A gas or vapor with a relative density of 2.5 will most likely be detected using a density meter or a gas detector. A density meter measures the density of a substance by comparing its mass to the mass of an equal volume of water. A gas detector is a device that detects the presence of gases or vapors in the air. Gas detectors can be used to detect a wide range of gases and vapors, including those that are flammable, toxic, or corrosive.