The fluid flow treated in this section is isothennal@ subsonic@ and incompressible. The effects of heat addition@ work on the fluid@ variation in sonic velocity@ and changes in elevation are neglected. An incompressible fluid is one in which a change in pressure causes no resulting change in fluid density. The assumption that liquids are incompressible introduces no appreciable error in calculations@ but the assumption that a gas is incompressible introduces an error of a magnitude that is dependent on the fluid velocity and on the loss coefficient of the particular duct section or piece of equipment. Fig. lA-1 shows the error in pressure drop resulting from assuming that air is incompressible. With reasonably small loss coefficients and the accuracy that is usually required in most calculations@ compressible fluids may be treated as incompressible for velocities less than Mach 0.2. At higher velocities and for large loss coefficients (Kt and 4fL/U D)@ compressible flow analysis should be used.