shaken vigorously at intervals; after each shaking cycle the gauge is read, and the procedure is repeated until consecutive readings are stable.
• The stable gauge reading, corrected for gauge error, is reported as the Reid vapor pressure of the sample.

The standard defines specific procedures for different product classes, including distillate fuels, gasoline and gasoline-oxygenate blends, and aviation gasoline, with corresponding sample-handling and air-saturation requirements.

Automatic Mini-Method (SH/T 0794)

The mini-method injects a small volume of sample, typically one to a few milliliters, into a temperature-controlled vacuum chamber at 37.8 °C. The pressure developed is measured by a calibrated transducer, and the total vapor pressure is mathematically converted to the dry vapor pressure equivalent (DVPE) through a defined correlation. The method offers substantially reduced sample consumption, faster turnaround, and excellent repeatability, making it well suited to high-throughput quality control laboratories, while remaining statistically consistent with the Reid method within stated precision limits.

Crude Oil Vapor Pressure — GB/T 11059

For crude oils, the sample is transferred under pressure into a floating-piston or pressurized cylinder to preserve dissolved light ends. The measurement is performed by controlled expansion of a known volume of sample into a measurement chamber at a constant temperature, allowing the vapor pressure at a defined vapor-liquid ratio to be calculated. This approach follows internationally recognized practice of the same type as ASTM D6377 and provides realistic data for custody transfer, tank breathing assessment, and cargo compatibility evaluations.

LPG Vapor Pressure — GB/T 12576 and ISO 4256

Under GB/T 12576, the composition of the LPG sample is determined by gas chromatography, and the gauge vapor pressure at the specified temperature is calculated from the composition using established equilibrium relationships. Alternatively, ISO 4256 describes a direct manometric method using a dedicated LPG test bomb immersed in a temperature-controlled water bath.

Quality Control and Precision

Reliable vapor pressure data depend on strict metrological control:

• Pressure gauges and transducers must be calibrated and traceable to national metrology standards.
• Bath temperature is verified with certified thermometers at the required tolerance of ±0.1 °C.
• Repeatability and reproducibility limits defined in each standard are monitored through duplicate determinations and control charts.
• Accredited third-party laboratories further validate performance through regular participation in interlaboratory proficiency testing programs.

Applications

GB standard vapor pressure determination supports a wide range of technical and commercial purposes:

• Compliance certification of motor gasoline and ethanol gasoline against the seasonal vapor pressure limits of GB 17930 and GB 18351, typically 40–65 kPa in summer and 45–85 kPa in winter.
• Refinery blending optimization, in particular the control of butane and light naphtha addition to achieve the target RVP while maximizing yield.
• Storage and transport safety assessments, including pressure build-up evaluation for tanks, drums, and sealed packages of volatile liquids.
• LPG quality acceptance against GB 11174 for commercial propane and propane-butane products.
• Customs clearance, trade settlement, and arbitration in quality disputes, where standardized GB methods provide a recognized technical basis.
• Environmental management programs aimed at reducing evaporative VOC emissions from fuels and solvents.
• Research and development of new fuel formulations, oxygenate blends, and volatility-adjusting additives.

FAQ

At what temperature is the vapor pressure measured?

The standard reference temperature for GB/T 8017, GB/T 11059, SH/T 0794, and most related methods is 37.8 °C (100 °F). LPG vapor pressure may additionally be reported at 45 °C depending on the specification involved.

What is the difference between RVP and DVPE?

RVP is the gauge pressure measured directly with the classical Reid apparatus under a fixed 4:1 air-to-liquid volume ratio, and it includes the partial pressure of dissolved air and water vapor effects. DVPE is a calculated value derived from the mini-method measurement that approximates the vapor pressure of the fuel in the absence of dissolved air; product specifications state clearly which parameter applies.

How much sample is required and how should it be collected?

A minimum of approximately one liter is recommended for GB/T 8017, collected in a sealed container filled to 70–80% of its capacity. The container must be kept cool, protected from heat and sunlight, and tested as soon as possible after sampling, because any loss of light ends will bias the result low.

Which products cannot be tested by GB/T 8017?

The Reid method under GB/T 8017 applies to liquids whose vapor pressure is below 180 kPa at 37.8 °C. Liquefied gases such as LPG and very light hydrocarbons require dedicated methods such as GB/T 12576 or ISO 4256, while crude oils are better evaluated under GB/T 11059 with pressurized sampling.

How long does a typical test take?

For a properly conditioned sample, a single GB/T 8017 determination can be completed within one hour in the laboratory, and the mini-method within minutes. Including sample receipt, conditioning, and duplicate confirmation, accredited third-party laboratories generally report results within three to five working days.

Why is vapor pressure regulated in gasoline specifications?

Vapor pressure must be high enough to ensure good cold-start performance, yet low enough to prevent vapor lock, minimize evaporative losses, and reduce VOC emissions. Seasonal limits in GB 17930 and GB 18351 therefore set both upper and lower bounds, making accurate GB standard vapor pressure determination essential for product release and market surveillance.