At given temperature and pressure, a fluid possesses a definite density, which in engineering practice is usually measured in kilograms per cubic meter. Although the density of all fluids depends on the temperature and pressure, the variation in density with changes in these variables may be small or large. If the density changes only slightly with moderate changes in temperature and pressure, the fluid is said to be incompressible; if the changes in density are significant, the fluid is said to be compressible. Liquids are generally considered to be incompressible and gases compressible. The terms are relative, however, and the density of a liquid can change appreciably if pressure and temperature are changed over wide limits. Also, gases subjected to small percentage changes in pressure and temperature act as incompressible fluids and density changes under such conditions may be neglected without serious error.

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