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

There are 2 kinds of differential pressure ranges: unidirectional and bi-directional.

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

A bi-directional pressure range starts at a negative pressure and goes to some positive pressure. For example: -2.5"WC to +2.5"WC, or +/- 2.5"WC.

Some areas require a room to have a negative differential pressure in relation to the reference area. A place you will see this set-up is in hospitals