How is the density of liquid fuel commonly measured?

The density of liquid fuel is commonly measured using a hydrometer. A hydrometer is a device specifically designed to measure the density or specific gravity of liquids. It operates based on Archimedes' principle, floating in the liquid at a level that corresponds to the density of that liquid. The scale on the hydrometer provides a direct readout of the density, allowing for precise measurements that are critical for various applications in the petroleum industry. This tool is essential for ensuring fuel quality, calculating fuel quantities, and understanding combustion properties.

Other options mentioned do not serve the purpose of measuring the density of liquid fuels. A thermometer measures temperature, a barometer measures atmospheric pressure, and a voltmeter measures electrical potential difference. Therefore, these tools would not provide relevant information regarding the density of liquid fuels.