What does specific gravity measure when applied to gas?

Specific gravity when applied to gas measures the weight of a gas volume compared to that of air. This concept is essential because it indicates whether a gas is lighter or heavier than air, which influences how the gas behaves in different environments. For instance, if a gas has a specific gravity less than 1, it will rise in the air, while a gas with a specific gravity greater than 1 will sink.

Understanding specific gravity is particularly important in plumbing and HVAC applications, as it impacts gas flow, ventilation requirements, and safety protocols. Knowing whether a gas is lighter or heavier than air directly affects how it disperses and can inform decisions about gas leak detection and ventilation systems.

The other options do not accurately reflect what specific gravity means in relation to gas. The measurement of volume relative to water is not applicable here since specific gravity for gases compares them to air. The temperature difference and pressure in relation to a vacuum are also unrelated to the concept of specific gravity concerning gas measurements.