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Lab Methods

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Automated Vapor Pressure

  • Method ASTM D5191
  • Repeatability 0.00807 (DVPE + 18.0psi)
  • Reproducibility 0.0161 (DVPE + 18.0psi)
  • Blending non-linear
  • Additive Correction No

Automated vapor pressure instruments are used to determine the total vapor pressure exerted in vacuum by air-containing, volatile, liquid petroleum products, including automotive spark-ignition fuels with or without oxygenates.

Vapor pressure is regulated by compliance agencies to ensure that product has the correct volatility characteristics for the climate it is being used in. It is also one of the main driving factors of the EPA complex emission calculation. A high rvp can negate the effects of emissions reductions in the calc. This is the reason why only the summer months are VOC controlled.

The gasoline is vaporized when it is introduced to a combustion engine to ensure a complete burn of the fuel when mixed with air. In general the higher the rvp of the fuel the more volatile it is considered to be. Using a low rvp fuel in the winter months can cause starting and drivability issues as the fuel is not volatile enough to properly vaporize. The inverse is true when using high rvp fuels during the summer months. The high rvp fuel can vaporize in the fuel lines and cause an engine to run too lean due to the lack of enough fuel being injected into the engine. The fuel can also cause a gas tank to expand and rupture if the pressure built up is high enough.

The method is performed by first air saturating and chilling the sample to 0°C(32°F). After this the automatic instrument will vacuum in a premeasure portion of sample and then air (usually at a ratio of 4 vapor : 1 sample). This mixture is then heated to 37.8°C(100°F) and the rise in pressure is measured by a transducer. This result is considered to be the PTOT. An equation is then applied to the PTOT to convert it to DVPE which is the result most often used by buyers/sellers/and compliance.