Petroleum product volatility metrics: How does the true vapour pressure (TVP) of a petroleum fraction compare with its Reid vapour pressure (RVP) measured by the standard Reid apparatus?

Introduction:
Volatility is a cornerstone property for handling gasoline, naphtha, and other light petroleum fractions. Two common measures are true vapour pressure (TVP) and Reid vapour pressure (RVP). This question checks conceptual understanding of how TVP compares to RVP under the standard test conditions used in fuel specification and safety practice.

Given Data / Assumptions:

• TVP is the equilibrium vapour pressure of the pure liquid hydrocarbon at a stated temperature with no air present.
• RVP is measured in a specific apparatus at 37.8°C (100°F) with a defined vapour-to-liquid volume ratio and includes dissolved/entrained air effects.
• We compare TVP and RVP for the same fuel near the same temperature reference.

Concept / Approach:
Because the Reid method uses a fixed expansion and includes a gas space that can contain air, the measured RVP tends to be slightly higher than the equilibrium pressure of the pure liquid (TVP) at the same temperature. The presence of air and the test geometry promote additional vapour formation, so RVP is generally a conservative (higher) value relative to TVP for specification and safety margins.

Step-by-Step Solution:

Verification / Alternative check:
Fuel testing guides routinely note that RVP overstates equilibrium vapour pressure; correlations exist to estimate TVP from RVP when needed for engineering calculations.

Why Other Options Are Wrong:

• Is more than / Is the same as: Opposite or rarely true; method bias makes RVP higher.
• May be either more or less than: Overly vague; the normal expectation is RVP > TVP.
• Unrelated: They are closely related measures of volatility.

Common Pitfalls:
Confusing RVP with “true” vapour pressure in process design; always confirm which metric is specified.

Final Answer:
Is less than