What's the difference between bi-directional and unidirectional pressure ranges?

压差范围有单向和双向两种。

A unidirectional pressure range starts at 0 differential pressure and goes to some positive pressure. For example: 0 to 5"WC.

双向压力范围从一个负压开始，到一些正压力。例如:-2.5“WC到+2.5”WC，或+/- 2.5”WC。