In a gradiomanometer tool or pressure derivative calculation, the apparent increased fluid density observed due to frictional pressure losses along the tool and casing in a fast-flowing fluid. The magnitude of the correction depends on the flow rate, tool geometry and the casing size, and is negligible in most casings below about 2000 B/D (318 m³/d). The fluid density will appear erroneously high unless this effect is corrected for.