问答题 翻译：The most common means measuring rate of flow of a fluid is based on the principle that if there is a contraction or constriction (a section of reduced diamater) in a pipe,the pressure of a fluid flowing down-stream from the contraction will be lower than that up-stream from the contraction.

问答题 翻译：There are special types of valves designed to be operated by control instrument rather than by hand.

问答题 翻译：Exploration methods falling in this category are those employing a physical measurement of subsurface conditions made from a surface location.