backpressure regulator self-contained是什么意思?

backpressure regulator self-contained是什么意思? A n-faced regulat polyhedron consists of 2n edges and (2n-4) vertices.find the n 这是我一个香港朋友的题, 英语翻译When open- or tandem-center valves are used in circuits requiring pilot pressure to shift the valves,there must be a means of maintaining pressure when the valves are in neutral.One method is to install a backpressure check valve in the t