Reid Vapor Pressure (RVP) is an empirical laboratory test measurement (ASTM D323 or ASTM D5191) conducted at a fixed temperature of 37.8°C (100°F) in a closed chamber with a 4:1 vapor-to-liquid volume ratio. True Vapor Pressure (TVP), on the other hand, is the actual physical thermodynamic equilibrium vapor pressure exerted by the liquid mixture at its true ambient storage temperature (which might be -10°C in winter or 45°C in a desert tank farm). TVP dictates actual volatile organic compound (VOC) emissions, flash fire risks, and pump cavitation.

EPA AP-42 Section 7.1 correlates TVP through semi-empirical Antoine-type log relations: ln(TVP) = A - B / (T + 459.67), where A and B are empirical polynomial functions of RVP, distillation slope (S), and stock hydrocarbon category (refined motor gasoline, crude oil, or natural gasoline). For crude oils, dissolved light ends (methane, ethane, propane) cause a steeper vapor pressure response with temperature compared to stabilized finished gasolines.

Atmospheric storage tanks (cone roof or floating roof) are designed for hydrostatic liquid containment with minimal internal gas overpressure (typically only 2 to 6 inches of water column above atmospheric pressure). If hot summer solar radiation raises liquid surface temperature such that TVP approaches or exceeds local barometric pressure (101.3 kPa / 14.7 psia), the liquid begins bulk boiling. The violent volume expansion of boiling vapors overwhelms conservation vents, causing tank over-pressurization, roof seal blowout, or catastrophic fire.

The distillation slope S = (T_15% - T_5%) / 10 is an indicator of product volatility distribution. For motor gasolines, ASTM 10% evaporation temperature typically ranges from 45°C to 65°C. A steeper distillation slope means a higher concentration of highly volatile butane and pentane fractions, which produces a much higher TVP for a given 100°F RVP measurement at lower temperatures.

In internal or external floating roof tanks, a buoyant pontoon deck rests directly on the liquid hydrocarbon surface, eliminating the large vapor headspace present in fixed-roof tanks. Mechanical shoe seals, liquid-filled wipers, or secondary elastomeric seals bridge the annular gap between the floating deck and tank shell. When TVP rises, vapor pressure under the deck increases, but emissions are restricted to narrow rim seal clearances, reducing VOC losses by 95% to 98% compared to fixed-roof breathing cycles.